(4ek) Directed Assembly of Colloidal Particles

The directed assembly of colloidal particles is important to not only the fabrication of advanced materials, such as colloidal crystals, but also to microelectronics and next generation electrochemical systems, particularly when the particle is a carbon nanotube (CNT) or a sheet of graphene oxide (GO). Both my PhD and postdoctoral research has focused on the rich science underlying the assembly and structure of colloidal particle ensembles. I will present a series of vignettes summarizing my recent work in this area.

My PhD work focused on the directed assembly of particles proximate to an electrode stimulated by an electric field. The process is reversible and depends on the strength and frequency of the applied electric field. Surprisingly, the structure of the assembly also depends on the dispersing electrolyte and, until recently, the origin of this electrolyte dependence was a mystery. Negatively charged polystyrene spheres 5.7 µm in diameter dispersed in 0.15 mM KCl or NaHCO₃ assemble into a tightly packed hexagonal array upon application of a ~1 kV/m 100 Hz electric field, but an ensemble dispersed in 0.15 mM NH₄OH or KOH assembles into an array with a larger inter-particle spacing despite all other experimental parameters being identical to the previous case. I discovered that electrolyte dependence arises in both the polarization of the particle’s diffuse layer and the conductivity of the dispersing electrolyte. My thesis included the first a priori model for electrolyte dependent particle motion normal to a polarized electrode.

My current work is focused on the assembly and structure of colloidal particles pinned at a fluid/fluid interface. Although it has been known for over a century that solid particles are effective at stabilizing a fluid/fluid interface, there remain several unanswered questions that are crucial to fully understanding this phenomenon. Two important research questions concern the correct expression for the interaction potential of particle pairs and how to predict surface pressure from this pair potential. In addition, I am interested in designing processes that direct the assembly of colloidal particles with bulk and interfacial flow. I am helping to develop a “capillary focusing” technique that uses the differences in capillarity experienced by fluid evaporating from a substrate with a heterogeneous surface to control the deposition and assembly of CNTs. I plan to use this technique to fabricate CNT based microelectronics in environmentally friendly conditions.

In the future, I will build on my training in electrokinetics and complex fluids to conduct research on responsive complex interfaces. In addition, I am interested in developing new techniques for the measurement of anisotropic colloidal interactions.