Critical thickness of microscopic thin liquid films

This paper outlines the progress achieved during the four decades of research on the spontaneous destruction of the thinning microscopic liquid films through rupture or black spot formation at the so-called critical thickness. Although most of both experimental and theoretical results are primarily related to the foam films that form between gas bubbles, in many respects they can be principally generalized for emulsion films, as well as the wetting films confined between a bubble and a solid surface. The paper focuses on the validation, application and extension of the theory of the phenomenon. The experimental results are analysed with respect to the frequently observed deviations from the widely used model of a planar circular film with tangentially immobile surfaces. The applicability of the new theory of accelerated drainage due to spatial variation in thickness is expressed. The effects of surface tension, surface mobility, variation of the film size, and spatial thickness heterogeneity on the critical thickness are compared.