Pressure-controlled formation of discontinuous clogs in tapered microchannels

In suspension flows through microchannels with parallel walls, rigid colloidal particles form clogs that grow continuously in the upstream direction. However, introducing a slight taper to channel walls leads to a qualitatively different clogging mechanism. Clogs of rigid particles do not grow continuously in these tapered pores. Instead, new clogs are initiated upstream of pre-existing clogs, truncating their growth and thereby creating multiple distinct clogs within a channel. We refer to this novel phenomenon as discontinuous clogging. Here, we investigate its features by analyzing the dimensions and locations of discontinuous clogs in parallel tapered pores. Measurements reveal the discontinuity of clog growth depends strongly on flow driving pressure and particle volume fraction. Increasing volume fraction increases clogging frequency and positions clogs upstream, in wider regions of the channels. Interestingly, two regimes of driving pressure are observed. Clogs become dramatically longer above a critical pressure. These long clogs are positioned downstream, towards the channel outlet, at the lowest volume fraction. However, long clogs are increasingly positioned upstream as volume fraction increases. The existence of this critical pressure lends insight into the bridging mechanism of clogging. Particles arriving simultaneously to a given location can span the channel width to form a bridge. Permanent clogs form when driving pressure is lower than the force the bridges can sustain. Above this limit, driving pressure overcomes the force chains, preventing the formation of new permanent clogs. Balancing the critical pressure with the stress on the particles in a bridge suggests each particle sustains 4 µN.