Session A25: Focus Session: Hydrodynamics of Particles and Macromolecules at Fluid Interfaces I

From Electrodiffusion Theory to the Electrohydrodynamics of Leaky Dielectrics through the Weak Electrolyte Limit

The Taylor-Melcher (TM) model is the standard model for describing the dynamics of poorly conducting leaky dielectric fluids under an electric field. The TM model treats the fluids as Ohmic conductors, without modeling ionic electrodiffusion. Mathematical reconciliation of the electrodiffusion picture and the TM model has been a major issue for electrohydrodynamic theory. Here, we derive the TM model from an electrodiffusion model the electrochemistry of ion dissociation is modeled explicitly. We introduce salt dissociation reaction terms in the bulk electrodiffusion equations and take the limit in which the salt dissociation is weak (i.e., poorly conducting media). Together with the assumption of small Debye length, we derive the TM model with the surface charge convection term. An important quantity that emerges is the Galvani potential (GP), the jump in voltage across the liquid-liquid interface between the two leaky dielectric media. When the GP is absent, we recover the TM model. In the presence of a non-zero GP, our model predicts that the liquid droplet will drift under an imposed electric field, the velocity of which is computed explicitly to leading order.   

Capillary-Induced Assembly of Janus Particles in Drying Films

We perform mesoscale simulations to study the self-assembly and deposition of amphiphilic Janus particles in a drying thin film. Many-body dissipative particle dynamics is applied to model the evaporation of particle-laden films. For charged particles, electrostatic interactions are calculated using a finite-difference Poisson solver, which takes account for the dielectric contrast across the liquid-vapor interface. Given the unique anisotropy of Janus particles, we probe their translational and rotational dynamics in a thin film and subsequent assembly after adsorption to the interface. The simulation provides quantitative insight into the interplay between the electrostatic interactions with substrate, image force due to dielectric discontinuity, and interparticle electrostatic and capillary interactions. Janus particles assemble into a large-scale open fractal structure with identical orientation. In contrast, homogeneous particles form only isolated small aggregates having random orientation. These structures agree well with the deposits observed in experiments. Simulation further indicates the dominant force is capillary attraction due to the interface pinning at rough Janus boundaries.   

Colloidal assembly on the surface of drying sessile droplets

The self-assembly of nano or microparticles on evaporating liquid droplets is an important transport process underpinning various applications, such as printing and surface patterning. By applying a recently developed lattice Boltzmann-Brownian Dynamics (LB-BD) method, we observe the aggregation of colloidal particles on the droplet surface for strong particle-fluid interactions. The effective attraction between particles is probed in detail and shows long-range nature. By simulating drying droplets with different geometries, we find that the aggregation spots relate to the local curvature. This indicates that the effective attraction in the simulation resembles the capillary interaction, which is induced by the disturbance of the fluid interface. In addition, we model the particle assembly under evaporation to reveal the interplay of the long-range particle attraction and the evaporation-induced flow field. Our study aims to provide greater insight into the capillary-like assembly of particles in evaporating liquid masses and its relationship to the final deposit.