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Image of Vitamix Blender

For a blender to be effective the particles must be small enough so that the mixture will flow. This is done first by cutting, smashing, chopping of the food. . Since it takes a lot of kinetic energy to break pieces down very small, the final size in this step depends on the power of the blender. A lot of the kinetic energy from the fast-spinning blades will cause the temperature of the mixture in the blender to rise through friction. At this point and another set of forces take over. A combination of compressive shearing forces and cavitation that causes the blender to rip pieces of food into bits so small that the end result is a smooth puree or a rich emulsion.

What is Cavitation?

Cavitation occurs when the local pressure in a liquid drops below the vapor pressure of the liquid, causing the liquid to form bubbles (similar to that in boiling). This occurs around the blades of both a propeller in a boat or the blades found in a food blender. As a liquid is accelerated over the blades, the fluid pressure decreases in response to the increase in fluid velocity (according to a relationship known as Bernoulli's principle).

The Bernoulli Equation

P is pressure, rho is the density of the fluid and u is its velocity. A consequence of this law is that if the velocity increases then the pressure falls.

The Bernoulli Equation can be considered to be a statement of the conservation of energy principle appropriate for flowing fluids.

In fluid dynamics, Bernoulli's principle states that for an inviscid flow of a nonconducting fluid, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738.

If the flow reaches a high enough velocity, the pressure can be reduced enough to cause cavitation. Additionally, when the bubbles move to regions of higher fluid pressure, they will collapse. The force of collapse will further break apart particles in a food blender.

In the case of a propeller or blade such as found in a blender the rotating blade will cause the fluid to accelerate in the axial direction, so that the fluid that has passed through the blade moves at a higher speed than that which is below the blade.

In fluid dynamics, a vortex is a region, in a fluid medium, in which the flow is mostly rotating on an axis line.

The vortex causes a vacuum at the center of the jar, which pulls the ingredients toward the middle and down into the blade.

A vortex is required for blenders to work properly. According to America’s Test Kitchen, a good vortex is formed when the blender’s blades have a “wingspan” that comes close to the sides of the blending jar.

Shown to left are the blades found in the Vitamix Vita-Prep-3


Video by ChefSteps shows a vortex being formed in a Waring Blender

Look for tiny bubbles being shown near the blades which is cavitation.

References and Selected Readings:

The Physics of Blending — Cutting Food Down to Size --ChefSteps

Bernoulli Equation -- Georgia State University Physics Department

Science of Cooking - Science of Blending

Wikipedia - Cavitation

Science of Fluids

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