Hydrodynamic force on a microparticle approaching a wall in a nanoparticle dispersion: observation of a separation-dependent effective viscosity.
Hydrodynamic force on a microparticle approaching a wall in a nanoparticle dispersion: observation of a separation-dependent effective viscosity.
复制标题
纳米粒子分散体中接近壁的微粒上的水动力：观察与分离相关的有效粘度。
作者：
G. James;J. Walz
使用胶体探针原子力显微镜测量了直径为30μm的二氧化硅颗粒在直径为22 nm(6%或8%)的水分散体系中朝向或远离二氧化硅平板移动时所受到的流体动力。将测量到的力与使用考克斯和布伦纳(Cox，R.G.；Brenner，H.Chem)的著名表达式所作的预测进行比较。英语。科学1967，22,1753-1777)假设恒定粘度等于主体分散体的粘度，在较小的颗粒-板分离距离(例如，500 nm)下，测量的阻力明显小于预测的阻力。Bhattacharya和Blawzdziewicz(Bhattacharya，S.；Blawzdziewicz，J.J.Chem.Phys.2008,128,214704)预测，在分散的纳米颗粒的溶液中，表征颗粒上流体动力的有效粘度应该从接触时的溶剂粘度变化到大分离时的本体分散体粘度。通过调整Cox和Brenner表达式中的粘度，使预测的流体动力与测量的流体动力(即有效粘度)相匹配，得到了反映这些精确特性的曲线。有效粘度分布不是颗粒速度的函数，有效粘度的变化延伸到长达2μm的分离距离(几乎是纳米颗粒硬直径的100倍)。这些结果表明，在典型的胶体力范围内(约100 nm)，这种体系中颗粒运动的动力学由溶剂的粘度决定，而不是由主体分散体的粘度决定。
Colloid probe atomic force microscopy was used to measure the hydrodynamic force exerted on a 30-μm-diameter silica particle being moved toward or away from a silica plate in aqueous dispersions of 22-nm-diameter silica nanoparticles (6 or 8 vol %). Upon comparing the measured force to predictions made using the well-known expression of Cox and Brenner (Cox, R. G.; Brenner, H. Chem. Eng. Sci.1967, 22, 1753-1777) assuming a constant viscosity equal to that of the bulk dispersion, the measured drag force was found to become significantly less than that predicted at smaller particle-plate separation distances (e.g., <500 nm). A recent theoretical paper by Bhattacharya and Blawzdziewicz (Bhattacharya, S.; Blawzdziewicz, J. J. Chem. Phys.2008, 128, 214704) predicted that in a solution of dispersed nanoparticles the effective viscosity characterizing the hydrodynamic force on the particle should vary from that of the solvent at contact to that of the bulk dispersion at large separations. By adjusting the viscosity in the Cox and Brenner expression to make the predicted hydrodynamic force match that measured (i.e., the effective viscosity), a curve showing these exact characteristics was obtained. The effective viscosity profile was not a function of particle speed, and changes in the effective viscosity extended to separation distances of as large as 2 μm (nearly 100 times the hard diameter of the nanoparticles). These results suggest that in the range of typical colloidal forces (on the order of 100 nm), the dynamics of particle motion in such systems are determined by the viscosity of the solvent and not that of the bulk dispersion.