title: Role of Particle Focusing in Resistive-Pulse Technique: Direction-Dependent Velocity in Micropores.
abstract: Passage time through single micropores is an important parameter used to quantify the surface charge and zeta potential of particles. In the resistive-pulse technique, the measured time of pressure- or electric-field-induced translocation is assumed to be direction independent. This assumption is supported by the low velocities of the particles and the supporting fluid such that the transport reversibility known for Stokes flow is expected to apply. In this article, we present examples of micropores in which passage time of ∼400 nm diameter particles becomes direction-dependent; that is, the particles' translocation times from left to right and right to left are different. These pores are characterized by an undulating inner diameter such that at least one wider zone called a cavity separates two narrower regions of different lengths. We propose that the observed direction-dependence of the translocation velocity is caused by an asymmetric efficiency of particle focusing toward the pore axis, which leads to a direction-dependent set of particle trajectories. The reported pores present the simplest system in which time-broken symmetry has been observed. The results are of importance for sensing of particles and molecules by the resistive-pulse technique since pores used for detection are often characterized by finite roughness or noncylindrical shape. This article also points to the role of particle focusing in the magnitude and distribution of the translocation times.
authors.affiliation: Department of Physics and Astronomy, University of California , Irvine, California 92697, United States.|School of Mechanical Engineering and Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University , Nanjing 211189, China.
authors.forename: Yinghua
authors.initials: Y
authors.lastname: Qiu
authors.affiliation: Oak Ridge National Laboratory , 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States.
authors.forename: Ivan
authors.initials: I
authors.lastname: Vlassiouk
authors.affiliation: Department of Physics and Astronomy, University of California , Irvine, California 92697, United States.
authors.forename: Preston
authors.initials: P
authors.lastname: Hinkle
authors.affiliation: Department of Materials Science, GSI Helmholtz Center for Heavy Ion Research , Darmstadt, Germany.
authors.forename: Maria Eugenia
authors.initials: ME
authors.lastname: Toimil-Molares
authors.forename: Alex J
authors.initials: AJ
authors.lastname: Levine
authors.affiliation: Department of Physics and Astronomy, University of California , Irvine, California 92697, United States.
authors.forename: Zuzanna S
authors.initials: ZS
authors.lastname: Siwy
keywords: asymmetric transport; micropore; particle focusing; resistive pulse