Collective behavior of colloidal particles on the fluid interface [electronic resource]
Keywords: Chemical Engineering

It is well known that charged colloidal particles may form an effective two dimensional suspension at a fluidic interface. Research toward the understanding of the dynamics and collective behavior of these suspended particles is at the core of engineering Pickering emulsions, which have a broad range of practical applications. In the first half of this dissertation, we will explore the single phase particle monolayer. Although microscopic images and rheological measurements have been obtained in abundance, the underlying physics behind the collective behavior of such systems is yet to be fully understood. We started by examining the attractive and repulsive potentials in this system. In the case of charged colloidal particles at a fluidic interface, capillary attraction rises from the meniscus deformation, which is predominantly due to the mismatch of dielectric constants between the aqueous phase and the non-polar phase. On the other hand, aggregation is prevented by the long range Coulombic repulsion through the non-polar phase as proposed by Aveyard and coworkers. With the basic knowledge of interaction potentials present, Monte Carlo simulation and Brownian dynamics simulation were performed, assuming pair-wise interactions with all physical constants of a system consisting of latex particles trapped at a water-decane interface. Microscopic images of such a system were recorded concurrently, serving as an internal verification of the numerical simulation. Furthermore, the pair distribution function in the radial direction and the angular order parameter, were extracted from the equilibrium configuration of the Monte Carlo simulation as well as real-time microscopic images to investigate the phase transition behaviour. The primitive simulation results agreed with the experimental observation qualitatively, showing a two-dimensional phase transition from a disordered phase to an ordered solid phase as the surface coverage of the particles increases. In the second half of the dissertation, we focus on the coalescence experiments involving a Pickering droplet. Many experiments have been performed where two particle laden interfaces have been brought into close contact in a controlled manner and various observations, including particle "bridging", have been made in an attempt to understand the stabilization mechanism of interfacial particles in a Pickering emulsion. One of the most interesting observations is the tendency for the particles on one interface to "evacuate" and those on the other interface to "aggregate" during the close approach of the surfaces. In this work, we propose to understand the mechanism of particle evacuation- aggregation via a combined experimental and theoretical approach. First, we performed real-time experiments where two particle-laden water-decane interfaces were brought into contact. Many phenomena including particle evacuation-aggregationand bridging were observed. We then developed a Brownian dynamics simulation of the evacuation-aggregation including the important relevant interparticle interactions that we presumed were important in describing the phenomena. In order to do so, we had to answer three questions. First, what are the relevant aspects of the charged particle interaction within the same interface? Second, what is the charge interaction across the two approaching interfaces? Third, what are the flow effects, including the flow between the two interfaces during approach, on the particle motion and how can we model such a flow? Toward this goal, we have incorporated both reasonable electric inter-particle interactions from available literature studies and flow interactions via a porous media model that relates the particle velocity to the local surface coverage through the effective permeability of a porous media. Thus the flow effects are captured in a mean field sense. The BD simulations were able to capture the evacuation-aggregation qualitatively and, in most instances, quantitatively. In particular the diameter of the evacuated area decreases with increasing surface coverage in both simulations and experiments, and we will describe the physical mechanisms leading to this behavior by analyzing the particle force balance in the BD simulations.