Layer-resolved relaxation dynamics of confined water analyzed through subnanometer shear measurement

The relaxation dynamics of thin water layers confined between hydrophilic surfaces was analyzed through shear relaxation measurement at room temperature. With the well-characterized shear motion of atomic force microscopy (AFM) probe with subnanometer amplitude, the viscoelastic response of water confined between hydrophilic substrate and probe was measured. Obtained data showed a stepwise variation with a typical spacing of water monolayer thickness. Relaxation time for water monolayer and bilayer derived from viscoelasticity exhibited a marked decrease with strain rate. A slightly faster relaxation of the bilayer than that of the monolayer was observed. The lower limit for the intrinsic thermal relaxation time was roughly evaluated to be as long as 0.2 ms. The possibility of a glassy state at room temperature is discussed considering the effects of intramolecular hydrogen bonds, of the hydrophilicity of the confining surfaces, and of the compressive force