Stability of high-aspect-ratio micropillar arrays against adhesive and capillary forces.

High aspect-ratio (HAR) micropillar arrays have many interesting and technologically important applications. Their properties, such as large mechanical compliance, large surface area, and a topography that is well-separated from the underlying substrate, have allowed researchers to design and explore biomimetic dry adhesives, superhydrophobic, superoleophobic, and tunable wetting surfaces, mechanical sensors and actuators, and substrates for cell mechanics studies. However, the mechanical compliance and large surface area of the micropillars also make these structures susceptible to deformation by adhesive and capillary surface forces. As a result such micropillars, particularly those made from soft polymers, can prove challenging to fabricate and to use in various applications. Systematic understanding of these forces is thus critical both to assemble stable micropillar arrays and to harness these surface forces toward controlled actuation for various applications. In this Account, we review our recent studies on the stability of HAR polymeric micropillar arrays against adhesive and capillary forces. Using the replica molding method, we have successfully fabricated HAR epoxy micropillar arrays with aspect ratios up to 18. The stability of these arrays against adhesive forces is in agreement with theoretical predictions. We have also developed a new two-step replica molding method to fabricate HAR (up to 12) hydrogel micropillar arrays using monomers or monomer mixtures. By varying the monomer composition in the fabrication process, we have fabricated a series of hydrogel micropillar arrays whose elastic moduli in wet state range from less than a megapascal to more than a gigapascal, and we have used these micropillar arrays to study capillary force induced clustering behavior as a function of the modulus. As a result, we have shown that as liquid evaporates off the micropillar arrays, the pillars bend and cluster together because of a much smaller capillary meniscus interaction force while the micropillar structures are surrounded by a continuous liquid body. Previously, researchers had often attributed this clustering effect to a Laplace pressure difference because of isolated capillary bridges. Our theoretical analysis of stability against capillary force and micropillar cluster size as a function of pillar elastic modulus agrees well with our experimental observations. The fabrication approaches presented here are quite general and will enable the fabrication of tall, stable micropillar arrays in a variety of soft, responsive materials. Therefore, researchers can use these materials for various applications: sensors, responsive wetting, and biological studies. The new insights into the capillary force induced clustering of micropillar arrays could improve rational design and fabrication of micropillar arrays that are stable against capillary force. In addition, these results could help researchers better manipulate capillary force to control the assembly of micropillar arrays and actuate these structures within novel devices.