Is vorticity-banding due to an elastic instability?

Abstract

A possible mechanism for the vorticity-banding instability is proposed on the basis of experiments with colloidal rod-like particles that exhibit an isotropic-nematic phase transition. The proposed mechanism is similar to the well-known elastic instability for polymer systems that is due to nonuniform elastic deformation of polymer chains as a result of gradients in the local shear rate (the Weissenberg effect). However, the role of polymer chains is now played by inhomogeneities that exist in systems exhibiting vortic- ity banding. For the rod-like colloidal system investigated here, inhomogeneities are formed during the early stages of phase separation. Nonuniform deformation of these inho- mogeneities are thus proposed to lead to hoop stresses which give rise to banded structures where there is secondary, weakly rolling flow within each of the bands. Many of the features found experimentally for the rod-like colloidal system can be understood on the basis of this proposed mechanism. For different types of systems that also show vorticity banding, inhomogeneities can be identified, which might lead to vorticity banding for the same reasons as for the rod-like colloidal systems studied here.