T-junction MicroChannel Devices

Three different types of microsystem have been reported for the emulsification of a polymerizable liquid (Figure 18.1), namely the terrace-like microchannel device, the T-junction microchannel device and the flow focusing device (FFD). The emulsification mechanism, which is similar for these three devices, proceeds from the breakup of a liquid thread into droplets when the phase to be dispersed is sheared by the continuous and immiscible phase. 

The terrace-like microchannel device (Figure 18.1a) consists of a main channel in which the continuous phase flows. Several microchannels deliver the dispersed phase at the top and from both sides of the main channel. Then terraces located just below the microchannels allow the break-up of the dispersed phase thread. In T-junction microchannel devices (Figure 18.1b), the phase to be dispersed is delivered through a microchannel perpendicular to a main charmel in which the continuous phase flows. Depending on the flow rates of the continuous and dispersed phases, the break-up is observed at the junction of the two microchannels... 

Figure 18.4 Top design and dimensions of the T-junction microchannel device used forthe synthesis of poly(l,6-hexanediol diaciylate) beads. Bottom images of droplet formation for different continuous phase flow rates at a flxed flow rate of the...

Figure 18.5 Top schematic depicting the T-junction microchannel device for production of non-spherical poiymer particles. Bottom SEM images of plug-shaped particles (a, c) and disk-like particles (b, d). From Ref [8].

Figure 2 Flow maps of T-junction microchannels. (A) Liquid/liquid two-phase flow in a T-junction microchannel, whose cross-section is 0.52 x 0.2 mm for the main channel and 0.27 x 0.2 mm in for the side channel. The solid dots are from the experiment with water/2 wt% spanSO-dodecane and the hollow dots are from the experiment with octane/3 wt% SDS (sodium dodecyl sulfonate)—water. (B and D) Gas/liquid two-phase and gas/liquid/liquid three-phase flows in a cross-junction microchannel. (C) Liquid/ liquid/liquid three-phase flows in a cross-junction microchannel in a flow-focusing microfluidic device. Panels (B and D) These figures are adapted from Wang et al (2013b) with permission of Wiley. Panel (C) Reprinted from Nieetal (2005) with permission of American Chemical Society.

The mechanisms of formation of discrete segments of fluids in microfiuidic flow-focusing and T-junction devices, that we outlined above point to (i) strong effects of confinement by the walls of the microchannels, (ii) importance of the evolution of the pressure field during the process of formation of a droplet (bubble), (iii) quasistatic character of the collapse of the streams of the fluid-to-be-dispersed, and (iv) separation of time scales between the slow evolution of the interface during break-up and last equilibration of the shape of the interface via capillary waves and of the pressure field in the fluids via acoustic waves. These features form the basis of the observed - almost perfect -monodispersity of the droplets and bubbles formed in microfiuidic systems at low values of the capillary number. 

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