Influence of Moisture and Grasping Force on Static Friction Force between Finger Pad and Object with Smooth Surface

To explore how moisture and grasping force affect static friction force when a person grasped a smooth object, a new equipment, capable of measuring deformation, grasping force and static friction force of a finger pad when the object’s weight continuously changed, was developed in this study. Using this equipment, the deformation and maximum static friction coefficient were measured and analyzed as the moisture and the object’s weight were varied. The adhesion force between the finger pad and the smooth object was assessed by the finger pad deformation, thereby analyzing the effect mechanism of moisture on the maximum static friction coefficient. The results revealed that the maximum static friction coefficient decreased according to the law of negative power function as the grasping force got larger, and then leveled off at a certain grasping force. Moisture affected the adhesion force between the finger pad and the smooth object, further affecting the maximum static friction force. The adhesion force value was greatest when contact surface was medium moisture content, which was the most labor-saving and difficult to slip when grasping smooth object. Besides, the effect of moisture on the maximum static friction coefficient was nonlinear. The maximum static friction coefficient showed a peak value at a certain moisture content.