The Physics of Whipped Cream

From NASA:

Let's do a little science experiment. If you have a can of whipped cream in the fridge, go get it out. Spray a generous dollop into a spoon and watch carefully.

Notice anything interesting? The whipped cream just did something rather puzzling. First it flowed smoothly out of the nozzle like a liquid would, and then, a moment later, it perched rigidly in the spoon as if it were solid. What made it change?

(While you're pondering this question, insert spoon into mouth, in the name of science.)

Whipped cream performs this rapid changing act because of a phenomenon called "shear thinning." When part of the foam is forced to slide or "shear" past the rest of the foam, the foam "thins." It becomes less like honey and more like water, allowing it to flow easily until the shearing stops.

Shear thinning occurs in many substances--e.g., ketchup, blood, motor oil, paint, liquid polymers such as molten plastic--and it is often crucial to how a substance is used. For instance, excessive shear thinning of motor oil is unwanted because it reduces the oil's ability to protect engines from wear, while shear thinning of paint allows it to flow smoothly from the brush but stay put on the wall. It also allows ketchup to flow from the bottle but not drip off your french fries.

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