Effect of stress state on wrinkle morphology

Abstract

Wrinkles in thin films on soft substrates have been shown to self-organize into topological patterns, providing a possible route towards inexpensive generation of surface microstructure. However, the effect of the magnitude of applied stress in relation to the critical buckling stress, or overstress, on the observed patterns has until this point been neglected experimentally. In this paper, we investigate the effect of overstress using poly(dimethylsiloxane) which has been surface-oxidized with a UV-ozone oxidation technique. Using a swelling-based stress application technique, where the applied swelling stress in the thin film is controlled by changing the concentration of vapor-phase swelling agent (ethanol) in a sealed swelling chamber, we are able to impart swelling stresses below, at, and well above the critical stress. We observe a transition from hexagonally packed dimples at low overstress to ridgebased morphologies (herringbone and labyrinth) at high overstress. The observed dimple structures are remarkably widespread, and the hexagonal arrangement of these dimples is confirmed using Fourier analysis. Although analytical results predict that a square arrangement of dimples is preferred to hexagonal for flat wrinkling surfaces, hexagonal arrays are nonetheless unilaterally observed at low overstress. We attribute this observation to an inherent curvature that develops in the swelling film. The overstress is quantified by measuring the radius of curvature of swelling bilayer beams, both confirming the preferential swelling of the surface oxide layer by ethanol and quantifying the swelling extent. Effects of non-equibiaxial stress are investigated by inducing a compressive prestress prior to swelling, and ''trapped'' non-equilibrium morphologies are discussed briefly.

Cephalopod-inspired design of electro-mechano-chemically responsive elastomers for on-demand fluorescent patterning

“…3b,dII, Movie 1). The pattern of wrinkles resembles those observed in bilayer systems induced by thermal expansion 41,42 , swelling 43,44 and ultraviolet or ion irradiation treatment 45,46 . As the applied electric field further rises, the valleys of the wrinkles are pinched down, forming a pattern of craters (Fig.…”

Curvature-induced stiffness and the spatial variation of wavelength in wrinkled sheets

“…In particular, we find a correction to the stress along the wrinkles' direction σ ð1Þ xx = −Y ζ 0 ″ðxÞf cosð2πy=λÞ, [7] which exists only if the mean shape, ζ 0 ðxÞ, is curved in the wrinkles' direction. The significance of the correction in [7] can be understood by substituting this stress component into the first FvK equation [3], where it gives rise to a new force that is proportional to f: an entirely new source of stiffness. In detail, the linearized normal force balance [3] reads "…”

“…The significance of the correction in [7] can be understood by substituting this stress component into the first FvK equation [3], where it gives rise to a new force that is proportional to f: an entirely new source of stiffness. In detail, the linearized normal force balance [3] reads "…”

Wrinkling Measurement of the Mechanical Properties of Drying Salt Thin Films

“…Spontaneous surface wrinkling of laminate films can occur as a result of mechanical stress 3,4,5,6 , expansion and contraction associated thermal deposition, or during solvent evaporation and shrinkage 7,8,9,10,11 , and is often undesirable 7,8,11,12,13,14,15 . The analysis of wrinkling topographies has also been shown to provide a powerful tool for the mechanical characterisation of thin and ultra-thin polymeric, composite and metallic supported thin films 2,7,16,17,18 .…”