Dip-Coating Crystallization on a Superhydrophobic Surface: A Million Mounted Crystals in a 1 cm2 Array

Silicon wafers (silicon dioxide surfaces) were patterned by photolithograpy to contain 3 mu m (width) x 6 pm (length) x 40 mu cm (height) staggered rhombus posts in a square array (20 mu m center-to-center spacing). These surfaces were hydrophobized using a vapor phase reaction with tridecafluorooctyldimethylchlorosilane and exhibit superhydrophobicity (water contact angles of theta(A)/theta(R) = 169 degrees/156 degrees). When a section of a wafer is submerged in and withdrawn from water, the superhydrophobic surface emerges, apparently completely dry. lithe same procedure is performed using aqueous sodium chloride as the liquid bath, individual crystals of the salt can be observed on the top of each of the posts. Dip-coating crystallization using an aqueous sodium chloride solution of 4.3 M produces crystals with similar to 1 mu m dimensions. A less concentrated solution, 1 M NaCl, renders crystals with similar to 500 nm dimensions. These experiments suggest that superhydrophobic surfaces that emerge from water and are apparently completely dry are, in fact, decorated with micrometer-size (several femtoliters) sessile water drops that rapidly evaporate. This simple technique is useful for preparation of very small liquid drops or puddles (of controlled composition) and for preparation of arrays of controlled size, crystalline substances (dip-coating crystallization).