Surface Tension and Surface Elasticity of Soft Solids

许钦 博士  Qin Xu, Ph.D.

瑞士苏黎世联邦理工学院材料系，软物质和生物材料实验室

Laboratory of Soft and Living Materials, Department of Materials

ETH Zurich, Swissland

2018年5月9日（周三）10:00-11:00

新主楼A849

Surface tension describes in-plane force per unit length required to deform a material interface. While the concept is widely used to characterize capillarity on liquid interfaces, solid surface tensions are overlooked as they are too weak to significantly deform bulk solids. However, recent works have shown that solid surface tension is essential to understand contact mechanics on soft solid interfaces at micro- to nano-meter scales. For a solid material with Young modulus, E, and surface tension, Υ, soild capillarity has been shown to dominate over bulk elasticity at scales smaller than the elastocapillary length scale, l_e=Υ/Ε.  In this regime, the significant force contribution from surface tension Υ fundamentally alters experimental behavior, and consequently the interpretation of many materials characterization techniques.  In this talk, by studying microscopic profiles of the contact-line geometry of a partially wetting droplet on a strained polymeric gels, we directly measure the strain dependent surface stress, and associated surface elasticity. We find the surface tension of soft polymeric gels significantly depends on applied strain.

         表面张力是用来描述材料界面上单位长度受力的物理量。这一概念对于液体表面力学性质的研究尤为重要并有广泛的实验应用。然而对于固体表面的力学问题，表面张力却很少被提及。这是因为相对于体弹性，固体表面张力对于固体形变的贡献通常可以忽略不计。然而，近期的各项研究发现：在软物质体系中，固体表面张力对各类接触力学的性质却起到了关键性作用。 当一个材料的体杨氏模量为E，表面张力为Y，我们可以定义一个新的长度尺寸：弹性-张力尺寸，l_e=Υ/E（Elasto-capaillary length）。在这个空间尺度范围内，固体表面张力将对体系的力学性质起到关键性的影响。很多软物质的实验现象和经典的材料表征技术都应该考虑固体表面张力从而进行修正。在这个报告中，通过研究液滴在高分子胶体表面的浸润，以及各相张力在固体微观尺寸的特征。我们在实验上系统研究了固体表面张力随材料表面形变的变化，从而发现了随应变调控的固体表面张力，同时揭示了软性高分子材料的表面弹性性质。

Reference (相关文献)：

[1] Direct measurements of surface stress of stretched soft solids
Qin Xu, Katharine E. Jensen, Rostislav Boltyanskiy, Raphael Sarfati, Robert Style, and Eric R. Dufresne
Nature Communications 8, 555 (2017)

[2] Surface Elastic Constants of a Soft Solid
Qin Xu, Robert W. Style and Eric R. Dufresne
Soft Matter, 14, 916-920 (2018)

[3] Strain-dependent solid surface stress and the stiffness of soft contacts
Katharine E. Jensen, Robert Style, Qin Xu and Eric R. Dufresne
Physical Review X, 7, 041031 (2017).

[4] The mechanical equilibrium of soft solids with surface elasticity
Robert W. Style and Qin Xu
arXiv:1801.07744
Accepted by Soft Matter