  In this work, we investigate crystallization from droplets formed on
micro-patterned surfaces. By solvent exchange in a micro-chamber, a ternary
solution consisting of a model compound beta-alanine, water, and isopropanol,
was displaced by a flow of isopropanol. In the process, oiling-out droplets
formed and crystallized. Our results showed that the shape and size of the
crystals on micro-patterned surfaces could be simply mediated by the flow
conditions of solvent exchange. Varying flow rate, concentration, or channel
height led to the formation of a thin film with micro-holes, connected network
of crystals, or small diamond-shaped crystals. Rough micro-structures on the
surface allowed the easy detachment of crystals from the surface. Beyond
oiling-out crystallization, we demonstrated that the crystal formation from
another solute dissolved in the droplets could be triggered by solvent
exchange. The length of crystal fibers after the solvent exchange process was
shorter at a faster flow rate. This study may provide further understanding to
effectively obtain crystallization from surface droplets through the solvent
exchange approach.

Soft Condensed Matter