Research Papers: Fundamental Issues and Canonical Flows

Effects of Junction Angle and Viscosity Ratio on Droplet Formation in Microfluidic Cross-Junction

[+] Author and Article Information
Ich-Long Ngo

School of Mechanical Engineering,
Yeungnam University,
Gyeongsan 712-749, South Korea
e-mail: longngoich@yahoo.com

Sang Woo Joo

School of Mechanical Engineering,
Yeungnam University,
Gyeongsan 712-749, South Korea
e-mail: swjoo@ynu.ac.kr

Chan Byon

School of Mechanical Engineering,
Yeungnam University,
Gyeongsan 712-749, South Korea
e-mail: cbyon@ynu.ac.kr

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 16, 2015; final manuscript received October 2, 2015; published online January 6, 2016. Assoc. Editor: Shizhi Qian.

J. Fluids Eng 138(5), 051202 (Jan 06, 2016) (9 pages) Paper No: FE-15-1403; doi: 10.1115/1.4031881 History: Received June 16, 2015; Revised October 02, 2015

This study describes the dynamic behaviors of droplet formation in microfluidic cross-junction devices (MFCDs) based on a two-dimensional numerical model using the volume of fluid (VOF) method. The effects of the junction angle (ϕ = 30 to 90 deg) between the main and side channels and the viscosity ratios (β = 10−5 to 2.0) are considered. The numerical results indicate that the active area for droplet formation in the alternating digitized pattern formation (ADPF) generally increases with the decrease of ϕ at the same water fraction (wf). A junction angle of around 60 deg was predicted as the most efficient angle at which alternating droplets are still formed at lower capillary numbers (Ca). In addition, the droplet size in ADPF decreases as ϕ increases with the same flow conditions. When ϕ is less than 90 deg and prior to approaching the equilibrium state, there always exists a periodic deviation in the relative distance between droplets. The frequency of droplet generation in ADPF decreases as ϕ decreases, and it decreases more quickly when ϕ is less than 60 deg. In addition, the throughput of MFCDs can be controlled effectively as a function of ϕ, wf, and Ca. The droplet formation in MFCDs depends significantly on the viscosity ratio β, and the ADPF becomes a jetting formation (JF) when β is greater than unity. Furthermore, the droplet size in ADPF decreases with the increase of β. The understanding of droplet formation in MFCDs is very useful for many applications, such as nanoparticle synthesis with different concentrations, hydrogel bead generation, or cell transplantation in biomedical therapy.

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Grahic Jump Location
Fig. 1

Schematic of MFCD. For simplicity, dispersed flow 1 (DP1) and dispersed flow 2 (DP2) are both water with the same fluid properties and inlet velocity, Ud. The length of side channels (Ls) is a parameter that depends on the junction angle (ϕ).

Grahic Jump Location
Fig. 2

Grid convergence study in comparison with the experimental study of Zheng et al. [13]. (a) Contours of volume fraction for various grid models for Ca = 0.038 and wf = 0.8 (red (or light gray): water, blue (or black): oil, and white: fluid-fluid interface). (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.

Grahic Jump Location
Fig. 3

Phase diagrams of droplet formation in MFCD as a function of junction angle and capillary number with various water fractions at λ = 1.0 and β = 0.0173. The upper bound confines the ADPF near the AJF and JF, and the lower bound confines the ADPF near the MDPF.

Grahic Jump Location
Fig. 4

Evolution of distance between droplets versus time at Ca = 0.1, wf = 0.2, and β = 0.0173. The droplet distances were calculated relative to the main channel width (Wc).

Grahic Jump Location
Fig. 5

Variation of droplet length as a function of junction angles (ϕ) at (a) various capillary numbers (wf = 0.2) and (b) various water fractions (Ca = 0.05). Ld is the droplet length, which was defined in Fig. 1.

Grahic Jump Location
Fig. 6

Frequency of droplet generation in ADPF as a function of junction angles (ϕ) at various water fractions and capillary numbers

Grahic Jump Location
Fig. 7

Droplet formation as a function of viscosity ratio: Ca = 0.05, wf = 0.2, and λ = 1.0

Grahic Jump Location
Fig. 8

Droplet size as a function of viscosity ratio at various water fractions: Ca = 0.05 and λ = 1.0, Re = 0.0369



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