带电微粒在滤膜附近的定向运动

Directional motion of charged particles near filter membrane

滤膜在过滤和分离领域具有非常广泛的使用,但滤膜对溶液中的组分具有吸引或排斥作用,导致粒子产生定向运动,进而在膜附近产生粒子富集或“粒子禁区”等截然不同的效应,目前对此认识尚不清楚,粒子浓度分布不能实时准确测量是其中最大的困难.本文以表面带负电荷的胶体粒子为模型物质,利用胶体粒子结晶后粒子浓度可以实时原位测量的优势,研究了纤维素滤膜对粒子运动行为的影响规律.研究结果发现带电微粒会自发地在滤膜表面产生富集,滤膜中微量阴阳离子释放导致的扩散泳效应是微粒朝向滤膜产生定向运动的主要原因,基于扩散泳和粒子扩散两种机制构建了粒子运动方程和模型,数值计算结果和实验结果定性一致.此外,粒子除了朝向滤膜的纵向运动之外,壁面电渗流和横向扩散还会使粒子朝向壁面产生迁移,导致管壁粒子数的增多.

Membrane has many applications in the fields of filtration and separation,but due to the attraction or repulsion exerted by the membrane,the particles will experience a directional motion.As a result,two totally opposite effects,i.e.particle enrichment and exclusion zone,take place in the vicinity of the membrane,and the underlying reason is still not clear.In this work,colloidal particles with negative surface charge are used as a model substance,with the advantages of monitoring the particle concentration in a real time and in situ way,to investigate the influence of cellulose membrane on the movement of particles.The experimental results show that the particles are enriched in the vicinity of the membrane.The diffusiophoresis effect originating from tiny number of ions released by the film is the main reason of the directional movement of the charged particles.Based on the two mechanisms of diffusiophoresis and diffusion,we construct a model and make relevant numerical calculation,and the numerical results are qualitatively consistent with the experimental results.Moreover,in addition to the longitudinal motion of the particles towards the filter membrane,diffusio-osmotic flow and particles lateral diffusion also result in the migration of particles towards the container wall,and further increasing the particle number near the wall.