Pressure-controlled formation of discontinuous clogs in tapered microchannels

In suspension flows through microchannels with parallel walls, rigid 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 form 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 the clogs upstream, in wider regions of the channels. Two regimes of driving pressure appear to exist as the discontinuous clogs are observed to become dramatically longer above a critical pressure. Interestingly, these long clogs are located downstream, towards the channel outlet, at the lowest volume fraction. However, they are increasingly located upstream as volume fraction increases. The dependence of clog location on pressure and volume fraction lends insight into bridging mechanism. Particles arriving simultaneously to a given location can span the channel width to form a bridge, which happens easily at higher volume fractions. Permanent clogs form when driving pressure is lower than the force the bridges can sustain. As driving pressure increases, however, it can overcome the force chains, preventing formation of new permanent clogs, and pushing particles further downstream.