Droplet Mixers

This concept is also used for kinetics study of fast homogenous reactions with characteristic reaction times in the range of milliseconds. Further examples are precipitations and the reactions that use reagents in the nanoliter scale. 

Another method of droplet-based mixing is via electrowetting, which is based on the change of surface energies by applying electric fields [44]. This technique differs from continuous flow systems in which discrete droplets are manipulated rather than continuous liquid streams and possess a number of advantages over the latter such as the ability to control each droplet independently, enabling 

There are several miscellaneous passive mixers that have been used for case-specific applications by modifying the abovementioned principles. Some of them are presented here. 

One of the passive mixers was developed using capillary forces to insert and hold the liquids in separate chambers, which are connected via a small gap [45]. It is a self-filling micromixer device and does not require micropumps referring it as an automixing device. This device was developed on a chip with two channels with variable volumes that are separated by a thick porous plate through which mixing takes place by diffusion. The idea was to use the capillary forces to fill one capillary with two liquids. 

One of the promising designs is the modified Tesla structures [46]. It uses the Coanda effect to split part of the fluid stream and direct it so that it recombines with the opposing flow of the other part of the stream. Coanda effect micromixer relies on the redirection of a flow by a special guiding structure that creates new interfaces within the flow [47]. This special passive structure provides good mixing at low flow rates. In this way the Coanda mixer can also be seen as a special realization of the SAR approach using recycle flows [32]. 

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Mixer type Moving- and oscillating-droplet mixer Insulator material Parylene C... 

This dielectrophoretic droplet mixer, called a programmable fluidic processor, contains two rows of 32 pads for the electrodes these rows being at the upper and lower edges of the substrate and connected to the 8x8 electrode array in the center of the chip [99], The electrodes form a square matrix with an upper and lower half owing to the connectivity to the pads. The electrodes have a square shape. 

P 15] The reservoir of the dielectrophoretic droplet mixer was filled with 1-bromo-dodecane, a low-permittivity, low-viscosity, water-immiscible hydrocarbon [99]. AC signals of up to 180 Vp p and frequencies between 5 and 500 kHz were formed using a programmable function generator and amplified by variable gain amplifiers. 

In addition to point-to-point movement of droplets, the dielectrophoretic droplet mixer could form droplets at pressurized orifices near electrodes [99], In the latter, the fluid is normally kept by a hydrostatic holdofP, which is then overcome by the dielectrophoretic action. 

Flow patterns in concentrically multi-layered droplet mixers... 

Pair, P., Pamula, V. K., Fair, R. B., Rapid droplet mixers for digital microfluidic systems, Lab Chip 2003, 3, 253-259. 

EWOD-based digital microfluidics have already been combined with MALDI-TOF-MS by Wheeler et al.13 However, the Wheeler et al. device would have several drawbacks when used in pre-steady state kinetic studies, including lower throughput - for reasons explained in the Section 12.3 - and the absence of a mixing element. Further, while Paik et al.5 have demonstrated droplet mixers for EWOD-based systems, their system is simply not fast enough to be of use in pre-steady state kinetics. 

Fig. 7.8 Moving droplet mixer. Two aqueous solutions were injected into immiscible oil in a serpentine channel 28 pm wide and 45 pm deep. Flow is from left to right [104. ...

Paik P, Pamula V, Pollack M, Fair R (2003) Electrowetting-based droplet mixers for microfluidic systems. Lab Chip 3(l) 28-33... 

Cheng J-Y, Hsiung L-C (2004) Electrowettmg(EW)-based valve combined with hydrophilic Teflon microfluidic guidance in controlling continuous fluid flow. Biomed Microdev 6 341-347 Paik P, Pamula VK, Pollack MG, Fair RB (2003) Electrowetting-based droplet mixers for microfluidic systems. Lab Chip 3 28-33... 

Rapid Droplet Mixers for Digital Microfluidic Systems. Lab Chip, 2003, 253-259. 

P. Paik, V.K. Pamula, M.G. Pollack, R.B. Fair, Electrowetting-based droplet mixers for microfluidic systems. Lab on a Chip, 3, pp. 28 (2003) V. Srinivasan, V. Pamula, M. Pollack, R. Fair, A digital microfluidic biosensor for multianalyte detection. Proceedings of the IEEE 16th Annual International Conference on Micro Electro Mechanical Systems, p. 327 (2003). 

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