Effects of ionic strength on nonlinear electrophoretic mobility of fd virus in solid-state nanopore*

We report an experimental study of electrophoretic mobility of rod-like \textit{fd} virus in solid-state nanopores. It is found that the velocity $v$ is a nonlinear function of the electric filed E, and can be described by $v = \mu ^{\mathrm{(1)}}$E $+ \mu^{(3)}$E$^{3}$. In addition to the linear Smoluchowski term, there is a second term with cubic dependence on E which has been described as a Stotz-Wien effect caused by the polarization of the Debye counter ion cloud. Here we report a study of this nonlinear electrophoresis effect under different salt concentrations. We found that at low ionic strength, the cubic mobility term becomes less pronounced. The origin of this observation will be discussed.