Calendar

Title: The denser the hotter: Emergent properties of reactive active matter

Abstract: Synthetic active matter provides a powerful framework for studying collective behaviour in controlled environments. Granular gases, in particular, offer an alternative to self-propelled particle systems. However, research on granular gases has primarily focused on vibrated beds, making it difficult to assess the universality of granular gas properties. Here, we introduce a novel granular gas driven by contact charge electrophoresis, revealing a range of collective behaviours previously unstudied. Notably, we observe a unique granular gas phase where the kinetic energy per particle increases with density due to super-elastic collisions, an effect not seen before. This property diminishes at higher densities, and the system exhibits behaviour akin to vibrated granular gases while exhibiting signs of hyperuniformity. These findings highlight the critical role of energy injection and dissipation mechanisms in granular gases and pave the way for exploring a broader class of such systems.

Title: Active hydraulics

Abstract:  We explore the behavior of active fluids confined in microchannel networks, demonstrating how they deviate from the predictable laws of classical hydraulics, which govern viscous, laminar flows. While traditional systems like blood vessels and porous media are described by robust, linear laws, active fluids exhibit bistable and non-deterministic dynamics. Through a combination of experiments, simulations and theory I will show how to  build a general framework for predicting the geometry of active-hydraulic flows in arbitrary networks.
First, focusing on square-grid networks, experiments with colloidal rollers reveal that the degenerate flows of active fluids correspond to configurations of the six-vertex model. This quantitative correspondence enables us to predict and control the Lagrangian trajectories of active particles, which form completely packed loops, as described by the Baxter-Kelland-Wu framework. This experiment teaches us how to map active-hydraulic flows on vertex physics and and how to lay out the first free rules obeyed by all active hydraulic flows.
We then show that a crucial additional rule is required to account for geometry of active flows in network having an odd coordination number. We focus on trivalent networks and show show that active hydraulic flows act as dynamical spin-1 ices, resulting in degenerate streamline patterns. These patterns split into two geometric classes of self-similar loops, reflecting the fractionalization of topological defects at subchannel scales. From our observation we explain the interaction rules between the vertices that define active-hydraulic flows.
Our findings offer new perspectives on active fluid dynamics and provide a foundation for the design of microfluidic devices and the study of biological transport in complex environments.

Title: Salt-rimmed glass: is it a creep or is it a weirdo?

Abstract: Confined evaporation of multiphase multicomponent systems has recently garnered significant attention not only due its numerous practical applications, but also as a model system to study complex interface growth and interfacial self-assembly dynamics. Here, we focus on a canonical system for confined evaporation – evaporation from one end of a liquid-filled capillary. In the first part of the talk, I’ll focus on the evaporation of a binary fluid mixture, where the composition-dependent evaporation kinetics was found to exhibit three different temporal regimes. Along the way, we revisit the age-old question: “Is evaporation really a slow process?” In the second part, we focus on the evaporation of salty liquids, where the liquid front exhibits non-monotonic dynamics, unlike multifluid systems. Curiously, we further observe that evaporation, in fact, is not the dominant mechanism of liquid loss in such systems. Finally, we will also attempt to answer the question: “Do salt crystals experience rush hour?”

Title: Exploring Geometric Frustration in Mechanical (Meta)material via a Generalized Elasticity Theory

Abstract: Mechanical metamaterials have been studied extensively to explore their unusual properties owing to instabilities arising from their micro-structures. Our simple continuum framework for the mechanics of a metamaterial thin sheet shows that in the Föppl–von Kármán limit, the elasticity of the sheet screens the curvature induced stress through soft deformation modes. We use this approach to explore the characteristic shapes of a metamaterial assembly system which is geometrically frustrated. In addition to the Föppl–von Kármán number, which dictates the competition between bending and stretching modes in a traditional elastic sheet, we show the existence of novel dimensionless numbers which dictate how the soft modes affect this competition and give rise to anomalous equilibrium shapes. Through this continuum model, we propose and uncover an anomalous phase of energy expenditure for a solid material which, in the course of screening dilational strains, demonstrates an energy cost which is akin to orientational strains in Liquid crystalline membranes.

Title: Detecting Local Pressure Gradients in the Viscous Fingering Instability

Abstract: Fluid fingers form when a lower viscosity fluid penetrates a higher viscosity one within a confined geometry such as in the gap between two parallel circular glass plates. This system has become iconic to the study of pattern formation because the elegant branching patterns that form are similar to those seen pervasively in nature. However, as these fingers grow their morphology is less well understood. For both miscible and immiscible fluid pairs the growth dynamics change after the instability onset, and global pattern features are governed by the viscosity ratio. Using a novel experimental technique we observe a new length scale related to the penetration depth of local pressure gradients both behind and in front of the interface at late times. We characterize this length scale and show the surprising validity of a simple model that was previously proposed to describe late-time pattern growth.

Title: Effective thermal equilibrium induced by crosslinking proteins in polymer chromosome model

Abstract: Biological systems under the influence of microscale active agents such as proteins are frequently modeled using stochastically-switching forces as the agents shift between different states.  These rapidly switching forces are often on timescales faster than the time to reach thermal equilibrium, thus the system is in a constant state of disequilibrium.  In one example system, a bead-spring polymer model of chromosomes with additional crosslinking stochastically-switching forces, slow crosslinking leads to no clusters, fast crosslinking produces rigid slowly changing clusters, while intermediate timescales produce flexible clusters that mediate gene interaction, consistent with experimental results. By mathematically analyzing different relative scalings of the two sources of stochasticity, thermal fluctuations and the force induced by the transient crosslinks, we predict these three distinct regimes of cluster behavior. In the two regimes with clusters, we find an effective energy or quasipotential for the system that accurately predicts the lifetime of clusters.

Title: Phase diagram and confining strings in nematopolar matter

Abstract: We investigate a minimal model of a nematopolar system. We analytically uncover a phase diagram consisting of a phase where the polar order and nematic order are locked, and unlocked phases which could be ordered or disordered. In particular, we develop two complementary perspectives on the locked phase: (i) the nematic order induces polar order, (ii) in the locked phase, all 1/2 integral nematic topological charges are confined. In particular, the cores of polar +1 defects fatten from a point to a string with constant tension and confine a pair of nematic +1/2 defects at the ends.

Title: Characterization of Fluctuations in an Active Crystal

Abstract:  This thesis investigates displacements observed in a unique type of active matter where particles are not self-propelled. By combining a molecular crystal with contact charge electrophoresis, we create a macroscopic active crystal, resulting in oscillatory particles whose behavior can be finely tuned under specific parameters. Notably, these particles exhibit intriguing displacements that do not occur in isolated oscillators. Our study identifies these displacements as arising from complex interactions between particles, not thermal effects typical of crystals but rather resembling an active spring-like system. We propose an antiferromagnetic model to characterize these phenomena, enabling precise control using intrinsic physical parameters in our experimental setup. This research contributes to the broader understanding of active matter systems, opening new avenues for exploring particle interactions and dynamic behaviors in non-equilibrium conditions.

Title: Modelling the Collective Migration in Tumoroids

Abstract: Understanding the collective migration in tumoroids, in-vitro tumour spheroids, provides insights into breast cancer metastasis. Recent observations by Schmidt et al. have revealed their intriguing mechanical and morphological properties. Here, we investigate the morphology, dynamics and mechanics of tumoroids using particle-based models of active dividing soft spheres. While cell motility has been modelled extensively using Active Brownian Particles (ABPs), tumoroids lack a 2D substrate and must obey action-reaction. We propose such a reciprocal activity, Pair ABP, and implement the extracellular matrix (ECM) as a soft passive gel. Pair ABP activity allows for cell-cell and cell-ECM "crawling". Simulations with SAMoS and wrapper cellSAMoS enable automated model execution, analysis, and visualisation. First, without the ECM, we compare ABPs to Pair ABPs and find 3D tumoroid reconfiguration driven by cell division and motility. We then study the clustering and glassy dynamics of a 2D plane filled with polydisperse active Pair ABPs. Upon implementing the 3D ECM gel, we find an asymmetric balance of cell division and ECM secretion. By varying the ECM density, cell-ECM stiffness, and cell motility, we analyse the mixing of an active-passive particle collective and spheroid unjamming. Our latest simulations resemble radial pressure and shear profiles from experiments. Overall, we demonstrate the potential of active matter for refining our understanding of tumoroid multicellular migration and potentially informing cancer treatment strategies.

 Title: Disorder and order of cell packing in developing fly wing

Abstract: Tightly packed epithelial layers of cells are one of the main types of biological tissues. During tissue development, cellular packings commonly remains disordered, with a broad distribution of cell sizes. This pattern is also evident during the development of the wing of the fruit fly D. melanogaster. We analyze the dynamics of individual cells in developing fly wing over 13 hours of development and find that a simple model of cell cycle can explain the observed cell size distribution with no fitting parameters. Therefore, the polydispersity of cell sizes can be fully accounted for by the cell cycle. Strikingly, at a later stage of development, as cell proliferation halts, cellular packing transitions to an ordered hexagonal packing. We propose that this disorder-to-order transition is controlled by the reduction of cell size polydispersity, which we also observe in the experimental data. To explore this hypothesis further, we investigate this disorder-to-order transition in the vertex model of epithelial tissues, which allows us to interpret experimental observations. Finally, we explore mechanically and genetically perturbed fly wings and we find that the only case in which no transition to ordered state is observed corresponds to a perturbation where cell size polydispersity remained high. In summary, our work shows that natural progress of cell cycle maintains high cell size polydispersity, and corresponding disordered cell packing in developing fruit fly wing. However, upon sufficient decrease in cell size polydispersity cellular packing transitions to an ordered, hexagonally packed state.

Title: Phase separation in the presence of active chemical reactions

Abstract: In recent years, many efforts have been devoted to characterizing liquid mixtures composed of molecules that interact and undergo chemical reactions, such as the cell cytoplasm or pools of short polymers that mimic the primordial soup. In such mixtures, a key role is played by condensates that form via phase separation. In this talk, I will review a mean-field model capable of describing phase separation kinetics in the presence of chemical reactions. I will focus on reactions that are kept away from equilibrium by continuous turnover of chemical fuel. This framework predicts the existence of out-of-equilibrium stationary states composed of spherical shells, which have recently been observed in experiments using active coacervates. In conclusion, I will discuss the impact of active chemical reactions on the scaling of condensate size and their coarsening behaviour.

Title: The Geometric Basis of Epithelial Convergent Extension

Abstract: Animal development requires large numbers of cells
to choreograph their force generation in order to sculpt tissues and
organs. Leveraging the fact that cellular forces equilibrate rapidly
compared to the speed of development, we formulate a geometrical model
for the network of balanced active tensions in an epithelial sheet.
Within this framework, we can investigate how cells remodel the tension
network to change tissue shape. A simple "winner-takes-all" mechanical
feedback loop can self-organize complex cell movement, matching
experimental data on the cell and tissue scale. We find that the ability
to self-organize depends on initial order in the cellular packing. Our
model explains how genetic patterning, embryo geometry, and cellular
packing geometry combine to determine tissue shape change.

Title: Combined multi-scale modeling and experimental study of regulation mechanisms of shape formation during tissue development

Abstract: The regulation and maintenance of an organ’s shape and structure is a major outstanding question in developmental biology. The Drosophila wing imaginal disc serves as a powerful system for elucidating design principles of the shape formation in epithelial morphogenesis. Yet, even simple epithelial systems such as the wing disc, are extremely complex. A tissue’s shape and structure emerge from the integration of many biochemical and biophysical interactions between proteins, subcellular components, and cell-cell and cell- extracellular matrix (ECM) interactions. Results obtained using iterative approach combining multi-scale computational modelling and quantitative experimental approach will be used in this talk to discuss direct and indirect roles of subcellular mechanical forces, nuclear positioning, cell growth and division and interaction with ECM in shaping the major axis of the wing pouch during the larval stage in fruit flies, which serves as a prototypical system for investigating epithelial morphogenesis. The research findings demonstrate that subcellular mechanical forces can effectively generate the curved tissue profile, while extracellular matrix is necessary for preserving the bent shape even in the absence of subcellular mechanical forces once the shape is generated [1]. The developed integrated multi-scale modelling environment can be readily extended to generate, and test hypothesized novel mechanisms of developmental dynamics of other systems, including organoids that consist of several cellular and extracellular matrix layers. [1]  Kumar et al., "Balancing competing effects of tissue growth and cytoskeletal regulation during Drosophila wing disc development", Nature Communications 15, 2477 (2024), https://doi.org/10.1038/s41467-024-46698-7.

Title: Pulsating active matter

Abstract: We propose two types of diffusive models where the
internal states of particles are subject to a periodic drive. In the
first model, inspired by biological tissues, the activity of dense
repulsive particles drives periodic change in size. We show that the
competition between repulsion and synchronization triggers an
instability which promotes a wealth of dynamical patterns, ranging from
spiral waves to defect turbulence. In the second model, the discrete
symmetry of states enforces an effective energy landscape which can
counteract the drive to arrest the dynamics, and leads again to the
emergence of spiral waves. In both models, using analytical
coarse-graining, we rationalize the emergence of dynamical patterns from
the breakdown of a specific gauge invariance at the hydrodynamic level.
[Refs: PRL 131, 238302 (2023); arXiv:2310.14370]

Title: Patterning, waves and synchronization in arrays of cilia

Abstract: Living organisms rely on flows to perform essential functions that range from swimming and feeding in unicellular organisms to mucus clearance in humans. These large-scale flows are generated by the inte­grated activity of thousands of microscopic beating filaments attached to cell surfaces (cilia). In cells, collections of cilia exhibit highly coordinated temporal patterns known as metachronal waves. While patterns of cilia temporal coordination have been observed for decades, the mechanisms underlying their formation and their contribution to flow generation remain unclear. The lack of measurements of the geometric and dynamic properties of cilia arrays has limited our ability to understand the mechanisms of pattern formation. In my talk I will discuss the advantages of ciliated swimmers as experimental model systems where such measurements can be readily performed. Performing precise measurements and perturbations of temporal patterning in cilia arrays will enable the identification of the physical mechanisms underlying the emergence of collective behavior. Beyond their biological significance, arrays of cilia provide an accessible experimental platform to explore the physics of multi-scale pattern formation.

Title: Programmable self-assembly of nanotubes using DNA origami colloids

Abstract: Nature is replete with self-assembled materials that have one or more self-limited dimensions, including shells, tubules, and fibers. Despite significant advances in making nanometer- and micrometer-scale subunits, the programmable assembly of similar self-limiting architectures from synthetic components has remained largely out of reach, mainly due to difficulty in tuning the valence, binding angle, and interaction specificity, simultaneously. In this talk, I will show how we create geometrically programmed subunits using DNA origami and study their assembly into tubules with a self-limited width. We show that the width and the helicity of tubules can be tuned by changing the local curvature encoded in a single subunit. Exploiting the programmability of our system, we further test the tradeoffs between two paradigms of self-limited assembly: self-closure through programmed curvature and addressable assembly through programmed, specific interactions. By increasing the number of unique subunits in the system, the allowed combination of width and helicities of the tubules decreases, leading to more specific tubule geometries. In an extreme case, we use 16 unique subunits to assemble a tubule, leading to almost a full selectivity of the tubule geometry. In this talk, I will also describe an inverse design method for assembling periodic tilings, which we used to generate the interaction matrix necessary to assemble the multicomponent tubules.

Title

A journey at the interface ofphotoresponsive liquid crystal systems: from propelling droplets to motile,soft microrobots.

Abstract

Movement is a hallmark of life aswe know it. Both abiotic and living systems propel at the microscale by overcomingfriction on land or displaying non-reciprocal motion in fluid (Purcell’sscallop theorem). While several modes of locomotion are well established inmicroorganisms, artificial micro- walkers or swimmers are currently facingmultiple challenges to achieve purposeful deformations and, therefore,motility. Liquid crystals (LC), ensembled of self-assembled molecules withanisotropic orientational order, have been exploited as synthetic mimics oflife forms to unravel the key principles of microscopic locomotion, while concomitantlyreproducing and engineering other complex processes such as shape-change andcommunication.

In this talk, I will discuss howthe degree of polymerization drastically affects the behavior of LC-basedsystems, from LC droplets to LC elastomers (LCE). By coupling these systemswith photo-responsive molecules at their interfaces, we investigated theemergence of morphogenesis and motility under different light conditions. Photo-inducedmultimodal deformations are encoded in an LCE-based soft microrobot, including non-reciprocalmotions, allowing walking on land and, at the same time, swimming in water,solving a grand challenge in advancing the field of soft microrobots andimparting it a whole class of new applications, including micromanipulation andcargo-transportation in confined spaces.

Title: Hydrodynamic Hamiltonians of active two-dimensional fluids

Abstract: I will describe two biologically inspired systems that can be analyzed using the same hydrodynamic Hamiltonian formalism. The first is ATP synthase proteins, which rotate in a biological membrane. The second is swimming micro-organisms such as bacteria or algae confined to a two-dimensional film. I will show that in both cases, the active systems self-assemble into distinct structural states – the rotating proteins rearrange into a hexagonal lattice, whereas the micro-swimmers evolve into a zig-zag configuration with a particular tilt. While the two systems differ both on the microscopic, local interaction, as well as the emerging, global structure, their dynamics originate from similar geometrical conservation laws applicable to a broad class of fluid flows. I will then show experiments and simulations in which the Hamiltonian is perturbed, leading to different and surprising steady-state configurations.

Bio-inspired Gaussian Morphing: From inflatable fabrics to synthetic fin rays

What causes pinecones to open and close? How do lily petals change shape as they bloom? General morphing of such surfaces into complex 3D shapes involves modifying in-plane distances (i.e., metric distortion). Cartographers have long understood this, as continents are inevitably distorted on flat maps of Earth. Carl Gauss further formalized this geometric constraint in his seminal theorem, referred to here as Gaussian morphing. In natural structures, metric changes result from differential growth, while engineered systems use nonhomogeneous transformations like hydrogel swelling, liquid crystal elastomer relaxation, or origami tessellation. However, simply imposing metrics is not enough to set the final shape precisely; additional control of local bending is necessary to select between different isometric shapes. In this talk, I will present four strategies for shape morphing: (1) inflatable zigzag meso-structures with biaxial active stretches; (2) pneumatic Gaussian cells with non-symmetric cross-section channels; (3) self-folding structures along curved creases via a classical bilayer effect; (4) fish fin inspired composite beam combining high flexural stiffness and high morphing efficiency. Central to all these strategies is the interplay between geometry and elasticity, offering avenues to manipulate both metric and/or bending for on-demand shape transformations.

Active Elasticity in non-reciprocal robotic metamaterials


Abstract:  
 
Non-reciprocal interactions in active elastic media cause work cycles and wave propagation forbidden in equilibrium. These linear phenomena offer a route to designing autonomous materials that spontaneously crawl, roll or swim. Yet these same phenomena also render non-reciprocal materials hard to design, and force us to reckon with active elastodynamics beyond the linear regime.
 
In this talk I will describe our current work on rationally designing non-reciprocal materials made of robots, and modelling their collective dynamics. First I will show that odd elasticity, the continuum hallmark of microscopic non-reciprocity, emerges in a broad range of lattices made of non-reciprocal springs. Strikingly however, we find that the strength of odd response strongly depends on the precise lattice geometry. Hyperstatic lattices are needlessly hard to actuate, leading to sub-optimal odd response. By contrast, we find that in overly floppy lattices, zero modes couple to microscopic non-reciprocity, destroying odd moduli entirely. By avoiding these pitfalls, we identify optimal design principles for building odd lattices.
 
After connecting microscale non-reciprocity to macroscale elasticity, I will then present a continuum model of nonlinear odd elasticity, benchmarked against microscopic simulation and table-top experiments. Combining non-reciprocity and non-linearity in momentum-conserving materials yields long-wavelength instabilities and travelling nonlinear patterns. Strikingly, momentum conservation causes these emergent patterns to coarsen over time. As a result, these active metamaterials spontaneously rid themselves of disorder in favour of coherent motion. We then explore how this coarsening can respond to environmental stimuli, leading to a toolkit of distinct patterns for designing locomotion and actuation.

Title: Self-organization of the cytoplasm in early embryogenesis

Title: Quantified membrane biophysics by means interfacial water imaging

Abstract: Although recognized as the key ingredient of life, water is usually treated as a background for biology. However, as a solvent, water is a crucial mediator of chemical change and determines the structure of cell and organelle membrane and likely also that of dissolved (macro)molecules. The study of lipid membranes is generally pursued by following either a top-down approach, introducing labels to living cell membranes or a bottom-up approach with well-controlled but over-simplified membrane monolayer or supported membrane models. In the first approach molecular level hydration information is lost, while in the second approach the connection with real bilayer membranes is limited.
We present an alternative route that ultimately envisions bringing together both top-down and bottom-up approaches. By using intermediate nano-, micro- and macroscale free-floating membrane systems in combination with novel nonlinear optical spectroscopy and imaging methods, we advance the understanding of realistic membranes on a more fundamental level, yet allowing for the complexity of living systems [1]. In this presentation I will introduce high throughput wide-field second harmonic imaging, which enables the label-free imaging of interfacial (< 1 nm thick) water [2], with a spatial resolution of ~370 nm and using ~100 ms acquisition times per image. I will also compare it to the more familiar method of resonant two photon imaging and second harmonic generation using fluorophores as probes. We obtain information about the orientational order of water and use this interfacial response to create spatiotemporal transmembrane potential maps of free-standing lipid membranes [3,4,5], giant unilamellar vesicles [6] or living mouse brain neurons [7]. These maps are used to quantify membrane-water interactions, which show surprisingly heterogeneous behavior that can be used to shed new light on processes such as ion transport, ion channel operation and membrane deformation.

[1] - Chemistry of Lipid Membranes from Models to Living Systems: A Perspective of Hydration, Surface Potential, Curvature, Confinement and Heterogeneity, Halil I. Okur, Orly B. Tarun, S. Roke, J. Am. Chem. Soc., (2019), 141, 31, 12168.
[2] - Optical Imaging of Surface Chemistry and Dynamics in Confinement, C. Macias-Romero, I. Nahalka, H. I. Okur, S. Roke, Science (2017) 357, 784.
[3] - A label-free and charge-sensitive dynamic imaging of lipid membrane hydration on millisecond time scales, O. Tarun, C. Hannesschläger, P. Pohl, and S. Roke, Proc. Nat. Acad. Sci. USA (2018) 115, 4081.
[4] - Transient domains of ordered water induced by divalent ions lead to lipid membrane curvature fluctuations, O.B. Tarun, H.I. Okur, P. Rangamani, S. Roke, Commun. Chem. (2020) 3 (1), 1-8.
[5] - Spatiotemporal Imaging of Water in Operating Voltage-Gated ion Channels Reveals the Slow Motion of Interfacial Ions, O. B. Tarun, M. Y. Eremchev, A. Radenovic, and S. Roke, Nano Lett. (2019), 19, 7608.
[6] - Ion induced transient potential fluctuations facilitate pore formation and cation transport through lipid membranes, D. Roesel, M. Eremchev, C. S. Poojari, J. S. Hub, and S. Roke, J. Am. Chem. Soc. (2022), 144, 51, 23352.
[7] – Membrane water for probing neuronal membrane potentials and ionic fluxes at the single cell level, M. Didier, O. Tarun, P. Jourdain, P. Magistretti, S. Roke, Nat. Commun. (2018), 9, 5287

Title: Size-dependent transition from steady contraction to
waves in actomyosin networks with turnover

Abstract: Actomyosin networks play essential roles in many
cellular processes, including intracellular transport, cell division
and cell motility, and exhibit many spatiotemporal patterns. Despite
extensive research, how the interplay between network mechanics,
turnover and geometry leads to these different patterns is not well
understood. We focus on the size-dependent behaviour of contracting
actomyosin networks in the presence of turnover, using a
reconstituted system based on cell extracts encapsulated in
water-in-oil droplets. We show that the system can self-organize
into different global contraction patterns, exhibiting persistent
contractile flows in smaller droplets and periodic contractions in
the form of waves or spirals in larger droplets. The transition
between continuous and periodic contraction occurs at a
characteristic length scale that is inversely dependent on the
network contraction rate. These dynamics are captured by a
theoretical model that considers the coexistence of different local
density-dependent mechanical states with distinct rheological
properties. The model shows how large-scale contractile behaviours
emerge from the interplay between network percolation, which is
essential for long-range force transmission, and rearrangements due
to advection and turnover. Our findings thus demonstrate how varied
contraction patterns can arise from the same microscopic
constituents, without invoking specific biochemical regulation,
merely by changing the system geometry.

Title: Interplay between lipid membrane mechanics and proteins' diffusion, clustering and function

Abstract: Cell membranes are highly deformable and have to be strongly curved, for instance upon trafficking when small buds form and eventually detach from cell membranes. Membrane-shaping processes always require proteins, in particular proteins with intrinsically-curved shapes or transmembrane proteins with conical shapes. Moreover, since cell membranes are fluid, proteins can diffuse on/in membranes, which allows them to redistribute depending on membrane shape changes. In vitro membrane systems with controlled curvature, combined to theoretical models, have been instrumental for understanding the rich interplay between membrane shape/tension, protein distribution and lateral diffusion. I will summarize some results we have obtained with in vitro membrane systems and reconstituted trans-membrane protein on membrane curvature-induced protein sorting, on protein diffusion and clustering and on the effect of membrane curvature on transport activity.

Title: Membrane-mediated cooperativity of proteins and particles

Abstract: Besides direct protein-protein interactions, indirect interactions mediated by membranes can play an important role for the assembly and cooperative function of proteins in membrane shaping and adhesion. Also particles that adhere to membranes or are trapped in membrane contact zones experience such membrane-mediated interactions. In my talk, I will  address the origin and relevance of these indirect interactions, with a focus on three cases: 
(1) The intricate shapes of biological membranes are generated by proteins that locally induce membrane curvature. Indirect curvature-mediated interactions between these proteins arise because the proteins jointly affect the bending energy of the membranes.  These curvature-mediated interactions are attractive for arc-shaped proteins and a driving force in the assembly of the proteins during membrane tubulation. 
(2) Cell adhesion results from the binding of receptor and ligand proteins that are anchored in the apposing cell membranes. The binding of these proteins depends on the shape and shape fluctuations of the membranes on nanoscales, which leads to binding cooperativity and to the segregation of long and short receptor-ligand complexes in cell adhesion zones.
(3) Particles are wrapped spontaneously by membranes if the adhesive interactions between the particles and membranes compensate for the cost of membrane bending. The interplay of adhesion and bending energies during wrapping can lead to attractive curvature-mediated interactions and to the cooperative wrapping of spherical or elongated nanoparticles in membrane tubules.

Title: Emergence of disordered collective motion in dense systems of isotropic self-propelled particles

Abstract: Active matter is a broad class of materials within which individual entities consume energy in order to perform movement. These are thus out of thermodynamic equilibrium and display a wealth of surprising phenomena which challenge our conception of equilibrium phases and dynamics. We pay specific attention to collective motion, which has been shown to emerge in systems as diverse as crowds, flocks, schools, or swarms, yet with common characteristics. We focus in particular on one of the simplest class of active matter models, namely athermal particles with isotropic self-propulsions in 2D, which is a good approximation for dense cell tissues or self-propelled colloids. We find in size-polydisperse systems that an homogeneous active liquid exists at arbitrary large persistence times, and is characterized by remarkable velocity correlations and irregular turbulent-like flows. At large density, it undergoes a nonequilibrium glass transition. This is accompanied by collective motion, whose nature evolves from near-equilibrium spatially heterogeneous dynamics at small persistence, to a qualitatively different intermittent dynamics when persistence is large. We show that these different collective phenomena are ruled by the competition between three fundamental time scales: the intrinsic persistence and interaction time scales, and the emerging relaxation time scale. (1) doi.org/10.1103/PhysRevLett.129.048002 (2) doi.org/10.1039/D3SM00034F (3) doi.org/10.48550/arXiv.2306.07172

Title:
From living cells to tissues: opportunities from mesoscale mechanobiology
 
Abstract:
When individual cells of the same type gradually assemble into a more confluent unit, they form a higher hierarchical structure known as a cellular collective. As physical connections develop among these identical cellular units, a cellular collective advances to a higher supracellular hierarchical level, often referred to as a proto-tissue in order to distinguish it from more complex tissue architectures. The simplest form of proto-tissue is the epithelial cellular monolayer, a key tissue for maintaining the physiological functions of organs and systems in our body and, as such, also playing a significant role in the progression of deadly diseases such as carcinomas. Mesoscale mechanobiology investigates how mechanics at the level of individual cells integrate at a supracellular scale, leading to the emergence of prototissues from assemblies of individual cells. This integration process gives rise to emergence of new functions and behaviours (both biological and mechanical) which are uniquely available at the higher hierarchical level but not at the lower one. In this presentation, I will provide examples from my research on in vitro supracellular systems to illustrate and discuss the opportunities offered by mesoscale mechanobiology for regenerative and clinical purposes.

Abstract: We study collections of self-propelled dipolar particles, which spontaneously form traveling strings at low packing fractions. We compare Brownian dynamics simulations using LAMMPS with an active Rouse model of flexible polymers. The collective speed and translational diffusion of the string decay with its length. By analyzing the local tangent vector correlation function, we find an active stiffness, which is only possible because the system is out of equilibrium. The stiffness which can be estimated from spatially correlated fluctuations, emerges because of the finite relaxation time of the active driving. We compare the theory and simulation to an experiment of electrically driven active dipolar colloids.

Speaker: Yoav Lahini -- School of Physics, Tel Aviv University


Title: Crumpled sheets reveal (some of) the secrets of glassy dynamics: Physical aging via avalanches of localized instabilities.


Abstract: When a thin sheet is crumpled, it undergoes irreversible plastic deformations, resulting in a permanent network of folds and creases. This violent process also changes the sheet’s mechanical properties, endowing it with a range of unusual behaviors. Most prominently, the crumpled sheet fails to reach equilibrium under constant external loading. Instead, it exhibits an ever-slowing logarithmic relaxation, spanning many time scales – from fractions of a second to several weeks. This process, termed physical aging, is characteristic of many nonequilibrium disordered and glassy systems, yet the microscopic processes underlying the dynamic slow-down during aging and the reason for its similar occurrence in different systems remain poorly understood.



Here, we leverage the macroscopic nature of crumpled sheets to reveal the mechanism underlying slow relaxations. Combining experiments and simulations of a minimal mechanical model, we show that during aging the system dwells at a marginally stable state, where it can stay for long but finite times. These quiescent dwell times are interrupted intermittently by local instabilities, which facilitate each other to form scale-free avalanches. We reconstruct the energy landscape and its evolution over time, showing that a slow increase of local energy barriers results in prolonged dwell times between avalanches, generically leading to logarithmic aging.  



The emerging picture is of a highly frustrated disordered system with a complex energy landscape, that self-organizes to a state which lies on the edge of stability. I’ll discuss the possible relevance of this picture to other disordered and glassy systems.  



Read more here:
 
Memory from coupled instabilities in unfolded crumpled sheets, PNAS (2022) 

Logarithmic aging via instability cascades in disordered systems, Nature Physics (2023)

NON-ABELIAN METAMATERIALS Emergent computing and memory

Title: "Few Experiments on Active Polymer-Like T. Tubifex Worms" Abstract: We propose a new 'active particle' system in which the particles are in fact polymer-like: the Tubifex tubifex or 'sludge' worm. I will discuss few recent experiments that highlight the richness of this active system. In the first experiment, we perform classical rheology experiments on this entangled polymer-like system. We find that the rheology is qualitatively similar to that of usual polymers, but, quantitatively, (i) shear thinning is reduced by activity, (ii) the characteristic shear rate for the onset of shear-thinning is given by the time scale of the activity, and (iii) the low shear viscosity as a function of concentration shows a very different scaling from that of regular polymers. The level of activity can be controlled by changing the temperature but also by adding small amounts of alcohol to make the worms temporarily inactive. In a second experiment, we disperse the worm in a quasi-2D aquarium and observe their spontaneous aggregation to compact, highly entangled blobs; a process similar to polymer phase separation, and for which we observe power-law growth kinetics. We find that the phase separation of active polymer-like worms does not occur through Ostwald ripening, but through active motion and coalescence of the phase domains. Interestingly, the growth mechanism differs from conventional growth by droplet coalescence: the diffusion constant characterizing the random motion of a worm blob is independent of its size, a phenomenon that can be explained from the fact that the active random motion arises only from the worms at the surface of the blob. This leads to a fundamentally different phase-separation mechanism, that may be unique to active polymers. Finally, in the remaining time, I will briefly show that we can efficiently separate by size and activity these living polymers using hydrodynamic chromatography technics.

Title: Putting a spin on metamaterials: Mechanical incompatibility as magnetic frustration

Abstract: 
Extraordinary responses of mechanical metamaterials often stem from incompatibility of their elementary building blocks. Relying on analogies to ferromagnetic and antiferromagnetic interactions, we describe the deformation fields in complex mechanical metamaterials by discrete spin states. We show how spin frustration relates to mechanical incompatibility, and we use this to design, anlyze, and realize comlex mechanical metamaterials with novel functionalities; We employ combinatorial strategies to construct metamaterials that can deform to arbitrary numbers of pre-defined textures. We use topological defects to steer deformations and stresses towards desired parts of the system. We construct topologically non-trivial knots and links in the defect pattern or in the deformation field. We use the degenerate and disordered manifold of mechanically stable states to detect the sequence of operations that a material underwent.

Speaker: Liam Holt (Associate Professor, Institute for Systems Genetics New York University School of Medicine)

Title: Crowding, compression, and condensation. 

Abstract: Thousands of biochemical reactions occur simultaneously in the cell. Small molecules are
channeled through metabolic pathways at blistering speed. Giant complexes assemble to
orchestrate transcription and translation. ATP fuels the active transport of organelles along
microtubules, and actin networks drive membrane remodeling and agitate the cytoplasm. All of
this occurs within a crowded cell interior that approaches the physical limits where molecular
jamming and glassy transitions can occur. This extreme physical environment is both essential
for life, and a potential liability. If cells become too dilute, they senesce and die. On the other
hand, mechanical compression increases crowding and eventually stalls growth. Perturbations
to crowding change the balance of reaction rates in the cell. Crowding also drives biomolecular
condensation, which in turn is thought to regulate myriad processes. We propose that
perturbations to the physical properties of the cell interior through metabolic changes and
mechanical compression play an important role in both normal cell biology and disease.

Short Bio:
Liam Holt completed his Ph. D. at UCSF in 2009, was a Bowes Fellow at UC Berkeley, and is currently Associate Professor of Biochemistry at New York University. His lab studies how mechanical compression affects cells, and how the physical properties of the cell interior affect biochemistry in both normal biology and disease. He is passionate about outreach and community: he co-founded Science Sketches (www.sciencesketches.org), an online dictionary of science videos partnered with the Explorer’s Guide to Biology (www.explorebiology.org) and MBoC Journal; and Inspire Science (www.inspiresci.org), a symposium about maintaining happiness in a challenging career. 

Website: www.liamholtlab.org, www.sciencesketches.org, www.inspiresci.org.

One sentence:
``Liam Holt's lab studies tissue mechanics and the biophysical properties of cells.''

Speaker: Amir Shee (Northwestern University)

Title:Noise-induced quenched disorder in active elastic systems

Abstract: Self-organization is often observed in active systems such as cell colonies, developing tissue, insect swarms, bird flocks, and groups of autonomous robots. In recent years, several minimal models have been introduced to understand the underlying mechanisms that can lead to the emergence of large-scale coherent patterns of motion in such systems for different types of individual dynamics and interactions.

In this work, we consider a dense sheet of self-propelled disks with elastic repulsive forces

that act on their positions and orientations. Each disk can move along its heading direction or rotate about a center of rotation located behind its barycenter. This system displays three phases: polar moving order, standard dynamic disorder, and a novel noise-induced state of quenched disorder, in which the disks are jammed, and orientations fluctuate about fixed random directions.

We explain the mechanism that leads to the quenched state by formulating an approximate analytical description in which the heading fluctuations follow an Ornstein–Uhlenbeck process. Finally, we argue that this state could be observed in a broad range of natural and artificial dense active systems with repulsive interactions.

Speaker 1: Leonardo Passerini (Huber Lab)
Title: Identifying a short lived intermediate of oxygen reduction in a copper enzyme by EPR
 
Speaker 2: Gert-Jan Kuijntjes (v. Noort Lab)
Title: Single-Molecule Nucleosome Mapping through Nanopore Sequencing

Speaker: Michael M. Lerch is currently an assistant professor at the University of Groningen (the Netherlands), working on self-regulated soft materials and soft robotics (www.lerchlab.com). He holds a PhD in supramolecular and photochemistry based on his work on ‘Donor–Acceptor Stenhouse Adducts’ with Prof. Feringa. During a NWO Rubicon postdoctoral fellowship in the Aizenberg group at Harvard University/WYSS Institute, he developed microstructured liquid crystalline elastomer surfaces with self-regulated motion and feedback-controlled optically active hydrogels.


Title: Operating Soft Matter Through Engineered Feedback

Abstract: Eliciting complex movements in soft matter generally requires extensive external control or complicated material architectures. Both are impractical, particularly at small scales. A promising approach to avoid laborious fabrication or operation is to engineer feedback within the material, so the material deformations become self-regulated.
While self-regulation is abundant in natural systems, it is surprisingly difficult to design for in synthetic systems. In this talk, I will present our recent work on creating opto-chemo-mechanical feedback in liquid crystalline elastomers and hydrogels. We show that using a light-responsive crosslinker and a simple symmetry argument, one can design microstructures that twist and bend on demand and at higher light intensity undergo power-stroke type deformations. Interestingly, such microstructures can form self-organized deformation patterns in arrays, based on the same opto-chemo-mechanical feedback mechanism. Overall, the simplicity of the system offers a new approach to motional complexity in soft matter.

Speaker: Iain Muntz
 
Title: A Minimal Tissue Model: The Cell as a Physical Object

Abstract: Human tissues are complex systems composed of cells embedded in an extracellular matrix (ECM), a network of polymers which confers mechanical and structural integrity on the tissue. In addition, the ECM provides physical cues to the cells that affect their biological activity. Conversely, cells affect tissue mechanics by a volume exclusion effect combined with biochemical remodelling. The biophysics of the ECM-cell interplay in tissue mechanics is challenging to study because of the huge molecular complexity of the ECM and the reciprocal mechanochemical crosstalk of cells and ECM.

To tackle this challenge, we develop a minimal tissue model to understand how cells affect tissue mechanics through purely physical effects, specifically the volume exclusion interaction. To do this we use poly(acryl amide) based microgels as a proxy for the cell. We show that networks of fibrin, which exhibit compression-softening, instead stiffen under compression when cell-mimetic microgels are embedded in the network. This effect is, at least partially, explained through stretching of the network in between the beads due to the inhomogeneities in the system. However, we observe stronger than expected stiffening effects at short times which we postulate arise from poroelastic effects in the system, where fluid flows through the porous network initially dominate the response. These results are important to understand not only native tissue mechanics, but also as a step to producing tissue engineered constructs with properties more closely resembling those of native tissue.

Speaker 1: Vasilii Akulov (v. Noort Lab)
Title: TBA
 
Speaker 2: Jacco Ton (Orrit Lab)
Title: TBA

Speaker 1: Nasrin Asgari (Orrit Lab)
Title: Burst-by-burst analyzes of the rotational diffusion of single gold nanorods
 
Speaker 2: Robert Smit (Orrit Lab)
Title: Spectroscopy of single molecules on the surface of hexagonal Boron Nitride

Speaker1: ThéoTravers,Orrit Lab
Title:Characterization of the free diffusion of light nanosources using nonlinear microscopy
 
Speaker2: Georgia Kefala,Schmidt Lab
Title:Interaction forces and cell sorting between two different cell types in hetero-spheroids.

Speaker schedule:

Speaker: Tannie Liverpool


Title: The mathematics of active matter

Abstract: A flock of birds, a shoal of fish,  a swarm of robots,  a colony of swimming bacteria; these are examples of systems composed of interacting units that consume energy and collectively generate motion and mechanical forces on their environment. They show a rich variety of collective behaviour, much of which remains mysterious. In recent years we have come to call such systems active matter. Clearly, biology (living systems)  provides numerous examples of these active matter systems.   

We call them active matter because they share some of the properties of the constituents of what we call matter, i.e. solids, liquids and gases in that they are made of  many interacting components. However they have fundamental differences in that many conservation laws that govern the interactions of normal (passive) matter are not obeyed by their active components.
 
(Equilibrium) statistical mechanics has formed the framework for how we understand the properties of matter. I will argue that ideas developed in statistical mechanics must be augmented by a number of new mathematical structures to describe these systems. Then I will describe some recent theoretical work developing this framework for characterising the behaviour of active matter systems. Finally I will  apply it to describe two examples of active systems, active Brownian particles and active nematics.

Speaker: Victor Yashunsky (Ben Gurion University, Be'er Sheva, Israel)


Title: Chiral edge current in chaotic nematic cell monolayers

Abstract: Collective migration of cancer cells in the body is routinely observed close to confining structures such as muscle fibers or blood vessels. In vitro studies re-create such behavior by showing that fibrosarcoma cells collectively migrate at the border of their colony, even though within the monolayer cell flows obey turbulent chaotic dynamics characterized by an irregular array of vortices generated by self-propelled units. Even more surprising is that the edge currents always flow in the same direction—somehow cells collectively distinguish between their left and right near the edge. To understand this situation, we looked deeper at the organization of the cells within the monolayers. Fibrosarcoma cells are elongated and align together, defining a patchwork of well-aligned domains between which orientational singularities (topological defects) position themselves. In the bulk of the monolayer, the position and orientation of these defects randomly change over time. However, close to the boundary, we find that comet-shaped "+½ defects" orient themself with an angle slightly smaller than 90° relative to the boundary, consistently tilting their tails to the right. Because of this left-right symmetry breaking, clockwise vortices are pushed closer to the border and generate the directed edge flow. Modeling the system as a chiral, active, nematic liquid crystal accounts well for our results and demonstrates that cell handedness is a critical ingredient for the emergence of the observed edge flows and not only for their direction.

Title: Non-orientable order and non-commutative response in frustrated metamaterials

Abstract: From atomic crystals to animal flocks, the emergence of order in nature is captured by the concept of spontaneous symmetry breaking. However, this cornerstone of physics is challenged when broken-symmetry phases are frustrated by geometrical constraints. Such frustration dictates the behavior of systems as diverse as spin ices, confined colloidal suspensions, and crumpled paper sheets. These systems typically exhibit strongly degenerated and heterogeneous ground states and hence escape the Ginzburg-Landau paradigm of phase ordering. 
Here, combining experiments, simulations and theory we uncover an unanticipated form of topological order in globally frustrated matter: non-orientable order. We demonstrate this concept by designing globally frustrated metamaterials that spontaneously break a discrete Z_2 symmetry. We observe that their equilibria are necessarily heteregeneous and extensively degenerated. We explain our observations by generalising the theory of elasticity to non-orientable order-parameter bundles. We show that non-orientable equilibria are extensively degenerated due to the arbitrary location of topologically protected nodes and lines where the order parameter must vanish. We further show that non-orientable order applies more broadly, to objects that are non-orientable themselves, such as buckled Mobius strips and Klein bottles. Finally, applying time dependent local perturbations to metamaterials enjoying non-orientable order, we engineer topologically protected mechanical memories, we achieve non-commutative responses, and show that they carry an imprint of the braiding of the loads’ trajectories. 
Beyond mechanics, we envision non-orientability as a robust design principle for metamaterials that can effectively store information across scales, in fields as diverse as colloidal science, photonics, magnetism, and atomic physics.

Speaker 1: Rick Rodrigues de Mercado, Schmidt Lab
Title: Single-cell stress analysis in spheroids using deformablehydrospheres and cell segmentation
 
Speaker 2: Jacqueline Labra Muñoz, Huber Lab
Title: A new perspective into the magnetism of ferritin

Speaker: Pau Guillamat (Institute for Bioengineering of Catalonia (IBEC), Barcelona,
Catalonia, Spain.)



Room: EM109 (New Gorlaeus building)
 


Title: Defect-mediated force generation in cellular nematics

Abstract: Tissue reshaping during embryogenesis, cell extrusion events in the gut or tumour growth are crucial biological processes that rely on large-scale mechanical patterns powered by forces from hundreds of cells. To organize these forces, nature has refined a wide palette of biochemical and mechanical cues able to organize single cells into collectively behaving groups. In tissues composed of elongated cells,the organization of these forces strongly depends on the orientational (nematic) order of cells and the presence of topological defects, which are discontinuities in the orientation field. On the one hand, nematic order directs migration of single cells throughout these tissues. On the other hand, robust arrangements of orientation around topological defect cores lead to reproducible force patterns at large scales. Here I will show how cellular topological defects are able to organize unique patterns of force, providing cellular nematics with fine mechanical tools for controlling tissue remodelling.

Sarah L. Keller
Duane and Barbara LaViolette Professor of Chemistry
University of Washington, Seattle
(currently on sabbatical with Marileen Dogterom’s lab at TU Delft)

Title:
Two mysteries in 2D phase separation: What drives large-scale phase separation in yeast
membranes, and what controls the length scale of small domains in model membranes?

Abstract:
At a particular stage in the growth cycle of yeast, membranes of the vacuole (an organelle) phase
separate. Here, we show that yeast actively tune the transition temperature of their vacuole
membranes to be close to the yeast's growth temperature, which implies that the membrane's
proximity to the miscibility transition is important for the cell's function. Indeed, in yeast,
demixing of vacuole membranes into large, micron-scale domains is correlated with cell survival
through extended periods of low nutrients. In living cells and artificial systems alike, phase-
separated membranes frequently have excess area (more membrane than is needed to enclose the
volume), which leads to patterns of dots or stripes. A persistent open question in the field is
what physical mechanisms give rise to these patterns. Here we show which aspects of current
theories of pattern formation are supported by our data, and where opportunities lie for
developing new models.

Bio:
Sarah L. Keller, the Duane and Barbara LaViolette Professor of Chemistry, is a biophysicist in
the U.S. at the University of Washington in Seattle. She investigates self-assembly, complex
fluids, and soft matter systems. Her research group’s primary focus concerns how lipid mixtures
within bilayer membranes give rise to complex phase behaviour. She is a Fellow of the American
Physical Society and a Fellow of the Biophysical Society.

Speaker 1: Jacco Ton, Orrit Lab

Title: Progress towards label-free detection of single nano-particles in a dielectrophoretic trap

Speaker 2: Jeremy Ernst, Van Noort Lab

Title: Single-pair FRET reveals nucleosome dynamics in folded chromatin fibers

Muhittin Mungan: Institute of Biological Physics, U. of Cologne
 
Title:  Memory Formation in Driven Disordered Systems


Abstract: Memory formation and ageing are abundant in many soft matter systems. The disorder underlying these
systems gives rise to a rich energy landscape, consisting of a large number of metastable states.
These landscapes are accompanied by a plethora of pathways, along which such systems can evolve when exposed to a
varying temperature or mechanical load. The resulting dynamics can be rather complex. For example,  
a crumpled sheet of paper, can evolve under periodic compression into a cyclic response
that is characterized by a hierarchy of nested response-cycles. Often such a response is  
realized through the emergence of a system of spatially localized bi-stable mechanical elements, which in turn
can be regarded as constituting a memory of the loading.  In this talk I will present a general framework to analyze
the dynamics and memory formation of driven disordered systems.  

Lennard
 
Title: Counting, Memory and Information Processing in a Mechanical Metamaterial


Abstract: Mechanical Metamaterials use their internal structure to realize desired material properties not seen in nature, including anomalous elastic properties and spatially textured shapemorphing. So far, these materials react directly to mechanical input. Here we introduce metamaterials whose configuration is updated sequentially and that store a memory of the number of compressive cycles in the past: materials that count. Combining counters, we realize complex metamaterials that store multiple memories and that filter sequential input: materials that detect sequential passwords. Our platform is scalable and extendable, and opens a new route towards computation in materia.

 
 
Yogesh
 

Title: Self Assembly of Rigid and Flexible Colloidal Molecules.

Abstract: Colloidal molecules are promising building blocks for assembling larger structures, but their preparation in precise forms and sizes is challenging. Here, using experiment and simulation,  we show self-assembly of colloidal molecules by exploiting geometric constraints imposed by particle shape and size. Using two different approaches, we create bothflexible and rigid colloidal molecules by assembling finite-sized clusters from spheres (S) and cubes (C). We prepareflexible colloidal molecules by the assembly of spheres and cubes functionalized with complementary DNA linkers that are mobile on the particle surface, andrigid colloidal molecules using oppositely charged spheres and cubes. We obtained high yields of colloidal molecules with valencies (CS_n) CS₆, CS₄, and CS₂ depending on the size ratio of the sphere and cube. These colloidal molecules can be used as building blocks for assembling higher-order structures.

Title:
Transcriptional control and optimal sensing of regulatory signals

Abstract:
Transcription factor concentrations can be seen as signals that need to be sensed by organisms in order for them to express their genes as precisely as is required during development or during adjustment to different conditions. The low concentration of all the relevant molecules means that these measurements will be noisy. In eukaryotes, a number of proteins and genomic regions contribute to ensuring a precise transcriptional response. Such a response is important especially in embryonic development, where the organism needs to obtain a minimum of information in a limited time in order to generate a complex body plan. I will present our work on how to extract this information for a series of transcription factors in the fly embryo using a “sensing scheme”, called the information bottleneck, which has recently gained popularity in neuroscience and machine learning. Indeed, the sensors we identify with this scheme have important features in common with the fly enhancers. Our method thus provides a complement to mechanistic, bottom-up approaches for understanding why transcriptional elements are structured the way they are. If there is time, I will also discuss issues around optimal sensing given specific mechanistic constraints.

Title: Many fire-ant systems

Abstract: Experiments with fire-ant columnsreveal similarities and differences with granular columns. For granularcolumns, we will show that narrowing the column diameter can result in novelbehavior relative to how we think of wider columns, which were studied morethan a century ago by German engineer Janssen; the key is the force-chainstructure. We will then show that fire-ant columns also exhibit force chains;however, as an active granular system, there are apparent fluctuations andactivity within the system. In 2D, these effects become most noticeable andmanifest in the form of activity waves that propagate along the verticaldirection. Ultimately, these waves reflect the existence of activity cycles,whereby the fire-ant system changes “state” from “active”, where all the antsmove, to “inactive”, where a large fraction of the ants cluster and remain stationary.Our findings indicate that while the “active” states correspond to collectivemotion, in the “inactive” states there is clustering reminiscent of amotility-induced attraction resulting from the social interactions between theants. 

Title: Morphing soft structures with instabilities. 

Abstract: Buckling in thin structures is generally considered as a first step towards failure. Instead, we view mechanical and interfacial instabilities in structures as opportunities for scalable, reversible, and robust mechanisms that must first be understood predictively, and then harvested for their function. This new design paradigm – building with instabilities – calls for an improved understanding of instabilities and pattern formation in complex media.  Three examples will be presented: (1) fluid-instability based approaches for digitally fabricating geometrically complex uniformly sized structures, (2) flexible fabric-based gripper that contracts radially upon inflation (3) deployable structures inspired by insect wing expansion. The main feature common to these different problems is the prominence of geometry, and its interplay with mechanics, in dictating complex mechanical behavior that is relevant and applicable over a wide range of length scales.