Small scale contact between a soft, liquid-coated layer and a stiff surface is common in many situations, from synovial fluid on articular cartilage to adhesives in humid environments. Moreover, many model studies on soft adhesive contacts are conducted with soft silicone elastomers, which possess uncrosslinked liquid molecules (i.e. silicone oil) when the modulus is low. We investigate how the thickness of a silicone oil layer on a soft substrate relates to the indentation depth of glass microspheres in contact with crosslinked PDMS, which have a modulus of <10 kPa. The particles indent into the underlying substrate more as a function of decreasing oil layer thickness. This is due to the presence of the liquid layer at the surface that causes capillary forces to pull down on the particle. A simple model that balances the capillary force of the oil layer and the minimal particle–substrate adhesion with the elastic and surface tension forces from the substrate is proposed to predict the particle indentation depth.

中文翻译：

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毛细管驱动的将微粒压入柔软的油膜基底中的压痕。

从关节软骨上的滑液到潮湿环境中的粘合剂，在许多情况下，柔软的液体涂层和坚硬的表面之间通常会发生小规模的接触。此外，许多关于软粘合剂接触的模型研究都是使用具有未交联液体分子（即模量低时）。我们研究了软基底上的硅油层的厚度如何与与交联的PDMS接触的玻璃微球的压痕深度有关，压模深度小于10 kPa。随着油层厚度的减小，颗粒更多地进入下层基材。这是由于在表面上存在液体层，该液体层导致毛细作用力向下拉到颗粒上。提出了一个简单的模型来平衡油层的毛细作用力和最小的颗粒-基底粘附力以及来自基底的弹性和表面张力，从而预测颗粒压入深度。