Elastic nanomembrane metrology at fluid-fluid interfaces using axisymmetric drop shape analysis with anisotropic surface tensions: deviations from Young-Laplace equationElectronic supplementary information (ESI) available. See DOI: 10.1039/c2sm26604k

Fluid-fluid interfaces are an attractive template for engineering nanomembranes via self-assembly. Although an increasing number of investigations of the local (∼10 −10 m 2 ) surface mechanics of a variety of materials, detailed studies of mechanical behavior and the constitutive parameters over larger areas (>10 −6 m 2 ) are relatively scarce. This is because of the limitations of quantitative experimental techniques due to the asymmetry of the length scales involved. Here, we discuss fabrication and characterization of a polysaccharide and polyamino acid-based nanomembrane having a thickness of approximately 200 nm and surface area of 25 × 10 −6 m 2 and present mechanical data for hyaluronic acid-poly- l -lysine nanomembranes using a modified pendant drop as a test frame. Covalent cross-linking of these molecules at fluid-fluid interfaces leads to the formation of supramolecular networks which confer properties such as mechanical rigidity that are outside of the description provided by equilibrium surface thermodynamics. A theoretical framework for data interpretation of purely elasticity interface systems is provided, and experimental signatures of anisotropic tension distributions on axisymmetric drop and bubble shapes are identified. By taking advantage of the mechanical transparency of the fluid-fluid interface, this method provides a means of accessing mechanical properties of ultrathin materials independent of artifacts, such as adhesion, introduced by a solid substrate. Nanostructured ultrathin films resulting from adsorption or deposition can render fluid-fluid interfaces capable of supporting anisotropic stress distributions, causing systematic deviation from the Young-Laplace equation.