Interfacial Rheology of an Ultrathin Nanocrystalline Film Formed at the Liquid/Liquid Interface

We report the interfacial properties of monolayers of Ag nanoparticles 10-50 nm in diameter formed at the toluene-water interface under steady as well as oscillatory shear. Strain amplitude sweep measurements carried out on the film reveal a shear thickening peak in the loss moduli $(G^{\prime}^{\prime})$ at large amplitudes followed by a power law decay of the storage $(G^{\prime})$ and loss moduli with exponents in the ratio 2:1. In the frequency sweep measurements at low frequencies, the storage modulus remains nearly independent of the angular frequency, whereas $G^{\prime}^{\prime}$ reveals a power law dependence with a negative slope, a behavior reminiscent of soft glassy systems. Under steady shear, a finite yield stress is observed in the limit of shear rate $\dot{\gamma}$ going to zero. However, for $\dot{\gamma} > 1 s^{-1}$, the shear stress increases gradually. In addition, a significant deviation from the Cox-Merz rule confirms that the monolayer of Ag nanoparticles at the toluene-water interface forms a soft two-dimensional colloidal glass