Measuring the nanomechanics of nanoconfined water layers

Nanoconfined water has been the subject of special interest due to its applications in various fields such as biology, geology, medicine, and engineering tribology. While there is a general agreement on the layering of water molecules along atomically smooth surfaces, the behavior and properties of nanoconfined water is still poorly understood. A significant controversy exists whether there is a phase transformation imposed by confinement. We have measured the stiffness and damping coefficient of nanoconfined water using a small amplitude (0.5-1 A) atomic force microscope. The results were analyzed with the help of two viscoelastic models, the Kelvin model and the Maxwell model. The stiffness and damping coefficient oscillate with period 2.7 +/- 0.8 A below 1 nm thickness of the water film. The retardation time and the relaxation time were measured as a function of both the strain rate and the film thickness. Above a critical strain rate, the retardation time shows valleys, and the relaxation time shows peaks commensurate with the stiffness peaks in the oscillatory profile. We call this phenomenon the Dynamic Solidification.;The relaxation time was also measured as a function of the concentration of sodium chloride. It was found that the critical strain rate for the dynamic solidification is a function of the strength of the molarity of the solution. We found that above a critical sodium chloride concentration, water shows the dynamic solidification, even at significantly lower strain rates.;To standardize the AFM measurements, we measured the effects of the tip size on the stiffness and damping of a nanoconfined model liquid tetrakis-2-ethyhexoxysilane (TEHOS) by using a number of tips of different sizes. We found that the stiffness and damping coefficient of the liquid increase linearly with the tip-size. We also measured an effective elastic modulus of the nanoconfined liquid and found it to be independent on the tip-size.