  Interfacial polar molecules feature a strongly anisotropic response to
applied electric field, favoring dipole orientations parallel to the interface.
In water, in particular, this effect combines with generic orientational
preferences induced by spatial asymmetry of water hydrogen bonding under
confined geometry, which may give rise to a Janus interface. The two effects
manifest themselves in considerable dependence of water polarization on both
the field direction relative to the interface, and the polarity (sign) of the
field. Using molecular simulations, we demonstrate strong field-induced
orientational forces acting on apolar surfaces through water mediation. At a
field strength comparable to electric fields around a DNA polyion, the torques
we predict to act on an adjacent nanoparticle are sufficient to overcome
thermal fluctuations. These torques can align a particle with surface as small
as 1 nm2. The mechanism can support electrically controlled ordering of
suspended nanoparticles as a means of tuning their properties, and can find
application in electro-nanomechanical devices.
