Surface condensation forces electric field

Work Function (WF) plays a key role in the physics and chemistry of materials. Phenomena such as the semiconductor field effect, photo- and thermionic electron emission (Allen and Gobelli, 1962), catalysis (Vayenas et al 1996), and the like are dominated by the WF. This fundamental property of electronic materials is defined as the minimum work required to extract an electron from the Fermi level Ep of a conducting phase, through the surface and place it in vacuum just outside the reach of the electrostatic forces of that phase (Trasatti and Parsons, 1986). The reference level for this transfer is thus called the vacuum reference level. Because even a clean surface is a physical discontinuity, a surface dipole t] with its associated electric field always appears at the surface of the condensed phase. Thus, the work of extracting the electron can be conceptually divided between the work required to... 

In order to stimulate condensate motion under zero-G conditions, other forces must replace the gravitational force. This may be done by centrifugal forces, vapor shear forces, surface tension forces, suction forces, and forces created by an electric field. McEver and Hwangbo [133] and Valenzuela et al. [134] describe how surface tension forces may be used to drain a condenser surface in space. Tanasawa [1] reviews electrohydrodynamics (EHD) enhancement of condensation. Bologa et al. [135] showed experimentally that an electric field deforms the liquid-vapor interface, creating local capillary forces that enhance the heat transfer. 

In an air (or gaseous) medium, microscopic particles adhere to a solid surface not only as a result of molecular forces, but also under the influence of capillary forces in the liquid condensing in the space between the contiguous particles, the double electric layer formed in the zone of contact, and also Coulomb interaction and other like causes. Coulomb forces arise between charged particles and may considerably exceed the molecular forces. This fact is used in particular for holding pesticide particles sprayed in an electric field on the leaves of plants. 

At ambient conditions the formation of a water bridge between two surfaces requires very small separations. For gap distances above 3 nm, the formation of a water bridge requires the participation of an additional force such as the one provided by an electric field. The process of forming a water bridge is a dynamic one, where water molecules condense and evaporate at the same time. In the absence of an electric field, the times needed to reach the equilibrium configuration for a capillary condensate vary with its size. For example, formation times of 5 ms have been reported for a water meniscus with a curvature radius in the sub-10 nm range. ... 

Two major concepts of ion formation and desorption have been suggested, but it has remained a matter of debate whether the concept of field-induced desolvation [96-98] or that of ion evaporation [99,100] mote appropriately describes the event. Although different in several aspects, the models are coherent in that ions are created in the condensed phase and subsequently desorbed into the gas phase. Both recognize the electric field as the driving force to effect extraction of ionic species after charge separation within the layer adsorbed onto the emitter surface. 

An alternative approach to this problem is to regard the double layer as a parallel plate condenser in which one plate is the particle surface and the other plate is a plane of counterions at a potential located a distance from the surface and moving with a velocity u relative to the particle surface. If the surface charge density is cr, the electrical force per unit area of the particle plate in a field of unit potential gradient will be a and this force will be balanced by the viscous resistance, which for an assumed Newtonian flow, leads to the equation ... 

Particle transport through boundary layers in the presence of thermophoretic and electric forces is complex but predictable [5]. For coarse particles, thermophoretic and electric forces can usually be ignored. For fine particles, they can be very important. If the material to be tested is expected to exist in the field at >10°C cooler or warmer than the ambient, or if the surface is expected to be >100 V above or below ground potential, these effects need to be considered. Cool surfaces increase the particle deposition rate while warm surfaces decrease it (analogous to condensation but for different reasons). Experience has shown that surfaces biased at a few hundred volts can collect fine particles at >5 times the rate of grounded surfaces. 

---