Off-axis response for particles passing through long apertures in Coulter-type counters

A novel modification of the resistive pulse technique (Coulter principle) has been used to investigate how the measured resistance pulse amplitude depends on the off-axis particle position in long pores. By pressure drive, a particle enters a current-carrying pore and an increase in resistance proportional to the particle volume is detected. When the particle exits the pore, the pressure is reversed such that the particle re-enters the pore and the same particle can thus be studied for a long time. In Poiseuille flow, solid spheres migrate to an off-axis equilibrium position and this non-linear hydrodynamic effect has been utilised to study how the measured pulse amplitude from a single particle flowing back and forth through a pore increases when the particle migrates closer to the pore wall. The increase in pulse amplitude corresponding to a radial particle displacement from the pore axis to the wall is found to be less than 10% for all particle and pore sizes studied. This is considerably less than predicted by the off-axis upper-limit theory of Smythe.