In general, the friction behavior of soft materials is sensitive to wettability. To examine the effect of a non-uniform distribution of wettability, the friction between a rubber hemisphere partially covered with polyvinyl alcohol hydrogel patch and a glass plate under water-lubricated conditions was investigated. Experimental results revealed that the real contact area and friction coefficient increased and decreased with patch size relative to no hydrogel coating. Considering PVA hydrogel has low friction, it would be reasonable that these parameters decreased with the hydrogel patch size. However, the result exceeds this prediction. This effect occurred because the free energy of the system during contact formation was changed by adding the hydrogel.
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