Interfacial activity dynamics of confined active droplets

Abstract

Oil droplets in aqueous surfactant solution show spontaneous self-propulsion and dynamics ranging from straight-line to unsteady motion with increasing Péclet number. Recent studies have shown that products of interfacial activity (oil-filled micelles) can lead to multistable higher-mode flow fields and chemorepulsive phenomena. To investigate such chemodynamic effects, we have studied disk-like flattened droplets that are pinned at the top and bottom of the reservoir and produce only pumping flow. With increasing droplet radius we observe the following: (i) vortical structures generated by the droplet migrating around the interface, (ii) bistability between a dipolar and quadrupolar flow mode, and, eventually, (iii) a transition to multipolar modes. For a deeper analysis, we measured flow fields by particle image velocimetry and compared them to a hydrodynamic model based on a Brinkman squirmer. A simultaneous quantification of the flow fields and oil-filled micelle distribution suggests that a local buildup of chemical products leads to a saturation of the surface, which affects the propulsion mechanism. Hence, we describe the interfacial activity dynamics in time and space.

Emergent rigidity in chemically self-interacting flexible active polymers

“…3A). Experimentally, such states may be accessed via tuning of the monomer propulsion [13,36] and developing the full theoretical framework for such dynamics is a future challenge.…”

“…The hydrodynamic flow fields around the oil droplet microswimmer (Fig. 1A, Supplementary Video SV1) depend in tunable ways on geometric and chemical conditions [34][35][36]. In addition to the hydrodynamic flows, these microswimmers leave a trail of chemical fields [12] comprised of oil-filled surfactant micelles formed by the transfer of oil molecules into empty surfactant micelles (Fig.…”

Active compound particles in a quadratic flow: hydrodynamics and morphology

“…Similar to the case of a passive compound particle, the presence of confining drop induces four circulations inside the droplet due to the velocity field of a force dipole (B 2 mode). 55,56 Furthermore, the force dipole's extensional axis is orthogonal to the squirmer's orientation vector. Therefore, the droplet interface gets stretched perpendicular to the orientation vector and compressed in the direction of the orientation vector resulting in a prolate shape of the confining interface.…”