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 such level of control, applications that would not have been possible before also emerged, such as digital PCR and single cell encapsulation within droplets.
Flow rate stability is critical for having repeatable reactor volume and reproducible results. Syringe pumps are commonly used for generating droplets in microfluidic experiments, but can show limited flow control. As a consequence, the droplet size, proportional to the flow rate, is affected. An alternative to syringe pumps are pressure-based flow controllers. These show high-precision flow control, high reaction time, and flow monitoring is possible.
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 instrument.
Droplet production under identical experimental conditions
Droplet stability over time
To determine the droplet stability over time with the different instruments, droplets are generated within a microfluidic chip, working with constant water and oil flow rates of respectively 2,0 µL/min and 1,5 µL/min.
The figure on the right shows the flow rate of water and oil as a function of time for standard syringe pumps, and the Flow-EZ, the pressure-based flow controller from Fluigent. 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.
To determine 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 on the 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 is in good agreement with the results obtained regarding the flow rate stability and confirms that the more precise the flow rate, the more monodisperse the droplets
Change of droplet diameter
It can also 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.
The figure on the right 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. Droplets of 89 µm, 101 µm and about 110 µm diameter are obtained using respectively 1 µL/min, 2 µL/min, and 4 µL/min when using the Flow EZ.
The figure on the left shows the water flow rate as a function of time using 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 respectively 6 s and 93 s. Thus, the Flow EZ is the instrument with the highest response time. This allows to rapidly change from one droplet size to another.
Conclusion
More stable flow rates were observed when generating droplets using the Flow EZ, consequently leading to more monodisperse droplets compared to syringe pumps, and confirming the relationship between flow rate and droplet size. The response time between the different devices was also determined. The higher response-time was 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. They allow for control of droplet size and frequency by adjusting flow parameters.
