The surface of a soft elastic film becomes unstable and forms a labyrinth pattern when a rigid flat plate is brought into adhesive contact, without application of any external pressure. These isotropic undulations have a characteristic wavelength, lambda approximately 3H, where H is the film thickness. We present here technique of ordering, aligning, and modulating these micro-labyrinth structures by using a patterned stamp, by varying the stamp-film inter-surface distance, by a lateral confinement of the instability and even by a simple shear motion of a flat stamp. Many complex structures, such as an array of femto-liter beakers and doubly periodic channels, are generated from a simple stamp consisting of parallel channels. The elastic nature of the patterns allows an in-situ tuning, manipulation, and reconfiguration of the microstructures. Regardless of their precise morphology, the structures continue to have the elastic length scale, lambda approximately 3H. The structures can also be made permanent as required by UV-ozone-induced oxidation of the structures. The underlying principles of the elastic contact instability presented here have the potential to develop into a new soft lithography technique-elastic contact lithography (ECL), allowing a simple, rapid and large area patterning of soft solids.
Self-organized polymer patterns resulting from the evaporation of an organic solvent drop on a soluble layer of polymer are investigated. The patterns can be modulated by changing the rate of evaporation and also the rate of substrate dissolution controlled by its solubility. Both of these affect the contact zone motion and its instabilities, leading to spatially variable rates of substrate etching and redeposition that result from a complex interplay of several factors such as Rayleigh−Benard cells, thermocapillary flow, solutal Marangoni flow, flow due to differential evaporation, osmotic-pressure-induced flow, and contact-line pinning−depinning events. The most complex novel pattern, observed at relatively low rates of evaporation, medium solubility, and without macroscopic contact-line stick−slip, consists of a regularly undulating ring made up of a bundle of parallel spaghetti-like threads or striations and radially oriented fingerlike ridges. Increased rate of evaporation obliterates the polymer threads, producing more densely packed fingers and widely separated multiple rings due to a frequent macroscopic pinning−depinning of the contact line. Near-equilibrium conditions such as slow evaporation or increased solubility of the substrate engender a wider and less undulating single ring.
Controlled dewetting of thin polymer films on physically heterogeneous substrates is employed as a new soft lithography route to obtain various types of ordered meso-scale structures, including nano-membranes and ordered arrays of nano-droplets. Dewetting of a thin polymer film on a defect free homogeneous surface occurs by a randomly placed collection of holes and droplets with a well-defined spacing. In contrast, on a physically patterned surface, strong influence of the underlying pattern is observed on the dewetting pathways as well as on the ordering and size of the resulting meso-scale structures. The imposed periodicity of the substrate pattern vis-à-vis the spinodal length scale of dewetting provides a powerful tool for the morphology and size control. The thickness of the film and the kinetics of dewetting are the other important parameters that govern the morphology of the resulting structures.
When a flat rigid stamp contacts the surface of a soft hydrogel film, a competition between the adhesive and elastic forces engenders an isotropic, labyrinth pattern on the length scale of ∼3H, where H is the film thickness, We present here a very simple, self-organized technique for aligning and modulating these patterns by using a patterned stamp and by changing the film thickness and the stamp−film intersurface distance. Many complex structures such as an array of femtoliter beakers and doubly periodic channels are generated from a simple stamp. The elastic nature of the patterns allows in-situ tuning, manipulation, and reconfiguration of structures. The patterns can also be made permanent by drying of the hydrogel.
Based on experiments and 3-D simulations, we show that a soft elastic film during adhesion and debonding from a rigid flat surface undergoes morphological transitions to pillars, labyrinths and cavities, all of which have the same lateral pattern length scale, λ close to λ/H ~ 3 for thick films, H > 1 µm. The linear stability analysis and experiments show a new thin film regime where λ/H ≈ 3+ 2π (γ/3µH) 1/4 (γ is surface tension, µ is shear modulus) because of significant surface energy penalty (for example, λ/H ≈ 6 for H = 200 nm; µ = 1MPa).
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