Most of the analyses on the behaviors of an electrolyte droplet (in a dielectric fluid or air) have been performed based on the model of perfect conductor droplet. However, if the thickness of the electrical double layer (EDL) is not negligibly small ...
Most of the analyses on the behaviors of an electrolyte droplet (in a dielectric fluid or air) have been performed based on the model of perfect conductor droplet. However, if the thickness of the electrical double layer (EDL) is not negligibly small with a finite thickness, the perfect conductor model must be corrected to some degrees. In the present thesis, EDL effects on a drop applied electric field are analyzed in analytical method.
In part I, steric effects of ions on the charge-related wetting phenomena are studied. Along with a general treatment, three specific problems in two-dimensional systems are considered: a droplet on an electrode, a droplet on a charged surface, and an electrowetting phenomenon on a dielectric. For computation of wetting tension, the electromechanical approach is adopted with the principle of mechanical force balance for each phase. The modified Poisson-Boltzmann equation, which was originally proposed by Bikerman (Philos. Mag. 33, 384 (1942)), is adopted for the analysis of the steric effects. It is found that the steric hindrance reduces significantly both the osmotic pressure and the electrical stress near the triple contact line. This reduction results in a considerable decrease in the wetting tension when the ratio of the capacitance per unit area of the electrical double layer to that of the dielectric layer is small.
In part II, two analyses are performed for an electrolyte droplet in non-uniform electric fields based on the electrokinetic model. Firstly, the exact solution of electric potential is obtained for a spherical electrolyte droplet in a non-uniform electric field (uniform field + general linear field) under the Debye-Huckel approximation. The solution is used to derive the formula of the electrical force exerted on the droplet. The solution is also used for prediction of the first order deformation of droplet in the limit of small Weber number. Secondly, a new method is proposed for obtaining the first order correction to the solution of conductor model. This method is applicable to any deformed droplet shape. The key idea is that the volume charge density inside the thin EDL is integrated in the normal direction to the surface to be treated as the effective surface charge density. From the analysis, important features of the thin EDL are also revealed. When the EDL thickness decreases with O(kappa^-1), the difference between the droplet surface potential and the bulk potential also decreases with O(kappa^-1). Therefore, the slope of the electric potential change in the EDL remains as O(1). As a first concrete application, an ellipsoidal electrolyte droplet under non-uniform electric field is studied.