Capillary-Force-Induced Clustering of Micropillar Arrays: Is It Caused by Isolated Capillary Bridges or by the Lateral Capillary Meniscus Interaction Force?

Because of their increased mechanical compliance, arrays of high-aspect-ratio microstructures are susceptible to deformation by capillary forces. In the literature, the collapse of a I D array of tall line patterns during liquid evaporation off of their surface has been attributed to the Laplace pressure difference due to isolated capillary bridges. The same argument has often been simply extended to 2D arrays of tall microstructures to explain the collapse behavior. Using a short-chain polystyrene (PS) melt as a wetting liquid on a 2D array of epoxy micropillars, we showed that the collapse occurred while the micropillars were still completely surrounded by liquid, thus the clustering of micropillars should be caused by the lateral capillary meniscus interaction force rather than by often-reported isolated capillary bridges. We showed that the capillary meniscus interaction force was more than an order of magnitude smaller than that calculated from the Laplace pressure difference due to isolated capillary bridges. This result suggested a much lower critical elastic modulus for stable micropillar arrays, which agreed well with our experimental observation.