Freezing of fluids confined between mica surfaces

Using grand ensemble simulations, we show that octamethyl-cyclo-tetra-siloxane (OMCTS) confined between two mica surfaces can form a variety of frozen phases which undergo solid-solid transitions as a function of the separation between the surfaces. For atomically smooth mica surfaces, the following sequence of transitions 1[triangle up] --> 1[triangle up]b --> 2B --> 2 square --> 2[triangle up] are observed in the one- and two-layered regimes, where n[triangle up], n[square], and nB denote triangular, square, and buckled phases, respectively, with the prefix n denoting the number of confined layers. The presence of potassium on mica is seen to have a strong influence on the degree of order induced in the fluid. The sequence of solid-solid transitions that occurs with the smooth mica surface is no longer observed. When equilibrated with a state point near the liquid-solid transition, a counterintuitive freezing scenario is observed in the presence of potassium. Potassium disrupts in-plane ordering in the fluid in contact with the mica surface, and freezing is observed only in the inner confined layers. The largest mica separations at which frozen phases were observed ranged from separations that could accommodate six to seven fluid layers. The extent of freezing and the square-to-triangular lattice transition was found to be sensitive to the presence of potassium as well as the thermodynamic conditions of the bulk fluid. The implications of our results on interpretation of surface force experiments as well as the generic phase behavior of confined soft spheres is discussed.     

Vibrational spectroscopy of mechanically compressed monolayers

Sum-frequency spectroscopy has been used to investigate the behavior of self-assembled monolayers in a solid-solid contact. Various alkanethiol layers on gold were observed before, during, and after compression to 660 MPa against a sapphire counterface. Well-ordered layers that differ only in the length of their alkane tails (C(8) versus C(18)) behave similarly. In contrast, defective and partly melted monolayers are more sensitive to stress than are their well-ordered analogues, and they are more prone to irreversible changes. In all cases, the intensity of methyl C-H stretching modes decreases with applied pressure, indicating a loss of net orientational order among the terminal methyl groups. The magnitude of this effect in well-ordered layers can be compared with the theoretical sensitivity of the resonant sum-frequency signal to molecular orientation. On these grounds, an increased population of terminal gauche conformers is identified as the disordering mechanism under pressure.     

Squeezing wetting and nonwetting liquids

We present molecular-dynamics results for the squeezing of octane (C8H18) between two approaching solid elastic walls with different wetting properties. The interaction energy between the octane bead units and the solid walls is varied from a very small value (1 meV), corresponding to a nonwetting surface with a very large contact angle (nearly 180 degrees), to a high value (18.6 meV) corresponding to complete wetting. When at least one of the solid walls is wetted by octane we observe well defined molecular layers develop in the lubricant film when the thickness of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers (n-->n-1 layering transitions). With increasing interaction energy between the octane bead units and the solid walls, the transitions from n to n-1 layers occur at higher average pressure. This results from the increasing activation barrier to nucleate the squeeze-out with increasing lubricant-wall binding energy (per unit surface area) in the contact zone. Thus, strongly wetting lubricant fluids are better boundary lubricants than the less wetting ones, and this should result in less wear. We analyze in detail the effect of capillary bridge formation (in the wetting case) and droplets formation (in the nonwetting case) on the forces exerted by the lubricant on the walls. For the latter case small liquid droplets may be trapped at the interface, resulting in a repulsive force between the walls during squeezing, until the solid walls come into direct contact, where the wall-wall interaction may be initially attractive. This effect is made use of in some practical applications, and we give one illustration involving conditioners for hair care application.     

Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity

The properties of butane (C4H10) lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider the linear n-butane and the branched isobutane. For the linear molecule, well defined molecular layers develop in the lubricant film when the width is of the order of a few atomic diameters. The branched isobutane forms more disordered structures which permit it to stay liquidlike at smaller surface separations. During squeezing the solvation forces show oscillations corresponding to the width of a molecule. At low speeds (<0.1 ms) the last layers of isobutane are squeezed out before those of n-butane. Since the (interfacial) squeezing velocity in most practical applications is very low when the lubricant layer has molecular thickness, one expects n-butane to be a better boundary lubricant than isobutane. With n-butane possessing a slightly lower viscosity at high pressures, our result refutes the view that squeeze-out should be harder for higher viscosities; on the other hand our results are consistent with wear experiments in which n-butane were shown to protect steel surfaces better than isobutane.     

受限空间内油水界面动态特性及粘附行为的荧光原位观测

水污染作为润滑油污染的常见形式,对润滑油本身以及机械系统都有巨大的危害。为了模拟实际非均匀多相系统中的界面行为,本文搭建了高精度点接触实验台来研究传统的不溶相替换问题。将目前静态平行受限空间内油水界面行为的研究推广到动态点接触楔形受限空间内,探究了游离水滴穿过点接触狭缝间毛细油池过程中的界面特性。重点关注固壁润湿性以及固壁的分离运动对整个侵入过程中液滴动态行为的影响。实验发现了铺展系数是决定油水界面融合和分离特性的关键因素,揭示了固壁润湿性和球盘间的相对分离运动会影响游离水滴穿过毛细油池之后的粘附行为。表面张力和液体与壁面之间的粘附功能够解释观测的实验现象。     

Interface analysis of Ti/Al/Ti/Au ohmic contacts with regrown n+‐GaN layers using molecular beam epitaxy

The regrowth technique of highly doped n‐type GaN layers is reliable and effective for lowering the ohmic contact resistance. The interface between metal contacts with Ti/Al/Ti/Au and regrown n⁺‐GaN/GaN layers were analyzed in detail with transmission electron microscopy. During the annealing process, Ti metals and N atoms diffusing from GaN layers formed TiN epitaxial layers between metal alloys and n⁺‐GaN layers. The orientational relationship between GaN and TiN was [1 0 0]_GaN//[?1 1 0]_TiN verified by nano‐beam diffraction. Al atoms diffused through the GaN layers and formed thin AlGaN phase. Al content was confirmed as 60% by high‐resolution transmission electron microscopy images. Electron energy loss spectroscopy showed that Si dopants were confined within n⁺‐GaN layers. These results show that in regrowth technique both TiN layers and Si dopants affect the contact properties because the formation of TiN layers can induce nitrogen vacancies from GaN, while Si‐doped GaN layers can enhance the tunneling effect through the metal contacts resulting in reduced contact resistance. Copyright © 2011 John Wiley & Sons, Ltd.     

Solvent-induced organization of squaraine dyes in solution capillary layers and adsorbed films

Unusual behavior of indolenine and hydroxyphenyl squaraines has been observed in solution capillary layers and adsorbed films. The confined solutions showed anomalous aggregation of squaraine molecules in contrast to their monomer behavior in the bulk solutions of the same concentration, along with formation of a macroscopic cell-like structure in the confined solution layer, with the diameter of cells being 3-5 microm. The aggregate structure, as observed through electronic absorption spectra, was strongly dependent on the chemical structure of squaraine used and solvent used, and it also was different from squaraine aggregates observed in aqueous solutions and films prepared by vacuum evaporation. It has been found that indolenine squaraine is capable of forming H-aggregates in confined dimethylformamide solutions and hydroxyphenyl squaraine is capable of forming J-aggregates in confined dimethylformamide solutions and adsorbed films. The results were compared with pseudoisocyanine, which forms J-aggregates in aqueous bulk solutions readily; however, no J-aggregates have been found in their capillary layers. The interplay of dye-dye, dye-surface, and dye-solvent interactions resulting in the above effects is discussed.     

Adhesion and stable low friction provided by a subnanometer-thick monolayer of a natural polysaccharide

Using a surface forces apparatus, we have investigated the adhesive and lubrication forces of mica surfaces separated by a molecularly thin, subnanometer film of a high-molecular-weight (2.3 MDa) anionic polysaccharide from the algae Porphyridium sp. adsorbed from aqueous solution. The adhesion and friction forces of the confined biopolymer were monitored as a function of time, shearing distance, and driving velocity under a large range of compressive loads (pressures). Although the thickness of the dilute polysaccharide was <1 nm, the friction was low (coefficient of friction = 0.015), and no wear was ever observed even at a pressure of 110 atm over 3 decades of velocity, so long as the shearing distances were less than twice the contact diameter. Atomic force microscopy in solution shows that the biopolymer is able to adsorb to the mica surface but remains mobile and easily dragged upon shearing. The adhesion (adsorption) of this polysaccharide even to negatively charged surfaces, its stable low friction, its robustness (high-load carrying capacity and good wear protection), and the weak (logarithmic) dependence of the friction force on the sliding velocity make this class of polyelectrolytes excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the remarkable tribological properties of the ultrathin polysaccharide monolayer are discussed and appear to be quite different from those of other polyelectrolytes and proteins that act as thick "polymer brush" layers.     

Unraveling the properties of octamethylcyclotetrasiloxane under nanoscale confinement: atomistic view of the liquidlike state from molecular dynamics simulation

We developed an atomistic model of octamethylcyclotetrasiloxane (OMCTS) liquid confined within the nanospace between two flat mica surfaces. Molecular dynamics simulation was carried out for the liquidlike state where OMCTS liquid is not frozen, while forming molecular layers parallel to the surface. With the aid of a layer by layer analysis of the intra- and interlayer microscopic structures and the dynamics, it is found that the difference in the properties of the inner layers and the bulk liquid are relatively small in spite of the clear differences in the structure. This leads to the conclusion that the layered structure itself is an appearance of the microscopic structure that already exists in the bulk liquid. The most striking difference from the bulk liquid is mainly seen in the contact layer, where characteristic molecular orientations that are not seen in the crystalline phase appeared, and the dynamics of the liquid becomes 2-3 orders of magnitude slower than that of the bulk.     

Surfactant boundary lubricant film modified by an amphiphilic diblock copolymer

The effect of the uptake of a low-molecular-weight amphiphilic diblock copolymer on the morphology of didodecyldimethylammonium bromide (DDAB) adsorbed layers on mica, the interactions between two coated surfaces, and the frictional properties of the boundary film have been studied using an atomic force microscope and a dynamic surface forces apparatus nanotribometer. When DDAB-coated surfaces in aqueous solution were compressed, hemifusion or removal of the adsorbed surfactant bilayers could not be induced, and no frictional force could be measured between the surfaces, which display superior lateral cohesion and lubricant properties. Coadsorbing octadecyl end modified poly(ethylene oxide) chains at low density facilitates hemifusion, generating significant shear stress and leading to stick-slip instabilities. The mixed films regain their lateral cohesion at higher adsorbed copolymer densities, but an extra short-range attraction brings the adsorbed layers into adhesive contact without causing bilayer hemifusion. Here, noticeable frictional forces are also measured.     

润滑剂对聚氨酯弹性体的反增塑效应

聚氨酯嵌段聚合物的硬链段易于聚集形成微晶相分散于软链段连续相中,原位组成橡塑合金使聚氨酸弹性体兼具高模量、高动态力学性能、高耐磨性等性质,但是决定这些性质的软、硬段形态结构强烈地受组成和制备工艺的影响,使聚合物具有优良的综合性能并不简单．影响软、硬段...     

First synthesis of silicalite-1 from a dry powder system via solid-solid transformation

Silicalite-1 has first been synthesized from a dry powder system with an adsorbedstate template, and is presented as a convincing example for direct solid-solid transformation process.     

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle theta>110 degrees ). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface-surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a "solid-like" elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at approximately 3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of mu approximately 1. A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and mu approximately 1.1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating properties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths.     

Probing double layer structures of Au (111)-BMIPF6 ionic liquid interfaces from potential-dependent AFM force curves

High quality AFM force curves are presented with detailed potential dependent layering behaviors of the ionic liquid molecules, from which charged interior and neutral exterior layers are distinguished. The electric double layer is confined within the interior layers of one to two molecular size within the potential range of up to 1 V negative of the PZC.     

Molecular modeling of freezing of simple fluids confined within carbon nanotubes

We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate molecules from the pore wall. The simulations show evidence of a rich phase behavior in confinement; a number of phases, some of them inhomogeneous, were observed for the pore sizes considered. The multiple transition temperatures obtained from the simulations were found to be in good agreement with recent dielectric relaxation spectroscopy experiments for CCl(4) confined within multiwalled carbon nanotubes.     

On scale dependence in friction: transition from intimate to monolayer-lubricated contact

Scale dependence in friction is studied in the present paper using the newly developed mesoscale friction tester (MFT). A transition in frictional shear strength from several hundreds of MPa to several tens of MPa was observed over a very limited range of contact radii (20-30 nm) in both ambient and dry environments. Thus, a single apparatus has been able to establish these two limits which are consistent with the values previously obtained from friction experiments using atomic force microscopy (AFM) and the surface force apparatus (SFA), respectively. Consequently, it is hypothesized here that a shear strength in the hundreds of MPa results from intimate contact (solid-solid) and a shear strength in the tens of MPa results from a monolayer-lubricated contact. Furthermore, both the probe size and the normal pressure govern the interfacial conditions in the contact zone and it is these conditions, rather than the nominal environment, which in turn determine the resulting shear strengths. A continuum analysis based on the Lifshitz theory for van der Waals interactions is used to explain the quantized shear strengths which were obtained from our experiments and previous AFM and SFA friction experiments. This quantized friction behavior [J.N. Israelachvili, P.M. McGuiggan, A.M. Homola, Science 240 (1988) 189] results from the discrete separation due to the different interfacial conditions that can arise between two sliding surfaces. The consistency between the analysis and the experimental results shows that this analysis is applicable for nonwear friction with single asperity contact.     

Adhesion of alumina surfaces through confined water layers containing various molecules

When two surfaces confine water layers between them at the nanoscale, the behavior of these confined water molecules can deviate significantly from the behavior of bulk water, and it could reflect on the adhesion of such surfaces. Thus, the aim of this study is to assess the role of confined water layers on the adhesion of hydrophilic surfaces and how sensitive this adhesion is to the presence of contaminants. Our methodology used under water AFM force measurements with an alumina-sputtered sphere-tipped cantilever and a flat alumina single crystal and then added fractions of ethanol, dimethylformamide, formamide, trimethylamine, and trehalose to water as contaminants. Such solutions were designed to illuminate the influences of dielectric constant, molecular size, refractive index, and number of hydrogen bonds from donors and acceptors of solutes to water. Apart from very dilute solutions of dimethylformamide, all solutions decreased the ability of confined water to give adhesion of the alumina surfaces. The predicted theoretical contribution of van der Waals and electrostatic forces was not observed when the contaminants distorted the way water organizes itself in confinement. The conclusion was that adhesion was sensitive mostly to the hydrogen-bonding network within water layers confined by the hydrophilic alumina surfaces.     

Structure of a cyclodextrin functionalized anionic clay: XRD analysis, spectroscopy, and computer simulations

Carboxy-methyl beta-cyclodextrin (CMCD) cavities have been intercalated within the galleries of anionic clay, Mg-Al layered double hydroxide (LDH). The cyclodextrin functionalized LDH has been reported to adsorb neutral and nonpolar guest molecules. X-ray diffraction, IR, and Raman vibrational spectroscopy and (13)C CPMAS NMR have been used to characterize the confined CMCD molecules, whereas molecular dynamics simulations have been used to probe the interlayer arrangement and orientation of the intercalated species. Spectroscopic measurements as well as MD simulations show that there is no significant change in the geometry of the CMCD cavity on intercalation. Within the galleries of the anionic clay, the CMCD anions are arranged as bilayers with the carboxy methyl substituents, located at the narrower opening of the bucket-like cyclodextrin toroid, anchored to the LDH sheet. This arrangement leaves the wider opening of the CMCD anion facing away from the layers allowing the interior of the cyclodextrin cavity to be accessible to guest molecules. Finally, the hydrophobicity of the anchored cyclodextrin cavity has been characterized using fluorescence from pyrene included within it.     

The Structure and Properties of Graphite Monofluoride Using the Three-Dimensional Cyclic Cluster Approach

In this paper, we present the theoretical study of the crystal and electron structure of an intercalated compound of graphite —the graphite monofluoride {CF}_n. The latter is widely used as a lubricant under extremely high temperatures and high vacuum, and as a successful cathodic depolarizer in batteries with high energy density. The layered structure of the graphite monofluoride has been confirmed, but statistical distributions of the individual layers are possible. This fact helps in understanding the problems linked to an experimental determination of the structure of this material. Small interlayer dissociation energies show that the bonding between the individual layers is mainly due to the weak interlayer electrostatic forces, which explains the excellent lubricant properties of this material. Band structure calculations reveal that, whereas some layer arrangements of the bulk material lead to insulating properties, others have a conductive character. This fact explains the weak overall conductive properties of synthetic graphite monofluoride.     

Relaxation and jump dynamics of water at the mica interface

The orientational relaxation dynamics of water confined between mica surfaces is investigated using molecular dynamics simulations. The study illustrates the wide heterogeneity that exists in the dynamics of water adjacent to a strongly hydrophilic surface such as mica. Analysis of the survival probabilities in different layers is carried out by normalizing the corresponding relaxation times with bulk water layers of similar thickness. A 10-fold increase in the survival times is observed for water directly in contact with the mica surface and a non-monotonic variation in the survival times is observed moving away from the mica surface to the bulk-like interior. The orientational relaxation time is highest for water in the contact layer, decreasing monotonically away from the surface. In all cases the ratio of the relaxation times of the 1st and 2nd rank Legendre polynomials of the HH bond vector is found to lie between 1.5 and 1.9 indicating that the reorientational relaxation in the different water layers is governed by jump dynamics. The orientational dynamics of water in the contact layer is particularly novel and is found to undergo distinct two-dimensional hydrogen bond jump reorientational dynamics with an average waiting time of 4.97 ps. The waiting time distribution is found to possess a long tail extending beyond 15 ps. Unlike previously observed jump dynamics in bulk water and other surfaces, jump events in the mica contact layer occur between hydrogen bonds formed by the water molecule and acceptor oxygens on the mica surface. Despite slowing down of the water orientational relaxation near the surface, life-times of water in the hydration shell of the K(+) ion are comparable to that observed in bulk salt solutions.