Abstract
In this article, we report detection of deformable, hydrogel particles by the
resistive-pulse technique using single pores in a polymer film. The hydrogels pass
through the pores by electroosmosis and cause formation of a characteristic shape of
resistive pulses indicating the particles underwent dehydration and deformation. These
effects were explained via a non-homogeneous pressure distribution along the pore axis
modeled by the coupled Poisson-Nernst-Planck and Navier Stokes equations. The local
pressure drops are induced by the electroosmotic fluid flow. Our experiments also
revealed the importance of concentration polarization in the detection of hydrogels. Due
to the negative charges as well as branched, low density structure of the hydrogel
particles, concentration of ions in the particles is significantly higher than in the bulk. As
a result, when electric field is applied across the membrane, a depletion zone can be
created in the vicinity of the particle observed as a transient drop of the current. Our
experiments using pores with openings between 200 and 1600 nm indicated the
concentration polarization dominated the hydrogels detection for pores wider than 450
nm.
The results are of importance for all studies that involve transport of molecules, particles
and cells through pores with charged walls. The developed inhomogeneous pressure
distribution can potentially influence the shape of the transported species. The
concentration polarization changes the interpretation of the resistive pulses; the
observed current change does not necessarily reflect only the particle size but also the
size of the depletion zone that is formed in the particle vicinity.
