Shock compression of precompressed deuterium

Micro-to-macro transitions for continua with surface structure at the microscale

An improved technique for characterizing the fracture toughness via scratch test experiments

We review in this article the current theoretical understanding of collective and single particle diffusion on surfaces and how it relates to the existing experimental data. We begin with a brief survey of the experimental techniques that have been employed for the measurement of the surface diffusion coefficients. This is followed by a section on the basic concepts involved in this field. In particular, we wish to clarify the relation between jump or exchange motion on microscopic length scales, and the diffusion coefficients which can be defined properly only in the long length and time scales. The central role in this is played by the memory effects. We also discuss the concept of diffusion under nonequilibrium conditions. In the third section, a variety of different theoretical approaches that have been employed in studying surface diffusion such as first principles calculations, transition state theory, the Langevin equation, Monte Carlo and molecular dynamics simulations, and path integral formalism are presented. These first three sections form an introduction to the field of surface diffusion. Section 4 contains subsections that discuss surface diffusion for various systems which have been investigated both experimentally and theoretically. The focus here is not so much on specific systems but rather on important issues concerning diffusion measurements or calculations. Examples include the influence of steps, diffusion in systems undergoing phase transitions, and the role of correlation and memory effects. Obviously, the choice of topics here reflects the interest and expertise of the authors and is by no means exhaustive. Nevertheless, these topics form a collection of issues that are under active investigation, with many important open questions remaining.

Multi-time-scale 3-D coordinated formation control for multi-underactuated AUV with uncertainties: Design and stability analysis using singular perturbation methods

We discuss a class of similarity solutions to the hydrodynamic equations that describe droplets spreading under capillarity. The spreading time scale of these solutions exhibits a subtle dependence on the microscopic length scale around the contact line. We show that such solutions are linearly stable to small perturbations away from the contact line, justifying the universality of experimental spreading laws. We discuss the transition between the two macroscopic spreading regimes. Our prediction for the transition time is consistent with experimental data.

Neutron inelastic scattering from molecular liquids and glasses

In Buckingham’s model of an unconsolidated sediment 4 3 5 , 6 , the dissipation of acoustic energy is caused primarily by the rubbing of grains against one another. Moreover, the thinness of the pore fluid sandwiched between the grains is thought to be responsible for strain hardening in the granular medium. When two grains begin to slide against one another first there is a period when the particles stick, or in Buckingham’s terminology their asperities are effectively pinned 6 . On being triggered into slipping the contact will become tighter and the effective viscosity of the lubricating fluid film will increase giving rise to strain hardening. The methodology we shall assume is that of homogenization where there is assumed a small parameter 6 which describes the microstructure. Compared to the previous work in this field, our approach is most reminiscent of that in the paper of Burridge and Keller 7. A microscopic length scale 6 is introduced by assuming that material properties are of the form f(z, t), where y = is the so-called fast variable. To develop such a framework, we sought a model in which the length scale separation is maintained by imposing more flexible assumptions than the overly restrictive periodicity. We shall refer to a material as being admissible, if these fluctuations are comparable to 6 in size.

We apply an anisotropic version of the polymer reference interaction site model (PRISM) integral equation description of flexible polymers to analyze athermal liquid crystallinity. The polymers are characterized by a statistical segment length, σo, and by a physical hard-core thickness, d, that prevents the overlap of monomers on different chains. At small segment densities, ρ, the microscopic length scale d is irrelevant (as it must be in the universal semidilute regime), but becomes important in concentrated solutions and melts. Under the influence of the excluded volume interactions alone, the chains undergo a lyotropic, first-order isotropic–nematic transition at a concentration dependent upon the dimensionless “aspect ratio,” σo/d. The transition becomes weaker as d→0, becoming second order, as has been previously shown. We extend the theory to describe the transition of rigid, thin rods, and discuss the evolution of the anisotropic liquid structure in the ordered phase.

Additive force fields are designed to account for induced electronic polarization in a mean-field average way, using effective empirical fixed charges. The limitation of this approximation is cause for serious concerns, particularly in the case of lipid membranes, where the molecular environment undergoes dramatic variations over microscopic length scales. A polarizable force field based on the classical Drude oscillator offers a practical and computationally efficient framework for an improved representation of electrostatic interactions in molecular simulations. Building on the first-generation Drude polarizable force field for the dipalmitoylphosphatidylcholine 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) molecule, the present effort was undertaken to improve this initial model and expand the force field to a wider range of phospholipid molecules. New lipids parametrized include dimyristoylphosphatidylcholine (DMPC), dilauroylphosphatidylcholine (DLPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), dipalmitoylphosphatidylethanolamine (DPPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). The iterative optimization protocol employed in this effort led to lipid models that achieve a good balance between reproducing quantum mechanical data on model compound representative of phospholipids and reproducing a range of experimental condensed phase properties of bilayers. A parametrization strategy based on a restrained ensemble-maximum entropy methodology was used to help accurately match the experimental NMR order parameters in the polar headgroup region. All the parameters were developed to be compatible with the remainder of the Drude polarizable force field, which includes water, ions, proteins, DNA, and selected carbohydrates.

From anti-counterfeiting to biotechnology applications, there is a strong demand for encoded surfaces with multiple security layers that are prepared by stochastic processes and are adaptable to deterministic fabrication approaches. Here, we present dewetting instabilities in nanoscopic (thickness <100 nm) polymer films as a form of physically unclonable function (PUF). The inherent randomness involved in the dewetting process presents a highly suitable platform for fabricating unclonable surfaces. The thermal annealing-induced dewetting of poly(2-vinyl pyridine) (P2VP) on polystyrene-grafted substrates enables fabrication of randomly positioned functional features that are separated at a microscopic length scale, a requirement set by optical authentication systems. At a first level, PUFs can be simply and readily verified via reflection of visible light. Area-specific electrostatic interactions between P2VP and citrate-stabilized gold nanoparticles allow for fabrication of plasmonic PUFs. The strong surface-enhanced Raman scattering by plasmonic nanoparticles together with incorporation of taggants facilitates a molecular vibration-based security layer. The patterning of P2VP films presents opportunities for fabricating hybrid security labels, which can be resolved through both stochastic and deterministic pathways. The adaptability to a broad range of nanoscale materials, simplicity, versatility, compatibility with conventional fabrication approaches, and high levels of stability offer key opportunities in encoding applications.

Magainin 2 and PGLa in bacterial membrane mimics III: Membrane fusion and disruption

Marked hydration changes occur during the self-assembly of the melittin protein tetramer in water. Hydrophobicity induces a drying transition in the gap between simple sufficiently large (more than 1 nm(2)) strongly hydrophobic surfaces as they approach each other, resulting in the subsequent collapse of the system, as well as a depletion of water next to single surfaces. Here we investigate whether the hydrophobic induced collapse of multidomain proteins or the formation of protein oligimers exhibits a similar drying transition. We performed computer simulations to study the collapse of the tetramer of melittin in water, and observed a marked water drying transition inside a nanoscale channel of the tetramer (with a channel size of up to two or three water-molecule diameters). This transition, although occurring on a microscopic length scale, is analogous to a first-order phase transition from liquid to vapour. We find that this drying is very sensitive to single mutations of the three isoleucines to less hydrophobic residues and that such mutations in the right locations can switch the channel from being dry to being wet. Thus, quite subtle changes in hydrophobic surface topology can profoundly influence the drying transition. We show that, even in the presence of the polar protein backbone, sufficiently hydrophobic protein surfaces can induce a liquid-vapour transition providing an enormous driving force towards further collapse. This behaviour was unexpected because of the absence of drying in the collapse of the multidomain protein 2,3-dihydroxybiphenyl dioxygenase (BphC).

Non-Debye relaxations in disordered ionic solids

When boundary conditions arising from the usual hydrodynamic assumptions are applied, analyses of dynamic wetting processes lead to a well-known nonintegrable stress singularity at the dynamic contact line, necessitating new ways to model this problem. In this paper, numerical simulations for a set of representative problems are used to explore the possibility of providing material boundary conditions for predictive models of inertialess moving contact line processes. The calculations reveal that up to Capillary number Ca=0.15, the velocity along an arc of radius 10Li (Li is an inner, microscopic length scale) from the dynamic contact line is independent of the macroscopic length scale a for a&amp;gt;103Li, and compares well to the leading order analytical “modulated-wedge” flow field [R. G. Cox, J. Fluid Mech. 168, 169 (1986)] for Capillary number Ca&amp;lt;0.1. Systematic deviations between the numerical and analytical velocity field occur for 0.1&amp;lt;Ca&amp;lt;0.15, caused by the inadequacy of the leading order analytical solution over this range of Ca. Meniscus shapes produced from calculations in a truncated domain, where the modulated-wedge velocity field [R. G. Cox, J. Fluid Mech. 168, 169 (1986)] is used as a boundary condition along an arc of radius R=10−2a from the dynamic contact line, agree well with those using two inner slip models for Ca&amp;lt;0.1, with a breakdown at higher Ca. Computations in a cylindrical geometry reveal the role of azimuthal curvature effects on velocity profiles in the vicinity of dynamic contact lines. These calculations show that over an appropriate range of Ca, the velocity field and the meniscus slope in a geometry-independent region can potentially serve as material boundary conditions for models of processes containing dynamic contact lines.

Natural systems have exceptional capabilities of constructing structures across multiple length scales for remarkable performances from limited choices of building blocks. However, simultaneous control over structures at both the macroscopic and microscopic length scales is restricted by the high requirements in the type and property of building blocks. Here, directed self‐assembly of microdroplets in an evaporating liquid is harnessed to fabricate large‐scale ordered films with precisely controlled microstructures. Evaporation‐induced variations in liquid confinement allow microdroplets to reassemble for large‐scale ordered patterns. Deformation of microdroplets into different shapes accurately defines multiple topographical structures at the microdroplet level. The fabrication process is facile and scalable, and works with various materials for manufacturing functional films with tunable properties. These results suggest a facile yet effective strategy for engineering microstructures by combining directed self‐assembly with manipulation of the property of building components for both structural fidelity and diversity.