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 (μl to fl) 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 (medium in which droplets flow) and dispersed phase (the droplet phase). In order for droplet formation to occur, two immiscible phases, referred to as the continuous phase (medium in which droplets are generated) and dispersed phase (the droplet 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 (electric, magnetic, centrifugal) 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: (i) Cross-Flowing, (ii) Flow focusing, and (iii) Co-Flowing. Droplet based microfluidics often operate under low Reynold’s numbers to ensure laminar flow within the system. Droplet size is often quantified with coefficient of variation (CV) 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 is a passive formation method that involves the continuous and aqueous phases running at an angle to each other.