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 microscope, has been used to investigate confined water films under shear. A cylindrically tapered glass probe was mounted perpendicularly to the sample 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 oscillation amplitude and relative phase lag, at different tip-sample separations, exhibited a clear step-like behavior. The periodicity, recorded over several curves, ranged between 2.4 and 2.9 Angstrom, which is similar to the diameter of the water molecule. The in-phase (elastic) and the out-of-phase (viscous) stress response of the confined water film was evaluated (from the experimental data) by assuming a linear viscoelastic behavior. Finally, by modeling the water film with the Maxwell mechanical model, the values for the shear viscosity and shear rigidity were obtained. (C) 2001 American Institute of Physics.