Numerical simulations of self-diffusiophoretic colloids at fluid interfaces.

T Peter,J Harting,S Dietrich,P Malgaretti,A Scagliarini,N Rivas

doi:10.1039/c9sm02247c

Abstract

The dynamics of active colloids is very sensitive to the presence of boundaries and interfaces which therefore can be used to control their motion. Here we analyze the dynamics of active colloids adsorbed at a fluid-fluid interface. By using a mesoscopic numerical approach which relies on an approximated numerical solution of the Navier-Stokes equation, we show that when adsorbed at a fluid interface, an active colloid experiences a net torque even in the absence of a viscosity contrast between the two adjacent fluids. In particular, we study the dependence of this torque on the contact angle of the colloid with the fluid-fluid interface and on its surface properties. We rationalize our results via an approximate approach which accounts for the appearance of a local friction coefficient. By providing insight into the dynamics of active colloids adsorbed at fluid interfaces, our results are relevant for two-dimensional self assembly and emulsion stabilization by means of active colloids.

Highlights

The dynamics of synthetic or biological, self-propelled objects is strongly affected by the presence of boundaries and interfaces.[1,2,3,4,5] For example, sperm cells have been observed to accumulate at solid walls[6] and bacteria swim in circles when close to substrates[7] or fluid interfaces.[8]
Since these active colloids attain their net displacement by generating local gradients of intensive thermodynamic quantities, such as temperature or thechemical potential,[25,26,27,28] their active displacement is sensitive to barriers and interfaces, which affect the profile of the local temperature and thechemical potential gradients
After validating our numerical scheme against analytical predictions for the velocity of a self-diffusiophoretic colloid in a homogeneous fluid[42], we study the dynamics of a self-diffusiophoretic colloid adsorbed at a fluid interface

Summary

Introduction

The dynamics of synthetic or biological, self-propelled objects is strongly affected by the presence of boundaries and interfaces.[1,2,3,4,5] For example, sperm cells have been observed to accumulate at solid walls[6] and bacteria swim in circles when close to substrates[7] or fluid interfaces.[8]. The phoretic slip velocity has been invoked in those cases in which the imbalance in the local chemical potential is confined to a thin shell around the particle where the Stokes equation is solved analytically.[25] This approach becomes more complicated if the active colloid is adsorbed at a fluid interface in the presence of a three-phase contact line In this context, we present a novel approach, based on numerical simulations, in which the motion of the self-diffusiophoretic colloid is obtained by using the lattice Boltzmann method in order to construct approximate solutions of the Navier–Stokes equation directly. Our results show that, even in this case, self-phoretic colloids trapped at fluid

Methods

Results

Conclusion