Leaping shampoo and the stable Kaye effect

Shear-thinning fluids exhibit surprisingly rich behaviour. One example is the Kaye effect which occurs when a thin stream of a solution of polyisobutylene in decalin is poured into a dish of the fluid. As pouring proceeds, a small stream of liquid occasionally leaps upward from the heap. This surprising effect, which lasts only a second or so, is named after its first observer, Kaye, who could offer no explanation for this behaviour. Later, Collyer and Fischer suggested from 250 frames s⁻¹ cine recordings that the fluid must be highly shear thinning as well as elastic and 'pituitous' (slimy or sticky). In addition, their results suggested that a rigid surface is required to back the reflected liquid stream. While the words bouncing and reflection are associated with elastic effects, we will show here that the Kaye effect is in fact a continuous flow phenomenon. We show that the Kaye effect works for many common fluids, including shampoos and liquid soaps. We reveal its physical mechanism (formation, stability and disruption) through high-speed imaging. The measurements are interpreted with a simple theoretical model including only the shear thinning behaviour of the liquid; elastic properties of the liquid play no role. We show that the Kaye effect can be stable and that it can be directed. We even demonstrate a stable Kaye effect on a thin soap film excluding the necessity of a rigid backing surface.