ICTS

In models of “scalar” active matter, activity is incorporated by introducing two or more species of particles coupled to thermal baths at distinct temperatures. The temperatures in this context are not thermodynamic but emergent from effective diffusivities of multiple motile species. We have studied, using molecular dynamics simulations, the steady-state properties of systems of Lennard-Jones (LJ) particles and soft repulsive spherocylinders (SRS) in which scalar activity is introduced by connecting half of the particles to a thermostat at a higher temperature, while the rest of the particles remain connected to a thermostat at a lower temperature equal to that of the initial equilibrium system. In all the studied systems, the particles separate into hot and cold regions if the temperature difference is sufficiently large. More importantly, the particles in the cold region exhibit enhanced order in comparison to their passive counterparts under equilibrium conditions. In the LJ system, the particles in the cold region exhibit crystalline order, although an equilibrium system under the same temperature and pressure is in the fluid state. In the SRS system that exhibits several liquid-crystalline phases in equilibrium, the cold particles exhibit a more ordered state compared to the initial equilibrium state. Another interesting result is that the active SRS system with small aspect ratio exhibits liquid-crystalline phases that are not present at equilibrium. The anomalous ordering in the cold zone appears to be driven by non-equilibrium features such as pressure anisotropy and heat current.

The investigation of collective behavior of self-driven or active particles, or agents, has been motivated by a wide range of biological phenomena, ranging in scale from bird flocks to the dynamics and organization of biomolecular assemblies in cells. In particular, dense assemblies of active particles have been of interest to comprehend transitions between jammed and unjammed states in cellular and sub-cellular biological assemblies such as microbial populations and confluent tissue, and to investigate motion in dense cellular environments, including the nuclear interior and the cytoskeleton. Motivated by observations of mechanically induced changes in dynamical state in such systems, and the apparent role of confinement geometry, we study the transition between jammed to fluidized states of assemblies of active particles, as a function of the strength and temporal persistence of the active forces, and in different confinement geometries. Our results show that the fluidization transition broadly resembles yielding in amorphous solids, along the lines of recent suggestions. More specifically, however, we find that a detailed analogy holds with the yielding transition under cyclic shear deformation, for finite persistence times. The fluidization transition is accompanied by driving induced annealing, strong dependence of the transition on the initial state of the system, a divergence of time scales to reach steady states, and a discontinuous transition to the diffusive state. We also observe a striking dependence of the transition on persistence times and the nature of the confinement, with potential implications for mechanically induced changes in differentiation states in cell nuclei.

I will show that two-dimensional crystals made of active particles can experience extremely large spontaneous deformations without melting. Such active crystals are able to maintain long-range bond order and algebraically decaying positional order, but with an exponent η not limited by the 1/3 bound given by the (equilibrium) KTHNY theory. I will show how these findings can be rationalized within linear elastic theory via the existence of two well-defined effective temperatures quantifying respectively large-scale deformations and bond-order fluctuations. I will finally discuss similar observations made in different situations.

Tissue dynamics are often classified as extensile or contractile active nematics. Tissue of neural progenitor cells (NPCs) are known to be classified in an extensile active nematic system judging from cell accumulation at +1/2 defect and the direction of flow around it [1]. The linear active force term commonly found in many of the various models of active nematics predicts that the cell flow in extensile tissue will be inward for aster-type defects and outward for circular-type topological defects. However, the latest our experiments on NPCs have surprisingly found that +1 topological defects of all kinds, including asters, spirals, and circular patterns, attract cells to the center of the defects. This phenomenon is counterintuitive and cannot be explained by conventional models of dry active nematics. From the analysis of the flow and orientation fields of the tissue we propose a new model of active nematics that goes beyond linear active force.

Reference:
1. Kawaguchi et al., Nature, 545, 327–331 (2017).
2. Saw et al., Nature, 544, 212–216 (2017).
 

This talk is about the role of active hydrodynamics of acto-myosin in organizing the fluid membrane that encases every living cell: The membrane of a living animal cell is a bilayer consisting of myriad lipids and proteins that are draped over an active cortical cytoskeleton. Yet despite being a fluid, the membrane exhibits local membrane domains of reproducible composition. The origin of these heterogeneities which resemble phase separated lipidic domains are difficult to explain, primarily because the lipid composition of the membrane bilayer (detached from the actin cortex) does not phase separate into liquid-ordered and disordered phases at physiological temperatures. The discovery that glycolipids and lipid-anchored proteins present at the outer leaflet of the cell membrane or membrane proteins that possess actin-binding motifs form dynamic nanoclusters when the membrane is juxtaposed to a dynamic cortical cytoskeleton, provides a resolution of this apparent paradox. A combination of theory and experimental verification of its predictions, as well as in vitro reconstitution suggest that these clusters are generated by the active mechanics of actomyosin at the inner leaflet of the cell. A consequence of this activity is the generation of regulatable meso-scale active emulsions with characteristics of phase separated domains. These constitute lateral heterogeneities in the cell membrane and I will also discuss how they are endowed with specific functions influencing cell physiology

More than two decades ago, Simha and Ramaswamy showed that flocks in fluids are unstable, at least in the viscous limit. This instability of uniaxial orientational order in bulk active fluids is an inescapable consequence of the conservation of total mass and momentum. In this talk, I will show that the very activity that conspires with conservation laws to destroy bulk nematic or polar ordering can instead promote it, with radically suppressed fluctuations, in a layer of active fluid in contact with a solid or fluid medium. These escapes from the bulk instability and active stabilisation of uniaxial order via dynamical, nonequilibrium analogues of the Anderson-Higgs mechanism, would be impossible in equilibrium systems in which the existence of order and all equal-time correlators are independent of dynamics and the presence or absence of conservation laws.
 

For some years now, the study of matter whose constituents -- active particles -- turn an energy supply into work has been a dominating presence on the landscape of soft-matter and nonequilibrium physics. My first lecture will introduce the subject and present highlights of recent as well as key earlier work with students and colleagues. The remaining lectures will discuss selected topics in more detail, draw connections to other problems in the physics of driven systems, and identify important future directions.

I will give describe experiments studying stem cell shape fluctuations and discuss the experimental signature of their out of equilibrium nature. Shifting to fully differentiated cells, I will show that a paradoxical observation according to which increasing active contractility may either result in an increase or a decrease of nucleus fluctuations. Eventually, I will discuss microphase separation in a random copolymer built of self-avoiding and helicase driven intersecting segments made of otherwise identical monomers.

 

The Hamiltonian generator for the precessional dynamics of classical Heisenberg spins is reciprocal in nature. Here, we study the dynamics of a classical Heisenberg spin chain where the neighbours interact through a purely non-reciprocal exchange coupling, which preserves rotational symmetry. The resultant dynamics conserves neither the magnetisation nor the energy. We uncover other local conservation laws in their place in the extreme case of a strictly antisymmetric coupling, namely the staggered magnetisation and a "pseudo-energy". We develop an effective hydrodynamic description from which we demonstrate the presence of diffusion in the system. This is corroborated by numerical calculations of the two point correlator of the conserved quantities supplemented by a calculation of the "decorrelator" to characterise the spreading of chaos in the system. Our results point to the existence of an analogue of thermalisation in the system even though it is not described by a Hamiltonian.

Non-reciprocal interactions between scalar fields that represent the concentrations of two active species are known to break the parity and time-reversal (PT) symmetries of the equilibrium state, as manifested in the emergence of travelling waves. We explore the notion of nonlinear non-reciprocity and consider a model in which the non-reciprocal interactions can depend on the local values of the scalar fields. For generic cases where such couplings exist, we observe the emergence of spatiotemporal chaos in the steady-state. We associate this chaotic behaviour with a local restoration of PT symmetry in fluctuating spatial domains, which leads to the coexistence of oscillating densities and phase-separated droplets that are spontaneously created and annihilated. We uncover that this phenomenon, which we denote as effervescence, can exist as a dynamical steady-state in large parts of the parameter space in two different incarnations, as characterized by the presence or absence of an accompanying travelling wave.

Living systems are chiral on multiple scales, from constituent biopolymers to large scale morphology, and their active mechanics is both driven by chiral components and serves to generate chiral morphologies. We describe the mechanics of active fluid membranes in coordinate-free form, with focus on chiral contributions to the stress. These generate geometric 'odd elastic' forces in response to mean curvature gradients but directed perpendicularly. As a result, they induce tangential membrane flows that circulate around maxima and minima of membrane curvature. When the normal viscous force amplifies perturbations the membrane shape can become linearly unstable giving rise to shape instabilities controlled by an active Scriven-Love number. We describe examples for spheroids, membranes tubes and helicoids, discussing the relevance and predictions such examples make for a variety of biological systems from the sub-cellular to tissue level. This is joint work with Sami Al-Izzi.

For some years now, the study of matter whose constituents -- active particles -- turn an energy supply into work has been a dominating presence on the landscape of soft-matter and nonequilibrium physics. My first lecture will introduce the subject and present highlights of recent as well as key earlier work with students and colleagues. The remaining lectures will discuss selected topics in more detail, draw connections to other problems in the physics of driven systems, and identify important future directions.

Collective behaviour is found in a startling variety of biological systems, from clusters of bacteria and colonies of cells, up to insect swarms, bird flocks, and vertebrate groups. A unifying ingredient is the presence of strong correlations: experiments in bird flocks, fish schools, mammal herds, insect swarms, bacterial clusters and proteins, have found that the correlation length is significantly larger than the microscopic scales. In the case of natural swarms of insects another key hallmark of statistical physics has been verified, namely dynamic scaling: spatial and temporal relaxation are entangled into one simple law, so that the relaxation time scales as a power of the correlation length, thus defining the dynamical critical exponent, z. Within statistical physics, strong correlations and scaling laws are the two stepping stones leading to the Renormalization Group (RG): when we coarse-grain short-scale fluctuations, the parameters of different models flow towards one common fixed point ruling their large-scale behaviour. RG fixed points therefore organize into few universality classes the macroscopic behaviour of strongly correlated systems, thus providing parameter-free predictions of the collective behaviour. Biology is vastly more complex than physics, but the widespread presence of strong correlations and the validity of scaling laws can hardly be considered a coincidence, and they rather call for an exploration of the correlation-scaling-RG path also in collective biological systems. However, to date there is yet no successful test of an RG prediction against experimental data on living systems. In this talk I will apply the renormalization group to the dynamics of natural swarms of insects. Swarms of midges in the field are strongly correlated systems, obeying dynamic scaling with an experimental exponent z=1.37 +/- 0.11, significantly smaller than the naive value z = 2 of equilibrium overdamped dynamics. I will show that this anomalous exponent can indeed be reproduced by an RG calculation to one-loop, provided that off-equilibrium activity and inertial dynamics are both taken into account; the theory gives z=1.35, a value closer to the experimental exponent than any previous theoretical determination and perfectly in line with the numerical value, z=1.35 +/- 0.04. This successful result is a significant step towards testing the core idea of the RG even at the biological level, namely that integrating out the short-scale details of a strongly correlated system impacts on its large-scale behaviour by introducing anomalies in the dimensions of the physical quantities. In the light of this, it is fair to hope that the renormalization group, with its most fruitful consequence -- universality -- may have an incisive impact also in biology.

Active solids consume energy to allow for actuation and shape change not possible in equilibrium. I will focus on the elasticity of systems as wide-ranging as living matter, nanoparticles, and mechanical structures composed of active robotic components. I will review our work on odd elasticity and its recent experimental observations. I will then discuss how in lattices of robots, inertia and elasticity conspire and give rise to new varieties of pattern formation. These results provide a theoretical underpinning for recent experiments and point to the design of novel soft machines.

Understanding the dynamics of microbes in confined flow channels is sought after for several medical and biotechnological applications. Here, I will present our work based on a simplified microswimmer model capturing a strong coupling of the active flows to the self-propulsion dynamics. We will discuss the fundamental physics behind the swimmers’ somewhat surprising dynamics inside a channel and point out a few novel controls predicted by our analyses.

I will discuss the interactions between passive inclusions in an active suspension, where passive particles couple to the active suspension and quickly react to the active particles rearrangements. Hence, their relative dynamics plays an important role in the features that characterize the emergent interactions among the inclusions. Moreover, for systems where active particles develop long range polar order, the presence of passive obstacles triggers spontaneous macroscopic structures that give rise to non-reciprocal interactions. I will also discuss the susceptibility of polar active systems to small inclusions and the implications this has on the nature of their ordered phases.

When driven out of equilibrium by the consumption of biochemical energy, cytoskeletal protein filaments alone and in combination with molecular motors are able to generate sufficient forces to deform and move cells. In particular the protein actin can polymerise into filamentous networks. Active growth of actin filaments and contractility of the netwok due to the action of the molecular motor myosin contribute to cell motility and deformation.

First I will discuss our work on polymerising branched actin, comparing in vitro data with simulations and analytical calculations. Then I will present stochastic simulations of polymerising branched actin exerting force to deform a model membrane in the context of phagocytosis, which is a process by which immune cells engulf pathogens.

I will conclude with some discussion of potential universal characteristics and general principles of active matter from the perspective of polymerising actin networks.

Inertial particles in flow centrifuge out of vortical regions and form caustics, where the number density diverges, in certain portions of the flow. We will discuss how active particles, which have no inertia, show some of this behaviour, and discuss the differences. Such congregations of active particles can significantly influence reproduction and other aspects relevant to life.