Electrokinetic instability

External energy sources for active mixing are, for example, ultrasound [22], acoustic, bubble-induced vibrations [23,24], electrokinetic instabilities [25], periodic variation of flow rate [26-28], electrowetting induced merging of droplets [29], piezoelectric vibrating membranes [30], magneto-hydrodynamic action [31], small impellers [32], integrated micro valves/pumps [33] and many others, which are listed in detail in Section 1.2. 

It is customary, mainly owing to fabrication needs, that for biological applications chip-like systems with two-layer construction are used, thus being in-plane. Mixers used for the same purposes have to adjust to this fact. Since multi-lamination typically needs several layers to achieve the proper feeding pathways, other mixers with simpler designs such as the electrokinetic instability mixer need to be applied [25], In contrast, chemical applications, where the mixer is only associated with part of the plant and not integrated in a small, flat device, do not pose such preferences indeed, multi-layer microfabrication architectures have been used. 

Electric fields may interact with flows fed by hydrostatic or pumping action [91]. Flows driven by electroosmotic means may be mixed as well by the action of fluctuating electric fields, which creates oscillating electroosmotic flows and has been termed electrokinetic instability (EKI) [25, 93], In this way, rapid stretching and folding of material lines are induced, not unlike the effect of stirring. In one realized example, comparatively low frequencies, below -100 Hz, and electric field strengths in excess of 100 V mm1 are applied for channels with dimensions of about 50 pm [25],... 

Mixer 3 [M 3] Electrokinetic Instability Electroosmotic Flow Micro Mixer, First-generation Device... 

Electrokinetic instability electroosmotic flow mixer, lst-generation device... 

Figure 1.7 Design of an electrokinetic instability EOF micro mixer, first-generation device [25] (by courtesy of ACS).

Figure 1.8 Design of an electrokinetic instability micro mixer, second-generation device, based on the results obtained with the first design given in Figure 1.7. The electrokinetic instability is confined to the square mixing chamber shown in the center of the schematic and, to a small extent, to fluid channel regions attached to it [25] (by courtesy of ACS).

Mixer 5 [M 5] Electrokinetic Instability Micro Mixer by Zeta-potential Variation... 

Mixer type Electrokinetic instability micro mixer with zeta potential variation Shielding electrode channel width, depth 250 pm, 25 pm... 

Figure 1.13 Ensemble-averaged temporal evolution of voxel-averaged spatial intensity PDFs for the electrokinetic instability micro mixer, second-generation device. Each ensemble consists of nine realizations.

Figure 1.14 Two-dimensional power spectra of various mixing chamber images for the electrokinetic instability micro mixer, second-generation device, (a) Large frequency components along the vertical direction owing to the initial layered distribution of the dye. (b) Larger spatial frequencies are introduced by the EKI stirring within the chamber, (c) The attenuation of large spatial frequencies corresponds to a nearly homogeneous intensity profile [25] (by courtesy of ACS).

E E(t) EDM EDTA EDX EHD EKI EO EOF ESI-MS Ez Activation energy Exit-age distribution function Electro-discharge machining Ethylene-diamine-tetraacetic acid Energy dispersive X-ray Electrohydrodynamic Electrokinetic instability Electroosmotic-Electroosmotic flow Electrospray ionization mass spectrometry Electric field... 

An active mixer based on an oscillating EOF induced by sinusoidal voltage ( 100 Hz, 100 V/mm) was devised and modeled for mixing of fluorescein with electrolyte solutions. This is termed as electrokinetic-instability micromixing, which is essentially a flow fluctuation phenomenon created by rapidly reversing the flow. Various microchips materials (PDMS, PMMA, and glass) and various electrolytes (borate, HEPES buffers) have been used to evaluate this method of micromixing [480]. 

Oddy, M.H., Santiago, J.G., Mikkelsen, J.C., Electrokinetic instability micromixers. Micro Total Analysis Systems, Proceedings 5th iTAS Symposium, Monterey, CA, Oct. 21-25, 2001, 34-36. 

Fig. 7.1 T-shaped micromixer for electrokinetic instability induced mixing [79. ...

These stirring transverse flows can be generated by channel shapes that stretch, fold, break, and split the laminar flow over the cross-section of the channel. This effect can be achieved using 2D curved [111-113], or 3D convoluted channels [114—116] and by inserting obstacles [117] and bas-reliefs on the channel walls [54,118,119]. It must be noted that such type of chaotic flow could also be achieved by an active mixing strategy such as one using electrokinetic instability (EKl), as described in Sect. 3.2.2. 

Oddy M, Santiago J, Mikkelsen J (2001) Electrokinetic instability micromixing. Anal Chem 73(24) 5822-5832... 

Active Mixer, Fig. 3 (a) Configuration of electrokinetic instability mixer and (b) time-stamped experimental images of mixing process... 

Fig. 2 Time series images of instability waves due to an electrokinetic instability in a conductivity gradient at an applied field of 1.25 kV/cm. The two solutions are Borate buffers of 1 and 10 mM which are introduced into the device at a stable electric field of 0.25 kV/cm. The high-conductivity stream is seeded with a neutral fluorescent dye [6]...

Oddy MH, Santiago JG (2005) A multiple-species model for electrokinetic instability. Phys Fluid 17 064108-1-064108-17... 

Chen CH, Lin H, Lele SK, Santiago JG (2005) Convective and absolute electrokinetic instability with conductivity gradients. J Fluid Mech 524 263-303... 

ZhoUcovskij EK, Vorotyntsev MA, Staude E (1996) Electrokinetic instability of solution in a plane-parallel electrochemical cell. Adv Colloid Interf Sci 181 28-33... 

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