Salt comets in hand sanitizer: A simple probe of microgel collapse dynamics

Arash Nowbahar (1) , Art O'Connor (1) , Vincent Mansard (2) , Patrick Spicer (3) , Todd Squires (1)

1 Department of Chemical Engineering [Santa Barbara]
2 LAAS-MEMS - Équipe Microsystèmes électromécaniques
3 School of Chemical Engineering, University of New South Wales, 2052 Sydney, Australia

Polyelectrolyte microgels find many uses as rheological modifiers and stimulus-responsive materials. Understanding their swelling and collapse dynamics therefore holds broad importance in science and technology. We report remarkably simple experiments, requiring little sophistication, that reveal the subtle physics of microgel collapse. Millimeter-scale bubbles, sugar grains, and other small particles remain suspended and supported by the yield stress of household hand sanitizer, which arises due to a jammed suspension of swollen microgels. By contrast, salt grains with almost identical physical properties sediment through the material, leaving milky "comet tails" behind. Remarkably, the settling speed of a salt crystal remains constant as it dissolves-completely independent of its size or shape until it completely dissolves. Because the settling speed does depend on the type of salt that sediments, we hypothesize that salt grains effectively bore holes through hand sanitizer, with a velocity that is limited by the salt-induced dynamic collapse of the individual microgel particles. A simple convection-diffusion-collapse model successfully relates sedimentation velocities to microgel collapse dynamics for various salts. This model and its predictions are consistent with other observations and with complementary microfluidic experiments.

Domaines: Matière Molle [cond-mat.soft]

Arash Nowbahar, Art O'Connor, Vincent Mansard, Patrick Spicer, Todd Squires. Salt comets in hand sanitizer: A simple probe of microgel collapse dynamics. Physical Review Fluids, 2019, 4 (6), pp.061301. ⟨10.1103/PhysRevFluids.4.061301⟩. ⟨hal-02391635⟩