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.

We investigate the use of the Wilhelmy plate method to measure the surface pressure in a solid-like Langmuir film under compression. Layers of the protein hydrophobin, which exhibits a high shear elastic modulus, are spread and compressed in a Langmuir trough. The resulting isotherms are classified according to the surface pressure and distance between the barriers measured at the onset of buckling. We find that the surface pressure measured in the centre of the layer at the onset of buckling decays with increasing distance between the barriers (which can be tuned by varying the amount of material spread initially). However, unlike the case of particle rafts, the length scale of this decay is not controlled by the width of the trough but rather by the size of the Wilhelmy plate used. We use experiments and a computational model to suggest that this independence of trough width may be attributed to the localised nature of the effect of the trough walls. Our work highlights the potential pitfalls of using the Wilhelmy method to characterize layers with high shear rigidity and may lead to a better understanding of the use of the Wilhlemy plate to measure the surface stress tensor.

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-The protein hydrophobin HFBII self-assembles into very elastic films at the surface of water; these films wrinkle readily upon compression. We demonstrate and study this wrinkling instability in the context of non-planar interfaces by forming HFBII layers at the surface of bubbles; the interfaces are then compressed by deflating the bubble. By varying the initial concentration of the hydrophobin solutions, we are able to show that buckling occurs at a critical packing fraction of protein molecules on the surface. Independent experiments show that at this packing fraction the interface has a finite positive surface tension, and not zero surface tension as is usually assumed at buckling. We attribute this non-zero wrinkling tension to the finite elasticity of these interfaces. We develop a simple geometrical model for the evolution of the wrinkle length with further deflation, and show that wrinkles start close to the needle used for deflation and grow rapidly towards the mid-plane of the bubble. This geometrical model yields predictions for the length of wrinkles in good agreement with experiments, independently of the rheological properties of the adsorbed layer.

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