Interfacial activity dynamics of confined active droplets

Abstract

Active emulsions can spontaneously form self-propelled droplets or phoretic micropumps. It has been predicted that the interaction with their self-generated chemical fields can lead to multistable higher-order flows and chemodynamic phenomena. However, it remains unclear how such reaction-advection-diffusion instabilities can emerge from the interplay between chemical reactions and interfacial hydrodynamics. Here, we simultaneously measure the flow fields and the chemical concentration fields using dual-channel microscopy for oil droplets that dynamically solubilize in a supramicellar aqueous surfactant solution. We developed an experimentally tractable setup with micropumps, droplets that are pinned between the top and bottom surfaces of a microfluidic reservoir, which we compare directly to predictions from a Brinkman squirmer model to account for the confinement. With increasing droplet radius, we observe (i) a migration of vortex flows from the posterior to the anterior of the droplet, analogous to a transition from pusher- to puller-type swimmers, (ii) a bistability between dipolar and quadrupolar flow modes, and, eventually, (iii) a transition to multipolar modes. We also investigate how the dynamics evolve over long time periods. 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

References in corpus (17)

• Propulsion of a molecular machine by asymmetric distribution of reaction--products
• Self-propulsion of pure water droplets by spontaneous Marangoni stress driven motion
• Odd dynamics of living chiral crystals
• Self-propulsion of chemically-active droplets
• Vortex arrays and ciliary tangles underlie the feeding-swimming tradeoff in starfish larvae
• Chemotactic self-caging in active emulsions
• Nonlinear dynamics of a chemically-active drop: from steady to chaotic self-propulsion
• Emergence of bimodal motility in active droplets
• We the Droplets: A Constitutional Approach to Active and Self-Propelled Emulsions
• Culinary fluid mechanics and other currents in food science
• Spontaneously rotating clusters of active droplets
• Collective entrainment and confinement amplify transport by schooling micro-swimmers
• Collisions and rebounds of chemically-active droplets
• Instability and self-propulsion of active droplets along a wall
• Alignment and scattering of colliding active droplets
• Swimming droplet in 1D geometries, an active Bretherton problem
• Pumping and mixing in active pores

Cited by in corpus (3)

• Emergent dynamics due to chemo-hydrodynamic self-interactions in active polymers
• Marangoni droplets of dextran in PEG solution and its motile change due to coil-globule transition of coexisting DNA
• Arrested on heating: controlling the motility of active droplets by temperature