2025 年 39 巻 3 号 p. 106-115
The friction of the poly(vinyl alcohol) hydrogel against a glass substrate in a solution of polysaccharide xanthan was investigated. Simultaneously with the measurement of the friction coefficient, the gel-substrate interface was also observed in situ. The gel friction against hydrophilic glass was low at low speeds and increased with sliding speed. The friction at low speeds, which is caused by the elastic force of polymer chains on the gel surface adsorbed onto the counter surface, decreased with increasing xanthan concentration due to the screening of adsorption of the gel on the substrate by xanthan molecules. The frictional behavior of the gel against hydrophobically modified glass varied significantly depending on the xanthan concentration. At low xanthan concentrations, the friction coefficient was high at low speeds and decreased with increasing sliding velocity, but then began to increase at a certain point. With increasing xanthan concentration, the friction coefficient at low speeds decreased substantially, so that at high xanthan concentrations the friction coefficient increased monotonically with sliding velocity, as in the case of hydrophilic glass. The friction at high speeds, which is attributed to the lubrication of the fluid layer between the gel and the counter surface, did not depend notably on xanthan concentration. The thickness of the fluid layer increased with xanthan concentration and was estimated to be 101–102 nm. Observation of the gel-substrate interface showed that at low xanthan concentrations, the entire gel surface was in contact with the substrate, either directly or via a very thin film, regardless of the speed, but at high xanthan concentrations, the xanthan solution was trapped near the center of the disk-shaped gel at low speeds, and the film between the gel and the substrate became thinner overall as the speed increased.
