Abstract
A soft solid is more easily sliced using a combination of normal and shearing deformations rather than diced by squeezing down on it normally with the same knife. To explain why this is so, we experimentally probe the slicing and dicing of a soft agar gel with a wire, and complement this with theory and numerical simulations of cutting of a highly deformable solid. We find that purely normal deformations lead to global deformations of the soft solid, so that the blade has to penetrate deeply into the sample, well beyond the linear regime, to reach the relatively large critical stress to nucleate fracture. In contrast, a slicing motion leads to fracture nucleation with minimal deformation of the bulk and thus a much lower barrier. This transition between global and local deformations in soft solids as a function of the angle of shear explains the mechanics of the paper cut and design of guillotine blades.
Highlights
We find that purely normal deformations lead to global deformations of the soft solid, so that the blade has to penetrate deeply into the sample, well beyond the linear regime, to reach the relatively large critical stress to nucleate fracture
A slicing motion leads to fracture nucleation with minimal deformation of the bulk and a much lower barrier. This transition between global and local deformations in soft solids as a function of the angle of shear explains the mechanics of the paper cut and design of guillotine blades
Experimenting on the kitchen table, we quickly learn that the easiest way to cut soft solids with a knife is by a slicing action, i.e., dragging the sharp blade over the soft surface without pushing too strongly into it; pushing the edge of a knife too strongly into a soft solid only squashes it
Summary
Shear is controlled via the kinematic angle between the direction of penetration by the wire and the normal to the surface of the gel: the normal velocity is kept constant (Vn 1⁄4 0:2 mm=s), while the tangential component Vt varies as changes. (b) The gel surface deforms strongly before the initiation of penetration, and rebounds partially; these phases correspond to the nucleation and growth phases of the fracture for purely normal indentation (top) and increased shearing motion (bottom).
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