Droplets, breakup microfluidics

S. Okushima, T. Nisisako, T. Torii, and T. Higuchi Controlled Production of Monodisperse Double Emulsions by Two-Step Droplet Breakup in Microfluidic Devices. Langmuir 20, 9905 (2004). 

Okushima S, Nisisako T, Torii T, Higuchi T (2004) Controlled production of monodisperse double emulsions by two-step droplet breakup in microfluidic devices. Langmuir 20(23) 9905-9908... 

In microfluidic devices, generally but not always (e.g., inertial effects can be significant in case of high-speed flows, for high production rates or droplet breakup situations), inertial effects are minimal, meaning that they work in small Reynolds number regimes. 

L. Mdndetrier-Deremble and P. Tabeling, Droplet breakup in microfluidic junctions of arbitrary angles. 

M. De Menech, Modeling of droplet breakup in a microfluidic t-shaped junction with a phase-field model. Physical Review E, 73, 031505, 2006. 

Q.Xu andM. Nakajima The Generation of Highly Monodisperse Droplets through the Breakup of HydrodynamicaUy Focused Microthread in a Microfluidic Device. Appl. Phys. Lett. 85, 3726 (2004). 

Following the early work by Thorsen et al., focused on the formation of monodisperse aqueous droplets in an organic carrier fluid performed on a microfluidic chip, and then followed by others works, the breakup mechanism responsible of droplet formation was later analyzed by Garstecki et al. ° showing that when is order of 1 the dominant contribution to the dynamics of breakup at low capillary numbers is not dominated by shear stresses, but it is driven by the pressure drop across the emerging droplet. 

In T-junctions, the strong effects of conflnement exerted by the presence of walls in the microchannels, coupled with the importance of the evolution of the pressure field during the process of formation of a droplet, confers a quasistatic character of the collapse of the dispersed streams, while the separation of time scales between the slow evolution of the interface during breakup and fast equilibration of the shape of the interface itself via capillary waves, and of the pressure field in the fluids via acoustic waves, are the basis of the observed monodispersity of the droplets and bubbles formed in microfluidic systems at low values of the capillary number. 

It is clear that using a T-junction or flow-focusing device, the breakup of the disperse phase by the continuous phase becomes periodic and predictable. The micro- or even NPs produced using microfluidics devices typically present a narrower size distribution than those produced by conventional methods,leading to consistent and regular droplets size, where control can be obtained by altering the flow rates ratio Qr of continuous and disperse phases. 

When immiscible fluid streams are contacted at the inlet section of a microchannel network, the ultimate flow regime depends on the geometry of the microchannel, the flow rates and instabilities that occur at the fluid-fluid interface. In microfluidic systems, flow instabilities provide a passive means for co-flowing fluid streams to increase the interfacial area between them and form, e.g. by an unstable fluid interface that disintegrates into droplets or bubbles. Because of the low Reynolds numbers involved, viscous instabilities are very important At very high flow rates, however, inertial forces become influential as well. In the following, we discuss different instabilities that either lead to drop/bubble breakup or at least deform an initially flat fluid-fluid interface. Many important phenomena relate to classical work on the stability of unbounded viscous flows (see e.g. the textbooks by Drazin and Reid[56]and Chandrasekhar [57]). We will see, however, that flow confinement provides a number of new effects that are not yet fully understood and remain active research topics. 

Q.Y. Xu, M. Nakajima, The generation of highly monodisperse droplets through the breakup of hydrodynamically focused microthread in a microfluidic device, Appl. Phys. Lett., 2004, 85, 3726-3728. 

Cardinaels R, Moldenaers P. Critical conditions and breakup of non-squashed microconfined droplets Effects of fluid viscoelasticity. Microfluid Nanofluid 2011 10(6) 1153-1163. 

The controlled formation of water-in-oil emulsion droplet stream in a T-junction microfluidic system was first demonstrated by Thorsen [34]. Since then, there has been a plethora of articles published in this area. The analysis of the mechanism for droplet formation in a T-junction device was proposed by Garstecki et al. [35]. At low capillary numbers, it was demonstrated that breakup... 

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