Weak Double Layers Research Articles

Weak Double Layers: Existence, Stability, Evidence

Two more classes of monotonic double layers complementing the class of beam-type double layers are investigated analytically, and their range of existence is explored in the small amplitude limit. One class preferentially exists for hot ions and electron drifts of the order of electron thermal velocity. The second one, instead, assumes hot electrons and needs almost current-free conditions. The first class, called SEADL, is based on the slow electron acoustic branch and exhibits a tuning-fork configuration in the electron phase space. Its density decreases with increasing potential. The second one (SIADL) rests on the slow ion acoustic branch and. consequently, has a tuning-fork pattern in the ion phase space. Its density increases with the potential. Both classes are found to be linearly stable with respect to one-dimensional, but unstable with respect to two-dimensional electrostatic perturbations. A comparison with experiments suggests an identification of the second type with the double layers obtained by Hollenstein

Weak double layers

The characteristics of weak double layers with potential steps approximately equal to five are investigated in a triple plasma device and compared with a kinetic model. The double layers considered here differ from those previously investigated in two significant ways: (1) the double layers are much weaker; and (2) trapped ions play a much less important role. In this study, two plasmas at different potentials but with similar characteristics are separated from a target chamber by grids so that ions enter only at the high potential side and electron beams enter at the low potential side. What is more, thermal electrons enter at the high potential side and are trapped by the double layer. The model calculations illustrate the role of the trapped ions in determining the position of the double layer, and they demonstrate the dependence of the length of the double layer on the entering ion and electron fluxes.

Wetting films on silica

Measurements are reported of the thickness of equilibrium wetting films of dilute solutions of KCl, LiCl, BaCl 2 , and LaCl 3 on polished vitreous silica. Under constant hydrostatic pressure, film thickness decreases with increase of salt concentration and with increase of valency of the cation. The results are in approximate agreement with Langmuir's theory of wetting films, confirming that the fundamental origin of the phenomenon is the diffuse electrical double layer. However, the experimental film thicknesses with KCl and BaCl 2 appear to be significantly greater than values calculated on the assumption that no charge exists at the air/water interface. The discrepancies could be accounted for by the presence of a relatively weak negative electrical double layer, itself depending on the nature and concentration of electrolyte in the solution. Reduction of the zeta-potential to approximately zero by addition of LaCl 3 led to a very thin residual film; such films are particularly susceptible to contamination by traces of hydrophobic impurities.