Slippery Diffusion Limited Aggregation of Attractive Nanodroplets

Featuring Thomas G. Mason, Ph.D.
Assistant Professor and John McTague Chair
Department of Chemistry and Biochemistry;
Department of Physics and Astronomy
University of California, Los Angeles

Location: CS 174
*Refreshments will be served after seminar

ABSTRACT:
Nanoemulsions are metastable emulsions of nanoscale droplets formed by rupturing larger droplets into smaller droplets using extreme shear. Once formed, a surfactant prevents coalescence of the emulsion, yielding a long-lived dispersion of liquid droplets. Using time-resolved small angle neutron scattering, we have measured the structure factor of monodisperse silicone oil-in-water nanoemulsions that aggregate and gel after we suddenly turn on a strong, short-range, slippery attraction between the droplets. At higher scattering angles, peaks in the structure factor appear as dense clusters of droplets form initially. By contrast, toward lower angles, the structure factor increases rapidly, as these dense clusters become locked into a rigid gel network, despite the fluidity of the films between the droplets. The long-time structure of nanoemulsion gels formed by slippery diffusion limited cluster aggregation is universal in shape and remarkably independent of the droplet volume fraction. We call this new aggregation paradigm, "Slippery Diffusion Limited Aggregation" and show how it differs from other classic aggregation schemes, such as simple diffusion limited aggregation and reaction limited aggregation.

ABOUT THE SPEAKER:
Professor Mason received his Ph.D. in Physics from Princeton University in 1995.  His present research interests include 1.) Microrheology: measuring the dynamic mechanical shear properties of viscoelastic soft materials over an extraordinarily large range of time scales by optically observing the diffusive motion of micron-sized probe particles embedded in these materials.  2.) Shear-induced structures in complex fluids: emulsification, shear-banding, shear-ordering; 3.) Neutron and light scattering from nanomaterials: asphaltenes, clays, droplets, polymers;  4.) Directed assembly of nanoparticles in solution using shape-dependent attractive interactions.