Capacitance analysis of a circular disk on a dielectric layer

∙Analytical expressions of the capacitance to the second approximation were derived.∙The capacitance of a circular disk on a semi-infinite dielectric can be used for a ratio of the layer thickness to the disk radius larger than or equal to 4.∙There exists an upper bound of the electric voltage above which the saturation of contact angle or local instability will occur.

Using the Hankel transform, the mixed boundary value problem of a conducting, circular disk on a dielectric layer in an electric field was reduced to the solution of dual integral equations. Analytical expressions of the capacitance of the electrical system to the second approximation were derived for the ratio of the layer thickness to the disk radius being larger than one and the ratio being less than one, respectively. Both expressions yield numerical results in accord with those reported in literature. The capacitance of a circular disk on a semi-infinite dielectric can be used for a ratio larger than or equal to 4. Using the results, the thickness effect on electrowetting of a disk-like conducting film on a dielectric substrate was discussed. There exists an upper bound of the electric voltage applied to the film above which the saturation of contact angle or local instability will occur.