Mechanical properties of C60 thin films at the air-water interface

Understanding the mechanical properties of ultra-thin films formed by the self-assembly of molecules/nanoparticles/colloids at fluid-fluid interfaces is central to many technological applications. Here, we have carried out interfacial rheology measurements to systematically investigate the concentration dependent viscoelastic response of 2D films of Fullerene C60 at the air-water interface. With increasing C60 concentration, amplitude sweep measurements show that the films undergo a transition from viscoelastic liquid-like to viscoelastic solid-like behaviour. Interestingly, for high C60 concentrations's, the loss modulus G′′ reaches a maximum before the onset of power-law shear-thinning in G′, the storage modulus, and G′′. The power-law exponents have a ratio 2. This response is typical of systems that show soft glassy behaviour. We also observe a power-law increase in G′ and G′′ at low frequencies in the frequency response measurements and a transition from Newtonian to shear-thinning behaviour, with increasing shear rate, in steady shear measurements. Our results are in qualitative agreement with the phenomenological soft glassy rheology model.