Salt Comets in Hand Sanitizer: Salt-Induced Microgel Collapse*

Yield stress fluids have seen widespread use, with many industrial and consumer products relying on this property for functionality, such as in toothpastes, hair gel, and paint. While some applications employ colloidal gels, e.g. clay particles, others use colloidal glasses such as microgel particles that swell and jam. For example, hand sanitizer is a form of alcohol that sits without falling from the user’s hands due to its yield stress. In this work, we propose a simple experiment that takes advantage of a gel’s yield stress to understand the dynamics of microgel collapse in the presence of salt. The sedimentation of salt particles in hand sanitizer is studied with a diffusion-relaxation model and corroborated with microfluidic experiments, thus revealing a timescale for microgel collapse.