Department of Mechanical Engineering, Imperial College London, SW7 2AZ, UK.
St. John's College, Oxford Thermofluids Institute, Department of Engineering Science, University of Oxford, Oxford OX2 0ES, UK.
Phys Chem Chem Phys. 2018 Dec 12;20(48):30267-30280. doi: 10.1039/c8cp05155k.
The viscosity of liquids is a strong function of pressure. While viscosity is relatively easy to measure at low pressure, high-pressure rheology presents significant experimental challenges. As a result, rheological models are often used to extrapolate viscosity from low pressure measurements to higher pressures. Techniques to obtain data over a wide range of pressures and shear rates, as well as understanding the validity and limitations of methods to fill the gaps in the available data, are therefore of crucial practical and theoretical importance. This work examines the viscosity of polyalphaolefin (PAO) by combining average global area averaged measurements at high pressure and local molecular viscosity measurements at moderate pressures. Viscosities spanning five orders of magnitude are examined at pressures up to 720 MPa. High pressure results were obtained with friction measurements where the fluid is sheared between two surfaces in a loaded point contact. The local molecular microviscosity at medium and low pressures was measured by applying a technique based on fluorescence anisotropy, which probes the rotational motion of dye molecules in a nanoscale film under shear. Both sets of measurements are taken in the same configuration, an elastohydrodynamic (EHD) contact. This is the first set of quantitative local viscosity measurements that have been verified against both friction and high pressure rheometry measurements. Commonly used rheological models were compared to experimental results. Our work shows that fluorescence anisotropy and friction measurements can be used to determine the viscosity of liquids over a wide range of conditions from a single experimental setup. The results obtained match results from low- and high-pressure rheometry for PAO. The importance of correcting friction data for pressure non-uniformity, temperature and shear thinning is also highlighted.
液体的粘度是压力的强函数。虽然在低压力下粘度相对容易测量，但高压流变学提出了重大的实验挑战。因此，通常使用流变学模型将粘度从低压力测量值外推到更高的压力。因此，获得在宽压力和剪切速率范围内的数据的技术，以及了解填补可用数据中的空白的方法的有效性和局限性，对于实际和理论都具有至关重要的意义。这项工作通过将高压下的平均全局平均测量值与中等压力下的局部分子粘度测量值相结合，来研究聚α烯烃（PAO）的粘度。在高达 720 MPa 的压力下，研究了跨越五个数量级的粘度。通过摩擦测量获得高压结果，其中在负载点接触处将流体在两个表面之间剪切。通过应用基于荧光各向异性的技术来测量中低压力下的局部分子微粘度，该技术在剪切下探测染料分子在纳米级薄膜中的旋转运动。这两套测量都是在相同的配置中进行的，即弹性流体动力（EHD）接触。这是第一组已通过摩擦和高压流变测量验证的定量局部粘度测量值。比较了常用的流变学模型与实验结果。我们的工作表明，荧光各向异性和摩擦测量可以用于从单个实验装置确定在广泛的条件下液体的粘度。对于 PAO，获得的结果与低和高压流变学的结果相匹配。还强调了对摩擦数据进行压力不均匀性、温度和剪切稀化校正的重要性。
