I'm still trying to get better and better data - it's tricky given the size and speed of the movement. But my working model is something like this. Cutting is made of of two phases: cut initiation, and cleaving.
Cut initiation is all about the cutting edge. In an ultrasonic blade, that edge oscillates and the tiny imperfections on the blade edge act like a saw to break the linking fibers in food. It's just like using a human-scale slicing motion, but at 40kHz, and with a microscopic stroke length.
Cleaving is mostly about friction. Cutting a block of cheddar is pretty much all cleaving, and a very sharp cutting edge doesn't provide much advantage. My blade vibrates along the blade face, so foods experience the coefficient of kinetic friction, not static friction. This reduces cutting forces, and does so in a way that's totally independent of the sharpness of the edge.
We experience different foods as more cut-initiation-centric or more friction-centric. Tomatoes are all about piercing the skin. Hard squash is a cleaving game. Bread is layers upon layers of initiating cuts in the bubbles of the crumb.
If you're interested, I published my testing on regular knives in the Quantified Knife Project by strapping 21 chef's knives to a robot arm and collecting data on cutting forces. The data are open-source on github, too. https://youtu.be/GUQy0Sdp8Hc
Cut initiation is all about the cutting edge. In an ultrasonic blade, that edge oscillates and the tiny imperfections on the blade edge act like a saw to break the linking fibers in food. It's just like using a human-scale slicing motion, but at 40kHz, and with a microscopic stroke length.
Cleaving is mostly about friction. Cutting a block of cheddar is pretty much all cleaving, and a very sharp cutting edge doesn't provide much advantage. My blade vibrates along the blade face, so foods experience the coefficient of kinetic friction, not static friction. This reduces cutting forces, and does so in a way that's totally independent of the sharpness of the edge.
We experience different foods as more cut-initiation-centric or more friction-centric. Tomatoes are all about piercing the skin. Hard squash is a cleaving game. Bread is layers upon layers of initiating cuts in the bubbles of the crumb.
If you're interested, I published my testing on regular knives in the Quantified Knife Project by strapping 21 chef's knives to a robot arm and collecting data on cutting forces. The data are open-source on github, too. https://youtu.be/GUQy0Sdp8Hc