287044 Porous Media Model and Collective Behavior of Colloidal Particles Trapped At a fluidic Interface

Monday, October 29, 2012
Hall B (Convention Center )
Eric S.G. Shaqfeh, Chemical Engineering and Mechanical Engineering, Stanford University, Stanford, CA, Shenghan Yan, Chemical Engineering, Stanford University, Stanford, CA and Gerald Fuller, Stanford University, Stanford University, Stanford, CA

It is well known that charged colloidal particles may form an effective two di-
mensional 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.
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 interest-
ing 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 per-
formed real-time experiments where two particle-laden water-decane interfaces were
brought into contact. Many phenomena including particle evacuation-aggregation
and 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 interac-
tions 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 evac-
uation/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.

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