Duffadar Ranojoy D, Davis Jeffrey M
Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA.
J Colloid Interface Sci. 2008 Oct 1;326(1):18-27. doi: 10.1016/j.jcis.2008.07.004. Epub 2008 Jul 9.
The dynamic adhesion behavior of micrometer-scale silica particles is investigated numerically for a low Reynolds number shear flow over a planar collecting wall with randomly distributed electrostatic heterogeneity at the 10-nanometer scale. The hydrodynamic forces and torques on a particle are coupled to spatially varying colloidal interactions between the particle and wall. Contact and frictional forces are included in the force and torque balances to capture particle skipping, rolling, and arrest. These dynamic adhesion signatures are consistent with experimental results and are reminiscent of motion signatures observed in cell adhesion under flowing conditions, although for the synthetic system the particle-wall interactions are controlled by colloidal forces rather than physical bonds between cells and a functionalized surface. As the fraction of the surface (Theta) covered by the cationic patches is increased from zero, particle behavior sequentially transitions from no contact with the surface to skipping, rolling, and arrest, with the threshold patch density for adhesion (Theta(crit)) always greater than zero and in quantitative agreement with experimental results. The ionic strength of the flowing solution determines the extent of the electrostatic interactions and can be used to tune selectively the dynamic adhesion behavior by modulating two competing effects. The extent of electrostatic interactions in the plane of the wall, or electrostatic zone of influence, governs the importance of spatial fluctuations in the cationic patch density and thus determines if flowing particles contact the wall. The distance these interactions extend into solution normal to the wall determines the strength of the particle-wall attraction, which governs the transition from skipping and rolling to arrest. The influence of Theta, particle size, Debye length, and shear rate is quantified through the construction of adhesion regime diagrams, which delineate the regions in parameter space that give rise to different dynamic adhesion signatures and illustrate selective adhesion based on particle size or curvature. The results of this study are suggestive of novel ways to control particle-wall interactions using randomly distributed surface heterogeneity.
针对低雷诺数剪切流在具有10纳米尺度随机分布静电不均匀性的平面收集壁上流动的情况，对微米级二氧化硅颗粒的动态粘附行为进行了数值研究。颗粒上的流体动力和扭矩与颗粒和壁之间空间变化的胶体相互作用相耦合。在力和扭矩平衡中纳入了接触力和摩擦力，以捕捉颗粒的跳跃、滚动和停滞。这些动态粘附特征与实验结果一致，并且让人联想到在流动条件下细胞粘附中观察到的运动特征，尽管对于合成系统而言，颗粒与壁之间的相互作用是由胶体力而非细胞与功能化表面之间的物理键所控制。随着阳离子斑块覆盖的表面分数（Θ）从零增加，颗粒行为依次从不与表面接触转变为跳跃、滚动和停滞，粘附的阈值斑块密度（Θcrit）始终大于零，并且在定量上与实验结果一致。流动溶液的离子强度决定了静电相互作用的程度，并且可以通过调节两种相互竞争的效应来选择性地调节动态粘附行为。壁平面内静电相互作用的程度，即静电影响区域，决定了阳离子斑块密度空间波动的重要性，从而决定了流动颗粒是否与壁接触。这些相互作用垂直于壁延伸到溶液中的距离决定了颗粒与壁之间吸引力的强度，该强度控制着从跳跃和滚动到停滞的转变。通过构建粘附状态图来量化Θ、颗粒尺寸、德拜长度和剪切速率的影响，该图描绘了参数空间中产生不同动态粘附特征的区域，并说明了基于颗粒尺寸或曲率的选择性粘附。本研究结果提示了利用随机分布的表面不均匀性来控制颗粒与壁之间相互作用的新方法。
