To understand the dissipative mechanisms in soft hydrogel lubrication, polyacrylamide (PAAm) hydrogels with two distinct surface structures were examined under various contact conditions. The characteristic speed-dependent friction of the self-mated, crosslinked hydrogel surfaces could be explained by hydrodynamic shearing of a thin water layer between two rather impermeable bodies. On the other hand, the frictional response of brushy hydrogel surfaces is dependent on the contact conditions and the level of surface hydration. In a migrating contact, brushy hydrogels showed low, speed-independent friction (µ ~ 0.01) likely due to a thick layer of shearing liquid trapped within the sparse surface network. In stationary contact, however, brushy hydrogel surfaces can partially exude water from the near-surface region over time, as shown by time-resolved Fourier-transform infrared (FTIR) spectroscopy. This is assumed to be reflected in a friction increase over time. Interfacial shearing appears to shorten the characteristic exudation times compared to those observed under static loading. Once fluid has been exuded, brushy surfaces were shown to reach similar friction values as their crosslinked analogs. The results thus indicate that the dominating dissipation mechanism during sliding at low contact pressures is shearing of the interfacial liquid film, rather than poro-elastic dissipation within the bulk. Maintenance of surface hydration is therefore crucial, in order to take advantage of the low friction of such systems.

中文翻译：

---

水合和表面结构对丙烯酰胺水凝胶摩擦的重要性

为了了解软水凝胶润滑中的耗散机理，在各种接触条件下检查了具有两种不同表面结构的聚丙烯酰胺（PAAm）水凝胶。自交联，交联的水凝胶表面的与速度有关的摩擦特性可以通过在两个相当不可渗透的主体之间的薄水层进行流体动力剪切来解释。另一方面，有刷水凝胶表面的摩擦响应取决于接触条件和表面水合程度。在迁移接触中，有刷水凝胶显示出低的，与速度无关的摩擦力（µ 〜0.01）可能是由于稀疏的表面网络中夹有厚厚的剪切液层。然而，在固定接触中，随着时间的流逝，如时间分辨傅立叶变换红外（FTIR）光谱所示，有刷水凝胶表面会随时间从近表面区域部分渗出水。假定这反映在随时间的摩擦增加中。与在静态载荷下观察到的相比，界面剪切似乎缩短了特征渗出时间。一旦渗出了液体，就表明刷状表面达到了与其交联类似物相似的摩擦值。结果因此表明在低接触压力下滑动期间的主要耗散机制是界面液膜的剪切，而不是主体内的孔隙弹性耗散。