Ultrathin, dynamic and groovy: the physics of Dutcher films

Abstract

Dutcher films are ultra-thin solid–fluid–solid films where van der Waals forces acting across the film encourage the growth of surface instabilities. In the absence of the solid layers, which sandwich the fluid layer, the fluid would coalesce into isolated droplets like a bursting bubble. However, the solid layers stabilize the fluid layer and oppose the growth of surface instabilities which, in turn, deform the solid layers. The elasticity of the solid layers also causes the preferential growth of larger wavelength undulations which involve less elastic deformation. This is contrasted by the fluid dynamics of the sandwiched fluid layer, which kinetically would prefer smaller wavelength undulations. For larger wavelength undulations the fluid has to flow over larger distances and this takes longer resulting in slower growth. A combination of these effects selects a characteristic wavelength of the undulations. In other words, the van der Waals forces cause the film to undulate (spontaneously form alternating thick and thin regions), the elasticity of the solid layers encourages the growth of larger wavelength undulations, while the time required to move fluid from thinning to thickening regions kinetically favors the growth of smaller wavelength undulations. This competition between kinetics and energetics results in a characteristic lengthscale for undulation growth. The lengthscale of the undulations is on the order of micrometers while the films are initially on the order of 100 nm. As the undulations grow they eventually reach a stage where the top and bottom solid layers are brought into contact. At this point the fluid is segregated into isolated pockets of fluid separated by regions where the solid layers are in contact. These disc-shaped pockets which are on the order of 100 nm thick and micrometers in diameter could offer a unique environment for microfluidics. Dutcher films can also allow us to probe the mechanical properties of thin solid layers. The wavelength of undulation growth is sensitive to the Young’s modulus of the solid layers and, by measuring this wavelength, information concerning the mechanics of the solid layers can be gleaned from these experiments. As the spread of nanotechnology results in smaller mechanical systems, and an increase in thin film technologies, understanding the rich physical behaviour of Dutcher films will become increasingly important.