Abstract
Self-organization in soft (shear modulus: μ < 1 MPa) thin films (thickness: h < 10 μm) like poly-dimethyl siloxane has been conducive in forming patterns at meso-nano lengthscales. When an elastically inhomogeneous thin film, with depthwise positive shear modulus variation, is near an external contactor, the instabilities ensue at a reduced lengthscale of λc=2.96-0.37logR-0.078Mh compared to λc=2.96h for homogeneous films (R=μbase/μfilm, M slope variation parameter, values of which are 1 and 0 for a homogeneous film). For long-ranged electrostatic interactions, for constant separation distances, the reduced patterns ensue at critical voltages of ~50 V (10 times lower), whereas for constant voltages, dense spiky patterns with 20 times increase in asperity ratio than homogeneous films of similar base modulus are observed in the present numerical studies. All these effects, favourable for creating enhanced functionalized surfaces are relatively more noticeable for those inhomogeneous films which possess high shear modulus gradient ΔμΔz/μ-20 near the free interface.
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