Aggregation of stable colloidal dispersion under short high-shear microfluidic conditions

We have designed and carried out intense shear-driven aggregation of charge-stabilized polystyrene nanoparticles (NPs) to big fractal clusters, in a single and a double Z-shape microchannels (SZ-MC and DZ-MC). Both the MCs have the same cross-section, but the length of DZ-MC is double of SZ-MC. It is found that when the NP dispersion repeatedly passes through each of the two MCs, the conversion of the NPs to big clusters can only reach a plateau value that is far smaller than full conversion. The plateau conversion at a given mean velocity (shear rate) is larger for the DZ-MC than for the SZ-MC, i.e., it increases as the MC length (or the residence time of one pass) increases. This surprising phenomenon can be explained upon considering that, as the conversion increases, the average NP separation distance increases, i.e., the time for the specific shear-driven bi-particle aggregation to occur increases. When the aggregation time becomes larger than the NP residence time in a MC, the NP dispersion passing through the MC does not lead to aggregation. The situation does not change if we let the NP dispersion repeatedly pass through the MC many times (i.e., by increasing the cumulative residence time), because if the residence time of one pass is not enough for the NPs to overcome the interaction energy barrier, when they come out from the MC, the intense shear force disappears and the NPs return to their original interaction state.