Gravitational settling of active droplets.

The gravitational settling of oil droplets solubilizing in an aqueous micellar solution contained in a capillary channel is investigated. The motion of these active droplets reflects a competition between gravitational and Marangoni forces, the latter due to interfacial tension gradients generated by differences in filled-micelle concentrations along the oil-water interface. This competition is studied by varying the surfactant concentration, the density difference between the droplet and the continuous phase, and the viscosity of the continuous phase. The Marangoni force enhances the settling speed of an active droplet when compared to the Hadamard-Rybczynski prediction for a (surfactant free) droplet settling in Stokes flow. The Marangoni force can also induce lateral droplet motion, suggesting that the Marangoni and gravitational forces are not always aligned. The decorrelation rate (α) of the droplet motion, measured as the initial slope of the velocity autocorrelation and indicative of the extent to which the Marangoni and gravitational forces are aligned during settling, is examined as a function of the droplet size: correlated motion (small values of α) is observed at both small and large droplet radii, whereas significant decorrelation can occur between these limits. This behavior of active droplets settling in a capillary channel is in marked contrast to that observed in a dish, where the decorrelation rate increases with the droplet radius before saturating at large values of droplet radius. A simple relation for the crossover radius at which the maximal value of α occurs for an active settling droplet is proposed.

[1]  Lauren D. Zarzar,et al.  Chemically Tuning Attractive and Repulsive Interactions between Solubilizing Oil Droplets. , 2022, Angewandte Chemie.

[2]  Lauren D. Zarzar,et al.  We the Droplets: A Constitutional Approach to Active and Self-Propelled Emulsions , 2022, Current Opinion in Colloid & Interface Science.

[3]  S. Michelin Self-Propulsion of Chemically Active Droplets , 2022, Annual Review of Fluid Mechanics.

[4]  Aditya S. Khair,et al.  Dynamics of forced and unforced autophoretic particles , 2022, Journal of Fluid Mechanics.

[5]  Lauren D. Zarzar,et al.  Chemical Design of Self-Propelled Janus Droplets , 2021, Matter.

[6]  R. Golestanian,et al.  Chemotactic self-caging in active emulsions , 2020, Proceedings of the National Academy of Sciences of the United States of America.

[7]  Ory Schnitzer,et al.  Isotropically active colloids under uniform force fields: from forced to spontaneous motion , 2021, Journal of Fluid Mechanics.

[8]  Lauren D. Zarzar,et al.  Predator–prey interactions between droplets driven by non-reciprocal oil exchange , 2019, Nature Chemistry.

[9]  S. Michelin,et al.  Self-propulsion near the onset of Marangoni instability of deformable active droplets , 2018, Journal of Fluid Mechanics.

[10]  D. Lohse,et al.  Flutter to tumble transition of buoyant spheres triggered by rotational inertia changes , 2018, Nature Communications.

[11]  D. A. Barry,et al.  Universal expression for the drag on a fluid sphere , 2018, PloS one.

[12]  S. Herminghaus,et al.  Dimensionality matters in the collective behaviour of active emulsions , 2016, The European physical journal. E, Soft matter.

[13]  William H. Mitchell,et al.  Sedimentation of spheroidal bodies near walls in viscous fluids: glancing, reversing, tumbling and sliding , 2014, Journal of Fluid Mechanics.

[14]  E. Lauga,et al.  Spontaneous autophoretic motion of isotropic particles , 2012, 1211.6935.

[15]  Stephan Herminghaus,et al.  Swarming behavior of simple model squirmers , 2011 .

[16]  M. LeVan,et al.  Motion of a droplet containing surfactant , 1989 .

[17]  M. LeVan,et al.  RETARDATION OF DROPLET MOTION BY SURFACTANT. PART 2. NUMERICAL SOLUTIONS FOR EXTERIOR DIFFUSION, SURFACE DIFFUSION, AND ADSORPTION KINETICS , 1983 .

[18]  M. LeVan,et al.  RETARDATION OF DROPLET MOTION BY SURFACTANT. PART 1. THEORETICAL DEVELOPMENT AND ASYMPTOTIC SOLUTIONS , 1983 .