Vortex-Force-Driven Particle Assembly around Electrode-Fouling Insulating Oil Droplets Is Influenced by Electrolyte Composition.

Our previous reports demonstrated augmented electrodeposition rates at the insulating oil droplet-electrolyte-electrode triple point. This enhancement was hypothesized to arise from convection that results from a localized increase in current density. Here, we use time-resolved microscopy to track gold microstructures migrating toward the triple point and to elucidate how electrolyte identity and concentration influence electrolysis-induced convection around a fluid insulating droplet fouling an electrode. By fitting a logistic model on the microscopy data, the maximum particle assembly rate (α) and saturation level (K) were estimated from aqueous samples that contained HAuCl4-Na2SO4, HAuCl4-K2SO4, and HAuCl4-CH3COONa. The α and K values were comparable in samples that were electrolyzed in 75-300 mM Na2SO4. However, the particle assembly parameters generally decreased in 600 mM Na2SO4 due to a less dispersed electric double layer and higher bulk solution viscosity. In terms of the effect of the electrolyte species, both α and K were remarkably lower in the acetate-containing samples than in the sulfate-containing samples. This phenomenon could be explained by pH effect, capping effect, and, more importantly, specific ion effects represented by the Hofmeister series. In the series, CH3COO- lies at the borderline of chaotropicity. For this reason, it tends to destabilize the growth or agglomeration of gold particles. The findings in this manuscript are useful for micropatterning in various devices and the emerging field of electrolyte engineering.