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Extreme Wetting Effects during Crystallization in Nanostructured Fluids

P.T. Spicer
Proctor & Gamble, US

Keywords:
extreme wetting effects, crystallization in nanostructured fluids

Abstract:
This talk concerns some fascinating and unexpected findings that sit uniquely at the interface between crystallization, wetting, surfactant self-assembly, materials templating, and consumer product rheology. Recent experiments have shown that new photonic and encapsulating materials can be templated using emulsion droplets by tailoring the wettability of adsorbed particles. Unique interfacial hydrodynamic effects have also been applied to induce spontaneous movement of droplets on surfaces with wettability gradients.
We show a way to combine the above processes by controlled surfactant effects on crystallizing oil-in-water emulsion droplets. Typically when crystals form in emulsion droplets, they are bounded by the oil-water interface. However, we show that when interfacial surfactant organization is optimized, the crystals that form within the emulsion droplets can be made ''antagonistic'' to their own liquid phase. The amazing result is that droplets actually de-wet and move away from their own crystals as they crystallize, creating a crystalline ''comet tail'' and resulting in the formation of a solid particle as much as 100 times larger than the parent droplet. The phenomenon is strikingly similar to the movement of bacteria propelled by actin polymerization, although much faster here. As a result of these unusual dynamics, droplets can be forced to ''turn themselves inside out'' and to actually move along surfaces while leaving a templated solid trail along their trajectory. The solid shape formed is a function of the relative rates of dewetting and crystallization as controlled by surfactant adsorption, cooling rate, and oil purity. For negligible dewetting rates, crystals nucleate and grow within the droplet. At similar crystallization and dewetting rates, the droplet is propelled around the continuous phase on a crystalline ''comet tail'' much larger than the original droplet. Rapid dewetting causes the ejection of small discrete purified crystals across the droplet's oil-water interface. The applications of this phenomenon span the areas of microfluidics, melt crystallization, chemical purification, and material synthesis. The phenomenon is documented using numerous microscopic movies to convey the exciting dynamics that are at work. Application of this technique to the creation of unique fluid microstructure in foods and other consumer products is also discussed.

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