Dynamics of a spherical colloid at a liquid interface: A lattice Boltzmann study

Abstract

We study the dynamics of a spherical colloidal particle pulled along fluid-fluid interface using lattice Boltzmann (LB) simulations. We consider an interface with a finite width and include both the effects of the thermodynamics of the interface and the particle wetting, characterised by the contact angle θ between the particle surface and the interface, in addition to the viscosity ratio λ between the two fluids. We characterise the particle dynamics by applying a constant pulling force along the interface and measure both the translational and the rotational dynamics as a function of the contact angle and the viscosity ratio. We observe that the hydrodynamic drag is reduced and the particle rotation is increased when the particle resides more in the low viscosity fluid, in agreement with previous hydrodynamic theories. We also present a case where the particle rotation is suppressed. Then, the drag is observed to increase as compared to the situation where rotation is allowed, while the general behaviour of reducing drag while the particle thermodynamically prefers the lower viscosity fluid, is retained.

Reorientation dynamics of microswimmers at fluid-fluid interfaces

“…The fluid-fluid interface is realized in terms of a Ginzburg-Landau free-energy functional [43], where −A = B > 0 and κ are constants, and c is the phase composition, where c * = ±1 are the equilibrium compositions. The temporal evolution of the phase field variable c is governed by a Cahn-Hilliard advection-diffusion equation, and the fluid velocity is obtained by solving the incompressible Navier-Stokes equation (for more details of the model, see, e.g., [43,44]). The coupled equations are solved using a hybrid finite-difference lattice Boltzmann scheme detailed in [43,44].…”

“…The temporal evolution of the phase field variable c is governed by a Cahn-Hilliard advection-diffusion equation, and the fluid velocity is obtained by solving the incompressible Navier-Stokes equation (for more details of the model, see, e.g., [43,44]). The coupled equations are solved using a hybrid finite-difference lattice Boltzmann scheme detailed in [43,44]. The phase-dependent viscosities are implemented through the relation [45]…”

Reorientation dynamics of microswimmers at fluid-fluid interfaces

“…The temporal evolution of the phase field variable c is governed by a Cahn-Hilliard advectiondiffusion equation, and the fluid velocity is obtained by solving the incompressible Navier-Stokes equation (for more details of the model see e.g. [42,43]). The coupled equations are solved using a hybrid finite difference lattice Boltzmann scheme detailed in [42,43].…”

“…[42,43]). The coupled equations are solved using a hybrid finite difference lattice Boltzmann scheme detailed in [42,43]. The phase dependent viscosities are implemented through the relation [44],…”

“…Results.-A neutrally wetting particle placed at the interface, adopts a symmetrical position (Fig. 1a) [43]. We start with the case B 1 = 0, where the particle has zero linear velocity yet is subject to a force dipole B 2 .…”