Solvent-induced surface instability of thin metal films on a polymer substrate

Telephone cord blister formation in solvent swollen elastomer films

Video imaging was used to study large-slope folded telephone cord blister formation in solvent swollen films of polydimethylsiloxane (PDMS) elastomers. Chlorobenzene, chloroform, heptane and toluene were used to swell PDMS films with thickness values in the range 15 μm < h < 223 μm supported on glass substrates. Measurements of the blister width, corrugation wavelength and blister growth speed were studied as a function of the film thickness for all four solvents. Modified theories of buckling were shown to accurately predict the film thickness dependence of the width and corrugation wavelength and a modified fracture mechanics approach was shown to reproduce the non-monotonic thickness dependence of the blister growth rates. Two critical thickness values were identified for telephone cord blisters formed on solvent swollen PDMS films-one corresponding to a lower critical thickness for blistering and the other corresponding to a peak in the thickness dependent blister growth rates. Blister formation is shown to be consistent with the existence of mixed modality in the growth of the crack tip at the film-substrate interface. Both critical thickness values are shown to depend upon the strength of the adhesive interactions between the film and the substrate and the Young's modulus of the films. A simple method of patterning surfaces with telephone cord blisters is also introduced.

Thickness dependence of telephone cord blister formation in solvent swollen films of polydimethylsiloxane

The thickness dependence of telephone cord blister formation in thin films of solvent swollen polydimethylsiloxane (PDMS) was studied using a simple imaging setup. Chloroform was deposited on top of PDMS that had been spin coated on to glass slides coated with a thin wax layer. After an initial thickness dependent nucleation time, straight-sided blisters were observed to form on the films. These later developed into sinusoidal telephone cord blisters. Movies of the growing telephone cord blisters were recorded at 200 fps for PDMS films with thickness values in the range 38 < h ≤ 223 μm. Software written in Python was used to analyse the movies and to extract the thickness dependence of the width of the telephone cord blisters as well as the wavelength of the sinusoidal corrugations and the blister growth rates. Data were interpreted in the context of theories of buckling and dynamic fracture mechanics.

Water nanolayer facilitated solitary-wave-like blisters in MoS2 thin films

Solitary waves are unique in nonlinear systems, but their formation and propagation in the nonlinear fluid-structure interactions have yet to be further explored. As a typical nonlinear system, the buckling of solid thin films is fundamentally related to the film-substrate interface that is further vulnerable to environments, especially when fluids exist. In this work, we report an anomalous, solitary-wave-like blister (SWLB) mode of MoS₂ thin films in a humid environment. Unlike the most common telephone-cord and web buckling deformation, the SWLB propagates forward like solitary waves that usually appear in fluids and exhibits three-dimensional expansions of the profiles during propagation. In situ mechanical, optical, and topology measurements verify the existence of an interfacial water nanolayer, which facilitates a delamination of films at the front side of the SWLB and a readhesion at the tail side owing to the water nanolayer-induced fluid-structure interaction. Furthermore, the expansion morphologies and process of the SWLB are predicted by our theoretical model based on the energy change of buckle propagation. Our work not only demonstrates the emerging SWLB mode in a solid material but also sheds light on the significance of interfacial water nanolayers to structural deformation and functional applications of thin films.

Drop Spreading and Confinement in Swelling-Driven Folding of Thin Films

Surface instabilities are a versatile method for generating three-dimensional (3D) surface microstructure. When an elastomeric film weakly bonded to a substrate is swollen with solvent, buckle delamination and subsequent sliding of the film on the substrate lead to the formation of tall, self-contacting, and permanent folds. This paper explores the mechanics of fold development when such folding is induced by placing a drop on the surface of the film. We show that capillary effects can induce a strong coupling between folding and drop spreading: as folds develop, they wick the solvent toward the periphery of the drop, further propagating radially aligned folds. Accordingly, a solvent drop spreads far more on films that are weakly adhered to the substrate. As drop size reduces and folding becomes increasingly confined, debonding propagates along the perimeter of the wetted region, thus leading to corral-shaped fold patterns. On the other hand, as drop size increases and confinement effects weaken, isotropically oriented folds appear at a spacing that reduces as swelling increases. The spacing between the folds and the size of the corrals are both determined by the extent to which a single fold relieves compressive stress in its vicinity by sliding. We develop a model for folding which explicitly accounts for the fact that folds must initiate with near-zero volume under the buckle. The model shows that folds can appear even at very low swelling if there are large pre-existing debonded regions at the film-substrate interface.