Tunable thin elasto-drops

We present an experimental method to fabricate centimetric thin elastic capsules with a highly uniform thickness and negligible bending stiffness using silicone elastomers. In our experiments, the capsule thickness is tunable at fabrication, while internal pressure and hoop (circumferential) stress are adjustable via hydrostatic inflation once the capsules are filled and immersed in water. Capsule mechanics are probed through hydro-elastic waves generated by weak mechanical perturbations at the capsule interface. By analyzing the surface wave dynamics in the Fourier domain, we extract the in-plane stress and demonstrate that the hydro-elastic waves are exclusively governed by hoop stress. This provides a controllable macroscopic analogue of liquid drops characterised by an effective surface tension, allowing the capsules to be modeled as large-scale "elasto-drops" with an inflation and thickness tunable effective surface tension. In this limit, bending stiffness is negligible over the experimentally relevant wavelengths, so that the shell dynamics are governed primarily by in-plane tension. Our work demonstrates that elasto-drops serve as a robust model system for parametric studies of large-scale analogues of liquid drops with experimentally adjustable surface tension.