Viscosity
Tomato ketchup is a pseudoplastic — or "shear thinning" substance — which can make it difficult to pour from a glass bottle. Often, the neck of the bottle will appear to be blocked. A common method to getting ketchup out of the bottle involves inverting the bottle and shaking it or hitting the bottom with the heel of the hand, which causes the ketchup to flow rapidly. A technique known widely among caterers involves inverting the bottle and forcefully tapping its upper neck with two fingers (index and middle finger together). Specifically, with a Heinz ketchup bottle, one taps the 57 circle on the neck. This helps the ketchup flow by applying the correct shearing force. These techniques work because of how pseudoplastic fluids behave: their viscosity (resistance to flow) decreases with increasing shear rate. The faster the ketchup is sheared (by shaking or tapping the bottle), the more fluid it becomes. After the shear is removed the ketchup thickens to its original viscosity.
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