On flow-blocking particle structures in microtubes

Given their small size, microchannels are susceptible to being blocked by small amounts of solid matter. The lifetime of certain microfluidic devices depends on their ability to maintain flow without interruption, and certain microfluidic applications require devices that include the transport of either liquids that may contain impurities or liquids that intentionally contain particles. Under the conditions studied in the present experiments, flow-stopping blockages were observed for the flow of liquids with the surprisingly low volume concentrations of phi = 0.005-0.056 of polystyrene particles (d(p)of the order of tens of microns) through circular microtubes with diameters (D) of the order of 100 mu m. The quantity of interest is the occurrence of a flow-stopping blockage in the flow of a suspension of hard spheres, over the range of d(p)/D from 0.2 to 0.5 and phi < 0.06. The percentage of runs for which a blockage did or did not occur versus nominal d(p)/D is presented. Given that higher concentrations of particles have been used in many previous studies of suspension flows, the occurrence of blockages at these concentrations was initially unexpected. Based upon the data and the parameters varied in the current study, the blockages are more likely to occur when the nominal particle-to-tube diameter ratios are approximately 0.3-0.4.