Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA, 18015, USA.
Department of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, 14853, USA.
Adv Mater. 2023 Apr;35(17):e2211044. doi: 10.1002/adma.202211044. Epub 2023 Mar 15.
Lubricated contacts in soft materials are common in various engineering and natural settings, such as tires, haptic applications, contact lenses, and the fabrication of soft electronic devices. Two major regimes are elasto-hydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, and mixed lubrication (ML), in which there is partial solid-to-solid contact. The transition between these regimes governs the minimum sliding friction achievable and is thus very important. Generally, the transition from EHL to ML regimes is believed to occur when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. Here, it is reported that in lubricated sliding experiments on smooth, soft, poly(dimethylsiloxane) substrates, the transition can occur when the thickness of the liquid layer is much larger than the height of the asperities. Direct visualization of the "contact" region shows that the transition corresponds to the formation of wave-like surface wrinkles at the leading contact edge and associated instabilities at the trailing contact edge, which are believed to trigger the transition to the mixed regime. These results change the understanding of what governs the important EHL-ML transition in the lubricated sliding of soft solids.
在各种工程和自然环境中，软物质的润滑接触很常见，例如轮胎、触觉应用、隐形眼镜和软电子设备的制造。两种主要的状态是弹性流体动力润滑（EHL），其中固体表面完全被流体膜隔开，以及混合润滑（ML），其中存在部分固体-固体接触。这些状态之间的转变决定了可实现的最小滑动摩擦，因此非常重要。通常，当润滑剂层的厚度与表面粗糙度的幅度相当时，认为从 EHL 状态向 ML 状态的转变会发生。在这里，据报道，在对光滑、柔软的聚二甲基硅氧烷（PDMS）基底进行的润滑滑动实验中，当液体层的厚度远远大于凸起的高度时，转变就会发生。对“接触”区域的直接可视化显示，转变对应于在前缘接触边缘形成波浪状的表面皱纹以及在尾缘接触边缘的相关不稳定性，这被认为触发了向混合状态的转变。这些结果改变了对软固体润滑滑动中重要的 EHL-ML 转变的控制因素的理解。
