Non-negligible Water-permeance through Nanoporous Ion Exchange Medium

Abstract

While the water impermeable constraint has been conventionally adopted for analyzing the transport phenomena at the interface of electrolyte/nanoporous medium, non-negligible water-permeance through the medium results in significant effect on ion and particle transportation. In this work, a rigorous theoretical and experimental analysis of the water-permeance effect were conducted based on a fully-coupled analytical/numerical method and micro/nanofluidic experiments. The regime diagram with three distinctive types of concentration boundary layers (ion depletion, ion accumulation, and intermediate) near the ion exchange nanoporous medium was proposed depending on the medium’s permselectivity and the water-permeance represented by an absorbing parameter. Moreover, the critical absorbing parameters which divide the regimes were analytically obtained so that the bidirectional motion of particles were demonstrated only by altering the water-permeance without external stimuli. Conclusively, the presenting analysis of non-negligible water-permeance would be a substantial fundamental of transport phenomena at the interface of the ion exchange medium and electrolyte, especially useful for the tunable particle/ion manipulations in intermediate Peclet number environment.

Spontaneous diffusiophoretic separation in paper-based microfluidic device

“…Both velocities (U DP and U μ ) are proportional to the time −1/2 [14]. Thus, they were plotted as a straight line with a negative slope in log scale graph as shown in Fig.…”

“…Our previous work presented the spontaneous preconcentration (or selective preconcentration) method in microfluidic chip with leveraging convective flow through microchannel over diffusiophoresis induced by imbibition and ion exchange of ion exchange medium, respectively [13][14][15][16][17][18]. Especially, we had demonstrated that the velocity of the convective flow by the imbibition of the medium deviated from original Darcy's law using non-uniformly patterned ion exchange medium, but this work has the limitation of time-consuming and complex process [13].…”

Controllable pH Manipulations in Micro/Nanofluidic Device Using Nanoscale Electrokinetics

“…Therefore, the width of hydrogel was almost unchanged at pH = 3.5, while it expanded by 23% after 1 h in the solution of pH = 5.5 as shown in Figure 6a. When the swollen hydrogel blocked a microchannel, the configuration of microchannel was changed into micro-nano-micro-connection because the hydrogel possesses a number of nanopores inside [26,29,51]. Consequently, one can expect ICP phenomenon with the swollen hydrogel.…”

“…The reason why it slightly dropped in case of pH = 3.5 was that ICP also formed when a micro-and a nano-junction coexist in parallel [55][56][57][58]. While one expects a higher initial current in a micro-micro-micro-connection than one in a micro-nano-micro-connection by considering cross-sectional area, it involves complex interactions such as Donnan concentration of nanoporous material [51,59], surface charge of microchannel [13], the concentration of electrolyte (i.e., Debye length) [2,55], and the transversal dimension of hydrogel itself, etc. Thus, normalized current values with respect to each initial current value were plotted in the inset of Figure 6c.…”

Effect of evaporation through nanoporous medium on diffusiophoresis

“…Diffusiophoresis is the electrochemical phenomenon that a charged particle moved from low concentration region to high concentration region along a concentration gradient in the case of electrolyte solution such as KCl solution where each diffusivity is almost identical so that one can ignore induced electrophoresis. This concentration gradient can be generated by two miscible liquids of different concentrations [1,2] or by ion exchange [3][4][5]. Especially, cation exchange phenomenon can occur near a nanoporous medium having protons with high diffusivity if the medium meets the solution with non-protonic cation, which means that the concentration gradient would be spontaneously generated by cations' difference of diffusivity without any external forces [4].…”