Figure 2. Thin film deposition and stress distribution over un-stretched and pre-stretched PDMS ridge.

(a) Schematic representation of formation of parallel buckles due to unidirectional stress distribution when thin film is deposited over a non-planar ridge topology. Un-stretched ridge PDMS showing (b) development of transition length and formation of small parallel buckles (see magnified image), (c) 3D surface profile of a corner of the ridge PDMS showing the initial transition length followed by generation of very fine buckles and (d) 2D line profile of the same. (e) MATLAB® simulated stress distribution along ridge width (σ_x) and ridge length (σ_y). Only σ_x (in red) becomes zero near the step walls which results in preferential stress distribution along y-direction (σ_y) (in blue). This increased stress component along y-direction leads to the formation of buckles oriented parallel to each other and to the width of the ridge and (f) variation of buckle wavelength and amplitude for different thicknesses of deposited thin film. Pre-stretched ridge PDMS showing (g) development of large parallel buckles upon release of the prestrain, (h) 3D surface profile of a corner of the pre-stretched ridge PDMS without any transition length followed by generation of large sinusoidal buckles and (i) 2D line profile of the same. (j) MATLAB® simulated stress distribution for a pre-stretched system shows huge difference in the either stress components with σ_y,pre ≫ σ_x,pre due to additional prestrain, resulting in formation of large parallel buckles. (k) Sequence of microphotographs of buckled nichrome thin film showing gradual formation of buckles with relaxing prestrain and (l) variation of λ_b and A_b upon gradual release of the 35% initial prestrain. Scale bar, 250 μm (b,g), 100 μm (k) and 100 μm (magnified images in b,g).