Hoda Nazish, Kumar Satish
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Chem Phys. 2007 Dec 21;127(23):234902. doi: 10.1063/1.2806187.
The adsorption of single polyelectrolyte molecules in shear flow is studied using Brownian dynamics simulations with hydrodynamic interaction (HI). Simulations are performed with bead-rod and bead-spring chains, and electrostatic interactions are incorporated through a screened Coulombic potential with excluded volume accounted for by the repulsive part of a Lennard-Jones potential. A correction to the Rotne-Prager-Yamakawa tensor is derived that accounts for the presence of a planar wall. The simulations show that migration away from an uncharged wall, which is due to bead-wall HI, is enhanced by increases in the strength of flow and intrachain electrostatic repulsion, consistent with kinetic theory predictions. When the wall and polyelectrolyte are oppositely charged, chain behavior depends on the strength of electrostatic screening. For strong screening, chains get depleted from a region close to the wall and the thickness of this depletion layer scales as N(1/3)Wi(2/3) at high Wi, where N is the chain length and Wi is the Weissenberg number. At intermediate screening, bead-wall electrostatic attraction competes with bead-wall HI, and it is found that there is a critical Weissenberg number for desorption which scales as N(-1/2)kappa(-3)(l(B)|sigmaq|)(3/2), where kappa is the inverse screening length, l(B) is the Bjerrum length, sigma is the surface charge density, and q is the bead charge. When the screening is weak, adsorbed chains are observed to align in the vorticity direction at low shear rates due to the effects of repulsive intramolecular interactions. At higher shear rates, the chains align in the flow direction. The simulation method and results of this work are expected to be useful for a number of applications in biophysics and materials science in which polyelectrolyte adsorption plays a key role.
利用包含流体动力学相互作用（HI）的布朗动力学模拟研究了剪切流中单个聚电解质分子的吸附。采用珠 - 杆链和珠 - 弹簧链进行模拟，并通过屏蔽库仑势纳入静电相互作用，同时用 Lennard - Jones 势的排斥部分考虑排除体积。推导了 Rotne - Prager - Yamakawa 张量的修正，该修正考虑了平面壁的存在。模拟结果表明，由于珠子与壁的流体动力学相互作用导致的远离不带电壁的迁移，会随着流动强度和链内静电排斥强度的增加而增强，这与动力学理论预测一致。当壁和聚电解质带相反电荷时，链的行为取决于静电屏蔽的强度。对于强屏蔽，链会从靠近壁的区域耗尽，并且在高魏森贝格数（Wi）下，这个耗尽层的厚度按 N(1/3)Wi(2/3) 缩放，其中 N 是链长，Wi 是魏森贝格数。在中等屏蔽情况下，珠子与壁的静电吸引与珠子与壁的流体动力学相互作用相互竞争，并且发现存在一个用于解吸的临界魏森贝格数，其缩放比例为 N(-1/2)kappa(-3)(l(B)|sigmaq|)(3/2)，其中 kappa 是反屏蔽长度，l(B) 是 Bjerrum 长度，sigma 是表面电荷密度，q 是珠子电荷。当屏蔽较弱时，可以观察到在低剪切速率下，由于分子内排斥相互作用的影响，吸附的链会在涡度方向排列。在较高剪切速率下，链会在流动方向排列。这项工作的模拟方法和结果预计将对生物物理学和材料科学中的许多应用有用，其中聚电解质吸附起着关键作用。
