Interfacial activity dynamics of confined active droplets

Abstract

Active emulsions can spontaneously form self-propelled droplets or phoretic micropumps. However, it remains unclear how these active systems interact with their self-generated chemical fields, which can lead to emergent chemodynamic phenomena and multistable interfacial flows. Here, we simultaneously measure the flow and chemical concentration fields using dual-channel fluorescence microscopy for active micropumps, i.e. immobilised oil droplets that dynamically solubilise in a supramicellar aqueous surfactant solution. With increasing droplet radius, we observe (i) a migration of vortices from the posterior to the anterior, analogous to a transition from pusher- to puller-type swimmers, (ii) a bistability between dipolar and quadrupolar flows and, eventually, (iii) a transition to multipolar modes. We also investigate the long-time dynamics. Together, our observations suggest that a local build-up of chemical products leads to a saturation of the surface, which controls the propulsion mechanism. These multistable dynamics can be explained by the competing time scales of slow micellar diffusion governing the chemical buildup and faster molecular diffusion powering the underlying transport mechanism. Our results are directly relevant to phoretic micropumps, but also shed light on the interfacial activity dynamics of self-propelled droplets and other active emulsion systems.

Emergent dynamics due to chemo-hydrodynamic self-interactions in active polymers

“…The hydrodynamic flow fields around the oil droplet microswimmer (Fig. 1 B, Supplementary Video SV1) depend in tunable ways on geommetry also, as we show later, chemical conditions 36 , 41 – 43 . 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—which can be visualised using an oil-soluble fluorescent dye (Fig.…”

“…The “dry” chemical active matter model we have considered here is in line with similar recent successful descriptions used to describe the scattering dynamics of monomer droplets due to their repulsive auto-chemotaxis 12 , 50 . However, the effects of hydrodynamic coupling are apparent in our system at high filled micelle concentrations, where we have further found that the monomer self-propulsion speed is suppressed 42 , 43 . While such coupling does not qualitatively effect the emergent rigidity and self-propulsion that we have reported here, feedback and time-delay effects from such coupling can give rise to steady-states such as oscillations.…”

“…First, we discuss a method to tune the monomer propulsion speed, and then in the next section discuss its implication on the chemo-hydrodynamic coupling. The propulsion of the monomer can be tuned by modulating the transport of fresh surfactant micelles to the droplet interface 42 , 43 , 49 . Specifically, such modulation affects the distribution of the surfactant gradient, and thereby the slip velocity on the surface of the droplet, and hence the chemical and hydrodynamic fields.…”

Marangoni Droplets of Dextran in PEG Solution and Its Motile Change Due to Coil–Globule Transition of Coexisting DNA

“…13,14 We analyzed the convections in detail based on the squirmer model in a Brinkman medium 23−25 and the fitting is shown as the blue arrows in Figure 3a (see also S3 in the Supporting Information for the analysis). 26 It suggests that the convection outside the droplets is dipolar (β = −0.1, nearly neutral). The speed of the convections is high near the droplet boundary and the center.…”

The motion of micro-swimmers over a cavity in a micro-channel

“…Goh et al 29 described the chase behavior between two squirmers and emphasized the significance of hydrodynamic sensing capabilities in microbial turning. In some cases, chirality is not the primary consideration, for example, in artificial Janus particles 30 and active liquid droplets, 31 and microswimming in flat walls and narrow microchannels. 32 In summary, the research on squirmers under complex flows and confined boundary conditions demonstrates the diversity of microbial locomotion in fluid environments, which deserves further study.…”