What is digital microfluidics?
Introduction to digital microfluidics
The aim of this technology is to create fluid-fluid dispersion into channels (principally water-in-oil emulsion). It allows the production of monodisperse drops/bubbles, or drops with a very low polydispersity: less than 3% (Fig.1).
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How to create droplets/bubbles inside micro-channels?
The most common method for generating drops/bubbles is the application and control of fluids flow rates at a microfluidic junction. This method is very simple to carry out. Emulsion processing in microfluidics is based on the control of flow rates at the injection of a continuous phase (Qc) and of a dispersed phase (Qd) at the junction of micro-channels. The connection of micro-channels at the junction defines the geometry of the drop/bubble maker. There are three main methods to generate and manipulate droplets/bubbles in microsystems . These methods, distinguished by their channels geometries, are:
- Cross-flowing or T-junction
- Flow focusing
The formation of droplets/bubbles depends on the shearing force applied by the continuous phase on the dispersed phase. This shearing force deforms the interface between the two fluids until a droplet or a bubble forms. The adimensional number which characterizes this phenomenon is the capillary index Ca:
It expresses the competition between shearing and interfacial forces. U is the characteristic speed of the continuous phase and μc its dynamic viscosity. γ is the interfacial tension. The interfacial force attempts to retain the dispersed phase into its channel.
When the dispersed phase penetrates the main channel (channel of the continuous phase) a droplet/bubble starts to grow. Thus the two non miscible fluids form an interface at the junction of the micro-channels. The interface moves in the direction of the continuous phase flow and forms a neck. By the movement of the interface, the neck becomes narrower and narrower until it breaks and forms a droplet/bubble  (Fig.2).
The emerging droplets/bubbles confine the flow of the continuous phase in the thin lubrication film, between the liquids interface and the microchannels walls. The more the drop/bubble grows, the thinner the lubrication becomes. This confining space increases locally the resistance against the flow of the continuous phase . Thus, it produces an increase of pressure upstream, which constricts the interface until the neck becomes too narrow and breaks.
It is obvious that special attention must be given to the choice of the tubing size and chemistry. Fluorinated material (e.g. Teflon tubing) can be wetted with fluorinated oils (e.g. FC40), therefore minimizing potential problematic interactions of water droplets with the tubing walls.
The principal advantages of using microfluidics to do emulsions are the strong control over the production of droplets/bubbles, the high monodispersity and the facility to obtain both. Indeed, digital microfluidics offers the possibility to manage with high precision:
- Droplets/bubbles size
- Production frequency
- Internal composition of drops
The key to understand the above parameters is the principle of tuning the size of drops/bubbles. There are different geometries, so we will only focus on one (Fig.3). To understand the principles of the other geometries, refer to . The size of a droplet/bubble is calibrated by many factors. First of all, when the dispersed phase enters into the main channel and fills it, the length L of the drop/bubble is equivalent to the width of the channel ω. We defined d as the size of the neck, and h the height of the micro-channels.
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When the drop/bubble enters the main channel, the pressure of the continuous phase increases. This pressure thus constricts the neck and decreases its size at a speed Vc, inferring with the flow rate of the continuous phase:During this time of neck compression, the dispersed phase progresses into the main channel. The size of the drop increases at a speed,Vd, inferring with the flow rate of the dispersed phase:Thus, the final length on the drop/bubble is the sum of the initial length before filling the main channel ω and the length accumulated during the elongation time into the main channel,te. This time is the time needed to compress (at a speed Vc) the neck from its initial size d until it breaks:The initial size d of the neck depends on the size of the dispersed phase’s micro-channel ωd. Depending on the scientist’s choice, the ratio d/ω is generally equal to one. Thus, the size of the drop is calibrated by the scientist with his experimental parameters and/or restrains. The model  defined above, is verified experimentally (Fig.4). This plot shows how the control of droplets’/bubbles’ size is easy to tune.
Figure 4: Reporting data of relative droplets sizes VS. flow rates of phases ratio
Frequency of drops/bubble production is also easily controllable. One can fix a droplet/bubble size with a certain ratio of flow rates, and increase, without changing size of drops/bubbles, the frequency of production by increasing flow rates. The most used parameter is the ability of controlling internal composition of drops. It offers many applications with microfluidics in diverse domains of physics, chemistry and microbiology .
Example of droplet generation in flow focusing geometry
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For more tutorials about microfluidics, please visit our other tutorials here: «Microfluidics tutorials». The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if precised. Article written by Fabien Bertholle and Guilhem Velvé Casquillas.
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