Oscillating-droplet mixing

Moving- and Oscillating-droplet Mixing by Electrowetting Most Relevant Citations... 

Surface-active contaminants play an important role in damping out internal circulation in deformed bubbles and drops, as in spherical fluid particles (see Chapters 3 and 5). No systematic visualization of internal motion in ellipsoidal bubbles and drops has been reported. However, there are indications that deformations tend to decrease internal circulation velocities significantly (MI2), while shape oscillations tend to disrupt the internal circulation pattern of droplets and promote rapid mixing (R3). No secondary vortex of opposite sense to the prime internal vortex has been observed, even when the external boundary layer was found to separate (Sll). 

These droplets are then mixed with more air to dry the particles. The rate of droplet formation is equal to the oscillation frequency (/) of the piezoelectric crystal. From the volumetric flow rate of the liquid and the frequency /, the volume of the individual drops and particle diameter can be calculated. 

Figure 1.39 Liquid mixing time as a function of the droplet oscillation frequency, given for two-, three- and four-electrode structures [97]...

M 14] [P 14] In order to combine the effect of efficient oscillation of the extended linear arrays and the avoidance of flow reversibility of the circular arrays, an extended circular array (2 x 4) of electrodes was developed [29], This has three possible pivot points with up to two translations. This enhances the number of possible paths for the droplet which may result in enhanced mixing. 

A micromixer in which the fluid can be stirred by periodically pumping through the side channels is shown in Figure 3.45 [481]. The periodic perturbation applied via the side channel allows liquids A and B to be mixed. Other mixers based on oscillating pressure-pumped flow have also been reported [482,483]. Two droplets (600 pL) were merged and mixed by a push-pull (shuttling) method in a PDMS device consisting of a hydrophobic microcapillary vent (HMCV) [364]. 

Very recently, EW has been employed to trigger self-excited oscillations of millimetersized sessile droplets of water-glycerol mixtures [13]. During these oscillations, contact angles of the droplets have been found to vary periodically between 130° and 80°, with a frequency between 10 and 125 s. The resultant mixing of fluids within the droplets has been found to be two orders of magnitudes faster than the conventional chaotic micromixing processes. 

Mugele F, Baret JC, Steinhauser D (2006) Microfluidic mixing through electrowetting-induced droplet oscillations. Appl Phys Lett 88(1-3) 204106... 

The focus of this entry is on some unique physical characteristics of electrowetting applied to droplets. In particular, electrowetting-induced droplet shape changes, mixing, and droplet translation are discussed and some novel results involving shape-mode oscillations with and without simultaneous droplet translation are presented. 

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