(78e) Tensiometry, Interfacial Rheology, and Strain Field Mapping of Complex Interfaces

Adsorbed surfactant, proteins, and macromolecules on the interfaces form thin viscoelastic layers with complex structures. By developing a new tensiometry and interfacial rheology approach, we have studied the complex material properties of adsorbed protein layers on the air-water interfaces. To study the rheology of a such complex systems, we have developed a new instrument and approach to allow the formation and manipulation of fluid interfacial systems, allowing simultaneous tensiometry to measure surface pressure and microrheology to measure the surface viscoelasticity. This new apparatus fits on the stage of a conventional inverted microscope, and allows compression and stretching of the interfaces at a constant bulk volume. The new approach can measure the interfacial tension with accuracy comparable to the Pendant Drop Apparatus and Wilhelmy plates. We study adsorbed layer of β-lactoglobulin on the air-water interfaces at different surface pressure. We extended two-point microrheology to characterize the viscoelastic response of the interfaces with a wide range of the shear modulus (10^-9-10^-1Nm^-1). At high surface pressure, the power law frequency dependence of the shear modulus reveals the soft glassy response of the protein-laden interfaces. While surface rheological approaches provide important insights into properties of this complex system, they do not reveal the complex flow fields induced by forces or imposed stresses. To understand complex flow fields established on fluids interface, which depend on interface viscoelasticity, (in)compressibility, and hydrodynamic coupling with bulk fluids, we measure 2d displacement fields induced by the thermal motion of passive particles in interfacial films. This measurement is compared to calculated displacement fields for hydrodynamic modes permitted in interfacial layers as a function of interface compressibility and dilatational viscosity.