Complex stain morphologies

We examine the morphology of crystal, precipitate, and colloidal aggregate stains left by microliter drops evaporated on hydrophilic or hydrophobic surfaces. An understanding of such morphologies is important in the design of high-density stain libraries for high-throughput drug screening. Unlike classical homogeneous crystallization patterns, we find that the film and solutal flux dynamics of such small drops at their contact lines can induce macroscopic concentration segregation and produce distinct large-scale stain patterns such as concentric rings on hydrophilic surfaces and latticed crystals on hydrophobic ones. Coupling between these bulk segregation instabilities and the classical Mullins-Sekerka crystallization instability results in a large variety of crystal patterns with interwoven complex structures of two length scales. Furthermore, low-density crystals can occupy a larger area than the initial drop, and gravitational drainage on inclined substrates can change the larger length scale. The most symmetric stains with the smallest stain or drop areas are produced on hydrophobic or partially hydrophobic surfaces either above or below two critical concentrations.