Potential surfaces external charge, effect

When two different metal surfaces are brought into contact, the surface space charges that were present at their interfaces with a vacuum will be modified. The electrons from the metal of lower work function will flow into the other metal until an interface potential develops that opposes further electron flow. This is called the contact potential and is related to the work-function difference of the two metals. The contact potential depends not only on the materials that make up the solid-solid interface but also on the temperature. This temperature dependence is used in thermocouple applications, where the reference junction is held at one temperature while the other Junction is in contact with the sample. The temperature difference induces a potential (called the Seebeck effect), because of electron flow from the hot to the cold Junction, that can be calibrated to measure the temperature. Conversely, the application of an external potential between the two Junctions can give rise to a temperature difference (Peltier effect) that can be used for heat removal (refrigeration). 

Appropriate elucidation of structure is not only relevant to describe sieving effects but also to study the interactions of the membrane material with the feed to be treated, as far as the corresponding interfaces are placed inside the pores as well as on the membrane external surface. Thus, electrically determined membrane properties act on the solutes inside the pores and transport is affected by these properties (zeta potential, surface charges, etc.) in such a way that makes necessary a detailed knowledge of the geometry of both the inner and external surfaces of the membrane to adequately correlate interactions with their effects on flux (see Chapter 9). 

Pt containing a network of nanometer-sized pores could generate reversible strain with amplitudes comparable with those of commercially available actuators through surface-charging effects under potentials of about 1 V [24]. The conversion of an external electrical signal into a volume change, and a mechanical force, known as actuation, is of importance to small-scale devices. 

Hydrophobic colloidal particles move readily in the liqnid phase under the effect of thermal motion of the solvent molelcnles (in this case the motion is called Brownian) or under the effect of an external electric field. The surfaces of such particles as a rule are charged (for the same reasons for which the snrfaces of larger metal and insnlator particles in contact with a solution are charged). As a result, an EDL is formed and a certain valne of the zeta potential developed. 

Modern theories of electronic structure at a metal surface, which have proved their accuracy for bare metal surfaces, have now been applied to the calculation of electron density profiles in the presence of adsorbed species or other external sources of potential. The spillover of the negative (electronic) charge density from the positive (ionic) background and the overlap of the former with the electrolyte are the crucial effects. Self-consistent calculations, in which the electronic kinetic energy is correctly taken into account, may have to replace the simpler density-functional treatments which have been used most often. The situation for liquid metals, for which the density profile for the positive (ionic) charge density is required, is not as satisfactory as for solid metals, for which the crystal structure is known. 

Separation into chemical and electrical terms is possible with gradients but not with quantities, i.e., p and < >, themselves. The reason is simple. The electrochemical potential p was only conceptually separated into a chemical term p and an electrical term z F< >. The conceptual separation was based on thought experiments in practice, no experimental arrangement can be devised to correspond to the thought experiment described in Section 6.3.13.1, Thus, e.g., one cannot switch off the charges and dipole layer at the surface of a solution as one can switch off the externally applied field in a transport experiment Only the combined effect of lj and ZjFij) can be determined. 

The surface field effect can be realized in a number of ways. The semiconductor can be built into a capacitor and an external potential applied (IGFET), or the field can arise from the chemical effects on the gate materials (CHEMFET). In both cases, change in the surface electric field intensity changes the density of mobile charge carriers in the surface inversion layer. The physical effect that is measured is the change in the electric current carried by the surface inversion layer, called the drain current. 

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