Electrophoresis of a soft sphere in a necked cylindrical nanopore

The influence of boundary shape on electrophoresis is modeled by considering a soft spherical particle comprising a positively charged rigid core and an uncharged membrane layer on the axis of a necked cylindrical pore with its throat positively charged. The presence of the throat makes the associated flow and electric fields nonuniform, yielding several interesting behaviors. In general, the reduction in the cross-section area of the pore intensifies the local electric field and, therefore, accelerates the particle. It also makes the ionic distribution nonuniform, and the electric field induced accelerates the particle. The maximum mobility occurs at the center of a throat, and the higher the charge density of the throat the larger the ratio of maximum mobility/mobility far away from the throat. This result is informative for the design of separation devices having variable cross sectional area.