ac electroosmosis in rectangular microchannels

Measurement of the amplitude and phase of the electrophoretic and electroosmotic mobility based on fast single-particle tracking

The electrophoretic mobility of micron-scale particles is of crucial importance in applications related to pharmacy, electronic ink displays, printing, and food technology as well as in fundamental studies in these fields. Particle mobility measurements are often limited in accuracy because they are based on ensemble averages and because a correction for electroosmosis needs to be made based on a model. Single-particle approaches are better suited for examining polydisperse samples, but existing implementations either require multiple measurements to take the effect of electroosmosis into account or are limited in accuracy by short measurement times. In this work, accurate characterization of monodisperse and polydisperse samples is achieved by measuring the electrophoretic mobility on a particle-to-particle basis while suppressing electroosmosis. Electroosmosis can be suppressed by measuring in the middle of a microchannel while applying an AC voltage with a sufficiently high frequency. An accurate measurement of the electrophoretic mobility is obtained by analyzing the oscillating particle motion for 1.5 s per particle with a high-speed camera measuring at 850 Hz , synchronized to the applied electric field. Attention is paid to take into account the effect of the rolling shutter and the non-uniform sampling in order to obtain the accurate amplitude and phase of the electrophoretic mobility. The accuracy of method is experimentally verified and compared with a commercial apparatus for polystyrene microspheres in water. The method is further demonstrated on a range of particle materials and particle sizes and for a mixture of positively and negatively charged particles.

Dynamic Electroosmotic Flows of Power-Law Fluids in Rectangular Microchannels

Dynamic characteristics of electroosmosis of a typical non-Newtonian liquid in a rectangular microchannel are investigated by using numerical simulations. The non-Newtonian behavior of liquids is assumed to obey the famous power-law model and then the mathematical model is solved numerically by using the finite element method. The results indicate that the non-Newtonian effect produces some noticeable dynamic responses in electroosmotic flow. Under a direct current (DC) driving electric field, it is found that the fluid responds more inertly to an external electric field and the steady-state velocity profile becomes more plug-like as the flow behavior index decreases. Under an alternating current (AC) driving electric field, the fluid is observed to experience more significant acceleration and the amplitude of oscillating velocity becomes larger as the fluid behavior index decreases. Furthermore, our investigation also shows that electroosmotic flow of power-law fluids under an AC/DC combined driving field is enhanced as compared with that under a pure DC electric field. These dynamic predictions are of practical use for the design of electroosmotically-driven microfluidic devices that analyze and process non-Newtonian fluids such as biofluids and polymeric solutions.

Complex Ohmic conductance of electrolytes in rectangular microchannels

Motivated by the interest that microelectrolytic systems are gaining in the development of the so-called lab-on-a-chip systems, i.e., miniature microfluidic devices for biochemical analysis, we present an analytical study of Ohmic conduction in rectangular charged microchannels filled with electrolytic solution. The study complements a previous one [M. Campisi et al., J. Chem. Phys. 123, 204724 (2005)], concerning ac electro-osmosis. The problem is framed within the theory of nonequilibrium thermodynamics and is based on the solution of the incompressible Navier-Stokes equation with an electrical body force due to the interaction of the applied electric field with the charged electric double layer (EDL) which forms at the solid-liquid interface. We analyze in detail the dependence of the system complex conductance on the ratio linear dimensions over Debye length with an eye on finite EDL effects, and compare its scaling properties with those of electrokinetic and hydraulic complex conductances.