Search

We derive the nonlinear equations governing vacancy-controlled interdiffusion in a two-component noninteracting lattice gas. We observe that when the diffusion constants of the two species are not equal, the problem cannot be reduced to a simple linear diffusion. In order to treat the strongly nonlinear limit of large kinetic asymmetry and small concentration of vacancies, we introduce an adiabatic approximation in which the ‘‘fast’’ species is in an equilibrium state defined by the instantaneous distribution of the ‘‘slow’’ species, which obeys a linear diffusion equation with a renormalized diffusion coefficient. Comparison with numerical solutions of the nonlinear equations shows that the adiabatic approximation captures the essential physics of the diffusion process. [ABSTRACT FROM AUTHOR]

Optical based methods for non-invasive measurement of regional blood flow tend to incorrectly assess cerebral blood flow, due to contribution of extra-cerebral tissues to the obtained signal. We demonstrate that spectral analysis of phase-coded light signals, tagged by specific ultrasound patterns, enables differentiation of flow patterns at different depths. Validation of the model is conducted by Monte Carlo simulation. In-vitro experiments demonstrate good agreement with the simulations' results and provide a solid validation to depth discrimination ability. These results suggest that signal contamination originating from extra-cerebral tissue may be eliminated using spectral analysis of ultrasonically tagged light.