Temperature-Dependent Electro-osmotic Flow Reversal in a Nanotube with High Surface Charge Density

Abstract

As the nanofluidic device undergoes different operating temperatures in real application, the thermal effect becomes one important factor in the evaluation of its performance. In this work, the electro-osmotic flow (EOF) was investigated with a potassium chloride (KCl) solution confined in a carbon nanotube (CNT) with high surface charge density, for which the actual elementary charge distribution was adopted. The results show that the curved electrical double layer (EDL) in the nanotube makes the distribution and movement of ions in the confined fluids more subject to the fluid–wall interactions. The bulk flow region shrinks, the EDL occupies the entire fluid region in the CNT, and the EOF reversal occurs in the overlapped diffuse layer. It is found that the degree of charge inversion, which is represented by the peak value of K+ ion number density in the Stern layer, is not a monotonic function of temperature because of the temperature dependence of ion concentration, especially the counterions in the Stern layer. As the temperature is below a certain temperature (300 K in the present study), it increases slightly; then, it drops significantly with increasing temperature. As the viscosity decreases with increasing temperature, the velocity magnitude of ions is in a linear relation with temperature; however, the velocity of reverse EOF is in a nonlinear (cubic) relation with temperature. The combined influence of viscosity, ion velocity, degree of charge inversion, and ionic hydration capacity leads to a slowing down of the rate of increase in reverse EOF velocity with increasing temperature.

Manipulation and Mixing of 200 Femtoliter Droplets in Nanofluidic Channels Using MHz‐Order Surface Acoustic Waves

“…For this purpose, a variety of pump designs that curiously employ carbon nanotubes have been proposed over the past 15 years, using temperature gradients, [14,15] Coulomb drag, [16] surface waves (in theory), [17] or static electric fields. [18][19][20][21] Passive fL-scale fluid handling has also been devised using surface modification or geometric channel design.…”