Session J30: Composite Active Materials

Comparison of different approaches to single particle tracking of enzymes displaying enhanced diffusion

Enzymes have been shown to perform faster diffusion with the presence of their substrate. Recently, we have revealed new insights of this emergent enzyme activity using single particle tracking (SPT). We found that while the overall mobility of enzyme is improved by 2-3 folds at saturated substrate concentration, the mode of diffusion remains Brownian. Meanwhile, in order to achieve long trajectories, a polymer brush coated surface and a large viscous polymer was utilized to slow down the diffusion, raising questions as to the effect of these additives. Here, we investigate the effect of the surface coating by replacing the polymer brush with a lipid bilayer; we also replace the crowding polymers with a smaller viscous molecule, glycerol. We make the same diffusion measurements and compare the results from both methods. We found a faster diffusion together with an artifactual anomalous exponent for enzymes diffusing on lipid bilayer. Also, the presence of high percentage of glycerol, leads to the failure in reproducing enhanced diffusion due to the low enzymatic reaction rate in such a high viscous environment. Our results indicate the critical responsibility of the polymer brush in slowing the diffusion and enabling the direct reporting on single enzyme enhanced diffusion.   

Vortices, space-time braids and loops in the membrane of a living cell.

Topological defects determine the structure and function of matter over a wide range of scales. Many advances have been made in understanding and controlling the defect dynamics in active and passive non-equilibrium fluids. Yet, it remains unknown whether the statistical laws which govern the dynamics of defects in classical or quantum fluids extend to active living matter. Here, we show a defect-mediated turbulence underlies the complex wave propagation patterns of Rho-GTP signaling proteins on the membrane of starfish oocytes. Our experiments reveal that the phase-velocity field extracted from Rho-GTP concentration patterns exhibits vortical defect motions and annihilation dynamics reminiscent of those seen in quantum systems. Space-time analyses of defect trajectories reveal the existence of two characteristic types of braids: loops braided by multiple pairwise creation and annihilation events, and long-lived defect pairs that wiggle and form braid groups. Several key statistics and scaling laws of the defect dynamics, braids and loops can be captured by a generic complex Landau-Ginzburg continuum theory, suggesting space-time braids and loops are useful topological measures for unraveling information scrambling and transmission in dissipative living systems.   

Creation and evolution of defects in composite biopolymer nematics

Structured soft materials have internal order that give rise to unusual mechanical properties and emergent organization of inclusions. Here we present a composite structured liquid formed from biopolymers of distinct rigidities, actin and DNA. Actin filaments, well below their persistence length, behave as rigid rods while long polymers of DNA, well above their persistence length, form globular molecules. Crowded into a thin layer, actin filaments form a nematic liquid crystal phase within the DNA domains. As these nematic phases grow and coalesce, defects in the actin nematic filled with DNA are created. We investigate the evolution of these soft, polymer filled defects and compare the dynamics to a continuum model of lyotropic liquid crystal. From the model, we use the defect shape to extract material properties of the liquid crystal. Our results suggest a novel structured soft composite, potentially informing physical mechanisms of controlling material properties and templating functional polymeric materials.   

Microtubule based composite active matter

Active matter dynamics is determined by the balance between active stresses generated by the motile component, and the passive viscous or elastic stresses that arise due to deformation of the background material. In many current formulations of active matter Independent control of active and reactive stresses is not possible. For example, in microtubule-based 2D active nematics, decreasing ATP concentration simultaneously decreases the magnitude of the active stresses and increases the elasticity of the liquid crystalline materials. This limits the range of accessible dynamical states, while also impeding quantitative tests of theoretical models. Recent advances in formulation of microtubule based active matter has enabled dispersion of these motile elements in various passive soft materials including phase separated suspensions, viscoelastic networks and 3D colloidal liquid crystals. These new model systems of composite active matter exhibit diverse dynamical states that are not accessible in previous one components systems.
  

Information and motility exchange in collectives of active particles

Active systems exploit the interplay of autonomous motility and mechanical interactions to spontaneously organize in complex patterns. In many situations, the collective behavior of active agents is driven by the exchange of information that can change the state of the agents. In a minimal model of information exchange, we have studied numerically run-and-tumble runners with an additional two-state internal variable that specifies their motile or nonmotile state. Motile particles change irreversibly into nonmotile ones upon collision with a nonmotile particle. Once turned non-motile, they can reacquire their motility or ``reawaken’’ at a rate m. When m=0, the system relaxes to an absorbing fractal aggregate of non-motile particles, with fractal dimension controlled by density and tumbling rate [1]. For finite reawakening, we find motile, non-motile and mixed states that can organize in complex spatial patterns. We characterize the kinetics of approach to the steady state and its structure in terms of tumbling and reawakening rates.

[1] M. Paoluzzi, M. Leoni, and M. C. Marchetti, Phys. Rev. E 98, 052603 (2018   

Corralling Active Brownian Particles With "Active Billiard" Particles

We examine the role of boundary conditions in a heterogenous active matter system in a bounded domain. The system consists of two types of particles: active Brownian particles, such as Janus particles, and "active billiard" particles, inspired by microorganisms that move in straight paths until they bounce off boundaries at a specific angle determined by their body morphology. This model has been of recent interest in robotics but also applies to microscale active matter systems. Here, we develop a parameterized model of particle type mixtures and characterize how "corralling" behavior emerges. We define corralling as system configurations where the billiard particles converge to a stable periodic orbit with a greater density of active Brownian particles within the convex hull of the orbit than its complement. We confirm that corralling behavior occurs in our model active matter system, in simulation. In previous work, we analytically determined the conditions guaranteeing such stable orbits, as a function of environment geometry and billiard departure angle. We will present extensions of this theoretical approach to statistical models of our example active matter system, with a focus on developing strategies for control.   

Spontaneous demixing of mixed active-passive suspensions.

Understanding the properties of active matter is currently driving a rapid growth in soft- and bio-physics. Some of the most important examples of active matter are at the microscale, and include active colloids and suspensions of microorganisms, both as a simple active fluid and as mixed suspensions of active and passive elements. In these systems, recent work has started to provide a window into new phenomena including activity-induced depletion interactions, phase separation, and the possibility to extract net work from active suspensions. Building on current research in our group exploring the physics of colloid-microswimmer interactions we are interested in understanding how external control of the dynamics of the active component can be used to alter the transport of passive cargo. Here we report on new experiments studying the behaviour of active-passive systems under spatial confinement. We show that the spatial inhomogeneity in swimmers’ distribution and orientation resulting from confinement has a dramatic effect on the spatial distribution of passive particles, with the colloids accumulating either towards the boundaries or towards the bulk of the sample depending on the size of the container. We show that this can be used to induce the system to de-mix spontaneously.   

Unifying descriptions of phase separation in multi-component driven, steady-state systems.

Combined experimental and computational work has shown that a nonequilibrium driven system of Janus particles exhibits phase behavior that is accurately described by the standard Ising model. This suggests that certain steady-state, yet nonequilibrium systems may be described by thermodynamic equilibrium universality classes. Such a description would allow precise control over the phase behavior of active particles, enabling us to design the next generation of active materials analogous to the widespread application of phase separation in creating conventional materials. We examine the generality of this proposition by considering multi-component systems of composite active particles that may belong to other universality classes.   

Phase Diagram of a 2D system of Granular Self-propelled Particles

We report experiments on the phase behaviour of granular squares as a function of number density and activity. The granular particles are energized by vibrating them on a horizontal plane and are designed to have polar activity along a body diagonal. The activity of particles (quantified by the persistence length of motion along the mobility direction) can be controlled by varying the gap between top cover and bottom base. We find that adding activity to the particles qualitatively modifies their phase diagram. At large enough activity, particles always migrate to the boundary and form a high-density ordered state. At smaller values of activity, different phases are seen as a function of density. At low density, the particles form an isotropic liquid. As the density is increased, particles separate into a high-density ordered region while the remaining particles remain in the fluid state. Above a finite density, the phase coexistence curve terminates and all particles freeze into an ordered state. The start and end density of the coexistence region is found to be a function of density. We also discuss dynamics within the dense, ordered state.   

Irreversibility in biological active matter

Cellular structures constantly consume and dissipate energy on a variety of spatiotemporal scales in order to function. While progress has been made in elucidating their organizing principles, much of their thermodynamics remains unknown. In this talk, I will address the question: why measure dissipation in such nonequilibrium systems? I will show that by measuring a multi-scale irreversibility (time-reversal asymmetry) one can extract model-independent estimates of the time-scales of energy dissipation based on time series data collected in an experimental biological system. I further demonstrate that the irreversibility measure maintains a monotonic relationship with the underlying biological nonequilibrium activity. The basic idea of estimating irreversibility for various levels of coarse-graining is quite general; we expect it to lead to important inferences whenever there is a well-defined notion of dissipative scale.