Particles moving close to a planar wall undergo a considerable decrease in velocity, because of the viscous forces of the underlying fluid. The previous studies on electrophoretic migration very close to charged walls indicate an increase in the electrophoretic velocity as the particle approaches the wall, because of an increase in the electric field in the narrow lubrication region. However, analysis of the electrohydrodynamic forces acting on a particle in the vicinity of a planar wall/electrode has been conducted only for the case of a rigid particle. Soft particles or fuzzy particles that appear in biological systems consist of a polymer layer covering a rigid colloid, and the polymer layer upon compression leads to steric repulsion forces, due to its brushlike structure. In this study, the electrohydrodynamic forces acting on a soft particle deposited in the vicinity of an nonhomogeneously charged electrode with nonhomogeneous slip is considered. The fluid flow within the polymer layer of the soft particle follows a modified Brinkman equation that intrinsically depends on the polymer brush structure. The fluid flow within the thin lubrication region, which consists of the ordered porous layer and is bounded below the electrode wall, is modeled by the lubrication approximation. An added assumption of the large porosity of the polymer brush structure helps in arriving at an analytical approximation. The lubrication flow in the confinement and the electric field generated from the constant current emitting electrode leads to the development of nonzero forces and torques. The forces and the torques depend on the separation distance, the jump in the slip conditions, the nonhomogenous flow rates, and the porosity of the brush structure. The greater the compression of the polymer layer, the steric force, which is purely dependent on the compression, is found to increase, and along with it, the electrohydrodynamic lift forces are also found to increase. In cases of extreme compression, when D → 0, the magnitude of the forces goes to ∞, due to the singular nature of the forces. The analysis shows that forces and torques acting on a bare colloid and a soft colloid are dissimilar, which can be harnessed in the separation process of colloids with dissimilar structures.

中文翻译：

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靠近不均匀带电的扁平电极的刚性芯圆柱形软粒子上的电动流体动力


由于底层流体的粘性力，靠近平面壁移动的粒子的速度会大大降低。先前对非常靠近带电壁的电泳迁移的研究表明，由于狭窄润滑区域中电场的增加，当颗粒接近壁时，电泳速度会增加。然而，仅针对刚性颗粒的情况，对作用在平面壁/电极附近颗粒上的流体动力的分析进行了分析。生物系统中出现的软颗粒或模糊颗粒由覆盖刚性胶体的聚合物层组成，由于其刷状结构，聚合物层在压缩时会导致空间排斥力。在这项研究中，考虑了作用在沉积在具有非均匀滑移的非均匀充电电极附近的软颗粒上的电动流体动力。软颗粒聚合物层内的流体流动遵循改进的 Brinkman 方程，该方程本质上取决于聚合物刷结构。薄润滑区域（由有序的多孔层组成，位于电极壁下方）内的流体流动通过润滑近似建模。聚合物刷状结构孔隙率大的额外假设有助于得出解析近似值。约束中的润滑流和恒流发射电极产生的电场导致非零力和扭矩的产生。力和扭矩取决于分离距离、滑移条件下的跳跃、非均匀流速以及刷结构的孔隙率。 聚合物层的压缩越大，完全取决于压缩的空间位阻力就会增加，随之而来的是，电动流体动力升力也会增加。在极端压缩的情况下，当 D → 0 时，由于力的奇异性质，力的大小变为 ∞。分析表明，作用在裸胶体和软胶体上的力和扭矩是不同的，这可以在具有不同结构的胶体的分离过程中加以利用。