Anomalous Transit Time and Pulse Amplitude of Highly Charged Particles in Resistive Pulsing

Abstract

With resistive pulse technology, three types of particles with similar size but different surface charge densities have been investigated in track-etched polyethylene terephthalate (PET) single micropores. We have shown that pores with axially undulating diameter constitute a sensitive tool for spherical particles’ detection and sizing. Shape of resistive pulses carries information on the shape and charge of passing particles, as well as shape and charge of pores. Passage of single particles causes a decrease of the recorded current whose amplitude is correlated with the particle volume. However, our experiments have revealed, the blockage current of the strongly charged particles can be significantly larger than those from the other two types of particles and this observation cannot be described by the classical resistive-pulse model. This finding is also in contrast to earlier reports showing that pulse amplitude created by a charged molecule/particle is smaller compared to an uncharged particle of the same volume. Our results are explained by the distortion of the electrical double layer adjacent to the charged particle, which create a region with depleted ionic concentrations. We have also found that the particle passage time results from an interplay of three effects: (i) ion condensation, (ii) formation of an asymmetric electrical double layer around the particle, and (iii) electroosmotic flow induced by the charges on the pore walls and the particle surfaces. Our results are important for applying resistive-pulse technique to determine surface charge density and zeta potential of the particles. This research has implications for separation between particles with different surface charge properties, particle detection and sizing, and for the fundamental understanding of fluid flow at the meso-scale.