Droplet-based microfluidics
Droplet-based microfluidics manipulate discrete volumes of fluids in immiscible phases with low Reynolds number and laminar flow regimes. Interest in droplet-based microfluidics systems has been growing substantially in past decades. Microdroplets offer the feasibility of handling miniature volumes of fluids conveniently, provide better mixing, encapsulation, sorting, sensing and are suitable for high throughput experiments. Two immiscible phases used for the droplet based systems are referred to as the continuous phase and dispersed phase, resulting in either water-in-oil or oil-in-water emulsion droplets.
Droplet formation methods
In order for droplet formation to occur, two immiscible phases, referred to as the continuous phase and dispersed phase, must be used. The size of the generated droplets is mainly controlled by the flow rate ratio of the continuous phase and dispersed phase, interfacial tension between two phases, and the geometry of the channels used for droplet generation. Droplets can be formed both passively and actively. Active droplet formation often uses similar devices to passive formation but requires an external energy input for droplet manipulation. Passive droplet formation tends to be more common than active as it produces similar results with simpler device designs. Generally, three types of microfluidic geometries are utilized for passive droplet generation: cross-flowing, flow focusing, and co-flowing. Droplet-based microfluidics often operate under low Reynolds numbers to ensure laminar flow within the system. Droplet size is often quantified with coefficient of variation as a description of the standard deviation from the mean droplet size. Each of the listed methods provide a way to generate microfluidic droplets in a controllable and tunable manner with proper variable manipulation.Cross-flowing droplet formation
Cross-flowing is a passive formation method that involves the continuous and aqueous phases running at an angle to each other. Most commonly, the channels are perpendicular in a T-shaped junction with the dispersed phase intersecting the continuous phase; other configurations such as a Y-junction are also possible. The dispersed phase extends into the continuous and is stretched until shear forces break off a droplet. In a T-junction, droplet size and formation rate are determined by the flow rate ratio and capillary number. The capillary number relates the viscosity of the continuous phase, the superficial velocity of the continuous phase, and the interfacial tension. Typically, the dispersed phase flow rate is slower than the continuous flow rate. T-junction formation can be further applied by adding additional channels, creating two T-junctions at one location. By adding channels, different dispersed phases can be added at the same point to create alternating droplets of different compositions. Droplet size, usually above 10 μm, is limited by the channel dimensions and often produces droplets with a CV of less than 2% with a rate of up to 7 kHz.Flow focusing droplet formation
Flow focusing is a usually passive formation method that involves the dispersed phase flowing to meet the continuous phase typically at an angle then undergoing a constraint that creates a droplet. .This constraint is generally a narrowing in the channel to create the droplet though symmetric shearing, followed by a channel of equal or greater width. As with cross-flowing, the continuous phase flow rate is typically higher than the dispersed phase flow rate. Decreasing the flow of the continuous phase can increase the size of the droplets. Flow focusing can also be an active method with the constraint point being adjustable using pneumatic side chambers controlled by compressed air. The movable chambers act to pinch the flow, deforming the stream and creating a droplet with a changeable driving frequency. Droplet size is usually around several hundred nanometers with a CV of less than 3% and a rate of up to several hundred Hz to tens of kHz.Co-flowing droplet formation
Co-flowing is a passive droplet formation method where the dispersed phase channel is enclosed inside a continuous phase channel. At the end of the dispersed phase channel, the fluid is stretched until it breaks from shear forces and forms droplets either by dripping or jetting. Dripping occurs when capillary forces dominate the system and droplets are created at the channel endpoint. Jetting occurs, by widening or stretching, when the continuous phase is moving slower, creating a stream from the dispersed phase channel opening. Under the widening regime, the dispersed phase is moving faster than the continuous phase causing a deceleration of the dispersed phase, widening the droplet and increasing the diameter. Under the stretching regime, viscous drag dominates causing the stream to narrow creating a smaller droplet. The effect of the continuous phase flow rate on the droplet size depends on whether the system is in a stretching or widening regime thus different equations must be used to predict droplet size. Droplet size is usually around several hundred nanometers with a CV of less than 5% and a rate of up to tens of kHz.Droplet manipulation
The benefits of microfluidics can be scaled up to higher throughput using larger channels to allow more droplets to pass or by increasing droplet size. Droplet size can be tuned by adjusting the rate of flow of the continuous and disperse phases, but droplet size is limited by the need to maintain the concentration, inter-analyte distances, and stability of microdroplets. Thus, increased channel size becomes attractive due to the ability to create and transport a large number of droplets, though dispersion and stability of droplets become a concern. Finally, thorough mixing of droplets to expose the greatest possible number of reagents is necessary to ensure the maximum amount of starting materials react. This can be accomplished by using a windy channel to facilitate unsteady laminar flow within the droplets.Surfactants
s play an important role in droplet-based microfluidics. The main purpose of using a surfactant is to reduce the interfacial tension between the dispersed phase and continuous phase by adsorbing at interfaces and preventing droplets from coalescing with each other, therefore stabilizing the droplets in a stable emulsion state, which allows for longer storage times in delay-lines, reservoirs, or vials. Without using surfactants, the unstable emulsions will eventually evolve into separate phases to reduce the overall energy of the system. Surface chemistry cannot be ignored in microfluidics as the interfacial tension becomes a major consideration among microscale droplets. Linas Mazutis and Andrew D. Griffiths presented a method that used surfactants to achieve a selective and highly controllable coalescence without external manipulation. They manipulate the contact time and the interfacial surfactant coverage of a drop pair to control droplet fusion. The larger the difference percentage of the interfacial surfactant coverage between two droplets, the less likely coalescence will occur. This method allowed researchers to add reagents to droplets in a different way and further study the emulsification.Microfluidics is widely used for biochemical experiments, so it is important that surfactants are biocompatible when working with living cells and high-throughput analysis. Surfactants used in living cell research devices should not interfere with biochemical reactions or cellular functions. Hydrocarbon oil is typically not used in cell microfluidic research because it is not compatible with cells and damages cell viability. Hydrocarbon oil also extracts organic molecules from the aqueous phase. However, fluorosurfactants with fluorinated tails, for example, are used as a compatible droplet emulsifier that stabilizes droplets containing cells inside without harming or altering the cells. Fluorosurfactants are soluble in a fluorinated oil but insoluble in the aqueous phase, which results in decreasing the aqueous-fluorous interfacial tension. For example, a triblock copolymer surfactant containing two perfluoropolyether tails and a polyethylene glycol block head group is a fluorosurfactant with great biocompatibility and excellent droplet stability against coalescence. Another example are the fluorinated linear polyglycerols, which can be further functionalized on their tailored side-chains and are more customizable compared to the PEG-based copolymer. Surfactants can be purchased from many chemical companies, such as RainDance Technologies and Miller-Stephenson.
Physical considerations
Upon addition of surfactants or inorganic salts to a droplet-based microfluidic system, the interfacial tension of individual droplets alters within the microfluidic system. These separatory components allow for the utilization of the droplets as microreactors for various procedural mechanisms. In order to describe the relationship between interfacial tension, concentration of dissociated surfactants/salts in the bulk droplet, Temperature, the Boltzmann constant, and the concentration of dissociated surfactants/salts at the interface, the Gibbs adsorption isotherm was created, a simplified section highlighting relevant information displayed to the right.This isotherm reaffirms the notion that while the inorganic salt concentration increases, salts are depleted from the droplet interface, and the interface tension of the droplet increases. This is contrasted by surfactants, which adsorb at the interface, and lower interfacial tension . At low surfactant concentrations, surface tension decreases according to the Gibbs adsorption isotherm, until a certain concentration is reached, known as the critical micelle concentration, when micelles begin to form. Upon reaching the CMC, the dissolved surfactant concentration reaches a maximum, where the surfactant monomers will aggregate to form nanometer sized micelles. Due to this potential for micelle formation, three steps can be utilized when analyzing the adsorption of the surfactants to the droplet's interface. First, the surfactant molecules adsorb between the surface layer and the subsurface layer. Second, the molecules exchange between the subsurface and the bulk solution. Third, the micelles relax, caused by the breaking of equilibrium between free molecules and micelles.
The molecules making up each micelle are organized depending on the solution they are suspended in, with the more soluble portions in contact with the solution, and the less soluble portions of the molecule in contact with each other. Depending on the ratio of volume of the polar heads and nonpolar tail, various surfactants have been found to form larger aggregates, hollow, bi-layered structures known as vesicles. A notable surfactant that has been witnessed to form vesicles is . These micelles and vesicles are relatively new discoveries; however, they have been utilized to transport agents within microfluidic systems, revealing future applications for microfluidic transports.