Mixing corn starch and water in appropriate amounts creates a solution that is liquid when slowly stirred but hardens when you press it—a substance colorfully referred to as “oobleck.” (The name is taken from Dr. Seuss’s 1949 children’s book, Bartholomew and Oobleck.) According to a new paper published in the journal Physical Review Letters, high-speed imaging and force measurements have revealed another surprising property of oob droplets hitting a flat surface.
As mentioned earlier, in an ideal fluid, viscosity depends largely on temperature and pressure: water will flow regardless of other forces acting on it, such as stirring or mixing. In a non-Newtonian fluid, viscosity changes in response to applied stress or shear force, straddling the boundary between liquid and solid behavior. Shaking a cup of water produces a shear force, and the water moves out of the way. The viscosity remains unchanged. But for non-Newtonian fluids like Oobleck, the viscosity changes when shear force is applied.
For example, ketchup is a shear-thickening non-Newtonian fluid, which is one reason why tapping the bottom of the bottle does not force the ketchup out faster; Viscosity increases by applying force. Curd, gravy, clay, pudding, and thick pie filling are other examples. And so is Oobleck.
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