Regulation of droplet size and flow regime by geometrical confinement in a microfluidic flow-focusing device

TL;DR

The paper addresses how geometrical confinement in a modified flow‑focusing microchannel influences droplet formation and flow regimes. A three‑dimensional CLSVOF CFD model resolves the oil–water interface while varying constriction width $w^{*}_{or}$, length $l^{*}_{or}$, continuous‑phase flow rate, and interfacial tension, and quantifies metrics such as droplet length $L_D$, volume $V$, velocity $U$, and deformation index $DI$. Three regimes—squeezing, dripping, and jetting—are identified and mapped in $Re_c$ and $Ca$ spaces along with geometry, showing that confinement can enhance dripping and enable high‑throughput monodisperse droplets. The results provide practical design guidelines for flow‑focusing microfluidic devices with confinement to optimize droplet production for diagnostic, drug delivery, and emulsion applications.

Abstract

We have developed a coupled level set and volume of fluid-based computational fluid dynamics model to analyze the droplet formation mechanism in a square flow-focusing microchannel. We demonstrate a flexible manipulation of droplet formation and flow regime based on the modified flow-focusing microchannel with a constricted orifice. Furthermore, we have systematically studied the influence of geometrical confinement, flow rate, and interfacial tension on the droplet formation regime, length, volume, velocity, and shape. Three different flow regimes, namely squeezing, dripping, and jetting, are observed, and the flow regime maps are formulated based on the Reynolds and capillary numbers. After an extensive numerical investigation, we described the boundaries between the different regimes. Droplet shape is also quantified based on the deformation index value. Plug-shaped droplets are observed in the squeezing regime, and near spherical droplets are found in the dripping and jetting regimes. Our study provides insights into the transition of a regime under various geometrical confinement and fluid properties. The results reveal that the modified flow-focusing microchannel can substantially enhance dripping while decreasing the squeezing regime, which is of paramount importance from the standpoint of producing high throughput stable and monodisperse microdroplets. Eventually, this work emphasizes the importance of geometrical confinement, fluid properties, and flow conditions on the droplet formation process in a flow-focusing microchannel that can effectively provide helpful guidelines on the design and operations of such droplet-based microfluidic systems.

Figures (17)

• Figure 1: (a) Schematic of the standard flow– focusing microchannel where CP: continuous phase flow rate and DP: dispersed phase flow rate, (b) three dimension view of computation domain with channel dimensions, (c) modified flow– focusing microchannels with different orifice width for a fixed orifice length of 300 $\mu$m, and (d) modified flow– focusing microchannels with different orifice length for a fixed orifice width of 300 $\mu$m
• Figure 2: Computational grid of different flow– focusing microchannels (a) standard flow– focusing microchannel without orifice, (b) microchannel with smaller orifice width and length, (c) microchannel with largest orifice length, and (d) three dimensions view computational grid for the largest orifice length. Mesh refinement near the wall and orifice region $X$– $X^{'}$ is magnified in the insets.
• Figure 3: Comparison of the dimensionless droplet length in a flow– focusing microchannel against the (a) experimental results of fu-2012s at fixed operating condition of $Q_o$= 400 $\mu$L/min, $\mu_o$= 11 mPa s, $\mu_o /\mu_w$ = 12, and $\gamma$ = 11.8 mN/m, and (b) LBM numerical results of wu2008three at a fixed operating condition of $U_o$ = 0.00084 m/s, $U_o/U_w$ = 0.33, $\mu_o$ = 24 mPa s, $\mu_o /\mu_w$ = 2.4.
• Figure 4: Temporal evolution of droplet formation process in different stages: I– expansion stage , II– necking, III– pinch– off stage, and IV– stable droplet formation regime. Droplet formation process in (a) standard flow– focusing microchannel without orifice ${w^{*}_{or}}=1$, (b) modified flow– focusing microchannel ${w^{*}_{or}}= 0.83$, (c) modified flow– focusing microchannel ${w^{*}_{or}}= 0.66$, and (d) modified flow– focusing microchannel ${w^{*}_{or}}= 0.5$ at a fixed operating condition of orifice length $l_{or}= 300~\mu m$, oil viscosity $\mu_o$ = 0.53 mPa s, $\mu_o /\mu_w$ = 0.59, interfacial tension $\gamma$ = 5.37 mN/m, $\theta$= 120°, $Q_w/Q_o=2$ and $Q_o$= 400 $\mu$L/min.
• Figure 5: Effect of orifice width on (a) non– dimensional droplet length and droplet formation frequency, (b) droplet velocity and droplet deformation index, (c) droplet volume, and (d) pressure profiles in the middle of the microchannel along the channel length at fixed operating condition of $\theta$= 120°, orifice length $l_{or}= 300~\mu m$, oil viscosity $\mu_o$ = 0.53 mPa s, $\mu_o /\mu_w$ = 0.59, interfacial tension $\gamma$ = 5.37 mN/m, $Q_w/Q_o=2$ and $Q_o$= 400 $\mu$L/min.