Emulsification in a microfluidic flow-focusing device: effect of the viscosities of the liquids

Abstract

We report the results of a comparative study of microfluidic emulsification of liquids with different viscosities. Depending on the properties of the fluids and their rates of flow, emulsification occurred in the dripping and jetting regimes. We studied the characteristic features and typical dependence of the size and of the size distribution of droplets in each regime. For each liquid, we identified a range of hydrodynamic conditions promoting generation of highly monodisperse droplets. Viscosity played an important role in emulsification: highly viscous liquids were emulsified into larger droplets with lower polydispersity. Although it was not possible to provide a unified scaling for the volumes of the droplets, our results suggest that the break-up dynamics of the lower viscosity fluids resembles the rate-of-flow-controlled break-up, as reported earlier for the formation of bubbles in flow-focusing geometries [Garstecki P, Stone HA, Whitesides GM (2005) Phys Rev Lett 94:164501]. The results of this study can be helpful for a rationalized selection of liquids for the controlled formation of droplets with a predetermined size and with a narrow distribution of sizes.

Porous polymer particles—A comprehensive guide to synthesis, characterization, functionalization and applications

“…Viscosity is found to be playing a role [206] in the formation of undesired satellites too, which can be defined as the formation of much smaller droplets accompanying the larger monodisperse droplets [151,153,155,[207][208][209]. Kumacheva et al reported for O/W emulsions that there are larger and narrower safe ranges (no satellite formation) of flow rate ratios for different viscosity values of oils emulsified [206]. It is also important to mention that the jetting regime is one of the main reasons of satellite formation [210].…”

“…Some reports emphasized the effect of continuous phase viscosity on droplet formation [204], but others also figured out that it does not have a significant effect [205]. Viscosity is found to be playing a role [206] in the formation of undesired satellites too, which can be defined as the formation of much smaller droplets accompanying the larger monodisperse droplets [151,153,155,[207][208][209]. Kumacheva et al reported for O/W emulsions that there are larger and narrower safe ranges (no satellite formation) of flow rate ratios for different viscosity values of oils emulsified [206].…”

Parallelization of microfluidic flow-focusing devices

“…Microfluidic devices 5 offer a good control over the drop size and allow production of monodisperse drops. For example, microfluidic flow-focusing devices enable the production of drops with a coefficient of variation, defined as the standard deviation of their size 10 distribution divided by the mean drop size, of 1%-2%. 4 Junctions of such flow-focusing devices typically consist of a pair of inlets intersecting the main channel at an angle of 90° or less.…”

“…In this case, drops form in the squeezing regime where the 20 inner phase penetrates into the junction and blocks it almost entirely, thereby preventing the outer phase from entering the main channel before the inner phase starts to neck. In this regime, the dynamics of drop breakup is entirely controlled by the flow rate of the 25 outer phase, 8 such that the drop size depends on the flow rate of the outer phase, 6, 9 its viscosity and surface tension, 10 and the channel geometry. 7,11,12 However, the necking process, which eventually leads to drop formation, is slow, limiting the maximum drop 30 generation frequency and hence the throughput.…”

“…This result is in good agreement with 60 the viscosity-dependence of the size of drops produced in other microfluidic flow-focusing devices with θ = 90°. 10 For the same flow conditions, the size of drops generated in the devices with θ = 45° and θ = 135° are expected to be the same because the q x of the outer phase flow rate are 65 the same. Indeed, the sizes of drops produced in the devices with θ = 45° and θ = 135° are very similar if the viscosity of the outer phase is lower than the inner fluid, as shown by the solid symbols in Figure 2 (c).…”