Photocurable pillar arrays formed via electrohydrodynamic instabilities

Low viscosity, photocurable liquids are demonstrated as ideal materials for the formation of pillar arrays generated spontaneously by field-assisted assembly. Pillars form spontaneously via electrohydrodynamic instabilities that arise from the force imbalance at a film-air interface generated by an applied electric field. Conventional polymer films form pillars slowly as a result of their relatively large viscosities and are often process-limited by a requirement of heat to modulate rheological properties. In contrast, low viscosity liquids require no heat and form pillars orders of magnitude faster, as predicted by theory. The resulting structures are preserved by photopolymerization, eliminating the lengthy heating-cooling cycle necessary to process most polymers. The combination of nearly instantaneous formation and rapid photocuring at room temperature is ideal for patterning. Epoxy, vinyl ether, acrylate, and thiol-ene systems were evaluated for pillar formation. Relevant material properties were characterized (viscosity, dielectric constant, interfacial energy, kinetics) to explain the phenomenological behavior of each system during electrohydrodynamic patterning. The thiol-ene system formed pillar arrays nearly instantaneously and cured rapidly under ambient conditions. These are nearly ideal characteristics for pillar formation.