Controllable Clustering Transition Based on Temperature-Response Hydrogel by Optimizing Elastic Modulus

High aspect ratio micro-pillar has used to fabricate functional and large scale microstructures due to clustering phenomenon with external forces. Micro-pillars were limited for application in various fields because once clustered, they were not de-clustered naturally. In particular, the clustering phenomenon of micropillars caused difficulties in the fields of wetting, adhesion, and energy collection, but it was the necessary phenomenon in the case of transporting small particles or microfluidic platform. In this paper, it was confirmed that micro-pillars composed of temperature-responsive hydrogels can naturally and reversibly control clustering transition (clustering and de-clustering phenomena) by changing temperatures in various environments. A poly(N-isopropylacrylamide-co-acrylamide) hydrogel, temperature-reactive hydrogel, had varying elastic modulus depending on the two elements (the density of the crosslinker and the temperature in surrounding environment). Based on the changed elastic modulus, reversible clustering in water and into evaporation of water. The reversible clustering transition of micro-pillars was theoretically analyzed to thermodynamic model and acting forces, and followed by experimental demonstration. The applicability in various fields was confirmed for micro grippers that freely hold and release microparticles in water and smart mobile camera lens covers.