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. 2015 Mar 24;9(2):024107.
doi: 10.1063/1.4916228. eCollection 2015 Mar.

A numerical study on the dynamics of droplet formation in a microfluidic double T-junction

Ich-Long Ngo  1 , Trung-Dung Dang  2 , Chan Byon  1 , Sang Woo Joo  1
Affiliations

A numerical study on the dynamics of droplet formation in a microfluidic double T-junction

Ich-Long Ngo et al. Biomicrofluidics. .

Abstract

In this study, droplet formations in microfluidic double T-junctions (MFDTD) are investigated based on a two-dimensional numerical model with volume of fluid method. Parametric ranges for generating alternating droplet formation (ADF) are identified. A physical background responsible for the ADF is suggested by analyzing the dynamical stability of flow system. Since the phase discrepancy between dispersed flows is mainly caused by non-symmetrical breaking of merging droplet, merging regime becomes the alternating regime at appropriate conditions. In addition, the effects of channel geometries on droplet formation are studied in terms of relative channel width. The predicted results show that the ADF region is shifted toward lower capillary numbers when channel width ratio is less than unity. The alternating droplet size increases with the increase of channel width ratio. When this ratio reaches unity, alternating droplets can be formed at very high water fraction (wf = 0.8). The droplet formation in MFDTD depends significantly on the viscosity ratio, and the droplet size in ADF decreases with the increase of the viscosity ratio. The understanding of underlying physics of the ADF phenomenon is useful for many applications, including nanoparticle synthesis with different concentrations, hydrogel bead generation, and cell transplantation in biomedical therapy.

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Figures

FIG. 1.
FIG. 1.
Schematics of computational domain for MFDTD. Dispersed flow 1 (DP1) and 2 (DP2) are both water with identical properties and inlet velocity Ud. P1 and P2 are two pressure measurement points used to check pressure difference between the two dispersed flows.
FIG. 2.
FIG. 2.
Comparison between present result and the result obtained by Bashir's study dealing with droplet formation in microfluidic T-junction for dispersed flow velocity Ud = 0.012 m/s and viscosity ratio β = 0.8.
FIG. 3.
FIG. 3.
Four regimes observed for the formation of alternating droplets as a function of the capillary number Ca. (a) Experimental results observed by Zheng et al. in case of λ = 1.0 and the carrier fluid and the aqueous streams were both viscous (μ = 16 mPa·s). (b) Present computational results.
FIG. 4.
FIG. 4.
Grid convergence study in comparison with the experimental study of Zheng et al. (a) Contours of volume fraction for various grid models for Ca = 0.038 and wf = 0.8 (red: water, blue: oil). (b) The dependence of droplet diameter on the mesh resolution, Ca = 0.015 and wf = 0.4. Normalized droplet diameter is defined by (4Ad/π)0.5/Wc, where Ad is the droplet area.
FIG. 5.
FIG. 5.
Phase diagrams of droplet formation in a MFDTD as a function of capillary number and water fraction for (a) λ = 0.5, (b) λ = 1.0, and (c) λ = 1.5; β = 0.0173.
FIG. 6.
FIG. 6.
Formation of droplets as a junction of channel width ratio at various water fractions in the regularly alternating regime with Ca = 0.12, Re = 0.0885, and β = 0.0173.
FIG. 7.
FIG. 7.
Pressure difference between two dispersed flows in term of time for (a) Ca = 0.04, (b) Ca = 0.06, (c) Ca = 0.08 with wf = 0.2 and λ = 1.0 fixed for all cases. Δp is calculated by subtracting pressure at measurement point P1 from P2 shown in Fig. 1. Highlighted smooth curves are fitted by wave function from forth droplet after alternating occurrence. The amplitude (A) and half period (T/2) of wave functions corresponding to Ca = 0.04, 0.06, and 0.08 are A = 34.26, 24.71, and 19.38 and T/2 = 4.07, 3.28, and 2.9, respectively.
FIG. 8.
FIG. 8.
Non-symmetrical breaking of merging droplet caused the phase discrepancy of two dispersed flow. (a) Merging droplet prior to alternating droplet formation with Ca = 0.04. (b), (c), and (d) Sequence of merging droplets breaks alternately, Ca = 0.02 and λ = 1.0.
FIG. 9.
FIG. 9.
Droplet formation as a function of viscosity ratio for Ca = 0.05, wf = 0.2, and λ = 1.0.
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