Flow characteristics inside shear thinning xanthan gum non-Newtonian droplets moving in rectangular microchannels

In this paper, the internal flow characteristics of shear thinning xanthan gum (0.02–0.08 wt %) non-Newtonian droplets moving in rectangular microchannels were studied by means of micro-PIV. When the capillary number Ca is less than 4.05 × 10^–3, there are four eddies inside droplets running in the low depth-to-width ratio channel. Once the aspect ratio gets higher, more oil film resistance would result in a pair of eddies inside droplets near the channel wall at the plane of half the channel height, which means six eddies exist inside droplets in total. As Ca gradually increased to 5.06 × 10^–2, due to the shear stress of continuous phase and the shear thinning of dispersed phase, the flow topologies inside droplets would change to symmetric butterfly wing double vortex and \(\dot{\mathrm{M}}\) structure. The uneven distribution of interfacial tension caused by the inertial aggregation of surfactants at the back end of the droplet transforms long droplets into a teardrop-like under continuous phase extrusion. Because of the continuous phase reflux in the middle of the droplet horizontal axis, an obvious pressure fluctuation is observed. At high xanthan gum concentration, the increase of viscosity leads to four eddies inside teardrop-like droplet. The essence of the flow fields inside non-Newtonian droplets is the redistribution of dispersed phase velocity with different degrees of shear thinning. The results are beneficial to the development of microfluidic flow cytometry, the control of biochemical reaction process inside droplets and the understanding of the complex logic behavior of non-Newtonian droplets in microchannels.