We use confocal microscopy to investigate the behavior of suspensions of attractive colloids during flow. We characterize the bulk rheological properties of attractive colloidal suspensions, and then directly image them during flow in microchannels. We investigate both the gel structure and the flow properties as a function of applied pressure and the colloid volume fraction. In addition, we also vary the geometry of the microchannel and investigate the flow in both straight channels and channels with constrictions. While quiescent, the gel structure is nearly homogeneous and isotropic, whereas flow leads to the formation of dense clusters separated by large voids. When the applied pressure or displacement is relatively small, the particle velocity is nearly constant in the center of the channel; near the walls the gel yields via breaking of clusters.

When the applied pressure or displacement is large, the gel structure is disrupted and the flow velocity changes across the channel width.