plastic flow

A plastic material behaves like a solid until you push it hard enough, after which it flows like a liquid. Toothpaste is the textbook example: it sits motionless on the brush and does not slump, but squeeze the tube past a certain effort and it streams out smoothly. Below that threshold it simply will not flow at all.

The threshold is called the yield value: the minimum shear stress that must be exceeded before any flow begins. Below the yield value the material has an internal structure, often a loose three-dimensional network of flocculated particles, that resists deformation entirely. Once the applied stress breaks that network, the material yields and then flows, often in a roughly straight-line fashion above the yield point (the idealised version of this is called Bingham flow).

Plastic flow is valuable wherever you want a product to hold its shape until deliberately disturbed. It explains why a well-formulated ointment or paste stays put on the skin, why a flocculated suspension resists settling yet redisperses on shaking, and why toothpaste keeps a ribbon shape. The yield value can be tuned with structure-building excipients to balance 'stays put' against 'easy to use'.