News & Seminars | HKUST Department of Physics

Abstract

Soft materials have attracted great attention in recent years due to their wide-ranging applications in fields such as the design of biomedical devices, tissue engineering, and microfluidics. Despite their extensive use in diverse applications, several crucial aspects of their surface mechanical features, including surface stress, surface energy, and relaxation mechanisms, have remained unclear. A comprehensive understanding of these fundamental properties is of great importance and is greatly desired from both the scientific and engineering perspectives. The thesis includes my research progress in three different aspects: (1) the surface stress and surface energy of soft gels with different crosslinking densities, (2) the spreading dynamic of liquid droplets on soft solids, and (3) the control of spontaneous droplet movements on gradient soft inter[1]faces. The key findings of my research works are outlined as follows: First, the surface stress and surface energy of the soft gels were characterized by mea[1]suring the wetting profiles at different length scales. We found that, above a critical crosslinking density (k0), the surface stress was found to increase significantly with crosslinking density while the surface energy remained unchanged. By comparing the surface mechanics of the soft gels with their bulk rheology, the surface properties near the critical density k0 were found to be closely related to the underlying percolation transition of the polymer networks. Second, the wetting dynamics of liquid droplets on a soft solid was also investigated systematically. In addition to viscoelastic relaxations, the spreading process of low[1]viscous silicone oils was found to be affected by poroelastic diffusion. By contrast, for highly viscous silicone oils spreading soft gels, their wetting dynamics follows the classical Tanner’s law. By using a newly designed interference microscope, we found that the viscoelastic dissipation from the substrates was greatly suppressed by the rapid decaying of the wetting ridges. Finally, we carefully designed soft gradient surfaces to drive spontaneous movements of droplets. We demonstrated that a spatial variation in crosslinking density within a soft gel can induce a gradient in surface energy. Combining confocal microscopy with contact angle measurements, we found that the maximum driving force was de[1]termined by the difference between equilibrium contact angles at the advancing and receding contact points. As the dissipations from the substrates were quantified, we demonstrated that the soft gradient surfaces can even realize spontaneous movements of droplets against their gravity.