Christopher L. Wirth

Weak Electrolyte Dependence in the Repulsion of Colloids at an Oil–Water Interface

The two-particle interaction between 3.1-μm-diameter polystyrene latex particles at a decane-water interface was measured with time-shared optical tweezers. The water subphase contained either 0.103 mM RbCl or 0.0342 mM MgCl2, which have hydrated cations of different size but identical anions. The choice of both the anion and the concentrations makes a comparison with published data on NaCl possible and also isolates the effect of the nature of the cation on the electrostatic interaction. The measured magnitude of the dipolar force and the relative changes as a function of electrolyte were in quantitative agreement with predictions from a recently published model that uses the Langevin-Poisson-Boltzmann equation including steric effects and the polarization saturation of the medium to predict the dipolar interaction (Frydel, D.; Oettel, M. Phys. Chem. Chem. Phys. 2011, 13, 4109-4118). These results support the hypothesis that a condensed layer of counterions contributes to the electrostatic interaction between colloidal particles at an oil-water interface. Although it has been suggested that the electrostatic interactions between particles at liquid interfaces could serve as a sensitive probe of the structural details of the electric double layer, both the model predictions and experimental measurements showed a maximum change of only ~25% in the magnitude of the interaction with a change in electrolyte under the conditions tested. The ability to resolve this small change was confounded by the heterogeneous nature of the interaction. Thus, despite the apparent importance of the choice of electrolyte, the subtlety of competing effects makes it unlikely that colloidal force measurements could be used to probe the fine structure of the electric double layer.

Fabrication of Planar Colloidal Clusters with Template-Assisted Interfacial Assembly

The synthesis of nanoparticle clusters, also referred to as colloidal clusters or colloidal molecules, is being studied intensively as a model system for small molecule interactions as well as for the directed self-assembly of advanced materials. This paper describes a technique for the interfacial assembly of planar colloidal clusters using a combination of top-down lithographic surface modification and bottom-up Langmuir-Blodgett deposition. Micrometer sized polystyrene latex particles were deposited onto a chemically modified substrate from a decane-water interface with Langmuir-Blodgett deposition. The surface of the substrate contained hydrophilic domains of various size, spacing, and shape, while the remainder of the substrate was hydrophobic. Particles selectively deposited onto hydrophilic regions from the decane-water interface. The number of deposited particles depended on the size of each patch, thereby demonstrating that tuning cluster size is possible by engineering patch geometry. Following deposition, the clusters were permanently bonded with temperature annealing and then removed from the substrate via sonication. The permanently bonded planar colloidal clusters were stable in an aqueous environment and at a decane-water interface laden with isotropic colloidal particles. The method is a simple and fast way to synthesize colloidal clusters with few limitations on particle chemistry, composition, and shape.

An Imaging Ammeter for Electrochemical Measurements

A method for imaging local electrochemical current density uses light scattered from probe particles proximate to an electrode. A 5.5 μm particle near a working electrode (WE) scattered light more intensely or less intensely as a function of local current density. The light intensity correlated with the measured current. A current density of 1 A/m 2 was easily resolved. Fair agreement between the measured superficial current density and current density calculated from the light scattering intensity was achieved. The results demonstrate a method for imaging spatially varying electrochemical reaction rates with high resolution.

Mechanisms of Directed Assembly of Colloidal Particles in Two Dimensions by Application of Electric Fields

When electric fields interact with particles immersed in liquids and levitated near electrodes, the particles assemble into structures such as ordered arrays or chains. For example, direct electric current flowing through an aqueous solution held between two parallel-plate electrodes produces two dimensional arrays of colloidal particles near one of the electrodes. A high frequency electric field imposed in-plane, by contrast, forms chains of particles. These phenomena have interested scientists and engineers for a century; the multiphysics of the phenomena make it a rich problem for both theoreticians and experimentalists. Experimental investigations into the translation of particles laterally along surfaces when electric fields are applied normally to those surfaces, and into chain formation when the field is applied tangentially, have led to proposed mechanisms and theory by which colloidal particles and even cells move relative to the nearby surface and relative to each other. These mechanisms-, electrophoresis, electroosmosis, electrohydrodynamics, induced dipole repulsion, and dielectrophoresis- and supporting experimental evidence are the main topics of this account.