Influence of geometry on hydrodynamic focusing and long-range fluid behavior in PDMS microfluidic chips

Details of hydrodynamic focusing in a 2D microfluidic channel-junction are investigated experimentally and theoretically, especially the effect on the focusing width of volumetric flow ratio r between main and side channels, as well as angle θ between channels. A non-linear relationship is observed where the focus width decreases rapidly with increasing r and levels off at higher values. For the dependence on θ, results from both experiments and modeling show that an increased focusing effect is obtained as θ approaches 90°. Long-range focusing is explored along a 1 cm long channel and it is observed that in the middle section of the channel, a smaller θ induces less divergence. This effect is of importance for microfluidic systems utilizing hydrodynamic focusing in long, straight channels.