A reversibly switching surface

We report the design of surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential. The change in wetting behavior was caused by surface-confined, single-layered molecules undergoing conformational transitions between a hydrophilic and a moderately hydrophobic state. Reversible conformational transitions were confirmed at a molecular level with the use of sum-frequency generation spectroscopy and at a macroscopic level with the use of contact angle measurements. This type of surface design enables amplification of molecular-level conformational transitions to macroscopic changes in surface properties without altering the chemical identity of the surface. Such reversibly switching surfaces may open previously unknown opportunities in interfacial engineering.     

Ultrafast dynamics at semiconductor and metal surfaces

A variety of important dynamical phenomena at metal and semiconductor surfaces are now being investigated with the use of new ultrafast measurement techniques involving lasers and nonlinear optics. Understanding of the rates and mechanisms for relaxation of optical excitations of the surface itself as well as those of adsorbates on the surface is providing new insight into surface chemistry, surface phase transitions, and surface recombination of charge carriers in semiconductors.     

Molecular crowding on the cell surface

Strong steric interactions among proteins on crowded living cell surfaces were revealed by measurements of the equilibrium spatial distributions of proteins in applied potential gradients. The fraction of accessible surface occupied by mobile surface proteins can be accurately represented by including steric exclusion in the statistical thermodynamic analysis of the data. The analyses revealed enhanced, concentration-dependent activity coefficients, implying unanticipated thermodynamic activity even at typical cell surface receptor concentrations.     

Fluctuations and bistabilities on catalyst nanoparticles

We show that coverage fluctuations on catalyst particles can drastically alter their macroscopic catalytic behavior. Scrutinizing the occurrence of kinetic bistabilities, it is demonstrated by molecular beam experiments on model catalysts that macroscopically observable bistabilities vanish completely with decreasing particle size, as previously predicted by theory. The effect is attributed to fluctuation-induced transitions between two kinetic reaction regimes, with a transition rate controlled by both particle size and surface defects. These results suggest that fluctuation-induced effects represent a general phenomenon affecting the reaction kinetics on nanostructured surfaces.     

Endotoxin-induced structural transformations in liquid crystalline droplets

The ordering of liquid crystals (LCs) is known to be influenced by surfaces and contaminants. Here, we report that picogram per milliliter concentrations of endotoxin in water trigger ordering transitions in micrometer-size LC droplets. The ordering transitions, which occur at surface concentrations of endotoxin that are less than 10(-5) Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the LC. The sensitivity of the LC to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet versus slab), supporting the hypothesis that interactions of endotoxin with topological defects in the LC mediate the response of the droplets. The LC ordering transitions depend strongly on glycophospholipid structure and provide new designs for responsive soft matter.     

Confinement-induced phase transitions in simple liquids

The liquid-to-solid transition of a simple model liquid confined between two surfaces was studied as a function of surface separation. From large surface separations (more than 1000 angstroms) down to a separation corresponding to seven molecular layers, the confined films displayed a liquid-like shear viscosity. When the surface separation was further decreased by a single molecular spacing, the films underwent an abrupt, reversible transition to a solid. At the transition, the rigidity of the confined films (quantified in terms of an "effective viscosity") increased reversibly by at least seven orders of magnitude.     

Dimensionality effects in the lifetime of surface states

A long-standing discrepancy between experimental and theoretical values for the lifetimes of holes in the surface-state electron bands on noble metal surfaces is resolved; previous determinations of both are found to have been in error. The ability of the scanning tunneling microscope to verify surface quality before taking spectroscopic measurements is used to remove the effects of defect scattering on experimental lifetimes, found to have been a significant contribution to prior determinations. A theoretical treatment of inelastic electron-electron scattering is developed that explicitly includes intraband transitions within the surface state band. In our model, two-dimensional decay channels dominate the electron-electron interactions that contribute to the hole decay and are screened by the electron states of the underlying three-dimensional electron system.     

Identification of a second dynamic state during stick-slip motion

Stick-slip, or interrupted, motion rather than smooth uninterrupted motion occurs in many different phenomena such as friction, fluid flow, material fracture and wear, sound generation, and sensory "texture." During stick-slip, a system is believed to undergo transitions between a static (solid-like) state and a kinetic (liquid-like) state. The stick-slip motion between various types of pretreated surfaces was measured, and a second, much more kinetic state that exhibits ultra-low friction was found. Transitions to and from this super-kinetic state also give rise to stick-slip motion but are fundamentally different from conventional static-kinetic transitions. The results here suggest practical conditions for the control of unwanted stick-slip and the attainment of ultra-low friction.     

A surface-tailored, purely electronic, mott metal-to-insulator transition

Mott transitions, which are metal-insulator transitions (MITs) driven by electron-electron interactions, are usually accompanied in bulk by structural phase transitions. In the layered perovskite Ca(1.9)Sr(0.1)RuO4, such a first-order Mott MIT occurs in the bulk at a temperature of 154 kelvin on cooling. In contrast, at the surface, an unusual inherent Mott MIT is observed at 130 kelvin, also on cooling but without a simultaneous lattice distortion. The broken translational symmetry at the surface causes a compressional stress that results in a 150% increase in the buckling of the Ca/Sr-O surface plane as compared to the bulk. The Ca/Sr ions are pulled toward the bulk, which stabilizes a phase more amenable to a Mott insulator ground state than does the bulk structure and also energetically prohibits the structural transition that accompanies the bulk MIT.     

Origin of stick-slip motion in boundary lubrication

Molecular dynamics simulations of atomically thin, fluid films confined between two solid plates are described. For a broad range of parameters, a generic stick-slip motion is observed, consistent with the results of recent boundary lubrication experiments. Static plates induce crystalline order in the film. Stick-slip motion involves periodic shear-melting transitions and recrystllization of the film. Uniform motion occurs at high velocities where the film no longer has time to order. These results indicate that the origin of stick-slip motion is thermodynamic instability of the sliding state, rather than a dynamic instability as usually assumed.     

Fractal surfaces of proteins

Fractal surfaces can be used to characterize the roughness or irregularity of protein surfaces. The degree of irregularity of a surface may be described by the fractal dimension D. For protein surfaces defined with probes in the range of 1.0 to 3.5 angstroms in radius, D is approximately 2.4 or intermediate between the value for a completely smooth surface (D = 2) and that for a completely space-filling surface (D = 3). Individual regions of proteins show considerable variation in D. These variations may be related to structural features such as active sites and subunit interfaces, suggesting that surface texture may be a factor influencing molecular interactions.     

哈尔滨热岛效应与植被指数关系的动态分析

采用哈尔滨2001年与1989的ETM+/TM影像,利用遥感影像所具有的热信息内容特点,以及地表温度反演原理建立了辐射温度空间模型(Radiant Temperature Spatial model),分别得到哈尔滨2001年与1989年热力场的分布情况.同样通过归一化植被指数(INDV)的原理也建立归一化植被指数空间模型,并用拟合法对热力场温度与归一化植被指数进行线性相关分析.结果表明:该区域2001年与1989的温度分布存在显著差异;地面温度与植被指数之间存在反相关关系.     

Designing superoleophobic surfaces

Understanding the complementary roles of surface energy and roughness on natural nonwetting surfaces has led to the development of a number of biomimetic superhydrophobic surfaces, which exhibit apparent contact angles with water greater than 150 degrees and low contact angle hysteresis. However, superoleophobic surfaces-those that display contact angles greater than 150 degrees with organic liquids having appreciably lower surface tensions than that of water-are extremely rare. Calculations suggest that creating such a surface would require a surface energy lower than that of any known material. We show how a third factor, re-entrant surface curvature, in conjunction with chemical composition and roughened texture, can be used to design surfaces that display extreme resistance to wetting from a number of liquids with low surface tension, including alkanes such as decane and octane.     

天然气吸附存储的实验研究方法

在车用天然气的吸附存储技术中，超临界温度下甲烷的物理吸附和天然气的各组分在多孔吸附剂上的吸附平衡具有重要的实际意义，而建立一个能清晰阐述气－固系统热力学特性的等温线模型是该领域的一个难点。由于模型的过于简化及忽略吸附剂表面能量不均一性及微孔分布等重要因素，使得分子模拟的结果也不够理想，因此开展这方面的实验与研究很有必要。综述了以车用存储为目的的天然气多组分气相吸附平衡的传统与最新的实验方法。     

Adhesion and friction mechanisms of polymer-on-polymer surfaces

The adhesion and friction of smooth polymer surfaces were studied below the glass transition temperature by use of a surface forces apparatus. The friction force of a crosslinked polymer was orders of magnitude less than that of an uncrosslinked polymer. In contrast, after chain scission of the outermost layers, the adhesion hysteresis and friction forces increase substantially. These results show that polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost polymer segments at shearing interfaces, and that both increase as a transition is made from crosslinked surfaces to surfaces with long chains to surfaces with quasi-free ends. The results suggest new ways for manipulating the adhesion and friction of polymer surfaces by adjusting the state of the surface chains.     

Excitation spectra of circular, few-electron quantum dots

Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow "atomic physics" experiments to be performed in magnetic field regimes not accessible for atoms.     

Steady-state coupling of ion-channel conformations to a transmembrane ion gradient

Under stationary conditions, opening and closing of single Torpedo electroplax chloride channels show that the number of transitions per unit time between inactivated and conducting states are unequal in opposite directions. This asymmetry, which increases with transmembrane electrochemical gradient for the chloride ion, violates the principle of microscopic reversibility and thus demonstrates that the channel-gating process is not at thermodynamic equilibrium. The results imply that the channel's conformational states are coupled to the transmembrane electrochemical gradient of the chloride ion.     

A cusp singularity in surfaces that minimize an anisotropic surface energy

A mathematical proof shows that a surface with a cusp-shaped singularity can arise from minimizing an anisotropic surface free energy for a portion of a crystal surface. Such cusps have been seen on crystal surfaces but usually have been interpreted as being the result of defects or nonequilibrium crystal growth. Our result predicts that they can occur as equilibrium or near-equilibrium phenomena. It also enriches the mathematical theory of minimal surfaces.     

Transformation of a simple plastic into a superhydrophobic surface

Superhydrophobic surfaces are generally made by controlling the surface chemistry and surface roughness of various expensive materials, which are then applied by means of complex time-consuming processes. We describe a simple and inexpensive method for forming a superhydrophobic coating using polypropylene (a simple polymer) and a suitable selection of solvents and temperature to control the surface roughness. The resulting gel-like porous coating has a water contact angle of 160 degrees. The method can be applied to a variety of surfaces as long as the solvent mixture does not dissolve the underlying material.     

Size-driven structural and thermodynamic complexity in iron oxides

Iron oxides occur ubiquitously in environmental, geological, planetary, and technological settings. They exist in a rich variety of structures and hydration states. They are commonly fine-grained (nanophase) and poorly crystalline. This review summarizes recently measured thermodynamic data on their formation and surface energies. These data are essential for calculating the thermodynamic stability fields of the various iron oxide and oxyhydroxide phases and understanding their occurrence in natural and anthropogenic environments. The competition between surface enthalpy and the energetics of phase transformation leads to the general conclusion that polymorphs metastable as micrometer-sized or larger crystals can often be thermodynamically stabilized at the nanoscale. Such size-driven crossovers in stability help to explain patterns of occurrence of different iron oxides in nature.