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

Bongsu Kim, Soyoung Kwon, Manhee Lee, QHwan Kim, Sangmin An, and Wonho Jhe

Viscoelastic fluids exhibit rheological nonlinearity at a high shear
rate. Although typical nonlinear effects, shear thinning and shear
thickening, have been usually understood by variation of intrinsic
quantities such as viscosity, one still requires a better understanding
of the microscopic origins, currently under debate, especially
on the shear-thickening mechanism. We present accurate measurements
of shear stress in the bound hydration water layer
using noncontact dynamic force microscopy. We find shear
thickening occurs above ∼106 s−1 shear rate beyond 0.3-nm layer
thickness, which is attributed to the nonviscous, elasticityassociated
fluidic instability via fluctuation correlation. Such a nonlinear
fluidic transition is observed due to the long relaxation time
(∼10−6 s) of water available in the nanoconfined hydration layer,
which indicates the onset of elastic turbulence at nanoscale, elucidating
the interplay between relaxation and shear motion, which also
indicates the onset of elastic turbulence at nanoscale above a universal
shear velocity of ∼1mm=s. This extensive layer-by-layer control paves
the way for fundamental studies of nonlinear nanorheology and
nanoscale hydrodynamics, as well as provides novel insights on viscoelastic
dynamics of interfacial water.