Digital microfluidics: microfluidic droplets & emulsion science

Digital microfluidics emulsion science droplet generation

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.

Droplet Geometry

The most common geometries used to generate droplets in microfluidic device are:

  • Cross flowing streams, most commonly called T-shaped junction or T-junction
  • Elongational flow, most commonly called flow focusing because of the used 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 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 enables the formation of drops. The application of a 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.

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Microfluidics droplet generation with T-junction

Microfluidics droplet T-junction emulsion science on chip

T-junction is the most 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 channel perpendicular to it.

There are three regimes of microfluidic droplet generation with T-junction:

  • The dripping regime
  • The squeezing regime
  • The jetting regime

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 it is the jetting regime that 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].

Dripping regime and microfluidic droplet generation with T-junction

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.

MICROFLUIDICS DROPLET DRIPPING REGIME T JUNCTION

Squeezing regime and microfluidic droplet generation with T-junction

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 lower the pressure in the channel. When the pressure drop across the disperse phase becomes too large, a microfluidic droplet detaches.

MICROFLUIDICS DROPLET SQUEEZING REGIME T JUNCTION

Jetting regime and microfluidic droplets generation with T-junction

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].

MICROFLUIDICS DROPLET JETTING REGIME T JUNCTION

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.

DROPLET GENERATION PACK

UP TO 10 000 DROPLETS/SEC

microfluidic-droplet-pack

This fully integrated solution includes all the necessary elements to easily create droplets

DROPLET GENERATION PACK

The droplet generation pack has been designed to fit most common droplet generation needs of researchers. Whatever you need, feel free to contact us to discuss about your exact need and if necessary we will adapt the pack and chips to your particular research application. Moreover for most common additional needs we already propose on the shelf option.

  • Up to 10 000 droplets/sec
  • Flow rate: from 0.1 µL/min to 5 mL/min
  • Droplet size dispersion: 0.3%
  • Change of droplets content : 100 ms

Piezo electric microfluidics flow control

ADVANTAGES
  • Easy to use
  • Very basic geometry (easy to design)
  • Very common geometry (many systems described in literature)
DRAWBACKS
  • Difficult to form tiny microfluidic droplets (smaller than channel size)
  • Not flexible (droplets’ size defined by channels’ size)

T-JUNCTION DROPLETS: PERFORMANCES

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

microfluidic droplet emulsion T-junction pressure control

T-JUNCTION DROPLETS: TIPS AND TRICKS

  • Overcome bubble perturbation
  • Control pressure, and keep track of the flow rate more precisely than with a syringue pump: use flow sensor
  • Easily stop droplets with our valves product line

Microfluidics droplet generation with Flow Focusing

Microfluidics droplet flow focusing emulsion science on chip

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:

  • The simple cross junction
  • The cross junction followed by a constriction

Microfluidic droplet formation in flow-focusing geometry cross junction followed by a constriction type

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 flow-focusing geometry simple cross junction type

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 dripping regime
  • The jetting system

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 .

Dripping regime and microfluidic droplets generation with flow focusing

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.

MICROFLUIDICS DROPLET DRIPPING REGIME FLOW FOCUSING

Jetting regime and microfluidic droplets generation with flow focusing

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].

MICROFLUIDICS DROPLET JETTING REGIME FLOW FOCUSING

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 with increasing 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.

ADVANTAGES
  • Easy to use
  • Simple geometry
  • Access to tiny droplets’ volume
  • Flexible droplet size
DRAWBACKS
  • Unpredicatble droplet size
  • Satellite droplets
  • Polydispersity

FLOW FOCUSING DROPLETS: PERFORMANCES

  • Sub-micron microfluidic droplets

microfluidic droplet emulsion T-junction pressure control

FLOW FOCUSING DROPLETS: TIPS AND TRICKS

  • Overcome bubble perturbation
  • Advantage in pressure control over syringes pumps for keeping track of the flow rate: use flow sensor
  • Easily stop droplets with our valves product line

Microfluidics droplet generation with Co-Flow

Microfluidics droplet Co-flowing emulsion science on chip

This method of droplet generation based on the principle of concentric microchannel 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].

ADVANTAGES
  • Very simple geometry
  • Access to tiny droplets
DRAWBACKS
  • Insertion of a small capillary into another one
  • Satellite droplet
  • Design of fluid connection on microfluidic chip
  • High dead volume (especially continuous phase)

CO-FLOW DROPLETS: PERFORMANCES

  • High throughput droplet generation: > 10kHz
  • Very high monodispersity: size polydispersity <2%
  • Very fast droplet content switch

microfluidic droplet emulsion T-junction pressure control

CO-FLOW DROPLETS: TIPS AND TRICKS

  • Overcome bubble perturbation
  • Pressure control: use a flow sensor to know the flow rate more accurately than with a syringe pump
  • Easily stop droplets with our valves product line

DROPLET GENERATION PACK

UP TO 10 000 DROPLETS/SEC

microfluidic-droplet-pack

This fully integrated solution includes all the necessary elements to easily create droplets

DROPLET GENERATION PACK

The droplet generation pack has been designed to fit most common droplet generation needs of researchers. Whatever you need, feel free to contact us to discuss about your exact need and if necessary we will adapt the pack and chips to your particular research application. Moreover for most common additional needs we already propose on the shelf option.

  • Up to 10 000 droplets/sec
  • Flow rate: from 0.1 µL/min to 5 mL/min
  • Droplet size dispersion: 0.3%
  • Change of droplets content : 100 ms

Piezo electric microfluidics flow control

References

[1] Christopher, G. F., and S. L. Anna. “Microfluidic methods for generating continuous droplet streams.” Journal of Physics D: Applied Physics 40.19 (2007): R319.

[2] T. Thorsen, R.W. Roberts, F.H. Arnold et S.R. Quake : Dynamic pattern formation in a vesicle-generating microfluidic device. Physical Review Letters, 86(18):4163–4166, 2001.

[3] Dreyfus, R., Tabeling, P., & Willaime, H. 2003. Ordered and disordered patterns in two-phase flow in microfluidics. Physical review letters, 90, 144505–144507.

[4] Tice, J.D., Lyon, A.D., & Ismagilov, R.F. 2004. Effects of viscosity on droplet formation and mixing in microfluidic channels. Analytica chimica acta, 507, 73– 77.

[5] Garstecki, P., Fuerstman, M. J., Stone, H. A., & Whitesides, G. M. (2006). Formation of droplets and bubbles in a microfluidic T-junction scaling and mechanism of break-up. Lab on a Chip, 6(3), 437-446.

[6] T. Cubaud and T. G. Mason, “Capillary threads and viscous droplets in square microchannels,” Physics of Fluids, vol. 20, p. 053302, 2008.

[7] A. M. Ganan-Calvo and J. M. Gordillo, 2001. “Perfectly monodisperse microbubbling by capillary flow focusing,” Physical Review Letters, vol. 87, p. 274501

[8] Anna, S. L., Bontoux, N., and Stone, H. A. 2003. “Formation of dispersions using “flow focusing” in microchannels”. Applied Physics Letters, 82(3) :364–366.

[9] J. Tan, J.H. Xu, S.W. Li et G.S. Luo, 2008. “Drop dispenser in a cross-junction microfluidic device : Scaling and mechanism of break-up”. Chemical Engineering Journal, 136(2- 3):306–311

[10] L. Yobas, S. Martens, W.-L. Ong, and N. Ranganathan, 2006. “High–performance flow–focusing geometry for spontaneous generation of monodispersed droplets,” Lab on Chip, vol. 6, pp. 1073–1079

[11] Abate, A. R., Poitzsch, A., Hwang, Y., Lee, J., Czerwinska, J., and Weitz, D. A. 2009. “Impact of inlet channel geometry on microfluidic drop formation”. Phys. Rev. E, 80(2) :026310.

[12] Utada, A. S., Fernandez-Nieves, A., Stone, H. A., and Weitz, D. A. 2007. “Dripping to jetting transitions in coflowing liquid streams”. Phys. Rev. Lett., 99(9) :094502.

[13] S.L. Anna et H.C. Mayer, 2006. “Microscale tipstreaming in a microfluidic flow focusing device”. Physics of Fluids, 18(12):121512

[14] L. Peng, M. Yang, S.-S. Guo, W. Liu et X.-Z. Zhao, 2011. “The effect of interfacial tension on droplet formation in flow-focusing microfluidic device.” Biomedical Microdevices, 13 (3):559–564

[15] P. B. Umbanhowar, V. Prasad, and D. A. Weitz, 2000. “Monodisperse emulsion generation via drop break off in a coflowing stream,” Langmuir, vol. 16, pp. 347–351

[16] C. Cramer, P. Fischer, and E. J. Windhab, 2004. “Drop formation in a co–flowing ambient fluid,” Chemical Engineering Science, vol. 59, pp. 3045–3058

[17] Y. Hong and F. Wang, 2007. “Flow rate effect on droplet control in a co-flowing microfluidic device,” Microfluidics and Nanofluidics, vol. 3, pp. 341–346

[18] R. Xiong, M. Bai, and J. Chung, 2007. “Formation of bubbles in a simple co–flowing microchannel,” Journal of Micromechanics and Microengineering, vol. 17, pp. 1002–1011,

[19] G. I. Taylor, “The formation of emulsions in definable fields of flow, 1934. ” Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, vol. 146 (858), pp. 501–523

[20] Suryo R and Basaran O A 2006 Phys. Fluids 18 082102

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