Microfluidic droplet generators

Y. Tan, V. Cristini, and A.P. Lee Monodispersed Microfluidic Droplet Generation by Shear Focusing Microfluidic Device. Sensors and Actuactors B Chem. 144,... 

A. Bransky, N. Korin, M. Khoury, and S. Levenberg, A microfluidic droplet generator based on a piezoelectric actuator, Lab on a Chip, 9, 516-520,... 

Li W, Young E, Seo M, Nie Z, Garstecki P, Simmons C, Kumacheva E (2008) Simultaneous generation of droplets with different dimensions in parallel integrated microfluidic droplet generators. Soft matter 4(2) 258-262... 

Keywords Droplet-on-demand (DOD) droplet generators Electrohydrodynamics (EHD) droplet generators Microfluidic droplet generators Piezoelectric droplet generators Pneumatic droplet generators Thermal or bubble jet droplet generators... 

Figure 14.2 Schematic representations of various types of microfluidic droplet generator stream geometries, (a) Flow-focusing (b) T-junction (c) Terrace-like (d) Co-flowing. The droplet and the disperse phases are labeled as A and B, respectively. (Reprinted with permission from Ref. [6] 2011, Royal Society of Chemistry.)...

Y.C. Tan, V. Cristim, A.P. Lee, Monodispersed microfluidic droplet generation by shear focusing microfluidic device. Sens. Actuators B, 2006, 114, 350-356. 

Microdroplets can be generated within microfluidic devices using different methods such as electric fields [134], micro-injectors [135] and needles [136]. However, the most widely used methods for droplet generation rely on flow instabdities between immiscible fluids that lead to the so-called multiphase flow. Any fluid flow consisting of more than one phase or component (e.g., emulsions and foams) are examples of multiphase fluids. Traditional emulsification methods are based on the agitation of immiscible fluids and result in the formation of a polydisperse collection of droplets. By... 

Fig. 1 Droplets generation in microfluidic devices, (a) Schematic channel layout of T-junction device, (b) Micrograph showing the formation of droplets in a T-junction device. Reproduced with permission from [1]. (c) Micrograph of the flow-focusing device, (d) Droplets generation at different flow conditions in flow-focusing device. Reproduced with permission from [2]...

One apparent feature of droplets generated in microfluidic devices is their mono-dispersity in size. Formation of droplets containing precursor solutions dispersed in a continuous phase and then initiation of crosslinking, polymerization or phase separation produces monodisperse microparticles of defined compositions. 

Fig. 3 Droplet-based microfluidic device integrated with a T-junction droplet generator and a droplet mechanical trapping array. With this device, individual C. elegans can be encapsulated into microdroplets and, after the droplets are trapped, the mobility behavior of the worms can be investigated at single-animal resolution [48]...

Hung, L.-H., et al.. Alternating droplet generation and controlled dynamic droplet fusion in microfluidic device for CdS nanoparticle synthesis. Lab on a Chip, 2006, 6 174-178. 

Abstract This chapter provides information on different types of drop-on-demand drop generators. It starts with thermal or bubble jets, in which a nucleation bubble is used to eject a droplet out of an orifice. This is followed by piezoelectric, pneumatic, microfluidic, electrohydrodynamics (EHD) and aerodynamic droplet generators. For each droplet generator, the principle of operation and major features and characteristics are described. 

Microfluidic generation of droplets is a method of droplet formation in microfluidic channels. It works by combining two or more streams of immiscible fluids and generating a shear force on the discontinuous phase causing it to break up into discrete droplets. In contrast to piezoelectric, pneumatic and acoustic forms of droplet generation, in this method, there is no need for an actuator to impose instabilities on the liquid jet. In the absence of an actuator, the size and polydisper-sity of the droplets are determined by the dimensions of microchannels, the flow rates of liquids, wetting properties of microchannels, etc. 

M. Seo, C. Paquet, Z. Nie, S. Xu and E. Kumacheva Microfluidic consecutive flow-focusing droplet generators, Soft Matter 3, 986-992 (2007). 

The droplet-based microfluidic platforms for Lab-on-a-Chip applications can be fundamentally divided into two basic setups, the channel-based and the planar surface approach [2]. The channel-based systems are mostly pressure driven with droplet generation and manipulation relying on actuation via liquid flows within closed microchaimels. For the planar surface-based platforms, droplets can be arbitrarily moved in two dimensions representing planar programmable Lab-on-Chips. They are actuated by electrowetting (EWOD) or surface acoustic waves (SAW). 

Various direct microfluidic emulsification geometries are discussed in literature, e.g., (straight-through) microcharuiels and T- and Y-junctions (see Eig. 1). Two droplet formation mechanisms can be distinguished one uses Laplace pressure differences for spontaneous droplet generation and the other uses shear to form droplets. 

Shui LL, Mugele F, van den Berg A, et al Geometry-controlled droplet generation in head-on microfluidic devices, Appi Phys Lett 93 15311315, 2008. 

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