According to Christopher and Anna [1] “microfluidic technologies offer an efficient means of producing highly uniform droplets and bubbles, and also a convenient mechanism for manipulating their downstream motion. These capabilities have led to the development of several novel applications that could not have been realized using other technologies”. The emulsion science field using microfluidics represents since the last past five years more than 25% of published papers. Digital microfluidics is one of the main application fields of microfluidics.
The microfluidic droplets generation methods are passive or active. Most are passive. We will study in here passive methods. They are based on the application of flow to deform the two fluids interface and generate microfluidic droplets.
The most common geometries used to generate droplets in microfluidic devices are:
The geometry of the intersection where the two phases meet, flow rates and fluid properties (surface tension, viscosity) determine the local stresses which deform the interface and lead to the production of drops.
In emulsion science with microfluidics, basically, the drops are formed at the intersection of two immiscible fluids streams. Physically, the influence of the interfacial tension between the two phases governed by Rayleigh-Plateau instability allows the formation of drops. The application of shear stress on an emulsion droplet causes its elongation, then its rupture. This observation has led Taylor to define the capillary number Ca as the ratio of shear stress on surface tension.
The two main methods used in microfluidics to deliver fluids at the intersection of any microfluidics droplets generation geometries are the syringe pump and the pressure pump (see more details here about researchers’ opinion on droplet generation in microfluidics).
We will describe in here all methods T-junction, Flow Focusing and Co-flowing for droplet generation in microfluidics with both pressure and syringe pump.
If you want to learn more about Droplet Detection and Measurement in Microfluidic Channels, click here.
You can also check our application note and our short review on monodisperse double emulsion production on microfluidic chips.
T-junction is the most popular microfluidic geometry used to produce microfluidic droplets initially proposed by Thorsen et al. in 2001 [2]. The formation of droplets in microfluidic T-junction is commonly defined as follow: the continuous phase flows through a channel, while the dispersed phase comes through a perpendicular channel.
There are three regimes of microfluidic droplet generation with T-junction:
These three regimes of microfluidic droplet generation with T-junction are defined by the predominance of one over the others of the shear stress and the surface tension. So the squeezing regime is the one of two-phase systems at low capillary number (Ca < 0.01). For large capillary numbers, the jetting regime applies. The dripping regime concerns intermediate capillary numbers. The microfluidic droplet generation with T-junction is also highly dependent on the nature of the wetting on the materials composing microfluidic channels [3].
Reference work in the field of digital microfluidics Thorsen et al. [2] and Tice et al. [4] show that the detachment of microfluidic droplets is related to the competition between the surface tension tending to keep the dispersed phase in its channel and viscous forces tending to detach the microfluidic droplet by the shear stress. From the moment the value of the capillary number (also dependent on the radius R of microfluidic droplets) reaches a critical number, the microfluidic droplet breaks.
Garstecki et al’s work [5] define the behavior of microfluidic droplets generation at low capillary number. The behavior is defined in the following way: during its progress, the dispersed phase tends to clog the main channel. 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.
The dispersed phase following the flow of the continuous phase and the two immiscible fluids, flow side by side in the main channel and break. The generation takes place beyond the intersection between the two phases. The jetting regime generates small microfluidic droplets. However, the jetting regime is not easily accessible. This regime is also in restricted parameter controls. In addition, when the microfluidic droplets become very small relative to the diameter of the microchannel, the jetting system tends to become chaotic [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.
The first microfabricated flow-focusing systems appeared in 2001 [7]. It was not until 2003 that the flow focusing geometry was applied in a microfabricated planar geometry for the formation of microfluidic water droplets in oil [8]. The manufacturing of microfluidic droplets is as follows: the continuous phase is introduced into two side channels and the dispersed phase is injected into a central channel. In general, the two streams of the continuous phase surround the stream of the dispersed phase which is immiscible with the first phase. The two streams of the continuous phase forcing the formation of microfluidic droplets by geometrical constraints.
One of the particularities of the flow-focusing geometry lies in the large amount of systems that can be observed during the formation of microfluidic droplets. This results in the access to a wide variety in the size of microfluidic droplets, and the existence of established regimes without droplet formation [8]. Unlike the T-junction geometry, there is no simple model that predicts the size of the microfluidic droplets according to the control parameters. On the other hand, the generation frequency of microfluidic droplets is generally relatively high, of the order of kHz. However, breaking the jet into drops often involves the formation of second microfluidic droplets, much lower than the main droplet size, which according to the case may be troublesome.
There are two variants of geometric microfluidic flow-focusing systems:
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With this variant, all of the three streams are directed to a narrowing thereby generating microfluidic droplets [8, 10]. The dispersed phase is stretched between two opposite flows of continuous phase. The co-laminar flow created passes through the constriction, which refines it into a jet and induced jet’s break in microfluidic droplets either in the constriction or outside the constriction. There are no simple law to estimate the size, distribution or production speed based on control settings. Indeed, other parameters have been added compared to the T-junction: the size of the constriction, its length and the width of the microfluidic droplets collection channel.
Microfluidic droplet formation in this cross junction geometry and the effects of the two-phases flow have been studied in 2008 [9]. There is no assimilation compared to the T-junction where we feel we only add a perpendicular channel. They attribute these differences between the variant of flow focusing and the T-junction to the fact that the T-junction is asymmetrical while the cross-junction is symmetrical: a wall was replaced by a water/oil interface. The effect of the walls is limited in the flow-focusing and the effects of the shear force of the continuous phase is increased.
The geometric variant of microfluidic flow-focusing systems with constriction is the most used. It allows easier access to tiny volume microfluidic droplets. In addition to the two geometric variants of microfluidic flow-focusing systems, there are two regimes of microfluidic droplets using flow-focusing:
The transition between these two regimes depends on the imposed intensity of flows and pressures. So it depends on the capillary number Ca and the ratio of the two phases’ flow rates .
In the regime of dripping, the microfluidic droplets break in or very close to the constriction, and after the break, the interface remains in the same place for the constriction. In this scheme, if the capillary number is quite high, the droplets have a smaller diameter than the dimension of the constriction, and microfluidic droplets’ size have a high monodispersity. A small capillary number implies emulsion with more polydispersity in size [11]. The diameter decreases as the capillary number increases and the ratio of flow decreases. It is further possible to form very small microfluidic droplets, i.e. whose size is much smaller than channels’ size in the flow-focusing junctions. The technique allowing access to these sizes of microfluidic droplets is called tip-streaming. The tip-streaming is a scheme for which the shape of the dispersed phase is pointed, and very small microfluidic droplets are detached from the tip.
As the capillary number increases, a transition from the dripping regime to the jetting regime operates. In this regime, the dispersed phase extends along a jet out of the orifice for a distance of at least three times the size of the constriction. The interface of this jet ripples and grows until it breaks into microfluidic droplets. The resulting microfluidic droplets are proportional to the size of the jet. The resulting microfluidic droplets are larger than those obtained in the dripping regime and are much less uniform in size, because after breaking, the interface position is not fixed [12].
Finally, during your work related to the formation of microfluidic droplets, you should also consider the influence of other parameters. The importance of surfactant concentration in the establishment of some flow regimes has been demonstrated for a flow-focusing system [13, 14]. These studies have confirmed that the droplet size decreases when you increase the amount of surfactant. Furthermore, the wetting of the walls by the disperse phase plays a more critical role than in the case of the T-junction strongly influencing the dynamics of formation and rupture of the jet.
This method of droplet generation based on the principle of concentric microchannels has been implemented for the first time in 2000 [15]. This is the least used method in microfluidics. It consists in injecting the dispersed phase in a central microchannel placed in the middle of another microchannel with a largest dimension. The dispersed phase becomes unstable and breaks up into droplets by Rayleigh-Plateau, and the formation of the droplet is dependent on the diameter of the microchannel containing the dispersed phase [16, 17, 18].
Droplet break-up from a capillary tip immersed in a continuous co-flowing liquid can be separated into two distinct break-up regimes: dripping, in which droplets pinch off near the capillary tip, and jetting, in which droplets pinch off from an extended thread downstream of the capillary tip [19]. The transition from dripping to jetting occurs as the continuous phase velocity increases above a critical value.
The droplet size has been characterized as a function of the control parameters. In general, droplets are smaller when the continuous phase velocity is faster, due to larger shear stresses exerted on the interface. Droplet size generally increases with increasing dispersed phase flow rate. An emerging droplet continues to fill during pinchoff, so a larger internal flow rate leads to more volume entering a droplet prior to pinch off.
In the experiments, droplet diameter decreases monotonically as the continuous phase velocity increases. Reducing the interfacial tension results in larger droplets, due to decreased resistance to break-up. On the other hand, variations in viscosity ratio have little effect on droplet size over a wide range of continuous phase velocities.
Like in flow focusing geometry, generating sub-micron droplets is possible with the co-flow method. It is also the tip-streaming technique that enables to generate tiny droplets. Suryo and Basaran demonstrated that this process occurs due to the presence of a nonlinear extensional flow near the tip of the forming droplet [20].
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