Patterning Through Evaporation

Researchers developing a new one-step method of lithographic patterning through evaporation of colloidal films have discovered that changing the colloid concentration in organic solution affects whether particles are deposited in exposed or covered regions of the mask (Langmuir 2008, 24, 3681). Working with silica microspheres suspended in ethanol and toluene and dried beneath a patterned mask, Daniel J. Harris and Jennifer A. Lewis of the University of Illinois, Urbana-Champaign, found that as the solvent evaporates through open areas of the mask, a temperature gradient develops across the film surface, leading to a surface tension gradient and the formation of localized recirculating flow cells. Particles then get deposited under the mask at the interfaces between the recirculating cells. If the mask contains an array of circular holes, this produces a pattern of empty wells surrounded by a honeycomb-like structure of silica (shown). The effect depends on concentration—above a critical volume fraction of silica, recirculating flows are suppressed, and the system switches to depositing particles under open regions of the mask, allowing the scientists to change the pattern simply by tuning the system's solids content.