Surface stress, also known as surface tension, is a fundamental material property of any interface. However, experimental study of solid surface stress in traditional engineering materials, such as oxides and metals, have proven to be extremely challenging. Current understanding of solid capillarity heavily relies on untested theories, especially regarding strain dependent surface stress and surface relaxation mechanism. Here, we take the advantage of high compliance and large deformability of soft polymer gels to study their surface mechanics. We develop advanced optical techniques, including in-situ confocal microscopy and surface interferometry, to precisely measure surface deformation and fast relaxation in wetting problems. We experimentally measured strain-dependent surface stress for the first time, and addressed the importance of surface elasticity of soft solids. Further, we show how bulk porosity couples with elasticity and surface tension, and affects surface relaxation. Our results extend current understanding of mechanics of soft interfaces, and can bring new insight into various soft membrane systems.
Dr. Qin Xu is currently a postdoctoral scholar in the Laboratory of Soft and Living Materials at ETH Zurich. His current research focuses on surface mechanics of soft materials, especially solid capillarity and poroelasticity. He is also interested in developing advanced optical techniques, such as confocal and interference microscopy, to characterize surface deformation of various soft materials.
Formerly, Dr. Xu completed his Master in Physics from New York University and Ph.D. in physics from University of Chicago. After leaving Chicago in 2015, he immediately joined the Soft Matter Lab led by Prof. Eric Dufresne at Yale University. In the summer of 2016, Dr. Xu moved together with Prof. Dufresne to Switzerland. Since then, he has been helping to set up the opto-mechanical measuring facilities at ETH Zurich, and is currently working as a Principal Investigator for studying interfacial dynamics of soft materials supported by ETH Seed Grant.