Molecular Dynamics Study of Nanoconfined Water Flow Driven by Rotating Electric Fields under Realistic Experimental Conditions

Abstract

In our recent work, J. Chem. Phys. 2013, 138, 154712, we demonstrated the feasibility of unidirectional pumping of water, exploiting translational-rotational momentum coupling using nonequilibrium molecular dynamics simulations. Flow can be sustained when the fluid is driven out of equilibrium by an external spatially uniform rotating electric field and confined between two planar surfaces exposing different degrees of hydrophobicity. The permanent dipole moment of water follows the rotating field, thus inducing the molecules to spin, and the torque exerted by the field is continuously injected into the fluid, enabling a steady conversion of spin angular momentum into linear momentum. The translational-rotational coupling is a sensitive function of the rotating electric field parameters. In this work, we have found that there exists a small energy dissipation region attainable when the frequency of the rotating electric field matches the inverse of the dielectric relaxation time of water and when its amplitude lies in a range just before dielectric saturation effects take place. In this region, that is, when the frequency lies in a small window of the microwave region around ∼20 GHz and amplitude ∼0.03 V Å(-1), the translational-rotational coupling is most effective, yielding fluid velocities of magnitudes of ∼2 ms(-1) with only moderate fluid heating. In this work, we also confine water to a realistic nanochannel made of graphene giving a hydrophobic surface on one side and β-cristobalite giving a hydrophilic surface on the other, reproducing slip-and-stick velocity boundary conditions, respectively. This enables us to demonstrate that in a realistic environment, the coupling can be effectively exploited to achieve noncontact pumping of water at the nanoscale. A quantitative comparison between nonequilibrium molecular dynamics and analytical solutions of the extended Navier-Stokes equations, including an external rotating electric field has been performed, showing excellent agreement when the electric field parameters match the aforementioned small energy dissipation region.

Challenges in Nanofluidics—Beyond Navier–Stokes at the Molecular Scale

“…This means that an external torque that spins the molecules can be used to pump a fluid. It has been demonstrated that a rotating electric field applied to polar molecules (such as water) under these conditions can generate a net flow in a nanochannel or nanotube, without the need for electrolyte or a pressure gradient [30,31].…”

Continuum Nanofluidics

“…Just recently De Luca et al [63] showed that the spin field does possess slippage and Badur et al [64] used spin slip to account for flow enhancement. As mentioned in the introduction, we will treat the problem in an ad hoc fashion and simply set the angular velocity slip in accordance with the MD data.…”

Effect of Hydrogen Bonds on the Dielectric Properties of Interfacial Water

“…Understanding the underlying physical mechanism of the dielectric response is essential in the solvation dynamics of aqueous solutions 11−13 that are primarily governed by the bulk dielectric properties of the solvent as well as in the development of nanoscale electromechanical devices. 14,15 Experimental investigations of the problem are very complicated. This has led to the development of theoretical and computer simulation techniques to calculate the molecular dielectric response in highly confined geometries.…”