The research team led by Prof. ZHENG Zhijun from the CAS Key Laboratory of Mechanical Behavior and Design of Materials of the Chinese Academy of Sciences (CAS) found that surface roughness can change the strength and adsorption mode of the contact interface, revealing the mechanical mechanism of interface strengthing / weakening and adsorption mode transition. The result was published on Journal of the Mechanics and Physics of Solids.

Many phenomena and problems in nature are related to the problem of adsorption and contact on rough surfaces, such as geckos walking on walls, cell adsorbing probes or other cells, and so on. As matter of fact, no ideal smooth surface exists and all the actual surfaces are considered rough. However, the mechanical mechanism under adsorption and roughness is still unclear.

Prof. Guduru from Brown University claimed that the surface roughness enhanced the material interface, however, his theory failed to explain the issue of weakening on adsorption by surface roughness.

Theory of Prof. Guduru: (a) three-dimensional view; (b) two-dimensional view; and (c) complete load-displacement curve under typical roughness

Facing this challenge, the research team investigated this issue at the micro-nano scale. From this scale, it is found the surface morphology and surface interaction of materials greatly affect the adsorption contact behavior of materials.

To solve the problem, the team developed a fully self-consistent model (FSCM) by establishing a self-consistent equation for the deformation and interaction of rough surfaces, which is the most accurate adsorption contact mechanics model within the framework of continuum mechanics. The self-consistent equation contains a strong nonlinear integral term, which needs to be solved by numerical iteration.

Based on the computational results, an adsorption mode partition map was proposed, providing the critical roughness of interface enhancement / weakening and revealing the mechanical mechanism of adsorption enhancement/weakening.

Pressure undulates in the contact area when surface is relatively smooth, increasing the extraction force; however, as there is an upper limit of the adhesion stress, the interface cavitation appears locally during the extraction process, forming an adsorption shielding area, so that the pull-out force is reduced.

These findings indicate that in the subsequent development of the simplified models of the rough surface adsorption contact, the close contact condition must be abandoned, and the maximum value of the adhesion stress must be restricted.

Effect of surface roughness on the adsorption contact behavior during pull-out: (a) pull-out force, (b) surface spacing distribution and (c) pressure distribution. (Image by ZHU Yudong et al.)

The development of the theory of adsorption contact mechanics, particularly in the micro-nano scale, plays a significant role for many fields such as biomechanics, micro-nano electromechanical and atomic force microscopy. It not only systematically revealed the mechanism of how surface roughness influences the adsorption behavior of the interface, but also provided new ideas for the study of adsorption contact mechanics models.

Paper link:

https://doi.org/10.1016/j.jmps.2021.104402

(Written by WENG Jingwen, edited by JIANG Pengcen, USTC News Center)