Elastometry of Deflated Capsules: Elastic Moduli from Shape and Wrinkle Analysis

TL;DR

This paper presents a novel in situ method combining shape and wrinkle analysis to determine elastic moduli, Poisson ratio, and bending stiffness of deflated elastic capsules, validated on different materials.

Contribution

It introduces a new approach that integrates wrinkle analysis with shape fitting to extract comprehensive elastic properties of capsules in situ.

Findings

Method accurately measures elastic moduli and bending stiffness.

Results agree with existing rheological data.

Reveals nonlinear behavior in hydrophobin-coated bubbles.

Abstract

Elastic capsules, prepared from droplets or bubbles attached to a capillary (as in a pendant drop tensiometer), can be deflated by suction through the capillary. We study this deflation and show that a combined analysis of the shape and wrinkling characteristics enables us to determine the elastic properties in situ. Shape contours are analyzed and fitted using shape equations derived from nonlinear membrane-shell theory to give the elastic modulus, Poisson ratio and stress distribution of the membrane. We include wrinkles, which generically form upon deflation, within the shape analysis. Measuring the wavelength of wrinkles and using the calculated stress distribution gives the bending stiffness of the membrane. We compare this method with previous approaches using the Laplace-Young equation and illustrate the method on two very different capsule materials: polymerized octadecyltrichlorosilane (OTS) capsules and hydrophobin (HFBII) coated bubbles. Our results are in agreement with the available rheological data. For hydrophobin coated bubbles the method reveals an interesting nonlinear behavior consistent with the hydrophobin molecules having a rigid core surrounded by a softer shell.