Technical note: The yield stress—An engineering reality

A simple experiment using the falling ball technique was used to demonstrate unambiguously that a 2500 wppm aqueous Carbopol 960 solution has a yield stress. A series of photographs taken over a period of several months revealed that a nylon ball with a density of 1.156 g/cm3 showed no measurable movement when placed well within a Carbopol solution having a density of 1.008 g/cm3.