Schedule Apr 14, 2005
From Proteins to Peas: Diffusion Across Scales
Dr. J.F. Brady, Caltech

Diffusion is one of the most basic and elemental transport processes and is responsible for the molecular mixing of different chemical species. For a protein molecule, the diffusivity is given by the familiar Stokes-Einstein formula relating the Brownian diffusivity to the thermal energy times the hydrodynamic mobility of the protein: D=kT/6 \pi \eta a, where \eta is the viscosity of the solvent and a is the protein size. The Brownian self-diffusivity decreases as the concentration of protein molecules increases owing to the crowding effect of near neighbors. As the diffusing species increases in size from a protein to a several micron-sized colloidal particle, the stirring of the background fluid can give rise to another mechanism of transport – ‘shear-induced’ diffusion. Here, hydrodynamic interactions among particles promote mixing and the self-diffusivity now scales as \gamma a2 , where \gamma is the shear rate. In this regime, the self-diffusivity is an increasing function of concentration since particle-particle ‘collisions’ are responsible for the diffusion motion. At still large particle size (millimeter or larger), the inertia of the particles becomes important, direct particle-particle collisions dominate the transport as opposed to the stirring of the background fluid, and the self-diffusivity now behaves like that in a dense gas: D ~ a Tg1/2 , where Tg is the ‘granular temperature’, which is set by the stirring motion and the energy dissipated upon particle-particle collision. As in a dense gas, the self-diffusivity now decreases with increasing particle concentration. The physical origin of these various behaviors and their implications for mixing and concentration distributions in flows will be discussed.

Begin WebCam and audio for the whole talk: high bandwidth or medium bandwidth.
Or, begin audio only for the whole talk: high bandwidth or low bandwidth. (Or, right-click to download the whole audio file.)

Begin QuickTime of the whole talk (high bandwidth).

To begin viewing slides, click on the first slide below. (Or, view as pdf.)

[01] [02] [03] [04] [05] [06] [07] [08] [09] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

Author entry (protected)