Abstract
Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.
Highlights
Nowadays, microfluidics can be found in numerous applications, such as emulsification, chemical synthesis, biomedical diagnostics and drug screening [1,2,3,4]
Compared to conventional techniques that use reaction vessels, test tubes or microtiter plates, microfluidic technology offers several unique advantages: (i) much less volume of sample or reagents is used, which is practical and reduces costs; (ii) the diagnostic results or the molecular products are obtained in a shorter time, because the high surface-to-volume ratios at the microscale lead to shorter heat and mass transfer times; (iii) miniaturization allows for an increase in parallelization and automation
If the capillary number is low, the formed droplets will obstruct the channel and restrict the continuous phase. This causes a dramatic increase of the hydrodynamic pressure in the upstream part, which in turn induces the pinch-off of droplets
Summary
Microfluidics can be found in numerous applications, such as emulsification, chemical synthesis, biomedical diagnostics and drug screening [1,2,3,4]. Discrete droplets are produced in a continuously flowing immiscible liquid, and manipulated by downstream changes in the flow, either passively via e.g., bifurcations or constrictions, or actively using e.g., valves or electric fields This platform, being a subset of two-phase flow (TPF) microfluidics, offers unique possibilities for producing droplets with sizes in the nanometer to micrometer range in a controlled and reproducible manner, with a high throughput. Electrical control is increasingly used in microfluidic devices, especially for generating and merging droplets While this contribution of physics is not limited to channel flow geometries, but can be found in planar geometries (so called digital microfluidics using embedded electrode patterns to actuate the drops), we will in the current review restrict ourselves to the former case. Microfluidic device consideration for various applications will be discussed
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