CONDITIONS FOR STABLE ATTACHMENT OF FLUID PARTICLES TO SOLID-SURFACES

Recently Eriksson and Ljunggren (Langmuir 1995, 11, 2325) established theoretically that in some cases the Young contact angle equation corr esponds to a maximium of the energy of the system (that is an unstable equilibrium) rather than to a minimum. In fact, these authors have fo und out an apparent paradox in the classical field of interfacial ther modynamics. Indeed, it is known from the experiment that bubbles or dr oplets attached to smooth solid surfaces tend to form an equilibrium c ontact angle, theta, whereas it is shown in ref 1 that under some cond itions the fluid particles should avoid formation of an equilibrium co ntact angle because of an unstable equilibrium. Below we extend the an alysis of Eriksson and Ljunggren to demonstrate that the conditions fo r the stability of particle attachment are different depending on whet her the fluid particle contains an independent component, i.e. a compo nent whose chemical potential is independent from those of the compone nts in the surrounding medium. The results show that the equilibrium c ontact angle corresponds to an unstable equilibrium only when (i) 90 d egrees < theta < 180 degrees and (ii) the independent component is mis sing. In all other cases the equilibrium contact angle corresponds to the stable configuration of the attached fluid particle. These conclus ions can be important not only for the specific problem about the brid ging cavities as an explanation of the hydrophobic surface force but a lso for the whole broad field of wetting and spreading.