  Adhesion requires molecular contact, and natural adhesives employ mechanical
gradients to achieve complete (conformal) contact to maximize adhesion.
Intuitively, one expects that the higher the modulus of the top layer, the
lower will be the adhesion strength. However, the relationship between the
thickness of the stiff top layer and adhesion is not known. In this work, we
quantified the adhesion between a stiff glassy poly (methyl methacrylate)
(PMMA) layer of varying thickness on top of a soft poly dimethyl siloxane
(PDMS) elastomer with a sapphire lens. We found that only about 90 nm thick
PMMA layer on a relatively thicker, softer, and elastic PDMS block is required
to drop macroscopic adhesion to almost zero during the loading cycle. This drop
in adhesion for bilayers can be explained using a conformal model developed by
Persson and Tosatti, where the elastic energy to create conformal contact
depends on both the thickness and the mechanical properties of the bilayer. A
better understanding of the influence of mechanical gradients on adhesion will
have an impact on adhesives, friction, and colloidal and granular physics.
