Experimental observations of the squeezing-to-dripping transition in T-shaped microfluidic junctions

Abstract

An experimental study of droplet breakup in T-shaped microfluidic junctions is presented in which the capillary number and flow rate ratio are varied over a wide range for several different viscosity ratios and several different ratios of the inlet channel widths. The range of conditions corresponds to the region in which both the squeezing pressure that arises when the emerging interface obstructs the channel and the viscous shear stress on the emerging interface strongly influence the process. In this regime, the droplet volume depends on the capillary number, the flow rate ratio, and the ratio of inlet channel widths, which controls the degree of confinement of the droplets. The viscosity ratio influences the droplet volume only when the viscosities are similar. When there is a large viscosity contrast in which the dispersed-phase liquid is at least 50 times smaller than the continuous-phase liquid, the resulting size is independent of the viscosity ratio and no transition to a purely squeezing regime appears. In this case, both the droplet volume and the droplet production frequency obey power-law behavior with the capillary number, consistent with expectations based on mass conservation of the dispersed-phase liquid. Finally, scaling arguments are presented that result in predicted droplet volumes that depend on the capillary number, flow rate ratio, and width ratio in a qualitatively similar way to that observed in experiments.

Coalescence and splitting of confined droplets at microfluidic junctions

“…Two upstream T-shaped junctions generate droplets of water in the cross-flowing stream of immiscible oil as shown in inset A. Cross-flowing streams is a common and well-characterized method to generate droplets, in which the droplet sizes are controlled by adjusting the volumetric flow rate of the continuous phase liquid, the ratio of the volumetric flow rates of the two immiscible liquids, and the ratio of the inlet channel widths w c and w d . [44][45][46] Using identical flow rates for each of the upstream T-junctions, droplets with nominally equal volume and speed are generated. Downstream, the two uniform droplet streams collide head-on at a second T-junction and flow together into a common outlet channel with width equal to the inlet channel width w o ¼ w c as shown in inset B.…”

Droplet formation in microfluidic cross-junctions

“…Many microfluidic devices have been designed to generate uniform droplets, including geometry-dominated devices, 13,14 flow-focusing devices, [15][16][17][18][19] T-junctions, [20][21][22][23][24][25][26] and co-flowing devices. 27,28 However, the underlying mechanisms of droplet formation in microchannels have not been well understood, which hinders device optimization and operation.…”

Physics of Multiphase Microflows and Microdroplets

“…When Ca is increased to 0.032 and 0.056, the detachment point will move from the T-junction corner to the downstream as Q increases. In addition, the droplet detachment point gradually moves downstream until a stable jet is formed when Ca and Q increase, which was also observed both numerically (De Menech et al (2008); Liu and Zhang (2009)) and experimentally (Christopher et al (2008)). …”

“…Liu and Zhang (2009) noticed that the two regimes become difficult to distinguish as Q decreases because the droplet detachment point is always close to the downstream corner of the T-junction at small Q. This may explain why Christopher et al (2008) did not observe the critical Ca during the squeezing-to-dripping transition because they performed experiments at small viscosity ratio 0f 0.01, where the droplet breakup always occurs at the downstream corner of the T-junction. According to Liu and Zhang (2009), there is a critical capillary number (see Fig.…”

“…For example, De Menech et al (2008), using the Navier-Stokes solver with a phase-field model, reported a critical capillary number of 0.015. However, the recent experimental study by Christopher et al (2008) did not observe the critical capillary number during the squeezing-to-dripping transition. Liu and Zhang (2009) noticed that the two regimes become difficult to distinguish as Q decreases because the droplet detachment point is always close to the downstream corner of the T-junction at small Q.…”