Self-Assembly of Microscale Objects at a Liquid/Liquid Interface through Lateral Capillary Forces

Abstract

Small (100−600 μm 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/water interface. These plates were fabricated from photoresist and patterned by shadow evaporation of gold onto selected faces. The arrays that assembled from the 100 μm objects were similar in structure to those that assembled from millimeter-sized objects with analogous patterns of hydrophobic and hydrophilic faces, but with three important differences. (i) The contribution of buoyancy forces in establishing the level at which the 100 μm objects floated relative to the interface was small compared to the contribution of the vertical capillary forces. (ii) As a result, the designs of hydrophobic edges necessary to generate menisci useful in self-assembly were different for 100 μm objects than for millimeter-sized objects. (iii) The arrays that formed from the 100 μm objects had higher densities of defects than the arrays that formed from the millimeter-sized objects; these defects reflected the increase in the strength of the capillary forces (which favored assembly) relative to the shear forces (which disrupted assembly). This work adds two new elements to the study of mesoscale self-assembly: (i) It describes a new method of fabrication of plates with faces patterned into regions of different hydrophobicity that is applicable to small (perhaps <10 μm) objects, and (ii) it describes the self-assembly of 100 μm plates made by this method into ordered arrays. The work also established the contours of the menisci on the separate 100 μm and millimeter-sized plates. The scaling of the lateral and vertical capillary forces and buoyancy forces acting on millimeter-sized objects, relative to those acting on 100 μm objects, is described.