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Seminars
Nanoemulsions: From Shear-Induced Vitrification to Slippery Aggregation
Professor Thomas Mason
Department of Chemistry and Biochemistry
University of California, Los Angeles
When: Tuesday, February 27, 2007
Time: 2:30 PM to 3:30 PM
Where: 301 Sewall Hall
Abstract:
Using extreme extensional flow produced by a high-pressure microfluidic device, we rupture microscale emulsions into nanoemulsions comprised of sub-100 nm droplets. For silicone oil droplets in an aqueous solution of the anionic surfactant, sodium dodecylsulfate (SDS), both shear-induced rupturing and coalescence play important roles in determining the final ruptured droplet size. Remarkably, at droplet volume fractions far below that associated with maximal random jamming of spheres, the extreme shear irreversibly transforms a liquid-like microscale emulsion into a vitreous elastic nanoemulsion. We have studied the increase of the shear modulus as a function of the history of applied shear and composition, and we show that the effect of shear-induced elastic vitrification arises from Debye-screened repulsions that become increasingly important as the droplet size approaches the screening length. Uniform nanoemulsions are ideal for structural studies involving small angle neutron scattering (SANS). Using time-resolved SANS, we have studied the formation of glassy networks of nanodroplets after inducing a strong short-range ‘slippery’ attraction that does not cause coalescence. Simulations of ‘slippery’ diffusion limited aggregation confirm our observations that rigid and tenuous space-filling fractal structures can still form from small dense clusters of droplets, even when the individual bonds between the droplets are not shear-rigid.
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