Sliding Friction of Hydrogels with Surface Hemispherical Dimples in Water

Surface geometry affects surface properties, such as sliding friction, contact, and adhesion. Herein, we focused on circular dimples on the surface of articular cartilages and examined how the controlled surface geometry affects the friction of hydrogels. Poly(vinyl alcohol) gels with micrometer-order hemispherical surface dimples were prepared, and then sliding friction measurements and <i>in situ</i> observation of the gel/glass interface dynamics were performed. The results show that the gel with surface dimples reduced friction in a wide range of sliding velocities. Especially in the high-velocity region, friction decreased as the dimple size increased. This is attributed to the reduction in contact area with the counter body by dimples, and water trapped in the dimples was supplied to the interface, serving as a lubricant.