Electrokinetic actuators

Minimizing Biomolecular Adsorption with Electrokinetic Actuation Electroosmotic Flow... 

The most common method of electrokinetic actuation is capillary electrophoresis (CE), in which electrophoresis of analytes and electroosmotic flow (EOF) of bulk fluid usually occur at the same time. Figure 4 illustrates the working principle of CE. The surface of fused silica capillaries is negatively charged at the pH values commonly... 

Biomolecular Adsorption in Microfiuidics, Table 3 Surface materials used for electrokinetic actuation... 

This equation is the basis of both the electrovibration sind vibration potential listed in the electrokinetic phenomena of Table 9.10. In these cases, the root mean square (rms) voltage i=E L) is either measured as in the case of the colloid vibration potential or induced by an electrode and the rms pressure fluctuations (= AP) at the same fi quency are either induced by an ultrasonic actuator or measured with a pressure transducer, as in the case of electrovibration. 

The electrokinetic platform uses electric charges, fields, field gradients or temporally fluctuating electrical fields for liquid actuation. The actuation is provided between different electrodes, and several effects (eletrophoresis, dieletrophoresis, osmotic flow, polarization) superimpose each other, depending on the sample liquid. Besides liquid actuation, the effects can also be used for separation of molecules and particles, detection, and catalysis. 

Dynamic pumps infuse energy to the fluid in a manner that increases either its momentum (centrifugal pumps) or its pressure (electroosmotic and electrohydro-dynamic pumps) as shown in Table 4. They involve centrifugal or hydrodynamic actuations and, more specifically, are driven by electro-/magneto-hydrodynamic [257], electroosmotic [254-256], electrokinetic [251-253], electrowetting [258], and acoustic forces [259]. Centrifugal pumps are typically less effective for fluids with low Reynolds numbers and have limitation in miniaturization. 

Currently, most microfluidic devices are based on continuous fluid flow, primarily using electrokinetic phenomena for actuation. An alternative approach towards microfluidics is to manipulate the liquid as unit-sized discrete microdroplets. Due to the architectural similarities with digital microelectronic systems, we refer to this approach as digital microfluidics. 

The first microvalve was introduced by Terry [1], in 1979, which was the first magnetic MEMS microvalve. After that, microvalves were improved in several ways. Around the year 2000 a revolution in fabrication of microvalves happened [2—4] which solved the problems of the various MEMS-based microvalves. However, many problems (such as (i) using moving parts that causes additional problems and difficulties, (ii) external actuation means, (iii) complex fabrication and installation processes, (iv) resistible flow and pressure, (v) considerable dead volume, (vi) long respmise time, (vii) leakage, and (viii) stability) remain unsolved when classical electrokinetic theory is used to design a microvalve. 

To prevent biomolecular adsorption and water stiction, the surface materials for a microfluidic device should be carefully chosen with consideration of the actuation method (hydrodynamic or electrokinetic) and analytes to be manipulated (small solutes, DNA, proteins, and/or cells). Tables 2 and 3 summarize the surface materials that have been used in microfiuidic applications. 

Min JY, Hasselbrink EF, Kim SJ (2004) On the efficiency of electrokinetic pumping of liquids through nanoscale channels. Stats Actuators B 98 368-377... 

Tang GY, Yan DG, Yang C et al (2007) Joule heating and its effects on electrokinetic transport of solutes in rectangular microchannels. Sens Actuator A Phys 139 221-232... 

The experimental results show that ferrofluids needs more research efforts in micro pumping, and magnetic actuation can be an appropriate alternative for more common techniques such as electromechanical, electrokinetic, and piezoelectric actuation methods. 

The design and implementation of mixers in microfiuidics differs considerably from that on the macroscale. The small length scale leads to different physical phenomena being dominant at the microscale First, inertial effects that typically result in turbulence and good mixing on the macroscale are weak in microfiuidics, while methods of actuation based on electrokinetics, surface tension, or other phenomena that are not relevant on the macroscale become feasible on the microscale. Secondly, many mechanical designs such as stirrers that can be easily implemented on the macroscale are very difficult to implement on the... 

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