A clean cut

Abstract

The effectiveness of slicing in cutting soft solids (i.e., transverse motion along the blade) is natural to anyone with even the slightest kitchen experience, but the underlying mechanism remains intriguing. This study seeks to unveil the mystic role of slicing by looking into the effect of friction in cutting soft materials. With the increase of indentation depth, the nonlinearity of the large deformation in the superficial layer diverts the lateral stress from the compression of the classic linear elastic solution and introduces a tension that accounts for the ultimate fracture. However, when friction is present between the blade and material, there is always a finite no-slip region on the contact surface, under which stress remains compressive in all directions. The slicing motion of the blade, on the other hand, directs the friction toward the horizontal direction, thus minimizing its contribution in resisting the cutting process in the vertical plane, and enabling a clean cut. Through a numerical model of frictional contact between a cutting wire and a hyperelastic solid, we show that the slicing action diverts the friction force, enables sliding, and facilitates the development of local tension. Without a trustworthy stress-based fracture criterion for soft solids, we then study the energetics of the cutting process. By introducing a small pre-existing crack underneath the blade, we compute the energy landscape of the system and show that the friction reduction in the vertical plane greatly decreases the energy barrier of crack opening and thus promotes cutting.