Yamamoto Tetsurou, Kurokawa Takayuki, Ahmed Jamil, Kamita Gen, Yashima Shintaro, Furukawa Yuichiro, Ota Yuko, Furukawa Hidemitsu, Gong Jian Ping
Graduate School of Life Science, Hokkaido University, Sapporo 060-0810, Japan.
Soft Matter. 2014 Aug 14;10(30):5589-96. doi: 10.1039/c4sm00338a.
Direct observation of hydrogel contact with a solid surface in water is indispensable for understanding the friction, lubrication, and adhesion of hydrogels under water. However, this is a difficult task since the refractive index of hydrogels is very close to that of water. In this paper, we present a novel method to in situ observe the macroscopic contact of hydrogels with a solid surface based on the principle of critical refraction. This method was applied to investigate the sliding friction of a polyacrylamide (PAAm) hydrogel with glass by using a strain-controlled parallel-plate rheometer. The study revealed that when the compressive pressure is not very high, the hydrogel forms a heterogeneous contact with the glass, and a macro-scale water drop is trapped at the soft interface. The pre-trapped water spreads over the interface to decrease the contact area with the increase in sliding velocity, which dramatically reduces the friction of the hydrogel. The study also revealed that this heterogeneous contact is the reason for the poor reproducibility of hydrogel friction that has been often observed in previous studies. Under the condition of homogeneous full contact, the molecular origin of hydrogel friction in water is discussed. This study highlights the importance of direct interfacial observation to reveal the friction mechanism of hydrogels.
直接观察水凝胶在水中与固体表面的接触对于理解水凝胶在水下的摩擦、润滑和粘附至关重要。然而，这是一项艰巨的任务，因为水凝胶的折射率与水非常接近。在本文中，我们提出了一种基于临界折射原理原位观察水凝胶与固体表面宏观接触的新方法。该方法应用于使用应变控制平行板流变仪研究聚丙烯酰胺（PAAm）水凝胶与玻璃之间的滑动摩擦。研究表明，当压缩压力不是很高时，水凝胶与玻璃形成非均匀接触，并且在软界面处捕获了宏观尺寸的水滴。预先捕获的水在界面上扩散，随着滑动速度的增加接触面积减小，这显著降低了水凝胶的摩擦力。研究还表明，这种非均匀接触是先前研究中经常观察到的水凝胶摩擦再现性差的原因。在均匀完全接触的条件下，讨论了水凝胶在水中摩擦的分子起源。这项研究强调了直接界面观察对于揭示水凝胶摩擦机制的重要性。
