Elucidating the roles of diffusion and osmotic flow in controlling the geometry of nanochannels in asymmetric track-etched membranes

Asymmetric membranes and asymmetric pores are broad classes of objects, the role of which is steadily growing in academia and industry. During the last two decades, asymmetric (often called "conical") track-etched nanopores have attracted increasing attention from the scientific community due to their great potential for accomplishing useful functions in nanofluidic devices. A great body of knowledge has been gained on the electrical and electroosmotic properties of track-etched asymmetric nanopores in electrolyte solutions. Less attention has been paid to the pore geometry, and practically no attempts have been made to explore their osmotic and hydraulic properties. The present study fills this gap by examining the interrelations between the electrical, osmotic, hydraulic and structural characteristics of an asymmetrically etched ion-track nanopore. Several consecutive phases of pore evolution are identified. In the last phase, the highly asymmetric membrane resembles a porous 2D material. The temporal dependences of the two counter-fluxes - the diffusional flux of the etchant and the volume flux of stopping media - are elucidated and analyzed in light of their effect on the nanopore configuration. The osmotic reflection coefficients of highly asymmetric pores are estimated under etching conditions and in diluted electrolyte solutions. Our results allow a new level of understanding of the phenomena underlying the development of an ion-track nanopore and pave the way for the controlled fabrication of a variety of nanopores of different shapes using one and the same principle of an asymmetric chemical treatment.