Direct measurement of surface elasticity of strained soft solids

Surface stress, also known as surface tension, is a fundamental material property of any interface. However, measurements of solid surface stress in traditional engineering materials, such as metals and oxides, have proven to be very challenging. Consequently, our understanding relies heavily on untested theories. Here, we take the advantage of high compliance and large deformability of a soft polymer gel to directly measure solid surface stress as a function of strain. Under both biaxial and uniaxial stretch, we find the surface stress depends on the strain via the surface elastic constants, which, remarkably, is even larger than the zero-strain surface tension [1, 2]. Further, we put forward a surface layer model to explain the origin of measured surface elasticity and strain-dependent surface stress for soft polymer gels. These results also suggest that solid surface stress, as a strain-dependent tensor, can play a dominant role in solid mechanics at much larger length scales than previously anticipated.

[1] Xu, et al. Nature Communication, 8, 555 (2017)
[2] Xu, et al. In preparation.