Buckled Particle Films

Here are two Laser Scanning Confocal Microscope images of a buckled monolayer of polystyrene particles. What should be clear is that there is a repeating pattern to the buckles (a wavelength). This is in remarkable contrast to what is expected - conventional elastic instabilities of this sort (known as wrinkles) require a bending modulus in the film. A traditional bending modulus cannot occur in the particle film, as the particles are free to rotate. The physics is about the 'torque chain', the way each sphere's rotation is matched by its neighbours. We are working on a very simple model, but are struggling with the difference between a well ordered hexagonally packed crystal of monodisperse spheres, and the disorder in the polydisperse sample.