Session B29: Active Matter in Complex Environments II

The Physics of Cold Active Matter: on how time-independent disorder affects the motion of self-propelled particles

Most examples of natural active matter systems, if not all, take place in heterogeneous media. Despite this, most experimental and theoretical efforts have focused on homogeneous media and the impact of environmental heterogeneities on individual and collective properties of active systems has remained, up to recently, unexplored. Here, we will see that the physics of active systems in heterogeneous media is fundamentally different from the one in homogenous environments. For instance, in heterogeneous environments, spontaneous particle trapping of particles and sub-diffusion can occur, while long-range order of two-dimensional polar active liquids is not possible. Furthermore, in the absence of dynamic noise, it is possible to show that when the equations of motion exhibit a Hamiltonian structure, particle trapping cannot occur, while the presence of attractors in these equations indicates the asymptotic convergence of particle trajectories to bounded areas in space, i.e. traps.

References:
F. Peruani and I. S. Aranson, Phys. Rev. Lett. 120, 238101 (2018)
O. Chepizhko and Fernando Peruani, Phys. Rev. Lett. 111, 160604 (2013)
O. Chepizhko, E.G. Altmann, and F. Peruani, Phys. Rev. Lett. 110, 238101 (2013)
  

The motion of active colloids and their induced flow field at fluid interfaces

Colloid motion is altered by adsorption to fluid interfaces, allowing for boundary guidance or directed assembly. This is particularly interesting for colloidal motion driven by external fields (driven colloids) or for self-propelled objects (active colloids or swimmers). However, the influence of a fluid interface on these hydrodynamic phenomena and the 2-d flow field induced by the motion of these active colloids are relatively unexplored. Here we image the motion of passive tracer probes at the interface to discern displacement fields generated by (1) a microbead undergoing thermal motion, (2) a magnetic disk forced to translate at constant velocity, and (3) actively swimming bacteria. We also theoretically quantify the flows generated by interfacially trapped colloids by developing an appropriate flow singularity model for both driven and active colloids. We consider an ideal flat "clean" interface characterized solely by a uniform interfacial tension. We also consider an incompressible fluid interface, as can occur even for trace surfactant adsorption. Theoretical results are compared with experimental flow field. Our results will be useful in future work on the use of active colloids to direct and enhance transport at interfaces   

Hydrodynamic attraction of bacteria to gas and liquid interfaces

Motile microorganisms often encounter solid and fluid boundaries. Understanding the underlying physics of their accumulation near these boundaries can impact a wide range of applications from biofilm formation to micro-robot design. We combine experiments and numerical simulations to investigate bacteria (Escherichia coli) attractions to various fluid-fluid interfaces. We show that the strongest cell accumulation occurs near the lowest viscosity ratio (gas interfaces). We develop a new theory based on Brownian dynamics including hydrodynamics and short-range physio-chemical interactions between bacteria and surfactant-laden interfaces and explain our experimental findings. We include higher order singularities in our hydrodynamic model and study their effects on bacterial orientations, trajectories, and accumulation.   

Non-Markovian active droplets

Active droplets are a model system for the chemotactic dynamics of microbes. They propel themselves through solute-mediated interactions. Previously, we showed that the motion of small droplets (~30µm diameter) is akin to active rotational diffusion whose speed and persistence time can be tuned by changing the salt and surfactant concentrations, respectively (Izzet et al., arXiv:1908.00581, 2019). Here we show that the trajectories of larger droplets display complex non-Markovian behaviors, such as self-interacting dynamics, that require a more complicated model than rotational diffusion. These results indicate that our tunable swimmers are ideal candidates for the study of the departure from equilibrium to high levels of activity, on both the single-particle level and their collective behavior, including the motility-induced phase separation (MIPS).   

Stigmergy in active furrowers

Stigmergy is the emergence of spatiotemporal coordination between agents through interactions mediated by their environment. Pheromone trail-following in ants is a well-known example. Recently, a mechanical form of stigmergy has been observed in colonies of motile bacteria that create networks of furrows as they advance over soft agar [Gloag et al., PNAS, 2015]. Vanguard rafts of bacteria mechanically deform the substrate to create a physical signal that other cells follow. Under certain conditions, extensive furrow networks emerge with a distinctive morphology that spans a broad range of length scales. It is possible that the formation of such a sparse network of furrows is a strategy to loosely colonize large areas with a small number of cells. We investigate the conditions under which extensive furrow networks emerge using simulations of self-propelled rods that furrow through a passive elastic or plastic medium. The structure of the furrow network appears to critically depend upon the clustering behaviour of active rods at the advancing edge, which, in turn, depends on substrate stiffness and the self-propulsion velocity of the rods. We attempt to explain our observations through a coarse-grained, but general, model of stigmergy.   

Slip and distance effects on the self-propulsion of catalytic microswimmers close to a wall

Catalytic microswimmers are typically found self-propelling parallel to substrates. Although experimental observations have so far hinted at non-negligible substrate effects on the swimmer velocity, a quantitative examination of the effect of the substrate is still lacking.
In this talk, we will present quantitative measurements of the effect of the substrate on the velocity of Pt-coated model microswimmers. We will show that under otherwise fixed conditions the velocity depends on the substrate wetting angle, which in turn relates to the associated hydrodynamic slip length of the substrate. We will further demonstrate that our hypothesis on slip dependent velocities is not only supported by qualitative and scaling arguments, but also by our measurements on swimmer-wall separation distance.
Our findings provide further understanding on the swimming behavior of synthetic microswimmers. Such understanding is desirable for future applications that may require microscopic devices to self-propel in liquid environments in confinement and along boundaries.
  

Measuring chaotic advection in a biological active nematic in viscous environments

Active flows are commonly found in nature ranging from macro-scale (bird flocks and schools of fish), to the micro-scale (cytoplasmic streaming and bacterial colonies). The common theme among these active materials is the individual entities consume energy leading to large-scale collective motion and flows. A widely studied example is the microtubule/kinesin based active network which is highly tunable. When this non-equilibrium system is confined in 2D at the oil/water interface, active topological defects emerge generating chaotic flows where the microtubules are advected within the network. An avenue of interest in nonequilibrium systems is to observe the material’s response to external changes in its environment. Prior research has shown that changing the viscosity of the oil in contact with the active network changes the velocity and morphology of the network. Increasing the viscosity increases the number density of defects and decreases the network velocity. We investigate how the external changes from viscosity affect various quantitative parameters from chaotic advection theory, such as local fluid stretching rates within the network and topological entropy calculated from defect braiding.   

Hydrodynamic interactions between passive colloids in an active bacterial bath

The dynamics of active particles has been a subject of intense scientific investigation owing to the complex nonequilibrium patterns observed in these systems. Nonequilibrium effects also have tangible consequences for interactions between active and passive objects, as evidenced by the spontaneous rotation of microscopic gears in suspensions of motile bacteria. Here, using video microscopy experiments and simulations, we show that motile bacteria have a profound impact on dynamical interactions between passive colloidal particles. By quantifying spatial velocity correlations and the short time pair diffusivity tensor for colloids, we show that bacterial motion leads to partial screening of long-ranged hydrodynamic interactions. Notably, for sufficiently dense bacterial suspensions, transverse velocity correlations develop a pronounced minimum at a characteristic length scale that is independent of the bacterial density, but depends on the bacterial species. We hypothesize that the observed length scale demarcates short-ranged interactions dominated by direct collisions between colloids and bacteria from long-ranged ones associated with hydrodynamics.   

Self-organization of swimmers drives long-range fluid transport in bacterial colonies

Motile subpopulations in microbial communities are believed to be important for dispersal, quest for food, and material transport. Here, we show that motile cells in sessile colonies of peritrichously flagellated bacteria can self-organize into two adjacent, centimeter-scale motile rings surrounding the entire colony. The motile rings arise from spontaneous segregation of a homogeneous swimmer suspension that mimics a phase separation; the process is mediated by intercellular interactions and shear-induced depletion. Our findings present a unique form of bacterial self-organization that influences population structure and material distribution in colonies.   

Linear instability and nonlinear dynamics of droplets and layers of active fluid

Active suspensions are fluids with extra stresses from the energy-consuming activity of suspended particles.
Coarse-grained continuum descriptions have successfully predicted instabilities and pattern formation observed in some experimental systems. In this work we focus on the effects of surface tension on the stability and nonlinear dynamics in droplets and layers of active fluid. Specifically we study the stability of a moving boundary between a viscous fluid and droplet- or layer-bound active suspension. Linear stability analyses predict parameter regimes for various dynamics such as rotation, self-propulsion, and chaotic dynamics. Weakly nonlinear analyses predict the equilibrium drop deformation as a function of activity magnitude in the suspension. Simulations of a small system of such active drops give insight into how the activity inside the drops dictates how they communicate with each other.   

Swimming of active drops in confinements with external flows

Biological microswimmers commonly navigate through liquid flows in confinements, e.g. locomotions of spermatozoa in the reproductive tract and bacteria in blood vessels. Artificial microswimmers designed for drug delivery also swim in confinements having flows. So far, there are theoretical works explaining the dynamics of microswimmers in Poiseuille flow. However, there is little quantitative experimental data regarding swimming of active matter in confined microflows. Here, we elucidate the swimming behaviour of active drops in Poiseuille flow. The Marangoni stress dominated `self-propulsion’ of these droplets serves as a model for studying the swimming hydrodynamics of microorganisms, specifically pushers like E. Coli. We quantitatively explain the upstream swimming and downstream tumbling motion of the active drops in externally imposed flows; we also provide a comparative analysis of the experimental data with the hydrodynamics based non-linear theory. We feel that understanding such swimming behaviour of active matter will make significant contribution towards better comprehension of certain important biological phenomena.   

Examining collective mechanical properties of fish schools using projected light fields

Social animals including insects, fish, birds, and even humans exhibit self-organized collective behavior. Models have shown that simple local interactions between individuals gives rise to the emergent self-organized macroscopic states such as flocks, swarms, or schools. We investigate the material properties of laboratory fish schools by exploiting the negative phototaxicity of Rummy-Nose Tetra (Hemigrammus blehri). To do this, we use an overhead high speed camera to record individual fish trajectories in a quasi-two-dimensional tank. By projecting two dark regions moving in opposite directions, the school of fish is strained as individuals use both social and environmental information to determine their behavior. The school undergoes a large deformation before snapping back into one of the dark regions. Our results show that the school exhibits a stress-stain relationship similar to Hooke's law.