Biomedical Engineering Reference
In-Depth Information
FIGURE 6.31
Bubble formation: (a) flow-focusing configuration and (b) T-configuration.
shearing regime was already described with a simple model in Section 6.4.1.1. In a T-configuration, the
ratio between the size of injection channel and carrier channel determines the formation regime
( Fig. 6.29 ). If D d / D c 1/2, the bubble size is large and breaks in the squeezing mode. If D d / D c <
1/2,
the bubble is stretched by the shear stress, allowing a large gap between the bubble and the top channel
wall. The formation process is then in the shearing regime.
In the squeezing regime, the length L of the bubble can be estimated as [47] :
L ¼ W 1 þ a
!
Q gas
Q liquid
(6.23)
Q gas and
Q liquid are the flow rates of
where W is the channel width, a is a constant on the order of 1, and
the gas and the liquid, respectively.
In mixing applications, bubbles are used as the immiscible phase for the segmented flow. The
internal flow field of the liquid plug may create chaotic advection. Fig. 6.32 shows the measured
velocity field inside a liquid plug, which is separated by gas bubbles [39] (micro-PIV).
6.4.1.3 Active control of microdroplet
Mixing in microdroplets can be realized passively in a pressure-driven continuous system. However,
droplets can be manipulated individually using different actuation schemes. Chaotic advection inside
a droplet can be achieved by controlling the motion of the droplet by an external actuation concept.
The most common actuation concepts for active control of microdroplets are
Direct electrowetting;
Electrowetting on dielectric; and
Thermocapillary actuation.
Direct electrowetting is the wetting effect between an electrolyte and the surface of an electrode. A thin
electric double layer (EDL) exists between the electrolyte and the electrode ( Fig. 6.33 (a). The EDL of
a thickness l D acts as a capacitor with the capacitance per unit surface:
c EDL ¼ 3 0 3 r
l D
(6.24)
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