Colloidal crystallization in the quasi-two-dimensional induced by electrolyte gradients.

We investigated driven crystal formation events in thin layers of sedimented colloidal particles under low salt conditions. Using optical microscopy, we observe particles in a thermodynamically stable colloidal fluid to move radially converging towards cation exchange resin fragments acting as seed particles. When the local particle concentration has become sufficiently large, subsequently crystallization occurs. Brownian dynamics simulations of a 2D system of purely repulsive point-like particles exposed to an attractive potential, yield strikingly similar scenarios, and kinetics of accumulation and micro-structure formation. This offers the possibility of flexibly designing and manufacturing thin colloidal crystals at controlled positions and thus to obtain specific micro-structures not accessible by conventional approaches. We further demonstrate that particle motion is correlated with the existence of a gradient in electrolyte concentration due to the release of electrolyte by the seeds.