Diffusiophoresis of a sphere along the axis of a cylindrical pore

The Effect of Axial Concentration Gradient on Electrophoretic Motion of a Charged Spherical Particle in a Nanopore

“…In the absence of the imposed electric field, the diffusiophoretic motion is generated by two mechanisms: one is induced electrophoresis generated by the induced electric fields arising from the difference of ionic diffusivities and DLP, and the other is chemiphoresis generated by the induced osmotic pressure gradient around the particle (Lou and Lee 2008a, b;Hsu et al , 2010Zhang et al 2009;Dukhin 1993Dukhin , 1995. The induced chemiphoresis always propels the particle towards higher salt concentration, regardless of the sign of charge on the particle.…”

“…Together with the charge of the particle and the charge of the EDL, a dipole moment is induced, which induces an electric field reaching beyond the limits of the EDL (Dukhin 1993(Dukhin , 1995. The concentration polarization inside the EDL by the imposed concentration gradient is named as the type I DLP, and its resulting electric field is named as E I−DLP , the direction of which is opposite to that of the applied concentration gradient when the particle is negatively charged (Lou and Lee 2008a, b;Hsu et al , 2010Zhang et al 2009). For example, the induced electric field due to the type I DLP, E I−DLP , is directed from higher salt concentration towards lower salt concentration (i.e., upward for α < 1 and downward for α > 1).…”

“…Meanwhile, the concentration of the coions near the outer boundary of the EDL on the high-concentration side is higher than that on the low-concentration side of the particle (i.e., the cases of α = 0.1 and 5 in Fig. 6), which is called the type II DLP, generating an electric field, E II−DLP , the direction of which is opposite to that established by the counterions inside the EDL, E I−DLP (Lou and Lee 2008a, b;Hsu et al , 2010Zhang et al 2009). Therefore, when a concentration gradient is imposed, an electric field is generated by three mechanisms: the first one, E diffusivity , is established by the difference in the ionic diffusivities; the second one, E I−DLP , is established by the type I DLP inside the EDL; and the third one,E II−DLP , is generated by the type II DLP near the outer boundary of the EDL.…”

“…The induced E I−DLP by the type I DLP always propels the particle towards higher salt concentration, while the E II−DLP generated by the type II DLP always drags the particle towards lower salt concentration, regardless of the sign of charge on the particle. For thin EDL (i.e., κa p is large), usually, the electric field generated by the type I DLP, E I−DLP , is stronger than that from the type II DLP, E II−DLP , if the ratio of the nanopore size to the particle size, a/a p , is very large (the boundary effect arising from the nanopore wall is insignificant), and the magnitude of the particle's surface charge is relatively low (Hsu et al 2010); however, the electric field arising from the type II DLP dominates over that from the type I DLP if the surface charge or surface potential of the particle is relatively high (Hsu et al 2010). If κa p is small (thick EDL), usually the electric field induced by the type II DLP dominates over that by the type I DLP.…”

“…In the present study, electrophoretic motion of a charged nanoparticle in a nanopore connecting two fluid reservoirs filled with different electrolyte concentrations is studied for the first time. Since the electrolyte concentrations on both sides of the nanopore are different, diffusiophoretic motion is also induced in addition to the electrophoresis (Anderson and Prieve 1984;Keh and Li 2007;Qian et al 2007;Lou and Lee 2008a, b;Keh and Wan 2008;Abecassis et al 2008Abecassis et al , 2009Prieve 2008;Lou et al 2009;Hsu et al , 2010Hsu and Keh 2009;Zhang et al 2009). Depending on the magnitude and direction of the imposed concentration gradient, the induced diffusiophoretic motion can enhance the particle's electrophoretic motion or slow down nanoparticle translocation in a nanopore, and thus can be used to regulate the nanoparticles translocation process to achieve a nanometer-scale spatial accuracy for DNA sequencing.…”