Interfacial activity dynamics of confined active droplets

Prashanth Ramesh*, Babak Vajdi Hokmabad, Dmitri O. Pushkin, Arnold J.T.M. Mathijssen, Corinna C. Maass*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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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.

Original languageEnglish
Article numberA29
Pages (from-to)1-17
Number of pages17
JournalJournal of fluid mechanics
Volume966
Early online date4 Jul 2023
DOIs
Publication statusPublished - 10 Jul 2023

Keywords

  • active matter
  • Hele-Shaw flows
  • microfluidics
  • UT-Hybrid-D

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