Similar to the human skin, a monolayer of packed particles capillary bound to a liquid interface wrinkles when subjected to compressive stress. The induced wrinkles absorb the applied stress and do not disappear unless the stress is removed. Experimental and theoretical investigations of wrinkle formation typically concern flat particle monolayers subjected to uniaxial stress. In this work, we extend the results on wrinkling of particle-covered interfaces to the investigation of mechanical properties of particle films on a curved interface, that is, we study particle shells formed on droplets and subjected to hoop stress. Opposed to flat particle layers where liquid buoyancy alone acts as the effective stiffness, the mechanical properties of particle layers on small droplets are also affected by the surface curvature. We show here that this leads to formation of wrinkles with different characteristic wavelengths compared to those found at flat interfaces. Our experimental results also reveal that the wrinkle wavelength of particle shells is proportional to the square root of particle size and the size of the droplets on which the shells are formed. Wrinkling of particle layers composed of microparticles with diameters ranging from around 1-100 μm was induced using a novel approach combining electrodeformation and electrohydrodynamic flows. We demonstrate that our contactless approach for studying the mechanical properties of particle shells enables estimation of elasticity, particle film thickness, and bending stiffness of particle shells. The proposed approach is insensitive to both particle coverage and electric field strength. In addition, it enables manipulation of particle packing that is intimately linked with formation of wrinkling patterns. With a wide range of applications depending on accurate mechanical properties (e.g., drug-delivery capsules to self-healing materials), this work provides a valuable method to characterize the mechanical properties of shells and tailor their surface properties (i.e., permeability and roughness).
Keywords: Janus shells; bending stiffness; droplets; elasticity; electric field; particle monolayer; particle shells; wrinkling.