Shear induced displacement of the spinodal of Brownian systems

The displacement of the gas-liquid spinodal of colloids on applying a shear flow is analyzed on the basis of the Smoluchowski equation. Relatively simple arguments predict that the off-critical part of the spinodal is displaced linearly with the shear rate gamma, while the shift of the critical temperature is similar to gamma(1/gamma) with gamma the critical exponent for the reciprocal equilibrium compressibility. This differs considerably from what is known for molecular systems. It is also argued that the cloud-point curve under shear no longer coincides with the spinodal, and is displaced far into the unstable region of the phase diagram. A sharp increase of forward scattered intensity in a sheared system does not probe the location of the spinodal.