Ultrafast Finger Snap is Mediated by a Frictional Skin Latch

The snap of a finger is a ubiquitous motion that has been seen across cultures and times. Using high-speed imaging, we analyze finger snap dynamics for the first time. We find that the mechanics of the snap are strongly mediated by human skin friction, which acts as a latch to generate rapid motion. The skin frictional latch is optimally tuned to enable maximum kinematic performance as the angular accelerations observed during a snap are one of the fastest human motions known. A simple scaling relationship is found that links the latch geometry to the performance of snapping motion across multiple organisms from termites to humans. Ultimately, our work reveals how friction between surfaces can be harnessed as tunable and scalable latching mechanism, with applications ranging from increasing grip in biomedical prosthetic surfaces to generating high force and accelerations in tiny robots.