Characterization of water confined between silica surfaces using the resonance shear measurement

We performed the resonance shear measurement (RSM) for evaluating the properties of water confined between silica surfaces with and without water vapor plasma treatment, which was used to increase the density of the silanol groups on the surfaces. We compared the properties of the confined water, such as viscosity and lubricity, by controlling the surface separation at a 0.1 nm resolution. The observed resonance curves for water between the plasma-treated and untreated silica surfaces showed the following results: (1) The viscosity of the water confined between the plasma-treated silica surfaces increased due to water structuring at separations less than 3 nm, while the value for the water between the untreated silica surfaces was 8 nm. (2) The water confined between the plasma-treated surfaces could maintain lubricity under the normal pressure of more than 1.7 MPa; however, the water confined between the untreated surfaces lost lubricity under the normal pressure of more than 0.4 MPa. To discuss these properties in terms of water structures on the silica surfaces, we performed sum frequency generation (SFG) vibrational spectroscopy for water on the plasma-treated and untreated silica surfaces. The main peak of SFG spectra for the water on the plasma-treated silica was around 3200 cm^-1, and that for water on the untreated silica was around 3400 cm^-1, indicating that the hydrogen bonding network of the water on the plasma-treated silica surface was stronger than that on the untreated one due to the higher silanol density. The strongly networked water could exhibit higher lubricity with the increased silanol density.