Three-dimensional self-assembly of millimetre-scale components

Abstract

The spontaneous association of molecules, termed molecular self- assembly, is a successful strategy for the generation of large, structured molecular aggregates. The most important source of inspiration for this strategy is the biological world, in which many processes involve interfacial interactions and shape selectivity that guide the formation of complex, multicomponent three-dimensional structures. The success of molecular self- assembly notwithstanding, many objectives in science and technology require the assembly of components that are much larger than molecules: examples include microelectronic and microelectro-mechanical systems, sensors and microanalytical and micro-synthetic devices. Photolithography, the principal technique used to make such microstructures, has certain limitations: it cannot easily form non-planar or three-dimensional structures; it generates structures that are metastable; and it can be used only for a limited set of materials. Here we describe an approach for the self-assembly of millimetre- scale components that uses two concepts to direct the assembly process: shape recognition and the minimization of liquid-liquid interfacial free energies. These play a role in other spontaneous self-assembly phenomena, such as the formation of bubble rafts, the patterned dewetting of surfaces, and the coalescence of liquid drops. We apply self-assembled monolayer molecular films to the surfaces of shaped macroscopic objects to render them hydrophilic or hydrophobic, depending on the terminal groups of the bound molecules. In aqueous solution, hydrophobic surfaces bearing a thin film of a hydrophobic, lubricating liquid adhere to similar surfaces with complementary shapes, while being able to adjust their relative alignment to ensure a good fit. In this way, the components assemble into well defined aggregates, which can be bound permanently when the hydrophobic liquid films consist of a polymerizable adhesive.