Nanoscale Wetting Under Electric Field from Molecular Simulations - Multiscale Molecular Methods in Applied Chemistry - page 173

5 Conclusions
We have reviewed computational studies of neat water under combined effect of
confinement and electric field. Molecular simulations in these systems were able
to demonstrate remarkable differences between field-enhanced wetting at the
nanoscale and in macroscopic systems. In particular, they highlighted the cou-
pling between interfacial hydrogen bonds [ 100 - 103 ] and molecular alignment in
the electric field. This coupling introduces a dependence of wetting on field
direction and polarity in contrast to the conventional picture in macroscopic
systems; system size plays a crucial role. The observed anisotropy in field-
induced wetting is a new nanoscale phenomenon that has so far been elusive as,
in the majority of current experimental setups, surface molecules represent a very
low fraction of the total number of molecules affected by the field [ 130 ]. It may
find applications, for example in the design of electrowetting techniques in
fabrication and property tuning nanomaterials. Likewise, these effects may play
a role in function of membrane proteins that are voltage sensitive, like pumps,
transporters, and channels (Roux B, private communication, [ 131 ]), including
artificial ones [ 132 , 133 ].
Another novel mechanism that originates from molecular anisotropy of polar
solvent molecules such as water reveals strong field-induced orientational forces
acting on apolar surface through water mediation, which operate regardless of
the presence or absence of solute/solvent permittivity difference. The findings
have applications in nanomaterials engineering, where direct interactions between
dipolar nanoparticles and applied electric field have been used to control and
explain nanoassembly processes [ 134 ]. The new mechanism [ 53 , 108 ] can be
used in a similar way, regardless of the electrostatic nature of a nanoparticle. The
water-mediated torques can act in concert with direct electrostatic interactions, and
can be of similar magnitude when the particles are of nanosize. The response to
the applied field takes place at an attractively short time scale [ 108 ]. Therefore, the
mechanism can be considered in the development of chemical and biosensors.
The examples presented highlight the importance and predictive power of
molecular modeling techniques in providing new insights into microscopic scale
phenomena not fully accessible in experiment. The novelty of the results should
have a broad impact in very active research fields of nano- and bioengineering,
physics at the cell scale [ 135 ], etc. The future outlook calls for natural extensions
of the work reviewed here: exciting new physics can emerge upon inclusion
of aqueous salt solutions [ 136 , 137 ] and evaluating the dynamic response of
nanoconfined water to field change [ 138 ], a critical dynamic property for electro-
switchable nanofluidic or optical devices.
Acknowledgements We thank Kevin Leung for his contribution to some of the work reviewed
here. We gratefully acknowledge the financial support from the National Science Foundation
(CHE-0718724) and the U.S. Department of Energy (DE-SC-0004406).