Showing papers in "Journal of Physics: Condensed Matter in 2005"

Electrowetting: from basics to applications

Abstract: Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects?rather than a unique one?are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices.

Effects of confinement on material behaviour at the nanometre size scale

Abstract: In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion.

Abstract: Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the atomic force microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.

The single-phase multiferroic oxides: from bulk to thin film

Abstract: Complex perovskite oxides exhibit a rich spectrum of properties, including magnetism, ferroelectricity, strongly correlated electron behaviour, superconductivity and magnetoresistance, which have been research areas of great interest among the scientific and technological community for decades. There exist very few materials which exhibit multiple functional properties; one such class of materials is called the multiferroics. Multiferroics are interesting because they exhibit simultaneously ferromagnetic and ferroelectric polarizations and a coupling between them. Due to the nontrivial lattice coupling between the magnetic and electronic domains (the magnetoelectric effect), the magnetic polarization can be switched by applying an electric field; likewise the ferroelectric polarization can be switched by applying a magnetic field. As a consequence, multiferroics offer rich physics and novel devices concepts, which have recently become of great interest to researchers. In this review article the recent experimental status, for both the bulk single phase and the thin film form, has been presented. Current studies on the ceramic compounds in the bulk form including Bi(Fe,Mn)O3, REMnO3 andthe series of REMn2O5 single crystals (RE = rare earth) are discussed in the first section and a detailed overview on multiferroic thin films grown artificially (multilayers and nanocomposites) is presented in the second section.

Transition metal-doped TiO2 and ZnO?present status of the field

Abstract: There has been considerable recent interest in the design of diluted magnetic semiconductors, with a particular focus on the exploration of different semiconductor hosts. Among these, the oxide-based diluted magnetic semiconductors are attracting increasing attention, following reports of room temperature ferromagnetism in anatase TiO2 and wurtzite ZnO doped with a range of transition metal ions. In this review we summarize the current status of oxide-based diluted magnetic semiconductors, and discuss the influence of growth method, substrate choice, and temperature on the microstructure and subsequent magnetic properties of thin films. We outline the experimental conditions that promote large magnetization and high ferromagnetic Curie temperature. Finally, we review the proposed mechanisms for the experimentally observed ferromagnetism and compare the predictions to the range of available data.

Negative thermal expansion

Abstract: There has been substantial renewed interest in negative thermal expansion following the discovery that cubic ZrW2O8 contracts over a temperature range in excess of 1000 K. Substances of many different kinds show negative thermal expansion, especially at low temperatures. In this article we review the underlying thermodynamics, emphasizing the roles of thermal stress and elasticity. We also discuss vibrational and non-vibrational mechanisms operating on the atomic scale that are responsible for negative expansion, both isotropic and anisotropic, in a wide range of materials.

Thermal quenching of Eu2+ 5d-4f luminescence in inorganic compounds

Abstract: The thermal quenching of Eu2+ 5d–4f emission on Ba, Sr, or Ca sites in compounds is often attributed to a large displacement between the ground state and excited state in the configuration coordinate diagram. This work will demonstrate that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism. A model is proposed to explain why in some types of compounds the quenching temperature decreases when going from the Ba variant via the Sr variant to the Ca variant and in other types of compounds the reverse behaviour occurs. The nature of the bottom of the conduction band plays an important role in this.

The frustration-based approach of supercooled liquids and the glass transition: a review and critical assessment

Abstract: One of the most spectacular phenomena in physics in terms of dynamical range is the glass transition and the associated slowing down of flow and relaxation with decreasing temperature. That it occurs in many different liquids seems to call for a 'universal' theory. In this article, we review one such theoretical approach, which is based on the concept of 'frustration'. Frustration in this context describes an incompatibility between extension of the locally preferred order in a liquid and tiling of the whole space. We provide a critical assessment of what has been achieved within this approach and we discuss its relation with other theories of the glass transition.

Computer simulations of supercooled polymer melts in the bulk and in confined geometry

Abstract: We survey results of computer simulations for the structure and dynamics of supercooled polymer melts and films. Our survey is mainly concerned with features of a coarse grained polymer model?a bead?spring model?in the temperature regime above the critical glass temperature Tc of the ideal mode-coupling theory (MCT). We divide our discussion into two parts: a part devoted to bulk properties and a part dealing with thin films. The discussion of the bulk properties focuses on two aspects: a comparison of the simulation results with MCT and an analysis of dynamic heterogeneities. We explain in detail how the analyses are performed and what results may be obtained, and we critically assess their strengths and weaknesses. In discussing the application of MCT we also present first results of a quantitative comparison which does not rely on fits, but exploits static input from the simulation to predict the relaxation dynamics. The second part of this review is devoted to extensions of the simulations from the bulk to thin films. We explore in detail the influence of the boundary condition, imposed by smooth or rough walls, on the structure and dynamics of the polymer melt. Geometric confinement is found to shift the glass transition temperature Tg (or Tc in our case) relative to the bulk. We compare our and other simulation results for the Tg shift with experimental data, briefly survey some theoretical ideas for explaining these shifts and discuss related simulation work on the glass transition of confined liquids. Finally, we also present some technical details of how to perform fits to MCT and give a brief introduction to another approach to the glass transition based on the potential energy landscape of a liquid.

Flow-induced structure in colloidal suspensions

Abstract: We review the sequences of structural states that can be induced in colloidal suspensions by the application of flow. Structure formation during flow is strongly affected by the delicate balance among interparticle forces, Brownian motion and hydrodynamic interactions. The resulting non-equilibrium microstructure is in turn a principal determinant of the suspension rheology. Colloidal suspensions with near hard-sphere interactions develop an anisotropic, amorphous structure at low dimensionless shear rates. At high rates, clustering due to strong hydrodynamic forces leads to shear thickening rheology. Application of steady-shear flow to suspensions with repulsive interactions induces a rich sequence of transitions to one-, two-and three-dimensional order. Oscillatory-shear flow generates metastable ordering in suspensions with equilibrium liquid structure. On the other hand, short-range attractive interactions can lead to a fluid-to-gel transition under quiescent suspensions. Application of flow leads to orientation, breakup, densification and spatial reorganization of aggregates. Using a non-Newtonian suspending medium leads to additional possibilities for organization. We examine the extent to which theory and simulation have yielded mechanistic understanding of the microstructural transitions that have been observed.

Slow dynamics in glassy soft matter

Abstract: Measuring, characterizing, and modelling the slow dynamics of glassy soft matter is a great challenge, with an impact that ranges from industrial applications to fundamental issues in modern statistical physics, such as the glass transition and the description of out-of-equilibrium systems. Although our understanding of these phenomena is still far from complete, recent simulations and novel theoretical approaches and experimental methods have shed new light on the dynamics of soft glassy materials. In this paper, we review the work of the last few years, with an emphasis on experiments in four distinct and yet related areas: the existence of two different glass states (attractive and repulsive), the dynamics of systems very far from equilibrium, the effect of an external perturbation on glassy materials, and dynamical heterogeneity.

Rheology of liquid foam

Abstract: Liquid foams can behave like solids or liquids, depending on the applied stress and on the experimental timescale. Understanding the origin of this complex rheology which gives rise to many applications and which resembles that of many other forms of soft condensed matter made of closely packed soft units requires challenging theoretical questions to be solved. We briefly recall the basic physics and physicochemistry of foams and review the experiments, numerical simulations and theoretical models concerning foam rheology published in recent years.

Magnetoelectronics with magnetoelectrics

Abstract: Magnetoelectric films are proposed as key components for spintronic applications. The net magnetic moment created by an electric field in a magnetoelectric thin film influences the magnetization state of a neighbouring ferromagnetic layer through exchange coupling. Pure electrical control of magnetic configurations of giant magnetoresistance spin valves and tunnelling magnetoresistance elements is therefore achievable. Estimates based on documented magnetoelectric tensor values show that exchange fields reaching 100 mT can be obtained. We propose a mechanism alternative to current-induced magnetization switching, providing access to a wide range of device impedance values and opening the possibility of simple logic functions.

A "magnetic" interatomic potential for molecular dynamics simulations

Abstract: We develop a semi-empirical many-body interatomic potential suitable for large scale molecular dynamics simulations of magnetic α-iron. The functional form of the embedding part of the potential is derived using a combination of the Stoner and the Ginzburg–Landau models. We show that it is the symmetry broken solutions of the Ginzburg–Landau model describing spontaneous magnetization of atoms that provide the link between magnetism and interatomic forces. We discuss a range of potential applications of the new method.

Crack propagation in rubber-like materials

Abstract: Crack propagation in rubber-like materials is of great practical importance but still not well understood We study the contribution to the crack propagation energy (per unit area) G from the viscoelastic deformations of the rubber in front of the propagating crack tip We show that G takes the standard form G(v,T) = G0[1+f(v,T)] where G0 is associated with the (complex) bond-breaking processes at the crack tip while f(v,T) is determined by the viscoelastic energy dissipation in front of the crack tip As applications, we discuss the role of crack propagation for adhesion, rolling resistance and sliding friction for smooth surfaces, and for rubber wear

Negative refractive index metamaterials from inherently non-magnetic materials for deep infrared to terahertz frequency ranges

Abstract: We present a new set of artificial structures which can exhibit a negative refractive index band in excess of 6% in a broad frequency range from the deep infrared to the terahertz region. The structures are composites of two different kinds of non-overlapping spheres, one made from inherently non-magnetic polaritonic and the other from a Drude-like material. The polaritonic spheres are responsible for the existence of negative effective magnetic permeability whils tt he Drude-like spheres are responsible for negative effective electric permittivity. The resulting negative refractive index structures are truly subwavelength structures with wavelength-to-structure ratio 14:1, which is almost 50% higher than has been previously achieved. Our results are explained in the context of the extended Maxwell–Garnett theory and are reproduced by calculations based on the layer Korringa–Kohn–Rostoker method, an ab initio multiple scattering theory. The role of absorption in the constituent materials is discussed. Effective medium computer F77 code is freely available at http://www.wave-scattering.com

Density minimum and liquid?liquid phase transition

Abstract: We present a high-resolution computer simulation study of the equation of state of ST2 water, evaluating the liquid-state properties at 2718 state points, and precisely locating the liquid–liquid critical point (LLCP) occurring in this model. We are thereby able to reveal the interconnected set of density anomalies, spinodal instabilities and response function extrema that occur in the vicinity of an LLCP for the case of a realistic, off-lattice model of a liquid with local tetrahedral order. In particular, we unambiguously identify a density minimum in the liquid state, define its relationship to other anomalies, and show that it arises due to the approach of the liquid structure to a defect-free random tetrahedral network of hydrogen bonds.

Transparent Cr-doped SnO2 thin films: ferromagnetism beyond room temperature with a giant magnetic moment

Abstract: Laser ablated Cr-doped SnO2 thin films grown on various kinds of substrates all show ferromagnetism well beyond room temperature. Surprisingly, films of Sn0.95Cr0.05O2 grown on LaAlO3 substrates have a giant magnetic moment of 6 μB/Cr, which is 20–30 times larger than that of films grown under the same conditions on SrTiO3 and R-cut sapphire substrates. All films are highly transparent.

Dynamics and structure formation in thin polymer melt films

Abstract: The stability of thin liquid coatings plays a fundamental role in everyday life. We studied the stability conditions of thin (3 to 300 nm) liquid polymer films on various substrates. The key role is played by the effective interface potential of the system air/film/substrate, which determines the dewetting scenario in case the film is not stable. We describe in this study how to distinguish a spinodal dewetting scenario from heterogeneous and homogeneous dewetting by analysing the emerging structures of the film surface by e.g. Minkowski measures. We also include line tension studies of tiny droplets, showing that the long-range part of does affect the drop profile, but only very close to the three phase boundary line. The dynamic properties of the films are characterized via various experimental methods: the form of the dewetting front, for example, was recorded by scanning probe microscopy and gives insight into the boundary condition between the liquid and the substrate. We further report experiments probing the viscosity and the glass transition temperature of nm-thick films using e.g. ellipsometry. Here we find that even short-chained polymer melts exhibit a significant reduction of the glass transition temperature as the film thickness is reduced below 100 nm.

Ultimate limits to thermally assisted magnetic recording

Abstract: The application of thermal energy to enable recording on extremely high anisotropy magnetic media appears to be a viable means of extending the density of stored information. The central physical issue facing the technology is what gain can be realized in writability along with long-term data stability using imaginable media materials. We reasonably expect the material properties M(T) and Hk(T) to determine this, since a stability metric for media with characteristic magnetization switching unit volume V is MV Hk/2kBT. This matter is controversial owing to still open questions related to thermomagnetic recording with temperature elevation above the Curie point and optimal cooling rates. There are indications that multi-component magnetic media may offer advantages in achieving performance goals. Beyond the physical issues lie engineering matters related to the correct system architecture to yield a practical storage device to meet future customer expectations. Here one must address a detailed means of delivering localized heating to the magnetic medium to perform efficient recording. To date, magnetic recording devices have been highly mechanical systems, so it is natural to inquire how a need for an aggressively heated head–medium interface could impact the evolution of future systems. Eventually elements of thermally assisted recording could be combined with patterned media approaches such as self-organized magnetic arrays to push toward ultimate limits where the thermal instability of bits overtakes engineered media materials. Finally, a practical recording system cannot be realized unless a means of finding, following, and reading the smallest bits with a usable signal-to-noise ratio exists—engineering issues separate from an ability to reliably record those bits.

Effects of dopant states on photoactivity in carbon-doped TiO2

Abstract: Recently an effective photoresponse in the visible-light region was experimentally observed in carbon-doped TiO2. We contribute a theoretical understanding of this phenomenon. Our ab initio density functional theory investigations show that substitutional carbon dopants incorporated into TiO2 drastically affect the electronic structure of the material, thus improving its photoactivity. The resulting bandgap of 2.35 eV predicted in this work agrees with the available experimental observations for carbon concentration around 5%. We also address the effects of doping concentration on the photoresponse of this material.

Velocity profile of granular flows inside silos and hoppers

Abstract: We measure the flow of granular materials inside a quasi-two-dimensional silo as it drains and compare the data with some existing models. The particles inside the silo are imaged and tracked with unprecedented resolution in both space and time to obtain their velocity and diffusion properties. The data obtained by varying the orifice width and the hopper angle allow us to thoroughly test models of gravity driven flows inside these geometries. All of our measured velocity profiles are smooth and free of the shock-like discontinuities ('rupture zones') predicted by critical state soil mechanics. On the other hand, we find that the simple kinematic model accurately captures the mean velocity profile near the orifice, although it fails to describe the rapid transition to plug flow far away from the orifice. The measured diffusion length b, the only free parameter in the model, is not constant as usually assumed, but increases with both the height above the orifice and the angle of the hopper. We discuss improvements to the model to account for the differences. From our data, we also directly measure the diffusion of the particles and find it to be significantly less than predicted by the void model, which provides the classical microscopic derivation of the kinematic model in terms of diffusing voids in the packing. However, the experimental data are consistent with the recently proposed spot model, based on a simple mechanism for cooperative diffusion. Finally, we discuss the flow rate as a function of the orifice width and hopper angles. We find that the flow rate scales with the orifice size to the power of 1.5, consistent with dimensional analysis. Interestingly, the flow rate increases when the funnel angle is increased.

Reverse Monte Carlo modelling of the structure of disordered materials with RMC++ : a new implementation of the algorithm in C++

Abstract: The basic reverse Monte Carlo algorithm, as applied primarily for the study of disordered systems, is introduced, using an example of a new reverse Monte Carlo computer code. RMC++ is a new implementation of the RMC algorithm in C++. Its main purpose is to provide the community with a fast, flexible and documented code for RMC simulations, compatible with the rmca distribution. The source code, the documentation and the executable files are made available through the Internet. The flexibility of the code is exemplified by the implementation of a 'molecular move' step in the Metropolis algorithm. This feature, as well as a performance comparison, is illustrated with simulations performed for molecular liquids such as CCl4 and C2Cl4.

Coherent x-ray scattering

Abstract: This is a tutorial paper on the properties of partially coherent hard x-ray beams and their use in the structural analysis of condensed matter. The role of synchrotron radiation in the generation of coherent x-ray beams is highlighted and the requirements on the source properties are discussed. The technique of phase contrast imaging is briefly explained, as well as diffraction in the Fresnel and Fraunhofer regimes. The origin of speckle is elucidated and it is shown how oversampling of the diffraction pattern by at least a factor of two enables retrieval of the phases of the waves scattered from different parts of the object. This in turn allows for a direct reconstruction of the object's structure. One-dimensional objects, such as a fluid confined between two surfaces, cannot be unambiguously reconstructed by phase retrieval without additional assumptions. A trial-and-error method based on the analysis of waveguiding modes within the confined geometry is discussed.

Alkali metal adsorption on graphite : a review

Abstract: The adsorption of alkali metals on graphite has been the subject of various studies for the past two decades. Briefly, two main reasons can be offered to justify the persisting interest in these adsorption systems. First, experiments have pointed out intriguing structural phase transitions of the adsorbed species, and, second, in an attempt to explain the experimental results, the more complicated question of the nature of alkali metal–graphite bonding arose. Despite the relative simplicity of the electronic structure of the alkali metals, their interaction with the graphite surface is still the subject of current debate. This review paper presents relevant experimental data and results of selected theoretical calculations that, in time, guided the process of scientific discovery towards the current understanding of the alkali metals/graphite adsorption systems.

High pressure study of structural and electronic properties of calcium chalcogenides

Abstract: The structural and electronic properties of calcium chalcogenides CaX (X = S,Se,Te) under high pressure have been investigated using the full potential linearized augmented plane wave method within density functional theory. We used both the local density approximation and the generalized gradient approximation (GGA) that is based on exchange?correlation energy optimization for calculating the total energy. Moreover, the Engel?Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The equilibrium lattice constant for CaX compounds agrees well with the experimental results. The pressures at which these compounds undergo a structural phase transition from NaCl-type to CsCl-type were calculated. A numerical first-principles calculation of the elastic constants was used to calculate C11, C12 and C44. The energy band gaps at ambient conditions in the NaCl-type structure and the volume dependence of band gaps in the CsCl-type structure up to the band overlap metallization were investigated. Besides this, the nature of the chemical bond in these compounds was analysed in terms of electronic charge density.

Cycle kinetics, steady state thermodynamics and motors—a paradigm for living matter physics

Abstract: An integration of the stochastic mathematical models for motor proteins with Hill's steady state thermodynamics yields a rather comprehensive theory for molecular motors as open systems in the nonequilibrium steady state. This theory, a natural extension of Gibbs' approach to isothermal molecular systems in equilibrium, is compared with other existing theories with dissipative structures and dynamics. The theory of molecular motors might be considered as an archetype for studying more complex open biological systems such as biochemical reaction networks inside living cells.

Density functional theory of the electrical double layer: the RFD functional

Abstract: Density functional theory (DFT) of electrolytes is applied to the electrical double layer under a wide range of conditions. The ions are charged, hard spheres of different size and valence, and the wall creating the double layer is uncharged, weakly charged, and strongly charged. Under all conditions, the density and electrostatic potential profiles calculated using the recently proposed RFD electrostatic functional (Gillespie et al 2002 J. Phys.: Condens. Matter 14 12129; 2003 Phys. Rev. E 68 031503) compare well to Monte Carlo simulations. When the wall is strongly charged, the RFD functional results agree with the results of a simpler perturbative electrostatic DFT, but the two functionals' results qualitatively disagree when the wall is uncharged or weakly charged. The RFD functional reproduces these phenomena of weakly charged double layers. It also reproduces bulk thermodynamic quantities calculated from pair correlation functions.

Ageing, fragility and the reversibility window in bulk alloy glasses

Abstract: Non-reversing relaxation enthalpies (� Hnr )a t glass transitions Tg(x) in the Px Gex Se1−2x ternary display wide, sharp and deep global minima (� 0) in the 0.09 < x < 0.145 range, within which Tgs become thermally reversing .I n this reversibilitywindow ,g lasse sa re foundnottoage ,i n contrast toageing observed for fragile glass compositions outside the window. Thermal reversibility and lack of ageing seem to be paradigms of self-organization which molecular glasses share with protein structures which repetitively and reversibly change conformation near Tg and the folding temperature respectively. Ageing occurs in many materials, both organic and inorganic. Inorganic crystals age under electrical, mechanical, or thermal stresses, ofte na s ar esultof dislocation motion. Ageing in organic materials is more complex, occurring as hydrogen bonding configurations are altered as ar esult of thermal cycling. The more complex the system, the less understood are the causes of ageing, and the causes of ageing in living systems are one of the greatest unsolved mysteries in science. Here we will show that inorganicnon-crystalline nanonetworks are universally divided into three regimes of composition, two of which age rapidly, while the third regime scarcely ages at all. The third regime defines a narrow window of composition that appears to have much in common mechanically with selected organic nanonetworks, namely the polypeptide chains that form proteins. Ageing is not evident in data obtained by conventional structural methods (diffraction, infrared and Raman spectra, magnetic resonance), but it is measured very accurately by modulated differential scanning calorimetry. New ideas on the nature of glass transitions (Tg )h av ee merged in recent years from examination [1–3] of the non-reversing relaxation enthalpy (� Hnr )a ssociated with Tg in temperature modulated differential scanning calorimetry (MDSC) measurements. In MDSC,

Mechanical properties of wet granular materials

Abstract: We elaborate on the impact of liquids upon the mechanical properties of granular materials. We find that most of the experimental and simulation results may be accounted for by a simple model assuming frictionless, spherical grains, with a hysteretic attractive interaction between neighbouring grains due to capillary forces.