Flow control for droplet generation using syringe pumps and pressure-based flow controllers

Micrometer droplets and particles are widely used in a broad range of industries. Droplet production using microfluidic systems was implemented for applications where monodispersity is of high importance. Within micrometer-sized channels, one droplet at a time is generated, allowing for the production of monodisperse droplets. With this level of control, applications that would not have been possible before, have also emerged, such as digital PCR and single cell encapsulation within droplets.

The importance of flow control for droplet production

Syringe pumps are commonly used for generating droplets in microfluidic experiments, but can show limited flow control. As a result, the droplet size, which is proportional to the flow rate, is affected. Therefore, flow control for droplet generation is critical for having repeatable reactor volume and reproducible results. An alternative to syringe pumps is pressure-based flow controllers. These show that high-precision flow control, high reaction time, and flow monitoring is possible.

Complete analysis comparing syringe pumps and pressure-based flow controllers

Analysis between Microfluidic syringe pumps and flow EZ — We compare the production of water-in-oil emulsions using microfluidic syringe pumps and Fluigent pressure-based flow controllers. Droplet size stability and the time required to reach several droplet diameters are determined for the two instruments.

Droplet production under identical experimental conditions

Flow rate stability over time when generating droplets

To determine the droplet stability over time achieved with the different instruments, droplets are generated within a microfluidic chip, working with constant water and oil flow rates of 2.0 µL/min and 1.5 µL/min, respectively.

Figure 3 shows the flow rate of water and oil as a function of time for standard syringe pumps and for the Flow-EZ, the pressure-based flow controller. The average flow rates, standard deviations, and coefficients of variation were also calculated. The water and HFE flow rate variations using the Flow EZ are lower than when using standard syringe, confirming the enhanced flow stability over time of pressure-based flow controllers.

Table 1: Water and HFE average flow rate, STD and CV with the Flow EZ and syringe pump

To determine the effect of flow control for droplet generation, and thus the effect of flow rate on droplet stability, a picture of the channels was taken every 10 seconds, and the average diameter over 5 minutes was calculated for each device. To better compare between the different instruments, we use normalized values.

The figure at left shows the normalized diameter variation over time. We can observe that droplets are more stable over time using the Flow EZ compared to the standard syringe pump. This aligns with the results obtained for flow rate stability and confirms that the more precise the flow rate, the more monodisperse the droplets will be.

Normalized droplet diameter variation over time — *Figure 5: Normalized droplet diameter as a function of time using the Flow EZ and syringe pump*

Impact of precise flow control on droplet diameter over time

It can also be of interest to be able to quickly move from one droplet size to another. We can determine the response time for reaching a specific droplet diameter. The water flow rate is varied, while the oil flow rate is kept constant at 1.5 µL/min. The set flow rates were the following: 1 µL/min – 4 µL/min – 1 µL/min – 2 µL/min.

Figure 6 shows the results obtained using the Flow EZ. We observe that the set flow rates are reached in a few seconds. After reaching a set flow rate, we can observe the droplet size that results.

In the analysis of the effect of flow control on droplet generation, droplets of 89 µm, 101 µm and approximately 110 µm in diameter were obtained via flow rates of 1 µL/min, 2 µL/min and 4 µL/min, respectively, when using the Flow EZ.

Droplet diameter change over time - Flow control for droplet — *Figure 6: Droplet diameter change as a function of time using the Flow EZ. The water flow rates are varied over time, while the HFE flow rate was kept constant*

droplet production with syringe pump vs pressure flow controller

Figure 7: Time required to change from one water flow rate to another using the Flow EZ and syringe pumps.

Figure 7 shows the water flow rate as a function of time for the syringe pump and the Flow EZ, and the table below shows the time required to move from one flow rate to the other after starting the system. We observe that the maximum time required to switch to another flow rate using the Flow EZ and the syringe pump are 6 s and 93 s, respectively.

We can therefore see that the Flow EZ is the instrument with the shortest response time. This allows users to rapidly change from one droplet size to another.

Conclusion

In analyzing the effect of flow control on droplet generation, more stable flow rates were observed when generating droplets with the Flow EZ, resulting in more monodisperse droplets compared to syringe pumps. These results confirm the relationship between flow rate and droplet size.

The difference in response time between the different devices was also determined. Better response times were observed when using the Flow EZ, allowing for responsive control over droplet size during an experiment.

Micrometer-size droplets and particles are widely used in a broad range of industries. Fluigent provides complete, cost-effective solutions for the production of monodisperse droplets., allowing users to control droplet size and frequency by adjusting flow parameters.