Wrinkle-Fold Coexistence and Stress Propagation in Nanoparticles at the Oil-Water Interface*

We present the behavior of nanoparticle surfactant monolayers, assembled at the oil-water interface, in response to uniaxial compression by a movable barrier. These systems are formed by the binding of nanoparticles and polymers at the oil-water interface. Upon compression, the films buckle with a characteristic wavelength and a spatially decaying amplitude. Increasing strain leads to the presence of folds near the barrier, and wrinkle formation further away from it. The size of the folded region grows with increasing strain, whilst the wrinkled region exists over a characteristic lengthscale. Remarkably, when compression ceases, the folded region expands and wrinkles propagate away from the compressing barrier. This relaxation occurs on the timescale of hours to days, depending on the size of the particles that make up the assembly. We combine theory and experiment to characterize this behavior using a range of model systems, including particles of differing sizes and chemical constituents.