Self-assembly of microscale objects at a liquid/liquid interface through lateral capillary forces

Small (100-600 nm in width) hexagonal polymeric plates with faces patterned into hydrophobic and hydrophilic regions, and interacting through lateral capillary forces, were allowed to self-assemble at the perfluorodecalin/wat er interface. These plates were fabricated from photoresist and patterned b y shadow evaporation of gold onto selected faces. The arrays that assembled from the 100 mum objects were similar in structure to those that assembled from millimeter-sized objects with analogous patterns of hydrophobic and h ydrophilic faces, but with three important differences. (i) The contributio n of buoyancy forces in establishing the level at which the 100 mum objects floated relative to the interface was small compared to the contribution o f the vertical capillary forces. (ii) As a result, the designs of hydrophob ic edges necessary to generate menisci useful in self-assembly were differe nt for 100 mum objects than for millimeter-sized objects. (iii) The arrays that formed from the 100 mum objects had higher densities of defects than t he arrays that formed from the millimeter-sized objects; these defects refl ected the increase in the strength of the capillary forces (which favored a ssembly) relative to the shear forces (which disrupted assembly). This work adds two new elements to the study of mesoscale self-assembly: (i) It desc ribes a new method of fabrication of plates with faces patterned into regio ns of different hydrophobicity that is applicable to small (perhaps <10 <mu >m) objects, and (ii) it describes the self-assembly of 100 mum plates made by this method into ordered arrays. The work also established the contours of the menisci on the separate 100 mum and millimeter-sized plates. The sc aling of the lateral and vertical capillary forces and buoyancy forces acti ng on millimeter-sized objects, relative to those acting on 100 mum objects , is described.