Milliscale Features Increase Friction of Soft Skin in Lubricated Contact

Abstract

Real world environments, such as kitchens, present objects covered in viscous fluids: soap, oil, water, etc. Understanding and designing for slippery and submerged contact, where fluid lubrication is present, is a continuing challenge in the robotics community. Contact area, bending stiffness, and the presence of a viscous fluid affect friction. This work focuses on milliscale features (3 to 20 mm in size) of soft urethane skin on smooth, flat surfaces. We characterize the friction of soft skins, with varying size, and therefore bending stiffness, of cylindrical features, all with the same nominal contact area. In addition, a new method of frustrated total internal reflection with dye is introduced to visualize lubricated contact. We find that a small number of milliscale fingertip features maximizes friction force in the presence of lubrication, as compared both to un-patterned and many-featured skin designs. This holds true for a robotic gripper test, when pinching glass submerged in oil.