Spatially Controlled Folding Instability of Moduli‐Patterned and Bilayered Membrane under Compressive Stresses

Abstract

systems consisted of thin fi lm bounded onto a moduli-patterned soft substrate under compressive stress. The spatial occurrence of wrinkles or folds is controlled by the induction of heterogeneous pre-strain distributions onto the moduli-patterned elastic substrate. Figure 1 A illustrates the experimental process in generating a spatially controlled wrinkle or fold pattern. Pre-patterned PDMS was prepared by using photolithography and soft lithography. To fabricate moduli-patterned elastic substrate, the trench of the pre-patterned PDMS elastomer was fi lled entirely with a 10:1 mixture of the PDMS prepolymer and cross-linker through a scraping-assisted micromolding process ( Figure S1, Supporting Information). After a certain curing time, the liquid PDMS in the trench was partially cross-linked. This provides a lower modulus region ( E s1 , Young's Modulus ≈ 30 kPa) than that of a groove-patterned PDMS ( E s2 , Young's Modulus ≈ 2 MPa) ( Figure S2, Supporting Information). By applying mechanical pre-stretching, the local strain was varied over the surface due to the modulus difference (Figure 1 B). Simplistically, the top surface of the moduli-patterned substrate can be assumed to be a serial arrangement of elastic materials with different spring constants ( k 1 < k 2 ) in the direction of pre-stretching. Next, the surface was exposed to UV light/ozone; under exposure, the top surface of the PDMS was converted into a silica-like stiff fi lm, with a fi lm thickness of around 4 µm ( Figure S3, Supporting Information). The fi lm thickness was obtained from model-based calculations. [ 27 ] The elastic modulus of the thin fi lm was ≈ 40 MPa, [ 28 ] corresponding to the moduli ratio that 1333 and 20 for the soft and hard regions, respectively. After the pre-stretching release, a certain level of compressive stress was conversely induced on the fi lm surface, resulting in selective wrinkle or fold pattern formation on the soft regions, while pattern formation did not occur on the hard regions. Figure 1 C shows the scanning electron micro scopy (SEM) images of each step in the procedure. For a 100 µm pitch groove-patterned PDMS substrate, the bilayered system buckled into wrinkles with a sinusoidal deformation of 50 µm in wavelength with a strain of 0.32. By increasing the strain acting on the fi lm, folds were formed in the soft regions. The curvature measurements of the folds were up to 0.3 µm −1 and that of the wrinkles were up to 0.2 µm −1 . The ratios of depth to width for these structures were coupled with curvature.

Magnetic and electric properties of wrinkled manganite films derived by sol‐gel method

“…The high density may be due to the low evaporation rate of solvents. Wrinkled films, as shown in Figure C,D, are prepared at high heating rate of 10°C/s when dried, and the isotropic wrinkled patterns are similar to those observed in other systems . The formation of wrinkled LSMO films derived by sol‐gel method may be ascribed to compressive stress generated from inhomogeneous evaporation of solvents at the drying stages.…”

“…The Figure 2C,D, are prepared at high heating rate of 10°C/s when dried, and the isotropic wrinkled patterns are similar to those observed in other systems. [14][15][16][17][18] The formation of wrinkled LSMO films derived by sol-gel method may be ascribed to compressive stress generated from inhomogeneous evaporation of solvents at the drying stages. Figure 2D shows wrinkled surface morphologies at high magnification.…”

“…Recently, Wrinkled films, [14][15][16][17][18] caused by compressive stress generated from varieties of instabilities, have been observed in many systems, including metal films deposited on polymer layers, 14,15 surface modified polymer films, 16,17 and sol-gel derived films. 18 The formation of wrinkles indicates the films are in compressive stress state, while stress is released in flat film.…”

Time-Dependent Retention of Nanotopographical Cues in Differentiated Neural Stem Cells

“…We fabricated structurally tunable nanowrinkle substrates by applying oxygen plasma oxidation to polydimethylsiloxane (PDMS) substrate. − As shown in Figure b, a PDMS substrate was uniaxially stretched from l 0 = 30 mm to l 0 (1 + ε) = 36 mm (strain (ε) = 0.2) and followed by oxygen plasma in the stretched state. After plasma treatment, the PDMS substrate was spontaneously released to obtain the wrinkle structure on its surface.…”