  We provide a C/C++ software for the shape analysis of deflated elastic
capsules in a pendant capsule geometry, which is based on an elastic
description of the capsule material as a quasi two-dimensional elastic membrane
using shell theory. Pendant capsule elastometry provides a new in-situ and
non-contact method for interfacial rheology of elastic capsules. Given an
elastic model of the capsule membrane, pendant capsule elastometry determines
optimal elastic moduli by fitting numerically generated axisymmetric shapes
optimally to an experimental image. For each digitized image of a deflated
capsule elastic moduli can be determined, if another image of its undeformed
reference shape is provided. Within this paper, we focus on nonlinear Hookean
elasticity because of its low computational cost its wide applicability, but
also discuss and implement alternative constitutive laws. For Hookean
elasticity, Young's surface modulus (or, alternatively, area compression
modulus) and Poisson's ratio are determined; for Mooney-Rivlin elasticity, the
Rivlin modulus and a dimensionless shape parameter are determined; for
neo-Hookean elasticity, only the Rivlin modulus is determined, using a fixed
dimensionless shape parameter. Comparing results for different models we find
that nonlinear Hookean elasticity is adequate for most capsules. If series of
images are available, these moduli can be evaluated as a function of the
capsule volume to analyze hysteresis or aging effects depending on the
deformation history. An additional wrinkling wavelength measurement allows the
user to determine the bending modulus, from which the layer thickness can be
derived. We verify the method by analyzing several materials, compare the
results to available rheological measurements, and review several applications.
We make the software available under the GPL license at
github.com/jhegemann/opencapsule.
