Electrothermal Effects

As we mentioned above, the electrokinetic mechanisms, which includes electrophoresis, DEP, ACEO, electrothermal effect, and electro-orientation, are main driving force for particle manipulation using an optoelectrofluidic device. In addition, we could also observe the electrostatic interactions due to the polarization of dielectric particles like cells. 

Dielectrophoresis has also been used to manipulate macromolecules such as DNA, viruses, proteins, and carbon nanotubes. The term colloids will be used here to generally describe a particle between 1 and 1,000 nm. At this scale we need to take into consideration additional parameters that will affect the efficiency and application of dielectrophoresis. The first is Brownian motion, or the random chaotic movement of molecules, which will introduce another destabilizing variable if we were to trap colloids. Second, electrostatic effects at the surface of colloids, created by the electrical double layer, will influence particle-particle interactions. Factors such as hydrodynamic drag, buoyancy, electrothermal effects, and a particle s double layer interactions need to be considered when applying dielectrophoresis to colloids. 

A detailed derivation of a comprehensive mathematical model of electrothermal effects has been presented in the review by Ramos... 

Electrothermal Effects, Fig. 1 Two parallel plates with a small inteielectrode gap which is covered by a dielectric. A potential V is applied across the gap in the direction as shown... 

Electrothermal Effects, Fig. 3 Flow patterns for frequencies below the charge relaxation frequency of the liquid... 

The electrothermal effects can also be exploited to design efficient microactuators, as... 

Induced-charge and second-kind electrokinetic phenomena arise due to electrohydrodynamic effects in the electric double layer, but the term nonlinear electrokinetic phenomena is also sometimes used more broadly to include any fluid or particle motion, which depends nonlinearly on an applied electric field, fit the classical effect of dielectrophoresis mentioned above, electrostatic stresses on a polarized dielectric particle in a dielectric liquid cause dielectro-phoretic motion of particles and cells along the gradient of the field intensity (oc VE ). In electrothermal effects, an electric field induces bulk temperature gradients by Joule heating, which in turn cause gradients in the permittivity and conductivity that couple to the field to drive nonlinear flows, e.g., via Maxwell stresses oc E Ve. In cases of flexible solids and emulsions, there can also be nonlinear electromechanical effects coupling the... 

There are many applications of nonlinear electro-kinetic phenomena in microfluidics. For example, the dielectrophoretic motion of particles and cells is used for separating and concentrating biological samples. Electrokinetic flow instability and electrothermal effects can be used for microfluidic mixing. 

The groups of Ramos [1] and Green [15] have pioneered the study of ACEO by developing theoretical models and measuring flows around pairs of micro-electrodes in dilute KCl. Their work has mled out electrothermal effects (below 5 V in dilute electrolytes) and clarified the capacitive charging mechanism described above. The basic model successfully predicts the scaling of ACEO flow at... 

A detailed derivation of a conprehensive mathematical model of electrothermal effects has been presented in the review by Ramos et al. [2]. A brief description of the mathematical model is outlined here, for the sake of completeness. Following the analysis by Ramos et al. [2], simple analytical estimates of the electrothermal forces on a liquid can be obtained by considering two thin parallel metaUic plates, with a very small interelectrode gap. The plates are covered in a dielectric liquid and are subjected to a potential difference of V across the gap, which sets up an electric field E (r, ) (see Fig. 1). Neglecting end effects, this electric field can be expressed as... 

The electrothermal effects in microflows can also be modulated by illuminating the electrodes with fluorescent light, as has been observed by Green et al. [3]. They described the importance of the illumination in generating the fluid flow, and found that the flow was dependent on both the intensity of illumination and the applied electric field. They also compared the theoretical estimates of the elec-trothermally induced flow velocities with their experimental observations. A reasonable agreement was found between the experiments and the theory, with the light... 

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