We describe a new experimental technique, colloidal probe atomic force microscopy (AFM) combined with reflection interference contrast microscopy (RICM), and accompanying theoretical framework for the mechanical characterization of soft materials with nanometer thickness; i.e. two dimensional continua. As a model system, a Pickering emulsion droplet consisting of perfluorooctane/water/Cow pea mosaic virus nanoparticles.

The emulsion droplets are compressed via the colloidal probe tip of the AFM, the deformed drop shape is reconstructed by RICM. To determine the elastic behavior of the material, equations of equilibrium are used to describe the shape of the membrane in terms of the applied force, the contact radius and the membrane elastic parameters; the elastic parameters of the material are then found by adjusting their values in the model until a minimum in the error between theory and experiment is achieved.

Non-crosslinked nanoparticle monolayers display the behavior consistent with an aqueous/oil interface, i.e. constant tension with increasing strain. When nanoparticles cross-linking is introduced on the surface, the material displays an initial region of linear elasticity for low deformation. For small surface strains, the surface elastic parameter is constant at approximately 0.7 N/m. As the strain increases further, the elastic parameter decreases, which indicates a loss of structure for higher, albeit small, deformations.