Session W13: Dense Active Matter: From Fluid to Solid

When active matter turns solid : from collective motion to collective actuation.

Active matter is made of a large number of out of equilibrium interacting units, which convert some source of energy into directed motion. In contrast with systems driven out of equilibrium by an external field, or through the boundaries, the departure from equilibrium is local and independent for each active unit. Not so surprisingly, active matter exhibits rich collective phenomena at large scales, such as new types of phase separation and/or collective directed motion.  The past 25 years have seen a surge of experimental observations and theoretical progress in the description of such phenomena in the realm of active liquids.

There are however a number of circumstances under which the description in terms of liquids is not suited. One can think of ants building solid bridges out of their bodies, meta-materials made of mechanically connected engines, cohesive cell layers or simply very dense assemblies of self propelled particles forming a glass or a crystal. In such cases a description in terms of elastic solid is likely to be more appropriate. However very little is known about the actuation of an elastic lattice by polar active particles, the orientations of which may couple to the displacement field.

In this talk, I will present very recent experimental and theoretical works aiming at developing this new path of research in active matter. Doing so, we shall unveil a new type of collective phenomena, namely collective actuation, and discover how the coupling between linear elasticity and activity leads to a rich selection mechanism of the actuated dynamics.

    

Active colloids that mix and mingle

Active matter describes nonequilibrium systems that consist of "elementary particles" that consume energy to move. Examples include dynamically self-organized molecular motors in the cytoskeleton, swarming bacteria, flocks of birds and crowds of people. However, most studies so far have focused on monodisperse systems. Active matter of mixtures of particles with distinct active driving forces remains surprisingly largely unexplored. Would materials scientists today fathom constraining themselves to purely single-component metals? Alloying active matter adds a whole new dimension to our exploration of mesoscopic building blocks. Today there is a striking gap both in our theoretical understanding and experimental endeavors in this domain. In this talk I will discuss my group's efforts in understanding active mixtures using the Active Brownian Particle (ABP) model. I will show results for systems where the concentrations of the two active species are 50:50, as well as the limiting cases of doping a passive material with a very small percentage of active particles in order to control the material's properties.   

Plugging active "hardware" into biological "software"

The actomyosin cytoskeleton is a naturally occurring active gel, which exhibits a density instability called contraction. This remarkable ability drives a wide range of autonomous mechanical behaviors in cells, allowing them to move, change shape, exert force, sense stiffness, and maintain constant tension. A thorough description of these behaviors requires a quantitative characterization of the mechanical properties of contractile active gels. However, mechanical properties are conventionally expressed as responses to external stresses and strains. Which physical quantities describe the response to molecular motor activity? Although researchers have long known that motors hydrolyze ATP to drive contraction, the relationship between motor activity and contraction remains poorly understood. Here we experimentally measure myosin ATPase activity and actomyosin contractile outputs in gels of reconstituted proteins, and express relationships between these quantities as response functions. Quantifying these response functions allows us to identify optimal operating conditions, such as maximum energy economy and maximum performance. We hypothesize that cells dynamically switch between these distinct optima to adapt to changing mechanical tasks, uncertain environments, and evolutionary pressures. Control over optimal contractile states could also be leveraged in the design of small robots actuated by actomyosin active gels. Finally, the response functions we introduce here are a necessary first step towards future studies which quantify how biochemical feedback loops connect information from contractile outputs back into ATPase input regulation.

    

Tissue models with active feecback

Epithelial tissues are one of the engines of development in multicellular organism, where they determine the emerging shapes, like during the gastrulation process where the embryo turns inside out. This feat is performed by the active stresses generated by the cytoskeleton, where cells coordinate their collective mechanics over large spatial distances. In contrast to chemical signalling, we do not understand how surch coordinated activity or mechanical signalling work.

Here I will present models of vertex models of epithelial tissues with active feedback. In the early stages of gastrulation in the chick embryo, tissue deformation is driven by convergent-extension flows where tissue flows through active T1 transitions, against the applied stress, and actomyosin localises along pronounced myosin tension chains. We first develop a single junction active element where catch-bond dynamics of actomyosin allows the junction to work as an egine that can contract to a single point under applied stress. At the tissue scale, such elements coupled to a vertex model enable active T1 transitions, which occur in an optimal applied tension and activity range and each contribute a discrete negative strain amount, thus acting like element of an active metamaterial. In a full, disordered tissue, active T1 transitions occur along pronounced tension chain structures, with emergent convergent-extension flow, i.e. a negative shear modulus, in agreement with experiment. We find that states of self-stress dominate the mechanics this model tissue, with stress localisation that resembles mechanical spring networks. I will conclude with recent results that show that tension chains emerge generically in such tissue models, in a process mirorring the compressive force chains of granular packings.   

Multiscale rheology and dynamics of topologically-active DNA solutions and composites

Topologically-novel polymers, such as rings with no free ends, give rise to complex scale-dependent rheological properties in entangled polymer solutions blends and composites. At the same time, non-equilibrium biopolymer networks, which undergo bulk rheological changes driven by macromolecular restructuring, are the topic of intense investigation. Yet, how macromolecular dynamics give rise to bulk viscoelasticity in such complex polymeric systems, and how local stresses are propagated and distributed across scales, are long-standing open questions. Here we present the design and rheological characterization of solutions of ring and linear DNA, as well as their composites with dextran crowders and stiff microtubules, that are pushed out of equilibrium by enzymatically-driven topological conversion of DNA. We use a robust characterization platform to elucidate the time-varying polymer dynamics and rheological properties of our engineered 'topologically-active' complex fluids across orders of spatiotemporal scales - from single polymers to the bulk over milliseconds to hours. Specifically, we couple particle-tracking and differential dynamic microscopy with optical tweezers microrheology and bulk rheology to reveal key discoveries including: viscous thickening and gated thinning of circular DNA solutions undergoing linearization and fragmentation; discrete state-switching of bulk rheological properties of topologically-active DNA-dextran composites; and emergent nonlinear stress responses of DNA-microtubule composites in which affine alignment, superdiffusivity, and elastic memory are maximized when the strain rate is resonant with the entanglement rate.