Session M19: Focus Session: Interfacial Active Matter II

Spontaneous polarization and locomotion of an active particle with surface-mobile enzymes

We study the locomotion of active colloids whose surface is uniformly coated with mobile enzymes, which can be realized experimentally by attaching enzymes to the surface of vesicles, polymerosomes or other particles with fluid-like surfaces. The enzymes can migrate and diffuse laterally along the surface and they and catalyze a reaction that generates surface phoretic flows. Since the  distribution of enzymes along the surface is homogeneous the locomotion of the active colloid is forbidden by symmetry. However, we find that the ability of the enzymes to migrate over the surface, combined with self-phoresis, can lead to a spontaneous symmetry-breaking instability whereby the homogeneous distribution of enzymes polarizes and the active particle propels. The instability is driven by the advection of enzymes by the phoretic flows and occurs above a critical Péclet number. The transition to polarized motile states occurs via a supercritical or subcritical pitchfork bifurcations, the latter of which enables hysteresis and coexistence of uniform and polarized states.   

Entrainment and confinement amplify transport by schooling micro-swimmers

Active agents serve an important function in the transport and distribution of drugs or chemical signals. While by intuition one might think of transport in the agent's interior, it can also be mediated by entrainment of the surrounding bulk liquid. Here, we investigate the fluid dynamics of entrainment by squirmers in confined geometries, represented by an active droplet, constituting a weak pusher, in a quasi-2D microfluidic environment. We compare experimental measurements of flow fields and the displacement of tracer colloids to a theory model of a swimmer in a Brinkman medium, which accounts for the quasi 2D-confinement. The model compares favourably with an unconfined Stokes description. We further extend this model by simulations of a school of squirmers collectively entraining particles in the bulk medium. We show that the entrainment is enhanced both by the collective action of the school and by the strong confinement in the presence of boundaries, giving the school a significant cargo capacity.   

Alignment and scattering of swimming droplets

Chemically-active droplets swim in viscous flows by exploiting the nonlinear hydro-chemical coupling between the transport of a solute produced at their surface and the surface-driven flows generated by its gradients. They are intrinsically anti-chemotactic, swimming away from regions of higher solute concentrations. This includes their own wake where the slowly-diffusing solute accumulates as they move, in a mechanism that is in fact critical to their self-propulsion. As a result, swimming droplets also avoid the proximity of other droplets and/or their chemical trails, leading to complex avoiding trajectories and collective dynamics.

Building upon our complete modeling of the nonlinear hydrochemical problem for axisymmetric droplet interactions, this presentation will introduce a reduced order analysis of general (oblique) droplet collisions, and show such interactions may lead to either alignment or scattering of their trajectories.   

Self-propulsion of chemically-active droplets along a wall

Chemically-active droplets exploit the non-linear coupling of the transport of a chemical solute produced at their surface to the Marangoni flows such solute generates, in order to swim at the micron scale. The onset of self-propulsion above a critical advective-to-diffusive transport ratio (Péclet number) is now well established in unbounded flows. Although it is an intrinsic feature of experimental systems, confinement by a rigid wall and its influence on the propulsion is systematically absent in active droplet models. Using non-axisymmetric bi-spherical coordinates, we therefore investigate the onset of self-propulsion of active droplets hovering near a rigid wall. To do so, we characterize the axisymmetric hovering state and analyse its stability with respect to horizontal (wall-parallel) swimming, for varying wall-to-drop distance and Péclet number.   

Swimming droplets in capillaries : Bretherton problem with active interfaces

We investigate experimentally the behavior of self-propelled water-in-oil droplets, confined in capillaries of different square and circular cross-sections. The droplet’s activity comes from the formation of swollen micelles at its interface. In straight capillaries, the velocity of the droplet decreases with increasing confinement. However, at very high confinement, the velocity converges toward a non-zero value, so that even very long droplets swim. Stretched circular capillaries are used to explore even higher confinement. The lubrication layer around the droplet then takes a non-uniform thickness which constitutes a significant difference to usual flow-driven passive droplets. A neck forms at the rear of the droplet, deepens with increasing confinement, and eventually undergoes successive spontaneous splitting events for large enough confinement. Such observations stress the critical role of the activity of the droplet interface in the droplet’s behavior under confinement. We then propose an analytical formulation by integrating the interface activity and the swollen micelle transport problem into the classical Bretherton approach. We further discuss on the saturation of the micelle concentration along the interface, which would explain the divergence of the lubrication layer thickness for long enough droplets, eventually leading to spontaneous droplet division.   

Self-propulsion of chemically-active droplets in a cylindrical capillary

Chemically-active droplets swim spontaneously  by exchanging chemical solutes with their environment and exploiting the non-linear coupling of their transport to the surface-generated flows created by their gradients. They always propel near boundaries, an experimental feature that was observed to significantly impact their dynamics.

Here, we present a novel numerical framework based on moving embedded boundaries to analyse the nonlinear self-propulsion dynamics of chemically-active droplets in complex spatial configuration. We  further use that framework to study the self-propulsion of an active droplet along a capillary tube, focusing specifically on the impact of the confinement intensity (i.e. the particle-to-tube radius ratio) on the dynamics and propulsion characteristics.   

The many behaviors of deformable active droplets

Active fluids consume fuel at the microscopic scale, converting this energy into forces that can drive macroscopic motion. In some cases, these phenomena have been well characterized, and theory can explain experimentally observed behaviors in both bulk fluids and those confined in simple stationary geometries. More recently, active fluids have been encapsulated in viscous drops or elastic shells so as to interact with an outer environment or a deformable boundary. Such systems are not as well understood. In this talk, I will discuss the behavior of droplets of an active nematic fluid. Through a mix of linear stability analysis and nonlinear simulations, we identify parameter regimes where single modes dominate and droplets behave simply: as rotors, swimmers, or extensors. When parameters are tuned so that multiple modes have nearly the same growth rate, a plethora of modes appears, including zigzaggers, washing machines, wanderers, and pulsators. The behavior of such droplets is not static, but is an emergent property of their internal dynamics and the environment that surrounds them. In particular, individual droplets in many-droplet suspensions can show qualitatively different dynamics than when isolated, as they respond to the stresses generated by those drops that surround them.   

Diffusiophoresis of particles under the influence of pH

We study experimentally and theoretically diffusiophoresis of colloidal particles with varying surface potential. When the system pH is varied, the ionization processes associated with acid-base equilibria change the fraction of charged groups on the surfaces of polymeric microspheres. Such change in the surface chemistry affects the diffusiophoretic motion of particles under a concentration gradient of ions. We demonstrate with microfluidic experiments how the solute and pH gradients, combined with the bulk flow, are manifested in the transient distribution of polystyrene particles. The ideas are tested with both positively and negatively charged particles, and compared with corresponding theoretical predictions.   

Interfacial agglomeration of self-propelled Janus particles in evaporating droplets

Living microorganisms are often found in confined systems, where they typically experience an affinity to populate boundaries, or at least their vicinities. For example, in the case of model microorganisms as the bacteria E. coli, the reasons for such affinity to interfaces is a combination of their directed motion and hydrodynamic interactions at distances larger than their own size. Here we will show that self-propelled Janus particles (polystyrene particles partially coated with platinum) immersed in droplets of water and hydrogen peroxide tend to accumulate on the vicinity of the liquid/gas interface. Interestingly, the interfacial accumulation occurs despite the presence of a Marangoni flow in the evaporating droplet, which tends to redistribute the particles within the bulk. By performing additional experiments with passive colloids behaving as flow tracers and by comparing with numerical simulations for the fluid flow and for the particle dynamics, we will disentangle the dominating mechanisms behind the interfacial particle accumulation observed. Our results show new unexpected parallelisms between active Janus particles and some biological microswimmers in the way they interact with their environment.   

Shape-induced pairing of metallo-dielectric active particles

Metallo-dielectric Janus particles can self-propel in liquid solution when exposed to AC electric fields via induced charge electrophoresis (ICEP). In recent experiments conducted in the Snezhko group, it was observed that ICEP Janus particles with discoidal shape can spontaneously form stationary bound pairs separated by a liquid-filled gap. Paired particles exhibit “head-on” alignment, i.e., the particles’ axes of symmetry are aligned with the center-to-center vector, and the gap width decreases with increasing frequency of the AC field. Strikingly, this pairing is not observed for spherical particles. In order to rationalize these observations, we develop a theoretical model for collisions between two ICEP particles interacting via self-induced hydrodynamic and electric fields. We recover the experimental observations, finding that hydrodynamic interactions dominate the pairing; effectively, the particles behave as field-tunable “squirmers.” In particular, we show that “head-on” alignment is driven by a coupling between the oblate shape of the particles and hydrodynamic flows generated by swimming activity. Our findings suggest that self-organization of ICEP particles could be “programmed” via particle shape and surface patterning.