(161f) Active Interfacial Microrheology of Protein Solutions

We present measurements of the surface shear viscosity of an albumin solution using a new nano-needle rheometer design, based on active microrheology techniques. The reorientation dynamics of Nickel nanorods(length=3µm, diameter =300 nm) placed at an air/albumin solution interface, in the presence of an externally applied magnetic field, is directly related to the surface shear viscosity of the monolayer. We present the surface viscosity of albumin monolayer both as a function of a range of bulk concentrations (0.02 mg/ml - 2 mg/ml) of albumin as well as a function of time. Our results indicate that the surface viscosity of albumin attains a near saturation above the bulk saturation concentration (0.1 mg/ml). However, it continues to increase by orders of magnitude (10-9 Ns/m-10-6 Ns/m) with time long after attaining the equilibrium surface pressure. This dramatic change in the surface viscosity of albumin with aging is a direct indication of the formation of complex protein networks at the interface which is likely a result of structural evolution of the albumin molecules with time. In addition to measuring the surface viscosity of a simple 2-d viscous system with the same order of precision as passive one and two particle microrheology techniques (10-9 Ns/m) (Prasad et al PRL 2006), this device is also capable of measuring surface viscosities of more complex systems including higher concentrations of albumin forming denser or more complex interfacial networks. This has grave implications in biology. Inhibition of lung surfactants by serum molecules is believed to be a consequence of lung injury resulting in adult respiratory distress syndrome. Surface viscosity measurements of the albumin molecules at the interface may lead to more insights into the mechanisms of inhibition reversal and re-spreading of lung surfactant in the presence of serum proteins.