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

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.

Figure 1

Steady shear rheology of hand sanitizer shows a finite yield stress ${\sigma }_Y\sim 10\mathrm{Pa}$, which decreases with added $\left[{\mathrm{CaCl}}_2\right]$, becoming Newtonian above $\left[{\mathrm{CaCl}}_2\right]\sim 2\text{–}5\mathrm{mM}$. Dashed lines indicate fits to a Herschel-Bulkley model (see Fig. S2 [14]). Inset: Millimeter-sized bubbles remain stably suspended in hand sanitizer, without rising, due to its yield stress.

Figure 2

(a) Snapshot of two sodium chloride particles falling in a vial filled with hand sanitizer. (b)–(d) Closeup snapshots of a calcium chloride particle falling in hand sanitizer, with the arrow indicating the downward direction of motion. Note the cloudy, “comet-tail” streak in the wake of the falling salt grain. (b) $t=0\mathrm{s}$, (c) $t=40\mathrm{s}$, (d) $t=75\mathrm{s}$.

Figure 3

Position of falling salt grains tracked with time. Triangles, squares, and circles correspond to KCl, NaCl, and ${\mathrm{CaCl}}_2$, respectively. Open circles correspond to a second distinct ${\mathrm{CaCl}}_2$ grain with a significantly different size and shape, showing reproducibility. Diamonds show the volume of the ${\mathrm{CaCl}}_2$ particle corresponding to experiment in solid circles. The sedimentation velocity does not depend on salt grain size (up until complete dissolution), but does depend on salt type.

Figure 4

Proposed mechanism for salt sedimentation through hand sanitizer. Hand sanitizer contains a jammed suspension of soft, swollen Carbopol particles. Dissolving salt diffuses into microgels, collapsing them and removing the yield stress. The particle settles through the fluid, sweeping fluid and collapsed microgels into wake, where they scatter light leaving a “comet tail.” The dashed line depicts the location where salt is concentrated enough to begin collapsing microgels. Right: Closeup of the leading edge of the falling grain, in the comoving reference frame, where dissolving salt establishes a steady convection-diffusion boundary layer into the microgel suspension. Microgels start to collapse at $x_c$, when local concentration exceeds $C_{*}$ and are advected a distance $x_c=V{\tau }_c$ during the collapse time.

Figure 5

(a) Schematic of microfluidic experiment. Salt solution is flowed in the left channel, and hand sanitizer flowed in the right channel. The hand sanitizer flow is turned off, with the yield stress holding it in place. The hand sanitizer/solution interface then propagates due to salt-induced collapse, as shown by the zoomed-in inset of the interface. (b)–(d) Phase contrast micrographs of the middle section of the channel at different times when flowing $0.37M{\mathrm{CaCl}}_2$. Collapsed microgels appear as bright spots. (b) $t=1\mathrm{s}$. (c) $t=5\mathrm{s}$. (d) $t=9\mathrm{s}$. (e) Position of the interface vs time for $70\%$ ethanol mixed with ${\mathrm{CaCl}}_2$ at $2.08M$ (circles), $0.37M$ (triangles), $0.032M$ (squares), $0.016M$ (diamonds), and $0M$ (line).

Figure 6

(a) Velocity of salt solution/microgel interface in microfluidic coflow increases with concentration of KCl (triangles), NaCl (squares), and ${\mathrm{CaCl}}_2$ (circles). (b) Using best-fit values for the microgel collapse concentration $C_{*}$ reveal a linear relation between the velocity of the microgel/solution interface and $\sqrt{ln(C/C_{*})}$. Dashed lines correspond to linear fits through the origin, with slopes 0.066, 0.057, and 0.034 mm/s. Error bars indicate the standard deviation of at least three replicate experiments, and are smaller than markers when not visible.