Cloaking by coating: How effectively does a thin, stiff coating hide a soft substrate?

TL;DR

This study investigates how a thin, stiff coating on soft materials affects their apparent stiffness during indentation tests, revealing that indenter size critically influences the cloaking effectiveness.

Contribution

The paper combines theoretical analysis and experiments to quantify how coating thickness and indenter size impact the perceived stiffness of coated soft materials.

Findings

Small indenters detect both coating and substrate properties.

Large indenters primarily sense the substrate.

Cloaking effectiveness depends on indenter size and coating parameters.

Abstract

From human tissue to fruits, many soft materials are coated by a thin layer of a stiffer material. While the primary role of such a coating is often to protect the softer material, the thin, stiff coating also has an important effect on the mechanical behaviour of the composite material, making it appear significantly stiffer than the underlying material. We study this cloaking effect of a coating for the particular case of indentation tests, which measure the `firmness' of the composite solid: we use a combination of theory and experiment to characterize the firmness quantitatively. We find that the indenter size plays a key role in determining the effectiveness of cloaking: small indenters feel a mixture of the material properties of the coating and of the substrate, while large indenters sense largely the unadulterated substrate.