In this Microdroplet Generation CFD Analysis tutorial, we explore a technology for making very small, perfectly even drops of liquid. Imagine making tiny, identical beads of water that float in oil. These droplets are used in important fields like medicine for drug delivery and in small “lab-on-a-chip” devices for chemical tests. Making these drops is hard because it happens very fast in tiny channels. It is expensive to build and test these devices in a real lab. Therefore, engineers use CFD simulation to test the designs on a computer first. We use ANSYS Fluent, a powerful software, to see how the droplets form. This helps us understand the physics and create better designs.

In this report, we perform a detailed VOF fluent simulation. We use a special model in Fluent called the Volume of Fluid (VOF) model. This model is perfect for tracking the border between two liquids that do not mix, like oil and water. The study shows how one liquid breaks up into tiny, regular droplets inside a T-shaped microchannel. This Microdroplet Generation fluent study helps designers make devices that produce droplets of the exact size and speed they need. For more details on fluid flow in small devices, please explore our Microfluids tutorials.

• Reference [1]: Feng, Zongrui, et al. “Numerical study of gas-liquid two-phase flow distribution of refrigerant mixtures in a vertically-upward T-junction.” International Journal of Refrigeration147 (2023): 48-59.

Figure 1: Schematic diagram illustrating the T-junction microchannel geometry used for the CFD simulation setup.

Simulation Process: VOF Model and Transient Setup for T-Junction in ANSYS Fluent

For this CFD simulation, we designed a 2D geometry of a T-junction microchannel. We used a 2D model because it is much faster to solve than a 3D model, but it still shows the important physics of how droplets form. The geometry has two inlets and one outlet. We created a high-quality mesh with 140,000 structured cells. This type of mesh is very accurate for problems like this where we need to see the sharp border between two liquids.

We used the Volume of Fluid (VOF) model in ANSYS Fluent to simulate the two liquids that do not mix. The VOF model works by calculating how much of each liquid is in every single cell of the mesh. This allows the software to track the exact location of the interface, or border, between the fluids. We also activated the Wall Adhesion model. This is important because it tells the simulation how the liquids touch the solid walls of the channel. We ran the simulation in Transient mode. This means the computer solves the problem step-by-step in time, which is necessary to watch the full, dynamic process of a droplet being born.

Figure 2: 2D Geometry and the structured computational mesh grid with 140,000 cells used for the ANSYS Fluent simulation.

Post-processing: Microdroplet Generation CFD Analysis of Pinch-Off

This section analyzes the engineering data to understand how the droplets are made. We look at the contours to see the physics and help designers improve their devices. The time-sequence contours in Figure 3 clearly show the entire droplet formation cycle. First (top image), the red liquid (dispersed phase) flows in from the side channel and completely blocks the main channel. This stops the blue liquid (continuous phase) from flowing. This is the start of the squeezing regime. Because the blue liquid is blocked, its pressure builds up. In the second image, this high pressure squeezes the red liquid from both sides. We can see a thin “neck” forming. Finally, in the bottom image, the neck becomes so thin that it breaks. This is called pinch-off. A new, separate red droplet is born and flows away. The simulation perfectly captures this repeating cycle.

Figure 3: Time-sequence of Volume Fraction contours illustrating the Microdroplet Generation CFD Analysis, showing the stages of droplet formation from channel blocking to final pinch-off.

For a designer or manufacturer, these Microdroplet Generation results are extremely valuable:

• Uniform Droplet Production: The simulation proves that this T-junction design creates droplets that are very uniform in size and spacing. This is a critical achievement for applications in medicine, where each droplet must carry the exact same amount of a drug. The contours show each droplet is about 2-3 times as long as the channel is wide.
• Design Optimization: A designer can now use this validated simulation to test new ideas without spending money on physical prototypes. They can change the inlet flow speeds in ANSYS Fluent and see how it affects the droplet size. This allows for rapid optimization of the microfluidic device.
• Understanding Wall Effects: The curved shape of the droplets near the top and bottom walls shows the effect of the Wall Adhesion model. This tells the engineer how the liquid interacts with the channel material. If the liquid sticks too much, it can ruin the droplet formation. This simulation allows designers to test different wall materials (by changing the contact angle) to find the best one for their application.
• Predicting Production Rate: Because the droplets are evenly spaced, the production is happening at a constant frequency. The simulation allows the manufacturer to precisely calculate this frequency (how many droplets are made per second). This is essential for scaling up production or for technologies like inkjet printing where timing is key. The VOF fluent simulation provides all the data needed to control the process.