Kim Bongsu, Kwon Soyoung, Lee Manhee, Kim Q Hwan, An Sangmin, Jhe Wonho
Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University, Seoul 151-747, Republic of Korea.
Department of Physics and Astronomy, Institute of Applied Physics, Seoul National University, Seoul 151-747, Republic of Korea
Proc Natl Acad Sci U S A. 2015 Dec 22;112(51):15619-23. doi: 10.1073/pnas.1515033112. Epub 2015 Dec 7.
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 ∼ 10(6) s(-1) shear rate beyond 0.3-nm layer thickness, which is attributed to the nonviscous, elasticity-associated 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 ∼ 1 mm/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.
粘弹性流体在高剪切速率下表现出流变非线性。尽管典型的非线性效应，即剪切变稀和剪切增稠，通常已通过诸如粘度等内在量的变化得到理解，但人们仍需要更好地理解目前仍在争论的微观起源，特别是关于剪切增稠机制。我们使用非接触式动态力显微镜对束缚水化水层中的剪切应力进行了精确测量。我们发现，在剪切速率超过10(6) s(-1)且层厚度超过0.3纳米时会发生剪切增稠，这归因于通过涨落关联产生的非粘性、与弹性相关的流体不稳定性。由于纳米受限水化层中水分子的长弛豫时间（约10(-6) s），观察到了这种非线性流体转变，这表明在纳米尺度上弹性湍流的开始，阐明了弛豫与剪切运动之间的相互作用，这也表明在约1毫米/秒的通用剪切速度以上纳米尺度上弹性湍流的开始。这种广泛的逐层控制为非线性纳米流变学和纳米尺度流体动力学的基础研究铺平了道路，同时也为界面水的粘弹性动力学提供了新的见解。
