Generation of Oil Droplets in a Non-Newtonian Liquid Using a Microfluidic T-Junction

Abstract

Abstract:We have compared the formation of oil drops in Newtonian and non-Newtonian fluids in a T-junction microfluidic device. As Newtonian fluids, we used aqueous solutions of glycerol, while as non-Newtonian fluids we prepared aqueous solutions of xanthan, a stiff rod-like polysaccharide, which exhibit strong shear-thinning effects. In the squeezing regime, the formation of oil droplets in glycerol solutions is found to scale with the ratio of the dispersed flow rate to the continuous one and with the capillary number associated to the continuous phase. Switching to xanthan solutions does not seem to significantly alter the droplet formation process. Any quantitative difference with respect to the Newtonian liquid can be accounted for by a suitable choice of the capillary number, corresponding to an effective xanthan viscosity that depends on the flow rates. We have deduced ample variations in the viscosity, on the order of 10 and more, during normal operation conditions of the T-junction. This allowed estimating the actual shear rates experienced by the xanthan solutions, which go from tens to hundreds of s´1.

Droplet breakup driven by shear thinning solutions in a microfluidic T-junction

“…In particular, A ∼ ϕ (0.32±0.02) , the exponent being similar to those reported by Refs. [35,63]. These observations provide a direct, experimental evidence of the validity of the Ca number, Eqs.…”

“…Images of the droplets are acquired and analyzed in with custom software, which also controls the syringe pumps. The length L of the droplets is measured, after breakup, in a region of interest downstream of the T-junction ( Fig.1-A), and is averaged over at least a hundred droplets [35]. Additional details on the experimental setup are reported in Sec.…”

“…The viscosity data of the Xanthan solutions are fitted according to the power law fluid model, similarly to that done in Refs. [30,35], where K and n, being the fluid consistency and the flow behavior index respectively [36], are used as fitting parameters. Their values are listed in Table II for the concentrations of Xanthan used in this study.…”

Experimental investigations of non‐Newtonian/Newtonian liquid‐liquid flows in microchannels

“…Chiarello et al 14 who studied the formation of organic droplets in a T-junction channel using different concentrations of xanthan gum in water, investigated the shear-thinning effect on the droplet size. They compared a xanthan gum/water solution to a glycerol/water (Newtonian) one with similar viscosity and found that the droplet lengths produced in the polymeric mixtures were longer than those obtained in the Newtonian one especially at a high flow rate ratio (under constant continuous phase flow rate).…”

“…Arratia et al 13 investigated the filament thinning and breakup of viscoelastic fluids in liquid-liquid flows in a crossslot microchannel inlet to obtain their extensional viscosity. Chiarello et al 14,15 studied the formation of oil droplets in shear-thinning, aqueous solutions and introduced an effective Capillary number that took into account the shear-thinning rheology. They observed that the droplet size in the nonNewtonian fluids increased with the flow rate ratio and decreased with increasing continuous phase flow rate.…”