Probing nonlinear rheology layer-by-layer in interfacial hydration water

Area of Science:

• Physics
• Chemistry
• Materials Science

Background:

• Viscoelastic fluids show nonlinear rheology at high shear rates.
• Microscopic origins of shear thickening, particularly in water, remain debated.
• Understanding nanoconfined water behavior is crucial for interfacial phenomena.

Purpose of the Study:

• Investigate shear stress and rheological nonlinearity in bound hydration water layers.
• Elucidate the microscopic mechanism behind shear thickening in nanoconfined water.
• Explore the onset of elastic turbulence at the nanoscale.

Main Methods:

• Accurate shear stress measurements using noncontact dynamic force microscopy (DFM).
• Analysis of bound hydration water layers at varying thicknesses.
• Fluctuation correlation analysis to understand fluidic instability.

Main Results:

• Shear thickening observed above ~10^6 s^-1 shear rate for water layers thicker than 0.3 nm.
• Attribution of shear thickening to nonviscous, elasticity-associated fluidic instability.
• Identification of long water relaxation times (~10^-6 s) in nanoconfined layers.

Conclusions:

• Nanoscale elastic turbulence occurs in hydration layers due to long relaxation times and shear motion.
• A universal shear velocity of ~1 mm/s marks the onset of elastic turbulence.
• Findings offer insights into nonlinear nanorheology, hydrodynamics, and interfacial water dynamics.