Probing mechanical properties of particle shells formed on droplets by electric field-induced wrinkling

Fabricating curved monolayers that mimic the properties from natural shells can help researchers to develop lighter, stronger and more flexible materials, better drug delivery and encapsulation systems, etc. Here we investigate synthetically made particle shells formed on droplets in a bulk fluid, in context of their mechanical properties. We used induced compressive stress on the particle shells by applying electric fields. In response to the stress, the particle monolayers folded and formed wrinkles with characteristic wavelengths. By deriving a simple model for particle shell wrinkling, we used these wavelengths to estimate the Young modulus and bending stiffness of the shells. Our results indicate that the elasticity of particle shells decreases with particle and shell size. We also show that deformation cycles induced by electric fields can be used to increase particle packing of monolayers on droplets and reduce wrinkle formation. These results suggest that in addition to probing mechanical properties, our approach can also be used to tailor the surface properties of shells, i.e. their permeability and roughness.