Dynamic Response in Nanoelectrowetting on a Dielectric.

Droplet spreading at an applied voltage underlies the function of tunable optical devices including adjustable lenses and matrix display elements. Faster response and the enhanced resolution motivate research toward miniaturization of these devices to nanoscale dimensions. The response of an aqueous nanodroplet to an applied field can differ significantly from macroscopic predictions. Understanding these differences requires characterization at the molecular level. We describe the equilibrium and nonequilibrium molecular dynamics simulations of nanosized aqueous droplets on a hydrophobic surface with the embedded concentric electrodes. Constant electrode potential is enforced by a rigorous account of the metal polarization. We demonstrate that the reduction of the equilibrium contact angle is commensurate to, and adjusts reversibly with, the voltage change. For a droplet with O(10) nm diameter, a typical response time to the imposition of the field is of O(10(2)) ps. Drop relaxation is about twice as fast when the field is switched off. The friction coefficient obtained from the rate of the drop relaxation on the nonuniform surface, decreases when the droplet approaches equilibrium from either direction, that is, by spreading or receding. The strong dependence of the friction on the surface hydrophilicity points to the dominance of the liquid-surface friction at the drop's perimeter as described in the molecular kinetic theory. This approach enables correct predictions of trends in dynamic responses associated with varied voltage or substrate material.

Dielectric properties of crystalline organic molecular films in the limit of zero overlap.

We present the calculation of the static dielectric susceptibility tensor and dipole field sums in thin molecular films in the well-defined limit of zero intermolecular overlap. Microelectrostatic and charge redistribution approaches are applied to study the evolution of dielectric properties from one to a few molecular layers in films of different conjugated molecules with organic electronics applications. Because of the conditional convergence of dipolar interactions, dipole fields depend on the shape of the sample and different values are found in the middle layer of a thick film and in the bulk. The shape dependence is eliminated when depolarization is taken into account, and the dielectric tensor of molecular films converges to the bulk limit within a few molecular layers. We quantify the magnitude of surface effects and interpret general trends among different systems in terms of molecular properties, such as shape, polarizability anisotropy, and supramolecular organization. A connection between atomistic models for molecular dielectrics and simpler theories for polarizable atomic lattices is also provided.

Wetting behaviour and contact angles anisotropy of nematic nanodroplets on flat surfaces.

We have studied the wetting behaviour of liquid crystal nanodroplets deposited on a planar surface, modelling the mesogens with Gay-Berne ellipsoids and the support surface with a slab of Lennard-Jones (LJ) spherical particles whose mesogen-surface affinity can be tuned. A crystalline and an amorphous planar surface, both showing planar anchoring, have been investigated: the first is the (001) facet of a LJ fcc crystal, the second is obtained from a disordered LJ glass. In both cases we find that the deposited nanodroplet is, in general, elongated and that the contact angle changes around its contour. Simulations for the crystalline substrate show that the angle of contact turns reversibly from anisotropic to isotropic when crossing the clearing transition. As far as we know this is a novel, not yet explored effect for thermotropic liquid crystals, that we hope will stimulate experimental investigations.

Dynamic Control of Nanopore Wetting in Water and Saline Solutions under an Electric Field.

Field-induced nanopore wetting by aqueous solutions, including electrolytes, provides opportunities for a variety of applications. Conflicting porosity requirements have so far precluded direct implementations of a two-way control: the pores have to be sufficiently wide to allow water infiltration at experimentally relevant voltages but should not exceed the kinetic threshold for spontaneous expulsion in the absence of the field. Applicable widths are restricted below a few nanometers. Only a narrow window of fields and pore geometries can simultaneously satisfy both of the above requirements. Accurate accounts of wetting equilibria and dynamics at nanoscale porosity require molecular level descriptions. Here we use molecular dynamics simulations to study dynamic, field-controlled transitions between nanoconfined liquid and vapor phases in contact with an unperturbed aqueous or electrolyte environment. In nanopores wetted by electrolyte solutions, we observe depletion of salt compared to the bulk phase. The application of a local electric field enhances the uptake of water and ions in the confinement. In systems prone to capillary evaporation, the process can be reversed at sufficient strength of the electric field. For alternating displacement field, we identify the conditions where O (ns) responses of the reversible infiltration/expulsion cycle can be secured for experimentally realizable field strengths, porosity, and salinity of the solution.

Wetting transparency of graphene in water.

Measurements of contact angle on graphene sheets show a notable dependence on the nature of the underlying substrate, a phenomenon termed wetting transparency. Our molecular modeling studies reveal analogous transparency in case of submerged graphene fragments in water. A combined effect of attractive dispersion forces, angle correlations between aqueous dipoles, and repulsion due to the hydrogen-bond-induced orientation bias in polarized hydration layers acting across graphene sheet, enhances apparent adhesion of water to graphene. We show wetting free energy of a fully wetted graphene platelet to be about 8 mNm(-1) lower than for graphene wetted only on one side, which gives close to 10° reduction in contact angle. This difference has potential implications for predictions of water absorption vs. desorption, phase behavior of water in aqueous nanoconfinements, solvent-induced interactions among graphitic nanoparticle and concomitant stability in aqueous dispersions, and can influence permeability of porous materials such as carbon nanotubes by water and aqueous solutions.

Nanoconfined water under electric field at constant chemical potential undergoes electrostriction.

Electric control of nanopore permeation by water and solutions enables gating in membrane ion channels and can be exploited for transient surface tuning of rugged substrates, to regulate capillary permeability in nanofluidics, and to facilitate energy absorption in porous hydrophobic media. Studies of capillary effects, enhanced by miniaturization, present experimental challenges in the nanoscale regime thus making molecular simulations an important complement to direct measurement. In a molecular dynamics (MD) simulation, exchange of water between the pores and environment requires modeling of coexisting confined and bulk phases, with confined water under the field maintaining equilibrium with the unperturbed environment. In the present article, we discuss viable methodologies for MD sampling in the above class of systems, subject to size-constraints and uncertainties of the barostat function under confinement and nonuniform-field effects. Smooth electric field variation is shown to avoid the inconsistencies of MD integration under abruptly varied field and related ambiguities of conventional barostatting in a strongly nonuniform interfacial system. When using a proper representation of the field at the border region of the confined water, we demonstrate a consistent increase in electrostriction as a function of the field strength inside the pore open to a field-free aqueous environment.

Wettability of pristine and alkyl-functionalized graphane.

Graphane is a hydrogenated form of graphene with high bandgap and planar structure insensitive to a broad range of chemical substitutions. We describe an atomistic simulation approach to predict wetting properties of this new material. We determine the contact angle to be 73°. The lower hydrophobicity compared to graphene is explained by the increased planar density of carbon atoms while we demonstrate that the presence of partial charges on carbon and hydrogen atoms plays only a minor role. We further examine the effects of graphane functionalization by alkyl groups of increasing chain lengths. The gradual increase in contact angle with chain length offers a precise control of surface wettability. A saturated contact angle of 114° is reached in butylated form. We find the saturation of contact angle with respect to the length of the functional groups to coincide with the loss of water's ability to penetrate the n-alkyl molecular brush and interact with carbon atoms of the underlying lattice. Since no experimental data have yet become available, our modeling results provide the first estimate of the wettability of graphane. The results also show how its alkyl functionalization provides the basis for a variety of chemical modifications to tune hydrophilicity while preserving the planar geometry of the substrate.

Tuning fluorescence lifetimes through changes in Herzberg-Teller activities: the case of triphenylene and its hexamethoxy-substituted derivative.

The fluorescence spectra of triphenylene (TP) and 2,3,6,7,10,11-hexamethoxy-triphenylene (HMTP) are measured in glass matrices, and the vibronic structure associated with the electronic spectra is simulated with the help of quantum chemically computed molecular parameters. Franck-Condon (FC) and Herzberg-Teller (HT) mechanisms are included. For excited-state calculations, both configuration interaction with single excitations (CIS) and time-dependent density functional theory (TDDFT) are employed. It is shown that the FC activity is associated with modes of similar shape and frequency in both molecules, while the HT-induced false origins with the largest activity are associated with rather different frequencies and normal coordinates as a result of the mixing and energy lowering of the low-lying allowed excited states in HMTP. The increased HT activity explains the reduced S(1) state lifetime in the substituted TP, in turn driven by the excited-state rearrangement occurring upon substitution of the TP core.