Microfluidic Droplet Production Method
Definition of droplet microfluidics
Droplet microfluidics is a powerful tool which consists of generating and manipulating micron sized monodispersed droplets. Microfluidic based droplet control and generation allows:
- Highly monodispersed droplet production contrary to other batch emulsion methods “In-line” continuous droplet production
- Single droplet manipulation as an individual pL scale biochemical reactor
- Miniaturization of production and bioanalytical devices thanks to droplet generation
A Microfluidic based droplet has many diverse and varied applications such as particle synthesis  and physicochemical analysis .
A good control of droplet production can also make single cell analysis , or drug testing possible .
Droplet generation principle
Droplet generation in microfluidics is based on the use of two immiscible fluids, oil and aqueous solutions are mostly used. The droplets are made in devices called microfluidic chips. Different physical aspects are involved in droplet generation depending on the design and material of the microfluidic chip. Among all chip designs to generate droplets, three are widespread in microfluidic area.
A co-flow, a T-junction and Flow Focusing design 
Figure 1: co-flow.
Co flow in droplet generation principle
The co-flowing design, in droplet generation, consists of two concentric capillaries with the inner capillary carrying the dispersed phase and the outer the continuous. As the dispersed phase enters the main channel, the viscous stresses created by the continuous phase stretch the interface until it breaks forming a droplet [ 10]   
One of the main advantages is the simple design of the co-flow.
Nevertheless, the droplet size and frequency remain limited in this kind of design
T-junction in droplet generation principle
Firstly highlighted by Thorsen et al in 2001 , this technique is the simplest and most used to generate droplets in a controlled way. In a T- junction, the dispersed/internal phase is injected perpendicularly to the flow of the continuous/external phase to generate microfluidic droplets. When the immiscible fluids arrive at the T-junction of the two channels, the dispersed phase progressively enters the main channel. The continuous phase shears at this point. The dispersed phase then forms a bend at the interface of the two fluids. The more the dispersed phase advances in the main channel, the more the elbow narrows to a break in the continuity of the interface. The break generates the detachment of a drop that continues to move in the direction of the flow of the channel. 
The advantages are the simplicity of the design, the great knowledge of the droplet “Break up” physical phenomena which allows for better understanding and control of the droplet generation process. In the T-junction it is really easy to control the droplet frequency and size. However, the frequency and size ranges remain limited by the chip design and materials.
Figure 2: T-junction.
Figure 3: flow focusing.
Flow focusing in droplet generation principle
As Demonstrated by Anna et. al. in 2003 
In a flow focusing design, the dispersed phase is introduced directly into the main channel while the continuous phase is injected by two branches placed perpendicularly. The dispersed phase is then pinched on both sides by the continuous phase, a droplet is formed due to the competition between the viscous force and the surface tension at the interface between the two phases . One of the big differences regarding the T-junction is the symmetry.
The advantage is that the symmetric design and physics effect allows one to have more flexibility in term of droplet size and droplet frequency. The symmetric design allows for droplet generation which is more sensitive to the flow of the two phases. The knowledge and control of the droplet “Break up” phenomena remain limited.
Droplet generation regimes
In a microfluidic droplet generation process, depending on the experimental conditions, several regimes of droplet formation are observable:
The “squeezing” regime leads to a plug-shaped drop flow that occupies the entire width of the channel with a ratio of their length to their width greater than 1. The size of plugs obtained depends mainly on the flow rates of the dispersed and continuous phases. The rupture of these plugs is due to the pressure drop in the main channel resulting from the presence of the dispersed phase.
The “dripping” regime leads to a flow of drops of spherical shape and dimension close to the width of the channel. In this case, the viscous shear forces cause the detachment of the drops.
The jetting regime is characterized by the generation of drops far from the intersection of the two phases. The drops generated are much smaller than the width of the channel and are generated at a very high frequency.
Figure 5: droplet generation with the Fluigent Droplet Starter Pack.
Overview of Dropseq application in microfluidics
DNA and its expression is at the heart of cellular mechanisms, nevertheless our knowledge of cells and their diversity remains limited and incomplete. Study is essential to understand the development, functioning and reproduction of living beings. Especially since mutations are at the origin of numerous pathologies (cancer, autoimmune disease, diabetes …). Nowadays the available methods to determine and sequence the genome do not allow us to fully understand the functioning of DNA.
Drop seq and In drop are methods / technologies that use microfluidics to tackle this issue by providing the ability to target thousands of individual cells simultaneously by encapsulating them in tiny droplets for parallel analysis.
Recent advances in dPCR for microfluidic application
PCR (Polymerase chain reaction) is a well-known and widespread technique in molecular biology. It allows for amplification (with a multiplication factor of the order of one billion) of a known DNA or RNA sequence.
Currently, microfluidics has enabled a new technique which consists of single cell and PCR mixture encapsulation into microdroplets. The high droplet generation frequency and the low volume of the microfluidic system allow one to increase the number of amplifications possible while reducing the cost of reagent consumption.
The Fluigent Droplet Starter Pack Fluigent Droplet generation pack allows you to easily generate droplet with a total control of the droplet size and frequency. The droplet generation pack provides all items needed to generate droplets, including pressure controllers to the chip and surfactant.
In this pack, Fluigent gives you:
Figure 7: the Fluigent Droplet Starter Pack system.
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