Slicing with friction
Evan Zabawski | TLT From the Editor March 2018
The physics of cutting begins with a crack.
The counterintuitive reason a sharp knife slices more easily through a tomato is that it has higher friction.
© Can Stock Photo / mflippo
Trying to slice a tomato
into thin slices with a dull knife can get a little scary. The harder the knife must be pressed downward before the skin is broken is directly related to the acceleration the knife experiences as it passes through the rest of the tomato but is indirectly related to the control of the path of the knife.
Moving the knife forward while pressing down can help break the skin with less pressure, however this does not completely eliminate the sudden burst of speed that diminishes control of the knife. The easiest and safest way to slice a tomato is to use a sharp knife. The counterintuitive reason a sharp knife slices more easily through a tomato is that it has higher friction, albeit only on the knife’s edge.
Slicing is actually stretching the tomato and, like most materials, a tomato is weaker when stretched than when compressed. Stretching the tomato skin creates a tearing force that opens a crack in the skin, thus beginning the slice.
A dull knife edge is relatively round and smooth, which glides smoothly across the skin as the knife is drawn across the tomato. Sometimes it can slide so smoothly near the edge of the previous slice that the knife slips off the tomato—hopefully away from the hand holding the tomato.
A sharp knife edge has a more jagged and peaked edge that generates much more friction as it is dragged across the tomato. A sharp edge is producing a shear force that reaches the same local critical stress using much less downward force, which enables greater control of the knife as it moves downward.
Once the crack has formed, the natural stress focused at the advancing crack is generally sufficient to propagate the cut at a much lower nominal stress, reducing the required force to slice the rest of the way through the tomato.
Other foods, like cheese, exhibit a nonlinear relationship between applied stress and the resulting deformation, meaning they can stiffen when deformed sufficiently. This nonlinear feedback leads to a frictionally locked situation when using a dull knife, where the knife progresses easily at first but then slows as both the cheese stiffens and the knife blade itself experiences the friction of the surrounding cheese. The locked state tends to release in the crack that penetrates the rest of the cheese, resulting in a messy cut.
This is why cheese is more easily sliced using a wire: the contact area with the cheese is much smaller. However, much like a knife edge, a wire becomes more effective as a cutting tool if the wire is not smooth. Experiments with roughened or sanded wires cut with less effort but produce a rougher cut that is less aesthetically pleasing.
Though cutting with a knife involves tearing, surgeons desire that the tearing be as minimal as possible; consequently, obsidian (volcanic glass) scalpels have been considered as an alternative to steel scalpels due to the narrowness of the cutting edge. When incisions made with each type of scalpel are viewed microscopically, one made with a steel scalpel will reveal tearing of individual cells, yet obsidian cuts between individual cells.
The healing of an incision made with an obsidian scalpel is believed to be much faster, and with less scarring, but obsidian tends to be used only where the patient might have an allergy to the metallic scalpel material. Obsidian scalpels do not have FDA approval, and they are fragile (pieces may break off if lateral force is applied), so they are unlikely to see widespread use.
Sharp knives make for better slicing, and narrow edges make smoother cuts, but (no matter how you slice it) friction always plays a key role.
Evan Zabawski, CLS, is the senior technical advisor for TestOil in Calgary, Alberta, Canada. You can reach him at firstname.lastname@example.org