What is a microfluidic T-junction?

This short note will introduce you to the key takeways about microfluidic T junction among other microfluidic junctions. A focus will be given to the use of microfluidic T junction for droplet generation.

The most common geometries for two-phase mixing (droplet or bubble generation) in microfluidic junction devices are:

• Cross flowing streams, most commonly called T-shaped junction or T-junction
• Elongational flow, most commonly called flow focusing because of the channels’ geometry
• Co-flowing streams

The geometry of the intersection where the two phases meet, flow rates and fluid properties (surface tension, viscosity) determine the local stresses that deform the interface and lead to the generation of the two-phase mixture (emulsion, foams).

Microfluidic T-junction devices can be made of various materials such as PDMS, Glass, Quartz or even Platinum and designed for a broad range of applications including mixing fluids, microreactions and droplet formation.

Beginner in the world of droplet microfluidics? please check our dedicated webinar!

How a microfluidic T-junction can be used for droplet generation ?

T-junction is one of the most common microfluidic geometry employed to generate two-phase mixture such as emulsion based on droplet-based microfluidics as initially proposed by Thorsen et al. in 2001 [2].

The formation of droplets in microfluidic T-junction can be described as the meeting of a continuous phase (Water or Oil) that flows through a channel, while a dispersed phase (Water or Oil) is flowing through a perpendicular channel. The resulting emulsion properties will reply importantly on the channel geometry, flow rate ratio and device surface properties [3].

There are three regimes for microfluidic droplet generation with a T-junction defined by the ratio of two forces acting on the interface deformation: the shear stress versus the surface tension, known as the Capillary number (Ca):

• Dripping regime: it implies intermediate capillary numbers.
• Squeezing regime: it occurs when one of two-phase system flows is at low capillary number (Ca < 0.01).
• Jetting regime: it applies for large capillary numbers.

What flow regimes can be obtained with a microfluidic T junction?

Find below a selection of three short videos representing droplet generation though microfluidic T or Y junction. If you’re interested in reproducing this methods, check our related application pack that contains all you need to start microfluidic droplet generation in your lab.

Dripping regime and microfluidic droplet generation with T-junction

Literature in the field of droplet microfluidics by Thorsen et al. [2] and Tice et al. [4] show that the detachment of microfluidic droplets is due to the existing competition between surface tension that keeps the dispersed phase in its channel and viscous forces that tend to detach the microfluidic droplet by the shear stress.

The evolution of the capillary number (also dependent on the radius R of microfluidic droplets) will determine when the droplet detaches from the liquid stream at a critical capillary number.

Squeezing regime and microfluidic droplet generation with T-junction

Garstecki et al’s work [5] described very well the behavior of microfluidic droplet generation at low capillary number. It was showed that during its droplet generation evolution, the dispersed phase tends to “clog” the main channel.

Therefore, the presence of the dispersed phase in the main channel locally lowers the pressure in the channel. When the pressure drop across the disperse phase becomes too large, a microfluidic droplet detaches.

Jetting regime and microfluidic droplets generation with T-junction

The dispersed phase following the flow of the continuous phase and the two immiscible fluids, goes side by side in the main channel and breaks.

The breaking and resulting droplet production occurs beyond the intersection between the two phases. The jetting regime generates small microfluidic droplets. However, the jetting regime is not reached easily.

This regime is also in restricted parameter controls. Furthermore, very small microfluidic droplets relative to the diameter of the microchannel tends to cause chaotic jetting regimes [6].

Regardless of the mode in which you work, the size of the microfluidic droplets formed is proportional to the flow rates of the two phases of the report. In general, it is preferable to be in a situation where the microfluidic droplets are completely non-wetting on the walls of the microfluidic channels. Flow control of each phase by the pressure controller can produce droplets with a high level of monodispersity. Piezoelectric pressure microfluidic generators are the most efficient pressure regulators to control the flow and the generation of droplets, if you want more information about it, click here.

What are the advantages of using a microfluidic T junction?

• Easy to use
• Very basic geometry (easy to design)
• Very common geometry (many systems described in literature)

• Difficult to form tiny microfluidic droplets (smaller than channel size)
• Not flexible (droplets’ size defined by channels’ size)

PERFORMANCES

TIPS AND TRICKS

• High throughput droplets generation: > 100Hz
• Very high monodispersity: size polydispersity down to 1%

• Overcome bubble perturbation
• Control pressure and flow rate with our pressure-driven flow controller while measuring the flow rate precisely with one of our fluid flow sensors (MFS, BFS).
• Easily stop droplets with our valves product line

How to perform droplet generation with a microfluidic T junction?

Standard droplet generation with a T-junction can be performed with Elveflow Droplet Pack. The Droplet Pack setup uses 2-channel pumping system to flow independantly dispersed and continuous phases inside the microfluidic chip, enabling water in oil (W/O) or oil in water (O/W) droplets to be generated. The droplet size will be determined by the chip channel size and the flow rate ratio of both phases. Flow rates can be monitored and controlled thanks to our versatile flow rate sensors (MFS or BFS series).

For more information about this setup, feel free to contact our team of experts!

References