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ABSTRACT: Electrorotation (ROT) data for solid titanium micrometer-sized spheres in an electrolyte are presented for three different ionic conductivities, over the frequency range of 10 Hz to 100 kHz. The direction of rotation was found to be opposite to the direction of rotation of the electric field vector (counter-field electrorotation), with a single rotation peak. The maximum rotation rate occurs at a frequency of the order of the reciprocal RC time constant for charging the particle double layer capacitance through the resistor of the electrolyte bulk. A model for the electrical torque acting on a metallic sphere is presented, using a constant phase element (CPE) impedance to describe the metal/electrolyte interface. The titanium spheres are much denser than the electrolyte and rest on the bottom substrate. Therefore, the electrical and viscous torques near a wall are considered in the analysis. Good agreement is found between the predicted and measured rotational speed as a function of frequency. Theory shows that there is no effect of induced charge electroosmotic flow on the ROT, as observed experimentally.
Electrophoresis 01/2013; · 3.30 Impact Factor
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ABSTRACT: We present dielectrophoresis (DEP) and electrorotation (ROT) measurements of gold-coated polystyrene microspheres as a function of frequency and for several electrolyte conductivities. Particle rotation was counterfield with a maximum rotation rate observed at a single characteristic frequency. Negative DEP was observed for frequencies lower than this characteristic frequency and positive DEP for signal frequencies higher than this. These experimental observations are in agreement with predictions for the force and torque on the induced dipole of a perfectly polarizable metal sphere. We present a theoretical model for this case, and good agreement is found for both ROT and DEP measurements if we take into account the viscous friction for a spherical particle near a wall. From the characteristic frequency for rotation, we obtain the capacitance of the electrical double layer at the electrolyte-particle interface. Remarkably, no effect of induced charge electroosmosis around the particles can be inferred from DEP measurements.
Langmuir 08/2012; 28(39):13861-70. · 4.19 Impact Factor
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ABSTRACT: AC electro-osmotic (ACEO) pumping is experimentally demonstrated on a symmetric gold electrode array. Using asymmetric connection
of electrodes to the applied AC voltage, spatial asymmetry along the array is created, which produces unidirectional flow
of electrolyte. An aqueous solution of 100μM KCl is selected as the pumping fluid. The liquid velocity obtained as a function
of voltage and frequency is compared to that generated using travelling-wave electroosmosis (TWEO) with the same electrode
array. The expected velocities from the linear electrokinetic models of ACEO and TWEO are computed numerically. The comparison
shows that TWEO generates greater velocity amplitudes and the streamlines are smoother than those generated by ACEO.
Microfluidics and Nanofluidics 04/2012; 7(6):767-772. · 3.37 Impact Factor
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ABSTRACT: Electric fields induce forces at the interface between liquids with different electrical properties (conductivity and/or permittivity). We explore how to use these forces for manipulating two coflowing streams of liquids in a microchannel. A microelectrode array is fabricated at the bottom of the channel and one of the two liquids is labelled with a fluorescent dye for observing the phenomenon. The diffuse interface between the two liquids is deflected depending on the ac signal and conductivity (or permittivity) ratio between the liquids. Only a few volts are needed for observing the interface destabilization, in contrast with other electrode configurations where hundreds of volts are applied.
Journal of Physics Conference Series 06/2011; 301(1):012031.
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ABSTRACT: Mixing within sessile drops can be enhanced by generating internal flow patterns using ac electrowetting. While for low ac frequencies, the flow patterns have been attributed to oscillations of the drop surface, we provide here the driving mechanism of the hitherto unexplained high-frequency flows. We show that: (1) the electric field in the liquid bulk becomes important, leading to energy dissipation due to Joule heating and a temperature increase of several degrees Celsius, and (2) the fluid flow at these frequencies is generated by electrothermal effect, i.e., gradients in temperature give rise to gradients in conductivity and permittivity, the electric field acting on these inhomogeneities induces an electrical body force that generates the flow. We solved numerically the equations for the electric, temperature and flow fields. The temperature is obtained from a convection-diffusion equation where Joule heating is introduced as a source term. From the solution of the electric field and the temperature, we compute the electrical force that acts as a body force in Stokes equations. Our numerical results agree with previous experimental observations.
Physical Review E 01/2010; 81(1 Pt 2):015303. · 2.26 Impact Factor
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ABSTRACT: We analyze the AC electro-osmotic motion of a 1:1 aqueous solution, taking into account the difference in mobilities and diffusion coefficients between positive and negative ions. This model serves to understand the behavior of common systems as a solution of NaCl in water. We pay special attention to two cases. First, the case of slightly different mobilities, that can model a KCl solution. Second, the case of a strongly asymmetric solution, with an almost vanishing mobility, applicable to the case of a salt where the negative ion is much more massive than the positive one. For all the cases, we perform the mathematical description and linear analysis of the problem, in order to establish the dependence of the induced velocity with the frequency, wavelength and amplitude of the applied voltage.
04/2008;
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ABSTRACT: Net fluid flow of electrolytic solutions induced by a traveling-wave potential applied to an array of co-planar interdigitated microelectrodes has been reported. At low applied voltages the flow is driven in the direction of the traveling-wave potential, as expected by linear and weakly nonlinear theoretical studies. The flow is driven at the surfaces of the electrodes by electrical forces acting in the diffuse electrical double layer. The pumping mechanism has been analyzed theoretically under the assumption of perfectly polarizable electrodes. Here we extend these studies to include the effect of Faradaic currents on the electroosmotic slip velocity generated at the electrode/electrolyte interface. We integrate the electrokinetic equations under the thin-double-layer and low-potential approximations. Finally, we analyze the pumping of electrolyte induced by a traveling-wave signal applied to a microelectrode array using this linear model.
Journal of Colloid and Interface Science 06/2007; 309(2):323-31. · 3.07 Impact Factor
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ABSTRACT: This paper describes an apparatus based on a novel use of a powder bed, whereby the relationship between consolidation stress, tensile strength, and free volume of fine powder is measured. The powder to be tested is first initialized to a reproducible state. The initialized powder is next consolidated either beyond its own weight or below its own weight by means of a controlled flow of gas. An ultrasonic device measures the height of the bed, thus providing an average value of the powder free volume. Next the consolidated bed of powder is subjected to a slowly increasing gas flow, so directed as to put the powder under tension. The overpressure causing the powder to break provides a measure of the tensile strength of the powder, which in turn is a function of the consolidation and free volume. The relationship between consolidation stress, tensile strength, and free volume is related to powder flowability. © 2000 American Institute of Physics.
Review of Scientific Instruments 06/2000; 71(7):2791-2795. · 1.37 Impact Factor
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ABSTRACT: The electrokinetic manipulation of particles suspended in a fluid medium is accomplished using microelectrodes that generate non-uniform fields of significant strength from low applied potentials. The high strength fields produce not only forces on the particles but also on the fluid medium used for suspension. This paper presents qualitative and semi-quantitative observations of the movement of the fluid at applied field frequencies of the order of 1MHz and higher. The importance of the illumination in generating the fluid flow is described, the flow depending on both the intensity of illumination and the applied electric field. The theory of electrothermally induced fluid flow is briefly described and compared with the experimental observations. Reasonable agreement is found between the experiments and the theory, with the light generating temperature gradients, and therefore gradients in fluid permittivity and conductivity, and the electric field responsible for the motive force.
Journal of Physics D Applied Physics 12/1999; 33(2):L13. · 2.54 Impact Factor
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ABSTRACT: Finite-amplitude bifurcation diagrams of axisymmetric liquid bridges anchored between two plane parallel electrodes subjected to a potential difference and in the presence of an axial gravity field are found by solving simultaneously the Laplace equation for the electric potential and the Young–Laplace equation for the interface by means of the Galerkin/finite element method. Results show the strong stabilizing effect of the electric field, which plays a role somewhat similar to the inverse of the slenderness. It is also shown that the electric field may determine whether the breaking of the liquid bridge leads to two equal or unequal drops. Finally, the sensitivity of liquid bridges to an axial gravity in the presence of the electric field is studied.
Journal of Fluid Mechanics 03/1993; 249:207 - 225. · 2.46 Impact Factor
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ABSTRACT: Recent experiments have shown that an AC electric field induces steady fluid flow at or near the surfaces of microelectrodes. The flow velocity is a function of frequency and applied voltage and deriving a full theoretical model has proved difficult. Recently in this journal, M. Scott, K. V. I. S. Kaler, and R. Paul (J. Colloid Interface Sci.238, 449 (2001)) presented a new theoretical model that not only is less comprehensive than previous ones but also is in direct contradiction to established fundamental principles of electrokinetics.
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ABSTRACT: Electrothermal motion in an aqueous solution arises from the action of an electric field on inhomogeneities in the liquid induced by temperature gradients. The temperature field can be produced by the applied electric field through Joule heating, or caused by external sources, such as strong illumination. Electrothermal flows in microsystems are usually observed at applied signal frequencies around 1 MHz and voltages around 10 V. In this work, we present self-similar solutions for the motion of an aqueous solution in a constant temperature gradient placed on top of: (a) two coplanar electrodes subjected to an a.c. potential difference, and (b) four coplanar electrodes subjected to a four-phase a.c. signal, generating a rotating field. The first case produces two-dimensional rolls whereas the second case produces a liquid whirl. Finally, we present experimental results of electrothermal liquid flows generated by alternating and rotating electric fields under strong illumination, and these experiments are compared to the analytical solutions. The induced rotating flow could be used in the mixing of analytes and of liquids in microsystems.
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ABSTRACT: In this paper we examine the motion and behavior of particles suspended in aqueous solutions subjected to non-uniform ac electric fields. The particles can move due to forces exerted over them, or due to the motion of the surrounding liquid. In the first case, we have dielectrophoresis, due the action of ac electric fields over polarizable particles. The high strength electric fields often used in separation systems can give rise to fluid motion, which in turn results in a viscous drag on the particle. The electric field generates heat, leading to volume forces in the liquid. Gradients in conductivity and permittivity give rise to electrothermal forces; gradients in mass density to buoyancy. In addition, non-uniform ac electric fields produce forces on the induced charges in the diffuse double layer on the electrodes. This gives a steady fluid motion known as ac electroosmosis. We also discuss the effects of Brownian motion in this context. We calculate the different forces and displacements and compare them for a simple system consisting of a saline solution subjected to a traveling wave electric field. This example provides scaling laws of a wider applicability.
