Evolution of structure in a quiescent electrorheological fluid

We report a real-time, two-dimensional light scattering study of the evolution of structure in a concentrated electrorheological fluid during the ``liquid-solid'' phase transition. We find that after particle chaining along the electric field lines, strong light scattering lobes appear at a finite scattering wave vector q orthogonal to the field lines, and then brighten as they move to q=0. This indicates the existence of an unstable concentration fluctuation that signifies the segregation of chains into columns. We suggest a theoretical explanation for our results.