Role of Particle Focusing in Resistive-Pulse Technique: Direction-Dependent Velocity in Micropores

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.