In modern medicine and biology, scientists use “lab-on-a-chip” technology. These are tiny plastic chips with microscopic channels inside. Instead of mixing chemicals in large glass tubes, these chips create thousands of microscopic, perfect fluid droplets. Each tiny droplet acts as its own individual test tube. To design these microscopic channels perfectly without wasting money on physical prototypes, engineers perform a Droplet Formation in Microfluidic Device CFD simulation.

This report is a highly detailed Droplet Formation in Microfluidic Device fluent simulation tutorial. We are recreating the T-junction geometry discussed in the reference paper by Qian et al. [1]. We will use ANSYS Fluent to visualize exactly how two unmixable liquids interact to create perfect spheres. By mastering this CFD Analysis of Droplet Formation in Microfluidic Device, you can design systems that generate exact droplet sizes. For more foundational lessons on simulating liquids that do not mix, please explore our Multiphase tutorials.

• Reference [1]: Qian, Jin-yuan, et al. “Mixing efficiency analysis on droplet formation process in microchannels by numerical methods.” Processes1 (2019): 33.

Figure 1: Diagram showing the T-junction geometry for the Microfluidic Device CFD simulation.

Simulation Process: The VOF Multiphase Model Setup

To build this Droplet Formation in Microfluidic Device ANSYS Fluent project, we used a 2D model of a “T-junction.” This is simply a main channel with a smaller side channel joining it at a 90-degree angle. We applied a very fine mesh (tiny calculation grid) directly over the junction because the physical changes happening here are microscopic.

The absolute most important physics setting for this Droplet Formation simulation is the Volume of Fluid (VOF) multiphase model. If you mix water and oil, they do not blend; they stay separate with a sharp border between them. The VOF model is a special mathematical tool inside ANSYS Fluent designed specifically to track and draw this sharp, moving border. Because making a droplet takes time, we also set this as a transient (time-dependent) simulation.

Post-processing: The Physics of the Pinch-Off Battle

To truly understand this Droplet Formation in Microfluidic Device fluent analysis, we must look at the visual story of the fluid phases. The creation of a droplet is a physical battle between two forces: Viscous Shear Force and Interfacial Tension. Look at the VOF Contours (Figure 2). The main channel contains the “continuous phase” fluid (colored Blue, like water). The side channel contains the “dispersed phase” fluid (colored Red, like oil).

The continuous blue fluid is flowing rapidly down the main pipe. When the red fluid enters the junction (Stage A: Intrusion), the blue fluid crashes into it. Because the blue fluid is sticky (viscous), it grabs the red fluid and violently drags it downstream. This dragging is called “viscous shear force.” You can see the direct result of this force in Stage B: Stretching, where the red fluid is pulled into a long, thin finger. However, the red fluid naturally wants to pull itself into a tight, round ball to protect itself. This natural inward pulling force is called “interfacial tension.” As the blue fluid pulls harder and harder, the red finger stretches too far. The interfacial tension fights back, squeezing the red fluid at its thinnest point. This squeezing is clearly visible in Stage C: Necking.

Finally, the stretching force and the squeezing force become too much. The thin “neck” snaps. This is shown perfectly in Stage D: Pinch-off. A perfectly round red droplet is released into the blue stream to flow away. This Droplet Formation CFD analysis proves exactly how microfluidic geometry works. The blue fluid provides the dragging force (the cause), and the red fluid’s surface tension provides the snapping force (the effect). By using ANSYS Fluent to see this invisible battle, engineers can simply change the speed of the blue fluid to create bigger or smaller droplets on demand.

Figure 2: A time-sequence of VOF contours from the Droplet Formation fluent simulation, showing the exact stages of (a) intrusion, (b) stretching, © necking, and (d) pinch-off.

Key Takeaways & FAQ

• Q: What is the VOF (Volume of Fluid) model?
  • A: It is a multiphase physics model in ANSYS Fluent used to track the sharp, distinct boundary between two unmixable liquids, which is absolutely required for any Droplet Formation simulation.
• Q: What causes the droplet to break off?
  • A: It is a battle of forces. The main fluid drags and stretches the injected fluid (Shear Force), while the injected fluid tries to squeeze itself into a ball (Interfacial Tension). When the neck gets too thin, it snaps.
• Q: Why use a fine mesh at the T-junction?
  • A: The exact moment of “necking” and “pinch-off” happens on a microscopic scale. A fine mesh ensures the Droplet Formation in Microfluidic Device CFD simulation calculates the curvature of the droplet perfectly.