Immobilization of Water Drops on Hydrophobic Surfaces by Contact Line Pinning at Nonlithographically Generated Polymer Microfiber Rings

Abstract

Water drops used as reaction compartments are commonly immobilized on hydrophilic areas bordered by hydrophobic areas. For many applications, such as the trapping of nonadherent cells, it is desirable to exploit the inertness and the antifouling behavior of hydrophobic surfaces as well as their repulsive behavior toward adsorbates in lab-on-chip configurations. However, the immobilization of water drops on hydrophobic surfaces has remained challenging. A nonlithographic approach is reported to arrest water drops on hydrophobically modified macroporous silicon (mSi) with perfluorinated surface. Contact line pinning at rings of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) fibers protruding from the mSi macropores immobilizes water drops when the hydrophobically modified mSi is moved or tilted and prevents dewetting within the PS-b-P2VP fiber rings. Without PS-b-P2VP fiber rings, water drops readily roll off. The PS-b-P2VP fiber rings are prepared by dropping PS-b-P2VP solution onto hydrophobically modified mSi. Selective swelling of the P2VP in the thus-formed circular PS-b-P2VP films with hot ethanol followed by detachment of the latter yields hydrophobically modified mSi exhibiting annular areas, in which ruptured PS-b-P2VP fibers protrude from the mSi macropores. For example, PS-b-P2VP fiber rings with diameters of 6.5 mm and widths of approximate to 0.2 mm immobilize water drops with a volume of 50 mu L.