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 earlystage dynamics of film evolution and near-collapse dynamics of rupture are understood, we match these regimes and find theoretically that the dimensionless rupture time, T r , scales with κ −10/7 . Here, κ is the dimensionless curvature given by the ratio of the Laplacepressure 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.

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, θ, and the strength of drainage, κ. For strong drainage (κ κ tr ), 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, t r , solely depends on κ, according to the earlier reported scaling, t r ∼ κ −10/7 . By contrast, for weak drainage (κ κ tr ), 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 θ as t r ∼ −(1/ω max ) ln( √ 2θ ) α , with α = 1.15. This scaling is rationalized using linear stability theory, which yields ω max as the growth rate of the fastest-growing wave and α = 1. 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.

Directed molecular self-assembly (DMSA) has emerged as a promising approach to achieve spatial organization of materials from molecular to macroscopic length scale, showing enticing applications in, e.g., molecular robots, [1] microelectronics, [2] and energy materials. [3] In recent years, some strategies toward DMSA have been developed. For instance, the employment of self-assembly systems that are sensitive to external stimuli such as light, [4] enzymatic action, [5] pH, [6] and nucleation seeds, [7] has led to DMSA by controlling the spatial distribution of these stimuli. Another example of DMSA is achieved by reaction-diffusion, [8] i.e., molecular reactants are separately distributed in space and allowed to react after meeting by diffusion, leading to local self-assembly at a certain preprogrammed location. Despite the recent progress, a major challenge for 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. Micropatterned Supramolecular Gelsfurther advance lies in control of the spatial parameters of the self-assembled structures. [9] We have recently proposed a catalysisresponsive supramolecular self-assembly system that involves an in situ formation of hydrazone-based gelator (HA 3 ) from water-soluble tris-hydrazide (H) and aldehyde (A) (Figure 1). The rate of HA 3 formation can be remarkably increased by acid catalysis, thereby providing a handle to control subsequent gelation. [10] Using this gelator system, we previously achieved surface confined formation of gels using surfaces modified with a monolayer of sulfonic acid that act as a catalyst for gelator formation. [11] That system has, however, several limitations which hamper further progress. First, the spatial resolution of gel formation along the surface plane is limited, presumably by the low interfacial catalytic activity. And second, the continuous growth of the gels along the surface normal to form 3D objects is hampered by adhesion of gel fibers to the surface, which blocks the influx of reagents to the catalytic surface. With these problem...

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 t r , with initial radius R film and thickness h o , 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 R film and h o 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 semiinfinite films that predicts t r ∼ R 0 film h 5/7 0. For small radii films, our numerical simulations show the opposite trend with lifetime being solely dependent on R film , in fair agreement with the simplified model that predicts t r ∼ R 10/7 film h 0 0 .

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