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In this method the first step identifies the tip surface as a superposition of infinitesimal surfaces (facets). The first hypothesis is that from an electrostatic point of view, for distances larger than the characteristic facet dimensions, the surface is regular. The second step, which is also the second hypothesis, evaluates the electric field created between the facetted conductor and the plane surface by postulating that the electric field on each infinitesimal surface of the tip is equal to that created by the dihedral capacitance constituted by two infinite planes in the same relative orientation.

DV : Consequently, d is located in the sphere by its distance r from O and the angle θ (the problem is symmetric as far as the azimutal angle φ is concerned). 11) and if we note x = D + R the integral of Eq. 14) and Eq. 3. Interaction potential between two spheres. Consequently, with the Hamaker constant A ≡ Kn1 n2 π 2 [J], the interaction potential W(S,dV ) of Eq. 2 Interaction Potential Between Two Spheres In order to determine the interaction potential W(S1 ,S2 ) between two spheres S1 (radius R1 , number density n1 , center O1 ) and S2 (radius R2 , number density n2 , center O2 ) separated by a distance D (see Fig.

48) Gutowsky [29] cited Johnson and Dettre [43] for a detailed study of the effect of roughness on contact angle hysteresis. This hysteresis implies that even at equilibrium, the contact angle value is not unique. The contact angle also depends on the velocity of the contact line. This phenomenon is described in Hoffman [36]. Due to the surface tension, there exists a pressure difference across the interface between a liquid and a gas. In the case of a soap bubble, for example, the pressure inside the bubble is bigger, to compensate the outside pressure and to overcome the tension effect.