Numerical Investigation of Diffusioosmotic Flow in a Tapered Nanochannel

Diffusioosmosis concerns ionic flow driven by a concentration difference in a charged nano-confinement and has significant applications in micro/nano-fluidics because of its nonlinear current-voltage response, thereby acting as an active electric gating. We carry out a comprehensive computation fluid dynamics simulation to investigate diffusioosmotic flow in a charged nanochannel of linearly varying height under an electrolyte concentration gradient. We analyze the effects of cone angle (alpha), nanochannel length (l) and tip diameter (d(t)), concentration difference (Delta c = 0-1 mM), and external flow on the diffusioosmotic velocity in a tapered nanochannel with a constant surface charge density (sigma). External flow velocity (varied over five orders of magnitude) shows a negligible influence on the diffusioosmotic flow inside the tapered nanochannel. We observed that a cone angle causes diffusioosmotic flow to move towards the direction of increasing gap thickness because of stronger local electric field caused by the overlapping of electric double layers near the smaller orifice. Moreover, the magnitude of average nanoflow velocity increases with increasing vertical bar alpha vertical bar. Flow velocity at the nanochannel tip increases when d(t) is smaller or when l is greater. In addition, the magnitude of diffusioosmotic velocity increases with increasing Delta c. Our numerical results demonstrate the nonlinear dependence of tapered, diffusioosmotic flow on various crucial control parameters, e.g., concentration difference, cone angle, tip diameter, and nanochannel length, whereas an insignificant relationship on flow rate in the low Peclet number regime is observed.

Automated summary

In this study, diffusionosmotic flow in a charged nanochannel with linearly varying height under an electrolyte concentration gradient is comprehensively investigated through computational fluid dynamics simulations. The effects of cone angle, nanochannel length, tip diameter, concentration difference, and external flow on the diffusioosmotic velocity are analyzed. The results show that the cone angle affects the direction of the diffusionosmotic flow, while the external flow velocity has a negligible influence. Furthermore, the nanochannel length and tip diameter also impact the flow velocity.