Observation of molecular layering in a confined water film and study of the layers viscoelastic properties

A transverse dynamic force microscope, more commonly known as shear force m icroscope, has been used to investigate confined water films under shear. A cylindrically tapered glass probe was mounted perpendicularly to the sampl e surface. Pure water was confined between the probe and a freshly cleaved mica surface and a sinusoidal shear strain was applied by setting the probe into transverse oscillation. Repeated measurements of the probe oscillatio n amplitude and relative phase lag, at different tip-sample separations, ex hibited a clear step-like behavior. The periodicity, recorded over several curves, ranged between 2.4 and 2.9 Angstrom, which is similar to the diamet er of the water molecule. The in-phase (elastic) and the out-of-phase (visc ous) stress response of the confined water film was evaluated (from the exp erimental data) by assuming a linear viscoelastic behavior. Finally, by mod eling the water film with the Maxwell mechanical model, the values for the shear viscosity and shear rigidity were obtained. (C) 2001 American Institu te of Physics.