Kinetics of colloidal deposition, assembly, and crystallization in steady electric fields

Abstract

We quantify and model the deposition and crystallization kinetics of initially dilute colloidal spheres due to application of a steady, direct current electric field in the thin gap between parallel electrodes. The system studied is poly(12-hydroxystearic acid) (PHSA)-stabilized poly(methyl methacrylate) (PMMA) spheres dispersed in a mixture of cyclohexylbromide (CHB), decalin, and a low concentration of the partially disassociating salt tetrabutylammonium chloride (TBAC). The temporal and spatial evolution of the colloidal volume fraction in the ∼1 mm gap between the electrodes is quantified under conditions of both deposition and relaxation by confocal laser scanning microscopy (CLSM). During deposition assembly, the spatial dependence of the colloid volume fraction approaches steady state at times between hundreds of minutes at the lowest electric field strength (as characterized by a Peclet number, Pe) and at tens of minutes at higher field strengths. During disassembly, the volume fraction relaxes nearly exponentially. The kinetics are modeled by adapting a treatment for sedimentation (Davis and Russel, Phys. Fluids A, 1989, 1, 82) to the case of steady electric fields. The model's predictions show good agreement with the measured kinetics at low Pe; however, agreement progressively deteriorates with increasing Pe. At low Pe the deposits are initially disordered. After an initial delay, 1D crystal growth propagates from the electrode surface at rates of several hundred nm min⁻¹. The sharp crystal boundary propagates as a characteristic of constant colloidal volume fraction, consistent with an equilibrium crystalline phase transition. The results inform operational ranges for devices that produce active colloidal matter by reversible assembly.