Electrocapillary flows

Spatial gradients in surface tension may arise from a variety of causes, including spatial variations at the interface in temperature (Eq. 10.1.3), in surface concentrations of an impurity or additive (Eq. 10.1.4), or in electric charge or surface potential. The resulting flows are termed, respectively, thermocapillary flows, diffusocapillary flows, and electrocapillary flows. We shall limit our discussion of electrocapillary phenomena because of space restrictions but instead refer the reader to Levich (1962) and Newman (1991). 

Similar to the thermocapUlarity (where temperature gradient creates the driving surface tension force), one can also exploit the effects of electrocapillarity or electrowetting, in which electric potentials can be employed to alter the surface tension and thereby cause a fluid motion. Compared to the thermocapiUary flows, electrocapillary flows are much more energy efficient, with a much faster speed... 

It is necessary that the mercury or other metallic surface be polarized, that is, that there be essentially no current flow across the interface. In this way no chemical changes occur, and the electrocapillary effect is entirely associated with potential changes at the interface and corresponding changes in the adsorbed layer and diffuse layer. 

It should be noted that the capacity as given by C, = a/E, where a is obtained from the current flow at the dropping electrode or from Eq. V-49, is an integral capacity (E is the potential relative to the electrocapillary maximum (ecm), and an assumption is involved here in identifying this with the potential difference across the interface). The differential capacity C given by Eq. V-50 is also then given by... 

Equations 49H and 50H explain why there has been little interest in obtaining the electrocapillary curve for ideally nonpolarizable interphases. On the other hand, this analysis can give us a feel for the type and magnitude of error that may arise when measurements are conducted with an electrode that is presumed to be ideally polarizable but in fact does allow some faradaic current to flow across the interphase. 

Several research groups have proposed using electric helds for controlling flow in microfluidic systems. The two techniques discussed here include electroosmotic valves [290] and electrocapillary force actuation [296]. Electroosmotic valving uses critical dimensions so that the valve is open to EOF flows and closed to pressure-driven flows. The electric held is turned off in channels where how is not desired, and the capillary forces prevent the huids from progressing. Electric held is applied to channels where how is desired. This systems appears to be a fairly simple method for valving in microHuidics. 

Potentials of zero charge of the interface can be found reliably by the same independent methods that are used at the metal-water interface. These include finding the differential capacitance minimum of the electric double layer, from electrocapillary curves, with a flowing-electrolyte electrode, with the vibrating boundary method, with radiotracers, or by measuring the second harmonic... 

When an electric field is applied to a system consisting of droplets of liquid phase B present in liquid A, electrocapillary forces can bring about the movement of these droplets. These forces can be used to recover trace metals and metal mattes from waste pyrometallurgical slags [47]. The driving force in thermocapillary flows is (dy/dT) i.e. the temperature dependence of the surface tension. Whereas in electrocapillarity the driving force is (dy/dE) where E is the electrical potential at constant chemical potential X, and (dy/dE) is equal [47] to the surface excess charge density (qi) at the droplet interface (Equation 18). 

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