Researchers’ opinion on droplet generation in microfluidics: syringe pumps or pressure control?
Droplet generation in microfluidics: the aim of this technology is to create fluid-fluid dispersion into channels (principally water-in-oil emulsion).
During the recent years, researchers have shown a greater interest in droplet-based microfluidics. There are many applications in domains as diverse as chemistry or biotechnology .
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Analogy with electrical circuit
There are two main ways to generate flows.
In the field of microfluidics flow behavior is quite similar to electrical current behavior. As well as current source and voltage source are the two main ways to generate an electrical current, microfluidic researchers have to choose between flow controller and pressure controller to command their flow in their experiments.
In microfluidics, the most widely used technology is the syringe pump. This method is strongly relevant for a lot of applications and is appreciated for its fast setup, simplicity and affordability. Pressure control is used for experiments requiring a short response time or a high level of stability and accuracy.
Micro-droplet generation and control
There are several ways to generate droplets.
3 categories of geometries are most commonly used :
- Co-ﬂowing streams
- Cross-ﬂowing streams (T or cross-shaped junction)
- Elongational ﬂow in a ﬂow focusing geometry
Most of the time, micro-droplet generation are performed in capillary tubes & junction or in on-chip channels. To shape droplet successfully it is necessary to control precisely flows of continuous phase and dispersed phase whatever the geometry chosen.
Microfluidic droplets generation
I asked to a hundred of researchers from laboratories working on droplet-based microfluidics which technology they used to control their flows in their latest papers and their opinion about it.
Choice of technology for droplet-based microfluidics
A significant majority of researchers uses syringe pump technology to control flows in their experiments related to droplet generation. Their choice of technology mainly depends on their opinion on syringe pumps, their habits of experimentation and equipment of their laboratory.
In the quarter of pressure control users, several of them have recently moved to pressure-driven control technology, or are still using the two technologies at the same time.
Lastly, some of researchers I interviewed built their own home made system using hydrostatic pressure or pressurized containers with valves.
(*)This study is based on the kind answers given by researchers in the field of droplets-based microfluidics [3-30]
Pros and cons of flow control and pressure control
- Ease of setup and control
- Using a syringe pump enables to know and specify the flow rate
- Commercial availability: variety of providers
- Ease of setup and control
- Fast equilibration / stabilization time (<50ms) for a large range of flow rate
- Pressure sources enables flow without oscillation
- The amount of dispensed fluid can vary between a few mL and several liters with the same performances
- Amenability to 2-layers PDMS devices
- Control of fluid in dead-end channels
- Long response time of the flow rate (seconds to minute)
- The piston of the syringe pump generates oscillations at low flow rate: droplets are irregularly dispersed 
- Fluidic resistance rises could lead to pressure increase and burst the device
- The amount of dispensed fluid is usually reduced around tens of mL
- Incompatible with a valve-based closing system
- Using a pressure controller does not enable to know the flow rate (*)
- The flow rate varies with fluidic resistance when controlling flow in pressure  (*)
Related tutorials about droplets generation
How to generate droplets in capillary tubes & junction step by step
How to generate droplets in on-chip channels step by step
For more details about flow control in microfluidics,also see.
(*) can be overcome with pressure source including flow rate feedback loop
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Article written by Thomas Grandry
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