Colloidal particles can be arrayed by evaporation of colloidal suspensions onto partially wetting substrates [1, 2]. When the substrate initially contacts the vapor-liquid interface, a meniscus forms with an advancing contact angle; the three phase contact line (CL) is pinned till the contact angle is reduced to its receding value. During this time, particles are convected to the CL, where they create a locally rough surface with a reduced receding contact angle. Further evaporation of solvent near the CL allows the colloidal particles to be drawn into crystalline order. Colloidal-rich CLs can depin, after which they hop to a new location; on vertical surfaces this depinning and hopping under the action of gravity can create regularly spaced striped patterns [3]. Here we consider the dip-coating of a vertical plate in contact with an evaporating colloidal suspension. The plate motion stretches the meniscus, so depinning frequency depends on the withdrawal velocity, U. A variety of deposition behaviors are observed for uniform wetting substrates as a function on U, ranging from periodic stripes whose thickness depends on particle diameter, to deposition of a disordered monolayer. Thereafter, we show that the use of surfaces of patterned wetting (created by soft lithography) with imposed dimensions can over-ride the natural length scale allowing control over particle positioning by forming daughter drops which can sequester particles on the wet features. Techniques to manipulate the heights of these drops, and therefore the sizes of the particles sequestered in them are developed to sort particles by size [4].

References:

1. N. D. Denkov, et al., “Two-dimensional crystallization”, Nature, 361 1993, 26. 2. Robert D. Deegan et al., “Capillary flow as the cause of ring stains from dried liquid drops”, Nature, 389 1997, 827-829. 3. Manouk Abkarian, Janine Nunes, and H.A. Stone, “Colloidal Crystallization and Banding in a Cylindrical Geometry “, J. Am. Chem. Soc., 126 2004, 5978-5979. 4. F. Fan and K.J. Stebe, “Assembly of Colloidal Particles by Evaporation on Surfaces with Patterned Hydrophobicity”, Langmuir, 20 2004, 3062-3067.