Imbibition and Flow of Wetting Liquids in Noncircular Capillaries

Micromolding in capillaries (MIMIC) has been used to study the dynamics of imbibition of liquid prepolymers in micrometer-scale, rectangular capillaries formed between an elastomer containing relief structures and a self-assembled monolayer (SAM) of alkanethiolates on gold supported on Si/SiO2. Self-assembled monolayers provide a wide range of solid/vapor interfacial free energies (γSV) and a control over wetting and spreading of liquids in the capillaries. The dynamic shapes of the liquids in the capillaries are obtained as solid polymers cross-linked photochemically and examined using scanning electron and atomic force microscopies. The shape of the imbibing liquid on a surface with high γSV shows that precursor structures precede macroscopic flow; on low γSV, these precursor structures are absent. Rates of capillary imbibition are linearly correlated to the cosine of static advancing contact angles (θa) of a liquid prepolymer on different SAMs. Different regimes of spreading are observed on a surface having constant, low γSV; in regions close to the origin, the liquid spreads with low θa and via precursor structures. As the distance relative to the origin increases, the liquid spreads with higher θa and the dynamic θa approaches static θa.