| The dynamics of formation and the pinch-off mechanism of droplets in microfluidic flow focusing generators was already studied in great detail. Depending on the rheology of the immiscible liquid phases used these studies can be divided into two groups: i) formation of Newtonian droplets in Newtonian ambient liquid [1] (overwhelming majority of the studies) and ii) formation of non-Newtonian (usually viscoelastic) droplets in Newtonian outer phase [2]. Here, for the first time, we present an experimental comparative study, where we generate Newtonian droplets in Non-Newtonian (viscoelastic) continuous phase. To investigate the effect of the elasticity on formation of droplets we use constant viscosity elastic liquids (also known as Boger fluids[3]) of various elasticity, shear viscosity and concentration of the polymer additives as the continuous phase along with a set of Newtonian liquids of the same shear viscosities. As the droplet phase we use silicon oils of a wide range of shear viscosities. Although we found similar regimes of operation in both Newtonian and viscoelastic systems: i) dripping regime without satellite droplets, ii) dripping with single satellites, iii) formation of multiple satellites and iv) jetting regimes, we found that elasticity of the continuous phase promotes “smoother” transition from dripping to jetting regime and facilitates formation of smaller droplets. We also show that in viscoelastic flow focusing the dripping-jetting transition occur at much lower capillary numbers (and thus rates of flow) of the continuous phase than in a Newtonian focusing liquid of the same viscosity and that this reduction of the capillary number is independent of the elasticity number of the viscoelastic continuous phase. Additionally we show that dispersion of the produced droplets can be controlled by adjusting the ratio of viscosities and ratio of flow rates of the two immiscible phases. Finally in flow focusing droplet generators with a contraction two classes of instabilities occur in the jetting regime which results in a systematic change of the size distribution of the produced droplets. [1] S. L. Anna, N. Bontoux, and H. A. Stone, Appl. Phys. Lett. 82, 364 (2003) [2] P.E. Arratia, J. P. Gollub, and D. J. Durian, Phys. Rev. E 77, 036309 (2008) [3] D. V. Boger, J. Non-Newtonian Fluid. Mech. 3, 87 (1977) |
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