Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. II. A linear double-layer analysis

Abstract

Frequency-dependent fluid flow in electrolytes on microelectrodes subjected to ac voltages has recently been reported. The fluid flow is predominant at frequencies of the order of the relaxation frequency of the electrodeelectrolyte system. The mechanism responsible for this motion has been termed ac electro-osmosis: a continuous flow driven by the interaction of the oscillating electric field and the charge at the diffuse double layer on the electrodes. This paper develops the basis of a theoretical approach to this problem using a linear double layer analysis. The theoretical results are compared with the experiments, and a good correlation is found.

Induced-charge electro-osmosis

“…The same time scale, τ c , also arises as the inverse frequency of AC electro-osmosis (Ramos et al (1999)) or AC pumping (Ajdari (2000)) at a micro-electrode array of characteristic length a, where again an RC-circuit analogy has been invoked to explain the charging process. This simple physical picture has been criticized by Scott et al (2001), but Ramos et al (2001) and Gonzalez et al (2000) have convincingly defended its validity, as in the earlier Russian papers on polarizable colloids.…”

“…In AC electro-osmosis at adjacent surface electrodes (Ramos et al (1999)) or AC pumping at an asymmetric electrode array (Ajdari (2000)), the inducing surfaces are the electrodes, and so the two time scales coincide to yield a single characteristic frequency ω c = 1/τ e . (Note that Ramos et al (1999) and Gonzalez et al (2000) use the equivalent form ω ∼ σλ D /ε w L.) Table 2 presents typical values for ICEO flow velocities and charging time scales for some reasonable microfluidic parameters. For example, an applied electric field of strength 100 V/cm across an electrolyte containing a 10 µm cylindrical post gives rise to an ICEO slip velocity of order 1 mm/s, with charging times τ c ∼ 0.1 ms and τ e = 0.1 s.…”

“…Ramos et al (1998; and Gonzalez et al (2000) theoretically and experimentally explored 'AC electro-osmosis', in which a pair of adjacent, flat electrodes located on a glass slide and subjected to AC driving, gives rise to a steady electro-osmotic flow consisting of two counter-rotating rolls. Around the same time, Ajdari (2000) theoretically predicted that an asymmetric array of electrodes with applied AC fields generally pumps fluid in the direction of broken symmetry ('AC pumping').…”

Numerical study of dc-biased ac-electrokinetic flow over symmetrical electrodes

“…7,8 AC-electrokinetic (ACEK) makes use of AC electric fields acting on AC charge density to induce rectified flow; two important examples of ACEK are AC-electroosmotic (ACEO) and AC-electrothermal (ACET) flows. [9][10][11][12][13][14][15][16] ACEO flow is based on the Coulomb force acting on the induced charge in the double layer in the presence of a tangential AC electric field. [10][11][12][13] ACET flow is based on the interaction of AC electric fields with conductivity gradients in the fluid induced by a thermal gradient.…”

“…[9][10][11][12][13][14][15][16] ACEO flow is based on the Coulomb force acting on the induced charge in the double layer in the presence of a tangential AC electric field. [10][11][12][13] ACET flow is based on the interaction of AC electric fields with conductivity gradients in the fluid induced by a thermal gradient. [14][15][16] More recently, a DC-bias AC voltage (V applied ¼ V DC þ V AC cos xt) has been employed effectively to concentrate particles/cells [17][18][19] and for pumping/mixing [20][21][22][23] applications in microfluidics.…”