Numerical modeling of sequential segmentation for enhancement of mixing inside microchannels

This study investigates sequential segmentation as an active mixing technique inside microchannels. In sequential segmentation, the solvent and anti-solvent streams are divided into segments in the axial direction. Due to the non-uniform velocity profile exhibited in laminar flow, Taylor-Aris dispersion can improve mixing by several orders of magnitude as compared with pure molecular diffusion. We built a 3D numerical model using COMSOL Multiphysics® to optimize this technique. We studied the effect of segmentation frequency, duty cycle (DC), flow velocity, microchannel aspect ratio and configuration of the inlet branches on the concentration distribution. We found that channel aspect ratio (H:W) and segmentation frequency had the most significant effect on mixing efficiency. Increasing segmentation frequency was found to increase the mixing efficiency up to a certain limit beyond which mixing efficiency decreased. This decrease was due to the non-complete segmentation of both streams at high frequencies and the formation of continuous trailing stream of both the solvent and anti-solvent. These trailing streams, which mitigate the segmentation effect, also appear in low aspect ratio (i.e. wider) channels and, therefore, reduces the mixing efficiency. Unequal mixing ratios were achievable by changing the segmentation duty cycle without changing the mixing efficiency considerably. Changing the flow velocity did not cause considerable changes in the mixing efficiency. Our 3D model confirms that sequential segmentation can considerably improve mixing inside microchannels and introduces the channel aspect ratio, for the first time, as an important parameter affecting the segmentation mixing efficiency.