Experimental Physics, Saarland University, Saarbrücken, Germany.
J R Soc Interface. 2013 Jan 6;10(78):20120587. doi: 10.1098/rsif.2012.0587. Epub 2012 Sep 19.
Surface energies are commonly used to determine the adhesion forces between materials. However, the component of surface energy derived from long-range forces, such as van der Waals forces, depends on the material's structure below the outermost atomic layers. Previous theoretical results and indirect experimental evidence suggest that the van der Waals energies of subsurface layers will influence interfacial adhesion forces. We discovered that nanometre-scale differences in the oxide layer thickness of silicon wafers result in significant macroscale differences in the adhesion of isolated gecko setal arrays. Si/SiO(2) bilayer materials exhibited stronger adhesion when the SiO(2) layer is thin (approx. 2 nm). To further explore how layered materials influence adhesion, we functionalized similar substrates with an octadecyltrichlorosilane monolayer and again identified a significant influence of the SiO(2) layer thickness on adhesion. Our theoretical calculations describe how variation in the SiO(2) layer thickness produces differences in the van der Waals interaction potential, and these differences are reflected in the adhesion mechanics. Setal arrays used as tribological probes provide the first empirical evidence that the 'subsurface energy' of inhomogeneous materials influences the macroscopic surface forces.
表面能常用于确定材料之间的粘附力。然而，源自长程力（如范德华力）的表面能分量取决于最外层原子层以下的材料结构。先前的理论结果和间接实验证据表明，次表面层的范德华能会影响界面粘附力。我们发现，硅片氧化层厚度的纳米级差异导致孤立壁虎刚毛阵列的宏观粘附力产生显著差异。当 SiO2 层较薄（约 2nm）时，Si/SiO2 双层材料表现出更强的粘附力。为了进一步探索层状材料如何影响粘附力，我们用十八烷基三氯硅烷单层对类似的基底进行了功能化处理，再次发现 SiO2 层厚度对粘附力有显著影响。我们的理论计算描述了 SiO2 层厚度的变化如何产生范德华相互作用势能的差异，这些差异反映在粘附力学中。用作摩擦学探针的刚毛阵列提供了第一个经验证据，表明不均匀材料的“次表面能”会影响宏观表面力。
