Short range attraction between two similarly charged silica surfaces

Using an atomic force microscope we measure the interaction between two identically charged silica surfaces in the presence of a saline solution. For pure NaCl the interaction is always repulsive. Upon addition of cobalt hexamine ions, Co(NH(3))(6)(+3), the repulsion is gradually suppressed and a pronounced attraction develops at distances much shorter than the screening length. Higher concentrations of cobalt hexamine turn the attraction back into repulsion. Measurements of surface charge renormalization by the trivalent cations provide their surface density and their association constant to the negatively charged silica surface. These estimates tend to exclude interaction between two condensed Wigner crystals as an explanation for the attraction.     

Inter-lamellar interactions modulated by addition of guest components

We have investigated the effects of a guest component (polymer or spherical colloidal particle) confined between flexible lamellar slits on the inter-lamellar interaction by means of a small-angle X-ray scattering technique and a neutron spin echo technique. The dominant interaction between flexible lamellar membranes without guest components is the Helfrich mechanism. The addition of a neutral polymer into the lamellar phase induces an attractive inter-lamellar interaction and finally destabilizes the lamellar phase. On the other hand, spherical colloidal particles confined between flexible lamellar membranes reduce the undulational fluctuations of lamellae and bring a repulsive inter-lamellar interaction. The behavior of the layer compression modulus of the lamellar membrane containing colloidal particles is well described by the entropical repulsive inter-lamellar interaction driven by steric hindrance.Received: 26 March 2004, Published online: 4 May 2004PACS: 82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions) - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 83.80.Hj Suspensions, dispersions, pastes, slurries, colloids - 89.75.Fb Structures and organization in complex systems     

Compositionally modulated ripples induced by sputtering of alloy surfaces

Sputtering of an amorphous or crystalline material by an ion beam often results in the formation of periodic nanoscale ripple patterns on the surface. In this Letter, we show that, in the case of alloy surfaces, the differences in the sputter yields and surface diffusivities of the alloy components will also lead to spontaneous modulations in composition that can be in or out of phase with the ripple topography. The degree of this kinetic alloy decomposition can be altered by varying the flux of the ion beam. In the high-temperature and low-flux regime, the degree of decomposition scales linearly with the ion flux, but it scales inversely with the ion flux in the low-temperature, high-flux regime.     

Nanowire manipulation on surfaces through electrostatic self‐assembly and magnetic interactions

A method for fabricating aligned nanowire arrays on surfaces is shown. Gold and segmented Au/Ni/Au nanowires of high aspect ratio have been prepared by template electrosynthesis, and functionalized with charged short alkanethiols that can be ionized in aqueous solutions. Different distributions of funtionalized nanowires could be obtained on large surfaces from nanowire aqueous suspensions, avoiding aggregation due to electrical repulsion. Due to the high magnetic anisotropy of segmented Au/Ni/Au nanowires chaining of aligned nanowires could be obtained by applying a low magnetic field. While electrostatics favours side wire interactions due to the high aspect ratio, concurrent electrostatics and applied magnetic field yields end‐to‐end interaction and linear alignment without bifunctionalization. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Emission of radiation by charged particles reflected from solid surfaces

It is predicted that charged particles reflected from solid surfaces could emit radiation due to the change in their direction of motion upon reflection. The theoretical spectrum of this radiation is obtained for specular reflection. The general concept of ‘transition radiation’ is reconsidered to include this new effect which is found to dominate for low-energy, non-penetrating electrons and for high-energy electrons at grazing incidence.     

Adsorption of polyampholytes on charged surfaces

We have studied the adsorption of neutral polyampholytes on model charged surfaces that have been characterized by contact angle and streaming current measurements. The loop size distributions of adsorbed polymer chains have been obtained using atomic-force microscopy (AFM) and compared to recent theoretical predictions. We find a qualitative agreement with theory; the higher the surface charge, the smaller the number of monomers in the adsorbed layer. We propose an original scenario for the adsorption of polyampholytes on surfaces covered with both neutral long-chain and charged short-chain thiols. Received 22 February 2002 and Received in final form 23 April 2002     

Adsorption of model charged proteins on charged surfaces with grafted polymers

The adsorption of model charged proteins on charged surfaces with and without grafted polymers is studied using a molecular approach. The ability of the polymer layer to reduce the amount of proteins adsorbed on top of the surface (primary adsorption) and at the same time to increase the adsorption of the proteins on top of the polymer layer (secondary adsorption) is presented. It is found that charging the free ends of the chains can result in an efficient way to enhance adsorption at the tip of the brush. Increasing the surface coverage of the polymers with charged free ends enhances the amount of proteins adsorbed at the tip of the polymer layer, while at the same time strongly reduces the number of proteins adsorbed directly onto the surface. The interplay between the attractive van der Waals protein-surface interactions, the steric polymer-protein interactions and the effect of the electrostatic interactions in determining the final adsorption is discussed. The manipulation of solution conditions to tune the amount of secondary adsorption is presented.     

Multiplicity distributions of charged hadrons invp and charged current interactions

Using data onvp and charged current interactions from a bubble chamber experiment with BEBC at CERN, the multiplicity distributions of charged hadrons are investigated. The analysis is based on 20000 events with incidentv and 10000 events with incident . The invariant massW of the total hadronic system ranges from 3 GeV to 14 GeV. The experimental multiplicity distributions are fitted by the binomial function (for different intervals ofW and in different intervals of the rapidityy), by the Levy function and the lognormal function. All three parametrizations give acceptable values forX ². For fixedW, forward and backward multiplicities are found to be uncorrelated. The normalized moments of the charged multiplicity distributions are measured as a function ofW. They show a violation of KNO scaling.     

晶体的双参量调制及其应用

利用折射率椭球分析法,分析了某些具有多重光学效应的光学晶体在两个外场同时作用下的一些特有调制规律.结果表明,当晶体的折射率椭球方程的三个交叉项中只有一项x1x2不为零时,可以得到其外场诱导新主轴折射率及其主轴取向的简单计算式.据此可以揭示出某些晶体在两个外加电场同时作用下将具有双横向电光Pockels效应,例如ˉ6点群的电光晶体.但一般晶体在双横向应力作用下不具有与双横向电光效应类似的调制特性,其弹光双折射大小与其应力差成正比,其双折射主轴方向一般为固定值.在相互垂直的外加应力和电场同时作用下,某些晶体(例如ˉ43m点群晶体)的双折射大小与外加应力和外加电场的加权几何平均值成正比,且新折射率主轴旋转角由外加应力和外加电场的比值来确定.晶体的上述双参量调制特性对设计新型光学调制器或传感器具有重要指导意义.     

Bound states and scattering resonances induced by spatially modulated interactions

We study the two-body problem with a spatially modulated interaction potential using a two-channel model, in which the interchannel coupling is provided by an optical standing wave and its strength modulates periodically in space. As the modulation amplitudes increase, there will appear a sequence of bound states. Part of them will cause a divergence of the effective scattering length, defined through the phase shift in the asymptotic behavior of scattering states. We also discuss how the local scattering length, defined through short-range behavior of scattering states, modulates spatially in different regimes. These results provide a theoretical guideline for a new control technique in the cold atom toolbox, in particular, for alkaline-earth(-like) atoms where the inelastic loss is small.     

Coherent electromagnetic radiation of a modulated beam of charged particles

The intensity of electromagnetic radiation produced by a modulated beam of charged particles is estimated. The coherence effect is due to the modulation, i.e. to periodicity in the particles distribution.     

Interaction of slow highly charged ions with surfaces

We show how a classical approach can be used to model electronic processes taking place when slow highly charged ions collide with surfaces. In particular we derive expressions for the distance of the first charge flow and for the image energy gain of an ion of chargeq outside a surface described by a workfunctionW and a static dielectric constant ?.     

Diffraction of light from weakly modulated silver surfaces

We have investigated the diffraction of light from sinusoidally modulated silver surfaces with grating periods from 4000 to 8000 Å for wavelengths of the visible spectrum. We compare our experiments with theoretical results from a light scattering theory in the first non-vanishing order of the modulation height. This theory includes polarization and the optical constants of the grating material. Good agreement of the theory with the experiments is found for weakly modulated gratings if grating anomalies are avoided.     

Adsorption of hydrophobic polyelectrolytes onto oppositely charged surfaces

We present a scaling theory for the adsorption of a weakly charged polyelectrolyte chain in a poor solvent onto an oppositely charged surface. Depending on the fraction of charged monomers and on the solvent quality for uncharged monomers, the globule in the bulk of the solution has either a spherical conformation or a necklace structure. At sufficiently high surface charge density, a chain in the globular conformation adsorbs in a flat pancake conformation due to the Coulombic attraction to the oppositely charged surface. Different adsorption regimes are predicted depending on two screening lengths (the Debye screening length monitored by the salt concentration and the Gouy-Chapman length monitored by the surface charge density), on the degree of ionization of the polymer and on the solvent strength. At low bulk ionic strength, an increase in the surface charge density may induce a transition from an adsorbed necklace structure to a uniform pancake due to the enhanced screening of the intra-chain Coulombic repulsion by the counterions localized near the surface. Received 12 April 2001     

Transverse polarizability of an aligned assembly of charged rods

The low-field transverse polarizability of the counterions condensed on an isolated charged rod is small. We show that it can be much larger if the rod is a member of an assembly of aligned rods. The polarization free energy of the assembly of rods in a transverse field is then similar to its polarization free energy in a field parallel to the rods. The polarization free energy of the assembly in a transverse field becomes lower than in a parallel field if the extent of the assembly (as measured, for example, by the diameter of a cylindrical assembly) is larger than the length of the individual rods. We suggest that this model may provide a reasonable explanation for the occurrence of “anomalous” birefringence in systems of interacting charged rodlike particles.     

偏心圆柱面与分离圆柱面带电导体等势面的统一描述

对偏心圆柱面带电导体和分离圆柱面带电导体的等势面的求解进行了统一的描述．采用保角映射法与分离变量法及计算工具软件Mathematica 4相结合,严格地求出了用初等函数表示的精确解,作出了相应的等势线簇图形,并且进行了必要的讨论．     

Creation of nanohillocks on CaF2 surfaces by single slow highly charged ions

Upon impact on a solid surface, the potential energy stored in slow highly charged ions is primarily deposited into the electronic system of the target. By decelerating the projectile ions to kinetic energies as low as 150 x q eV, we find first unambiguous experimental evidence that potential energy alone is sufficient to cause permanent nanosized hillocks on the (111) surface of a CaF(2) single crystal. Our investigations reveal a surprisingly sharp and well-defined threshold of potential energy for hillock formation which can be linked to a solid-liquid phase transition.     

Atomic assembly of metal surfaces and interfaces

Inert gas ion impacts can be used to manipulate atomic assembly processes during the growth of metallic superlattices but the detailed mechanisms are not well understood. Molecular dynamics simulations are used to investigate the effects of ion incident angle and fluence upon the reassembly and structure of a copper surface partially covered with cobalt asperities. In the low ion energy regime, increasing the ion fluence decreases the cobalt layer surface roughness while gradually leading to an increase in the degree of interfacial mixing. The flattening of asperities occurs by direct (athermal) ion activation of an Ehrlich-Schwoebel mechanism of atom jumping. Intermixing of cobalt surface atoms in an underlying copper layer is found to occur by a knock-on process and the lowest energy barriers for this occur in low-index 〈1 1 0〉 and 〈1 1 2〉 crystal (channeling) directions. The mechanistic insights gained from the study are used to simulate the ion assisted growth of a Cu/Co/Cu multilayer system. Using ion parameters chosen to selectively activate atomic assembly mechanisms that promote flat, unmixed interfacial structures, it is shown that Cu/Co/Cu multilayer structures with high quality, smooth and chemically sharp interfaces can be obtained by using oblique, low energy, moderate fluence ion assistance with an ion mass that is similar to the atomic mass of the metals.