A molecular dynamics study of nanoconfined water flow driven by rotating electric fields under realistic experimental conditions.

Langmuir : the ACS journal of surfaces and colloids

PubMedID: 24575940

De Luca S, Todd BD, Hansen JS, Daivis PJ. A molecular dynamics study of nanoconfined water flow driven by rotating electric fields under realistic experimental conditions. Langmuir. 2014;.
In our recent work, J. Chem. Phys. 138, 154712 (2013), 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 uid is driven out of equilibrium by an external spatially uniform rotating electric eld and conned 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 VĂ…-1, the translational-rotational coupling is most effective, yielding fluid velocities of magnitudes of ~ 2 ms-1 with only a 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 $\beta$-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 non-contact 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.