Langmuir 2001, 17, 2059-2062
2059
Film Stratification in the Presence of Colloidal Particles Gopi Nath Sethumadhavan, Alex Nikolov, and Darsh Wasan* Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 Received July 3, 2000. In Final Form: January 4, 2001 This is probably the first study about the drainage of curved liquid films in the presence of colloidal particles. The systems did not contain any surfactant. In the presence of monodispersed colloidal particles, thinning occurs in a stepwise manner (stratification). It has been shown that the size of the film is an important parameter in the stepwise thinning process. This investigation found a critical film size below which at least one layer of particles stays in the film at equilibrium; a “spot” (a thinner section of the film), even if formed, does not expand and is in equilibrium with the film. The area of the spot expands linearly with time. The rate of spot-area expansion increases linearly with the film perimeter and can be increased or decreased merely by changing the film size. The stepwise film thinning and the effects of film size and particle concentration on film stability are discussed on the basis of the diffusive-osmotic mechanism.
Introduction
Experimental Section
The phenomenon of particle layering between flat hard walls has been observed1,2 and understood through computer simulations3-6 and theoretical modeling.7,8 When a colloidal solution is present between two close flat walls (for example, a thin liquid film), the confinement causes the particles to form a layered structure parallel to the walls. Layering is an entropic phenomenon and occurs because the particles have a finite size that limits the number that can occupy the layer closest to the walls. Such layering naturally produces oscillatory decay in the number-density of particles near the walls. The concentration of colloidal particles between the flat walls (i.e., in the film) is higher than that in the bulk and produces a stabilizing pressure (“disjoining pressure”) that has the same oscillatory nature. Even though the phenomenon of particle layering is well understood, its implications to the mechanism of stepwise film thinning are not clear. The two widely discussed mechanisms, (1) the “holesheeting” model9,10 and (2) the diffusive-osmotic model,11 predict different stepwise behavior. This study is aimed at checking their validity against experimental observations. In colloidal dispersions such as foams and emulsions, curved films are susceptible to earliest rupture (for example, on the top of a foam). Therefore, we have used curved films, free of surfactants, to study solely the effect of hydrophilic colloidal particles on the stratification of thin liquid films.
Materials. Dispersion of monodispersed 8 nm silica particles at 5% and 80 nm particles at 10% v/v from Nalco Chemicals, Naperville, IL, were used in these experiments. The 80 nm system produced a total of four step-transitions during thinning and was found to have an effective particle volume fraction of 0.35, accounting for the double layer thickness (for details on methodology refer to ref 12). By use of a similar method, the 8 nm system was found to produce a total of three step-transitions during thinning and had an effective particle volume fraction of 0.25. The foam films in both the systems were stabilized only by colloidal particles. Experimental Methods. All experiments were carried out at room temperature (24 °C). We used an experimental technique (Figure 1) similar to one developed recently13 to investigate film thinning of curved films. An air bubble is formed from a tiny capillary (radius ≈ 0.5 mm) and the solution level around the bubble is lowered until the film size is of the required dimensions (diameter
