The initial thickness and radius of the film that forms upon close contact of two foam bubbles are known to influence the thinning dynamics and lifetime of the film. Various scalings of lifetime tr, with initial radius Rfilm and thickness ho, have been proposed in literature. In this paper, we present a hydrodynamic thin-film model that includes both surface tension, van der Waals forces, and drainage and that clarifies the various proposed scalings of lifetime. Our model equations were solved numerically for a range of Rfilm and ho as direct input parameters. Films with a large radius are found to thin locally at a dimple, while films with a small radius thin across the entire film. The observed dynamics and lifetime were interpreted by developing a simplified model that describes the early stage dimpled drainage and the late stage van der Waals thinning, using known similarity solutions. For large radii films, our simulations confirm earlier theoretical work on semi-infinite films that predicts tr∼Rfilm0h05/7. For small radii films, our numerical simulations show the opposite trend with lifetime being solely dependent on Rfilm, in fair agreement with the simplified model that predicts tr∼Rfilm10/7h00.

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Thin liquid films are fluid structures with perpendicular length scale, typically of the O(< 10 μm), being much smaller than the lateral length scale, typically of the O(> 1 mm). From foams and emulsions to tear films on eyes, they widely occur in industrial processes and natural phenomena. Depending on the wetting energies between its different interfaces, it is susceptible to developing an instability which can lead to its subsequent rupture. It is a great example of how dynamics at microscopic scale influence large scale physical behaviour, with instabilities at micron scale influencing a foam collapse or the blinking action of an eye. The subject of this thesis focuses on non-planar thin liquid films that are found, for instance, in between two foam bubbles or in partial wetting systems in microfluidic channels. The dynamics of such non-planar films is governed by two thinning mechanisms. The first mechanism involves drainage due to curvature differences, and results in a localized depression, commonly referred to as a dimple, at the connection between the planar and curved regions. The second thinning mechanism involves growth of a fluctuation originated instability arising from the competition between a stabilizing surface tension and destabilizing van der Waals forces. For this second thinning mechanism to manifest, the film’s lateral length (radius) needs to be large enough to accommodate unstable waves to fit within the film. We study thin film dynamics, by performing numerical simulations that incorporate all these crucial physical processes in the thin film equation...@en

Thermal fluctuations have been shown to influence the thinning dynamics of planar thin liquid films, bringing predicted rupture times closer to experiments. Most liquid films in nature and industry are, however, non-planar. Thinning of such films not just results from the interplay between stabilizing surface tension forces and destabilizing van der Waals forces, but also from drainage due to curvature differences. This work explores the influence of thermal fluctuations on the dynamics of thin non-planar films subjected to drainage, with their dynamics governed by two parameters: The strength of thermal fluctuations, , and the strength of drainage, . For strong drainage , we find that the film ruptures due to the formation of a local depression called a dimple that appears at the connection between the curved and flat parts of the film. For this dimple-dominated regime, the rupture time, , solely depends on , according to the earlier reported scaling, . By contrast, for weak drainage , the film ruptures at a random location due to the spontaneous growth of fluctuations originating from thermal fluctuations. In this fluctuations-dominated regime, the rupture time solely depends on as , with . This scaling is rationalized using linear stability theory, which yields as the growth rate of the fastest-growing wave and . These insights on if, when and how thermal fluctuations play a role are instrumental in predicting the dynamics and rupture time of non-flat draining thin films.

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Herein, the micropatterning of supramolecular gels with oriented growth direction and controllable spatial dimensions by directing the self-assembly of small molecular gelators is reported. This process is associated with an acid-catalyzed formation of gelators from two soluble precursor molecules. To control the localized formation and self-assembly of gelators, micropatterned poly(acrylic acid) (PAA) brushes are employed to create a local and controllable acidic environment. The results show that the gel formation can be well confined in the catalytic surface plane with dimensions ranging from micro- to centimeter. Furthermore, the gels show a preferential growth along the normal direction of the catalytic surface, and the thickness of the resultant gel patterns can be easily controlled by tuning the grafting density of PAA brushes. This work shows an effective “bottom-up” strategy toward control over the spatial organization of materials and is expected to find promising applications in, e.g., microelectronics, tissue engineering, and biomedicine.

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We experimentally and theoretically describe the dynamics of evolution and eventual rupture of Landau-Levich-Bretherton films of partially wetting liquids in microchannels in terms of nonplanar interface curvatures and disjoining pressure. While both the early-stage dynamics of film evolution and near-collapse dynamics of rupture are understood, we match these regimes and find theoretically that the dimensionless rupture time, Tr, scales with κ-10/7. Here, κ is the dimensionless curvature given by the ratio of the Laplace-pressure discontinuity that initiates film thinning to the initial strength of the disjoining pressure that drives the rupture. We experimentally verify the rupture times and highlight the crucial consequences of early film rupture in digital microfluidic contexts: pressure drop in segmented flow and isolation of droplets from the walls.

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