Nonequilibrium energy dissipation at the interface of sliding model hydroxylated alpha-alumina surfaces

Nonequilibrium molecular dynamics simulations were performed to study the dynamics of energy transfer at the interface of a small nanoscale hydroxylated alpha-alumina surface sliding across a much larger surface of the same material. Sliding velocities of 0.05, 0.5, 5, and 50 ms and loads of 0, 0.0625, 5, 15, 25, and 100 nN were considered. Nonequilibrium energy distributions were found at the interface for each of these conditions. The velocity distribution P(v) for the atoms in a sublayer of the smaller surface oscillates during the sliding, reflecting the periodicity of the interfacial intermolecular potential. When averaged over the sliding, this P(v) for each of the sublayers is bimodal with Boltzmann and non-Boltzmann components. The non-Boltzmann component, with temperatures in excess of 1000 K and as high as 2500 K, is most important for the interfacial H-atom sublayer and becomes less important in moving to a sublayer further from the interface. Similarly, the temperature of the Boltzmann component decreases for sublayers further from the interface and approaches the 300 K temperature of the boundary. The temperature of the Boltzmann component decreases, but the importance of the non-Boltzmann component increases, as the sliding velocity is decreased. The temperature of the non-Boltzmann component is relatively insensitive to the sliding velocity. Friction forces are determined by calculating the energy dissipation during the sliding, and different regimes are found for variation in the friction force versus sliding velocity v(s) and applied load. For v(s) of 0.05, 0.5, and 5 ms, the friction force is inversely proportional to v(s) reflecting the increased time for energy dissipation as v(s) is decreased.     

Monitoring gold nanorod synthesis by localized surface plasmon resonance

Surfactants can direct the growth of gold nanoparticles to create anisotropic structures in high yield by simple means, yet the exact roles of surfactants and other reactants are not entirely understood. Here we show that one can exploit the geometrical dependence of the localized surface plasmon resonant extinction spectrum of gold nanorods to monitor their synthesis kinetics. By using quantitative measurements of nanorod extinction cross sections, Gans' theory for the spectral extinction of prolate spheroids can be normalized to provide values for the nanorod length and diameter from extinction spectra measured during growth. The nanorod length growth rate was first observed at 0.15 nm/s and decayed during the growth reaction. The rate dependence on nanorod size did not correspond to any simple reaction-limited or diffusion-limited growth mechanisms.     

Grazing incidence surface-induced dissociation: molecules sliding along a surface

Principles of grazing incidence SID and a brief overview of previous works are summarized and the experimental setup for grazing incidence SID experiments is described. New results with fullerene C₆₀ are presented; these demonstrate that grazing incidence SID is not a special case of the conventional SID.     

Nanoscale probing of the enamel nanorod surface using polyamidoamine dendrimers

Although it is known that noncollagenous proteins of dental origin bind to the hydroxyapatite crystal surfaces, no measure of their binding strength has been calculated. This experiment used -COOH-capped generation 7 PAMAM dendrimers as nanoprobes of the biological hydroxyapatite nanorod surfaces. Dendrimer distribution was characterized using AFM. The results showed dendrimers to be spaced at intervals along the c-axis of the crystals. From these observations and assuming a fully ionized -COOH dendrimer, a mathematical model of the binding capacity of the crystal surface with the dendrimer was developed. The Monte Carlo method was used to simulate the binding process between the dendrimer and crystal surface, and the binding strength of the -COOH groups to the surface was calculated to be 90 +/- 20 kJ/mol. These results support the CFM studies which have described alternating bands of charge domains on the crystal surface and that the binding strength will be dependent on both the intensity of the charge on the protein and the crystal surface.     

Porous three-dimensional nanorod arrays through selective chemical etching of nanocomposites

Three-dimensional Cu-Si and Cu-SiO(2) nanorod arrays containing ~68 at% Cu have been fabricated by a glancing angle co-deposition technique. By selectively etching Cu in 0.05 M KCN methanol solution, porous nanorods with different shapes form, which are promising for applications in sensors, catalysts, and as medical capsules that are able to be loaded with functional materials.     

Nonequilibrium Fermi golden rule for electronic transitions through conical intersections

We consider photoinduced electronic transitions through conical intersections in large molecules. Starting from the linear vibronic model Hamiltonian and treating linear diabatic couplings within the second order cumulant expansion, we have developed a simple analytical expression for the time evolution of electronic populations at finite temperature. The derived expression can be seen as a nonequilibrium generalization of the Fermi golden rule due to a nonequilibrium character of the initial photoinduced nuclear distribution. All parameters in our model are obtained from electronic structure calculations followed by a diabatization procedure. The results of our model are found to agree well with those of quantum dynamics for a test set of systems: fulvene molecule, 2,6-bis(methylene) adamantyl cation, and its dimethyl derivative.     

Polarized surface enhanced Raman and absorbance spectra of aligned silver nanorod arrays

Polarized surface-enhanced Raman scattering (SERS) and UV-vis absorbance spectra were measured for a nonplanar Ag nanorod array substrate prepared by oblique angle vapor deposition. The anisotropy of the SERS polarization was shown to differ from that of the polarized UV-vis absorbance. The maximum SERS intensity was observed in the polarization direction perpendicular to the long axis of the Ag nanorods, while the UV-vis absorbance was strongly polarized along the direction of the long axis of the nanorod array. Analysis of the polarization data showed that molecular orientation was not the cause of the anisotropic SERS scattering. Rather, the SERS anisotropy was primarily attributed to the lateral arrangement of the three-dimensional tilted nanorod lattice in which highly localized plasmon modes are created by strong electromagnetic coupling between adjacent metallic nanorods.     

Ultrafast dynamics of heat flow across molecules

An ultrafast flash thermal conductance apparatus is used to study heat flow through aliphatic and aromatic molecules arranged in self-assembled monolayers (SAMs). The apparatus consists of a thin metal film which can be flash-heated by many hundreds of degrees in 1 ps using a femtosecond pulse. Heat flow from the metal surface into the SAM molecules is detected using vibrational sum-frequency generation (SFG) spectroscopy. The SAMs studied were alkanethiolates (AT) ranging from C6 to C24, benzenethiolate (BT) and benzylmercaptide (BMT). SFG in the CH-stretch region selectively probes transitions of the terminal methyl groups of AT and the CH moiety at the 4-position of the phenyl ring of BT and BMT (opposite the thiolate-surface bond). The SFG signal is sensitive to temperature-jump induced thermal disorder of the SAM and also to vibrational frequency shifts induced by the changing intramolecular vibrational populations. The SFG probe functions as a thermometer, and this thermometer is 1.5 Å thick with a response time of 1 ps. In the AT chains, a study of the length dependence is used to determine the rate heat flows across the metal–SAM interface and the rate of heat flow through the AT chains. The interface thermal conductance is 220 GW m⁻² s⁻¹. The AT molecular conductance is 50 pW K⁻¹ or 0.3 eV s⁻¹ K⁻¹. Heat flow through the AT chains is ballistic with a velocity of 1 km/s. Heat flow into BMT is slower than in BT because BMT has one additional methylene linker group. The BT and BMT structures evidence a thermally-initiated surface rearrangement occurring in a few tens of picoseconds. These SAMs are strained and the phenyl rings cannot adopt the most stable staggered herringbone structure. After the T-jump, the SAM molecules have enough freedom to relax into more favorable configurations.     

Electrically controlled director slippage over a photosensitive aligning surface; in-plane sliding mode

We have studied the electro-optical characteristics of a homogeneously aligned nematic liquid crystal (LC) with weak planar anchoring of the director at the bounding substrates. By using the in-plane switching (IPS) of the LC which is achieved by an in-plane electric field, the driving voltage was confirmed to be far less than that of the conventional IPS mode in which both substrates possess strong anchoring characteristics. Moreover, because of the absence of strong subsurface director deformations, the cell could operate optically in the Mauguin regime. Using these features we propose a new type of LC switching mode - in-plane sliding (IPSL) mode. We have realized this mode in a LC cell comprising one reference substrate with strong director anchoring and one substrate covered with photoaligning material with weak anchoring. In order to clarify the switching process, we derived a simplified expression for the threshold voltage on the assumption of uniformity of the in-plane electric field. For the dynamical response of the LC to the in-plane electric field, the switching on and off relaxation times of the IPSL mode were found to be longer than for the traditional IPS mode. However, we have proposed an optimized cell geometry for the IPSL mode with a response time comparable to that of the IPS mode.     

Effect of surface mobility on the particle sliding along a bubble or a solid sphere

The sliding velocity of glass beads on a spherical surface, made either of an air bubble or of a glass sphere held stationary, is measured to investigate the effect of surface mobility on the particle sliding velocity. The sliding process is recorded with a digital camera and analyzed frame by frame. The sliding glass bead was found to accelerate with increasing angular position on the collector's surface. It reaches a maximum velocity at an angular position of about 100 degrees and then, under certain conditions, the glass bead leaves the surface of the collector. The sliding velocity of the glass bead depends strongly on the surface mobility of a bubble, decreasing with decreasing surface mobility. By a mobile surface we mean one which cannot set up resistive forces to an applied stress on the surface. The sliding velocity on a rigid surface, such as a glass sphere, is much lower than that on a mobile bubble surface. The sliding velocity can be described through a modified Stokes equation. A numerical factor in the modified Stokes equation is determined by fitting the experimental data and is found to increase with decreasing surface mobility. Hydrophobic glass beads sliding on a hydrophobic glass sphere were found to stick at the point of impact without sliding if the initial angular position of the impact is less than some specific angle, which is defined as the critical sticking angle. The sticking of the glass beads can be attributed to the capillary contracting force created by the formation of a cavity due to spontaneous receding of the nonwetting liquid from the contact zone. The relationship between the critical sticking angle and the particle size is established based on the Yushchenko [J. Colloid Interface Sci. 96 (1983) 307] analysis.     

Amplification of localized surface plasmon resonance signals by a gold nanorod assembly and ultra-sensitive detection of mercury

Controlled assembly of gold nanorods induced by Na(3)PO(4) leads to a significant amplification of localized surface plasmon resonance (LSPR) signals. The strong affinity between Au and Hg alters the coupled LSPR signals due to the amalgamation of Hg and Au. This allows detection of Hg in aqueous solutions with ultra-high sensitivity and excellent selectivity, without sample pretreatment.     

Near-room-temperature production of diameter-tunable ZnO nanorod arrays through natural oxidation of zinc metal

A simple, low-temperature strategy has been developed for the low-cost and large-area fabrication of ZnO nanoarrays on a zinc substrate by the natural oxidation of zinc metal in formamide/water mixtures. The one-step, wet-chemical approach has exhibited well-controlled growth of highly oriented and densely packed ZnO nanoarrays with large-area homogeneity and consisting of nanorods or nanowires with predictable morphologies, such as tunable diameters and identical lengths. The chemical-liquid-deposition process, an analogue to the widely used chemical-vapor-deposition technique, has been used for the near-room-temperature production of ZnO nanoarrays through continuous supply, transport, and thermal decomposition of zinc complexes in a liquid phase.     

Lateral diffusion in lipid membranes through collective flows

There is no comprehensive model for the dynamics of cellular membranes. Even mechanisms of basic dynamic processes, such as lateral diffusion of lipids, are poorly understood. Our atomic-scale molecular dynamics simulations support a novel, concerted mechanism for lipid diffusion. We find that a lipid and its nearest neighbors move in unison, forming loosely defined clusters. What is more, the motions of lipids are correlated over tens of nanometers: the lateral displacements of lipids in a given monolayer produce striking two-dimensional flow patterns. These flow patterns should have wide implications, affecting, for example, the formation of membrane domains, protein functionality, and action of lipases and drugs on membranes.     

Nonequilibrium bubbles in a flowing langmuir monolayer

We investigate the nonequilibrium behavior of two-dimensional gas bubbles in Langmuir monolayers. A cavitation bubble is induced in liquid expanded phase by locally heating a Langmuir monolayer with an IR-laser. At low IR-laser power the cavitation bubble is immersed in quiescent liquid expanded monolayer. At higher IR-laser power thermo capillary flow around the laser-induced cavitation bubble sets in. The thermo capillary flow is caused by a temperature dependence of the gas/liquid line tension. The slope of the line tension with temperature is determined by measuring the thermo capillary flow velocity. Thermodynamically stable satellite bubbles are generated by increasing the surface area of the monolayer. Those satellite bubbles collide with the cavitation bubble. Upon collision the satellite bubbles either coalesce with the cavitation bubble or slide past the cavitation bubble. Moreover we show that the satellite bubbles can also be produced by the emission from the laser-induced cavitation bubbles.     

Aspect ratio dependence on surface enhanced Raman scattering using silver and gold nanorod substrates

Silver and gold nanorods with aspect ratios from 1 to 16 have been used as substrates for surface enhanced Raman spectroscopy (SERS) in colloidal solution. The nanorod aspect ratio is varied to give different degrees of overlap between the nanorod longitudinal plasmon band and excitation source in order to determine its effect on overall surface enhancement. Results suggest that enhancement factors are a factor of 10-10(2) greater for substrates that have plasmon band overlap with the excitation source than for substrates whose plasmon bands do not.     

First crystallographic signature of an acyclic peptide nanorod: molecular mechanism of nanorod formation by a self-assembled tetrapeptide

A terminally protected acyclic tetrapeptide Boc-Aib-Val-Aib-beta-Ala-OMe 1 (Aib: alpha-aminoisobutyric acid, beta-Ala: beta-Alanine) self-assembles into a continuous hydrogen-bonded supramolecular helix with an average diameter of 10 A (1 nm) starting from a double bend molecular conformation in crystals and further self-assembly of this supramolecular architecture leads to the formation of polydisperse nanorods of diameters 10-40 nm.     

Numerical study of forced convection heat transfer across a cylinder with various cross sections

Journal of Thermal Analysis and Calorimetry - The thermal performance of an isothermal horizontal cylinder with different selected cross sections in cross-flow was investigated numerically. These...     

Simultaneous electroosmotic and permeation flows through a Nafion membrane 2. Methanol-water electrolyte solutions

The volume flow of methanol-water potassium chloride solutions through a Nafion membrane originated by the simultaneous action of electric potential and pressure gradients has been measured at different percentages of methanol. Measurements were conducted when both gradients act in the same and in the opposite directions under different experimental conditions. The results indicate that the simultaneous action of the pressure and potential differences originates a total flow different from the sum of the individual electroosmotic and permeation flows due to each force acting separately. The application of the irreversible thermodynamics theory, which includes second-order terms, allowed the study of the influence of the composition of the solutions on the determination of the different phenomenological coefficients.     

The specific heat of surface layers

Summary The well-known relation ofW. Thomson for the latent heat of surface expansion was obtained by applying the second law to a cyclic surface expansion and heating process. A simple derivation of an expression for the excess heat capacity of the surface layer is obtained by applying the first law to the same cyclic process.If we do not include the amount of potential energy converted to heat,A /TdT in the definition of the excess heat capacity, the resultant relation between the specific heat of the surface layers and their excess heat capacity is of the same form as that expressing the latent heat of vaporization in terms of the differential heat capacities of the liquid and vapor phases.Numerical calculation shows that the excess heat capacity for the surface layer of water amounts to 24 percent of the heat capacity of the molecular layer in the bulk liquid phase.     

Nonequilibrium molecular dynamics simulation of water transport through carbon nanotube membranes at low pressure

Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate pressure-driven water flow passing through carbon nanotube (CNT) membranes at low pressures (5.0 MPa) typical of real nanofiltration (NF) systems. The CNT membrane is modeled as a simplified NF membrane with smooth surfaces, and uniform straight pores of typical NF pore sizes. A NEMD simulation system is constructed to study the effects of the membrane structure (pores size and membrane thickness) on the pure water transport properties. All simulations are run under operating conditions (temperature and pressure difference) similar to a real NF processes. Simulation results are analyzed to obtain water flux, density, and velocity distributions along both the flow and radial directions. Results show that water flow through a CNT membrane under a pressure difference has the unique transport properties of very fast flow and a non-parabolic radial distribution of velocities which cannot be represented by the Hagen-Poiseuille or Navier-Stokes equations. Density distributions along radial and flow directions show that water molecules in the CNT form layers with an oscillatory density profile, and have a lower average density than in the bulk flow. The NEMD simulations provide direct access to dynamic aspects of water flow through a CNT membrane and give a view of the pressure-driven transport phenomena on a molecular scale.