Physical Review Fluids

A highly accurate approximate solution is developed for the problem of diffusive mass transfer from a two-sided sphere. The problem is primarily motivated by the Marangoni-driven motion of a chemically active spherical surfer along a water-air interface, but it is also related to the self-diffusiophoresis of catalytic colloids, as well as to the conduction heat transfer and electrostatics problems involving partially insulated spheres.

Fire ants are a notorious invasive insect found all over the world. Like flocks of birds and schools of fish, fire ants can be found in large aggregations. Here, we show that fire ants avoid each other on the water surface. As a result, small rafts quickly separate. To aggregate together in larger numbers, they rely on surface tension forces, the same ones that bring Cheerios together in a bowl of milk, to overcome their exploratory instincts.

Chemically active drops are not neutrally buoyant and thus often swim along a rigid wall; yet the influence of such confinement on self-propulsion is generally overlooked in theoretical studies. Using a model system for the active drop, we solve here numerically the nonlinearly coupled hydrochemical problem for the chemical transport and flow field around the moving drop in order to unveil mechanisms governing droplet propulsion parallel to the wall. We show that proximity to a rigid wall increases the drop’s swimming speed by focusing the strongest interfacial flows to the thin gap between the drop and the wall.

Diffusiophoretic motion of particles is influenced by the pH of surrounding liquid, due to the varying zeta potential of surfaces at different pH. By using the particles with an isoelectric point (pI), we study pH-dependent diffusiophoresis under a pH gradient. Various particle behaviors in the absence and presence of wall diffusioosmosis are demonstrated using a dead-end pore geometry.

Many living microorganisms experience an affinity to populate boundaries. The reasons for such affinity can be complex. Here we show that a simple synthetic microswimmer (Janus catalytic colloidal particles) tends to accumulate in the vicinity of liquid interfaces in sessile droplets. We show that the main mechanism is related to their active swimming motion, which is dominating even in the presence of evaporation-driven flows within the sessile droplet.

Understanding active colloidal systems with nonequilibrium self-organization is a long-standing, challenging area in material sciences and biology. In this work we use a colloidal suspension of Janus particles to investigate the hydrodynamics that underlies their various morphologies (unilamellar, multilamellar, and striated structures) and rheological properties.