Common Experimental Observation Research Articles

Brittle-fracture simulations of curved cleavage cracks in α-iron: A molecular dynamics study

Although body-centered-cubic (bcc) metals and alloys are ubiquitous as structural materials, they are brittle, particularly at low temperatures; however, the mechanism of their brittle fracture is not fully understood. In this study, we conduct a series of three-dimensional molecular dynamics simulations of the cleavage fracture of α-iron. In particular, we focus on mode-I loading starting from curved crack fronts or the so-called penny-shaped cracks. In the simulations, brittle fractures are observed at cleavages on the {100} plane, while the initial cracks become blunted on other planes as a result of dislocation emissions. Our modeling results agreed with a common experimental observation, that is, {100} is the preferential cleavage plane in bcc transition metals. In addition, dislocation emissions from the crack front were analyzed; the result supported the notion that plasticity in the vicinity of the crack front determines the preferential cleavage plane.

Transition between Swimming and Crawling: A Model of Eukaryotic Cell Motility

Eukaryotic cells can migrate spontaneously or in response to external stimuli such as chemical gradients and may travel either individually or collectively. Eukaryotic cell motility is crucial during development, wound healing, the immune response, and cancer metastasis. In liquids, eukaryotic cells can swim by pushing on the surrounding fluid, but they more often crawl along the extracellular matrix. We are interested in studying the relationships between hydrodynamics and adhesion that regulate a cell's transition between swimming and crawling. Therefore, we develop a simple model of a cell capable of both kinds of motion based on the three-sphere swimmer proposed by Najafi and Golestanian. The model includes additional adhesive friction terms, geometry motivated by cells crawling on fibers, substrate surface hydrodynamic effects, and parameters from experimental data. Hydrodynamic effects diminish in the high adhesion limit, and the crawling cell can achieve significant center-of-mass velocity compared to its swimming counterpart. The crawling cell maintains the same net displacement per cycle regardless of its sequence of motions, including reciprocal cycles that would be unproductive in the frictionless limit, which constrains cell motion by the Scallop Theorem of low-Reynolds number hydrodynamics. The center-of-mass velocity of the cell plateaus at a maximum value as adhesion increases until the frictional drag exceeds the largest force the cell can generate, after which velocity decreases with increasing adhesion. This recapitulates the common experimental observation that cell speed is not monotonic with adhesion strength.

Neoclassical toroidal torque generation by auxiliary heating in non-axisymmetric tori

In conditions of ideal axisymmetry, for a magnetized plasma in a generic bounded domain, necessarily toroidal, the uniform absorption of external energy (e.g. rf or isotropic alpha heating) clearly cannot give rise to net forces or torques. A rather common experimental observation on contemporary tokamaks is that the near central absorption of auxiliary heating power (often ICH, ECH, and LHCD) and current drive in presence of non axisymmetric magnetic perturbations, including tearing modes, drives a bulk plasma rotation in the co − Ip direction. Also growing tearing modes provide a nonlinear magnetic braking that tends to flatten the rotation profile and clamp it at the q-rational surfaces. The physical origin of the torque associated with Paux absorption could be due the effects of asymmetry in deposition or in the equilibrium configuration, but here we consider also the effect of the response of the so called neoclassical offset velocity to the power dependent heat flow increment. The neoclassical toroidal viscosity (NTV), due to error fields, internal magnetic kink or tearing modes tends to relax the plasma rotation to this asymptotic speed, which in absence of auxiliary heating is of the order of the ion diamagnetic velocity. It can be shown by a kinetic calculation, this offset velocity is a function of the absorbed heat and therefore of the injected auxiliary power, thereby forcing the plasma rotation in a direction opposite to the initial, to large values. The problem is discussed in the frame of the theoretical models of neoclassical toroidal viscosity.

Superfluid density in the slave-boson theory

Despite of the success of the slave-boson theory in capturing qualitative physics of high-temperature superconductors like cuprates, it fails to reproduce the correct temperature-dependent behavior of superfluid density, let alone the independence of the linear temperature term on doping in the underdoped regimes of hole-doped cuprate, a common experimental observation in different cuprates. It remains puzzling up to now in spite of intensive theoretical efforts. For electron-doped case, even qualitative treatment is not reported at present time. Here we revisit these problems and provide an alternative superfluid density formulation by using the London relation instead of employing the paramagnetic current-current correlation function. The obtained formula, on the one hand, provides the correct temperature-dependent behavior of the superfluid density in the whole temperature regime, on the other hand, makes the doping dependence of the linear temperature term substantially weaken and a possible interpretation for its independence on doping is proposed. As an application, electron-doped cuprate is studied, whose result qualitatively agrees with existing experiments and successfully explains the origin of $d$- to anisotropic $s$-wave transition across the optimal doping. Our result remedies some failures of the slave-boson theory as employed to calculate superfluid density in cuprates and may be useful in the understanding of the related physics in other strongly correlated systems, e.g. Na$_{x}$CoO$_{2}$$\cdot$yH$_{2}$O and certain iron-based superconductors with dominating local magnetic exchange interaction.

Photobioreactors for microalgal cultures: A Lagrangian model coupling hydrodynamics and kinetics.

Closed photobioreactors have to be optimized in terms of light utilization and overall photosynthesis rate. A simple model coupling the hydrodynamics and the photosynthesis kinetics has been proposed to analyze the photosynthesis dynamics due to the continuous shuttle of microalgae between dark and lighted zones of the photobioreactor. Microalgal motion has been described according to a stochastic Lagrangian approach adopting the turbulence model suitable for the photobioreactor configuration (single vs. two-phase flows). Effects of light path, biomass concentration, turbulence level and irradiance have been reported in terms of overall photosynthesis rate. Different irradiation strategies (internal, lateral and rounding) and several photobioreactor configurations (flat, tubular, bubble column, airlift) have been investigated. Photobioreactor configurations and the operating conditions to maximize the photosynthesis rate have been pointed out. Results confirmed and explained the common experimental observation that high concentrated cultures are not photoinhibited at high irradiance level.

A New Model for the Mechanism of Operation of Scandate and Refractory Oxide Cathodes

Refractory oxide thermionic cathodes are modeled as a top layer of high-band-gap semiconducting cubic nanocrystals with surface donor sites activated by a monolayer of barium dispensed from a substrate. An interpretation of the common experimental observation of an apparent deviation from Child's law for the space-charge-limited region for scandate cathodes in simple diodes as a temperature-dependent resistance of the top layer of the cathode supports this model. Using a previously developed phenomenological theory based on the uncertainty principle, it is shown that the work function depends on the number of donor sites per unit area of the nanocrystals that comprise the top layer and the diameter of the metal atomic component of the oxide. The model is applied to provide quantitative values for the basic semiconductor properties of a top-layer scandate cathode from published experimental data.

Effect of reactivity loss on apparent reaction order of burning char particles

Considerable debate still exists in the char combustion community over the expected and observed reaction orders of carbon reacting with oxygen. In particular, very low values of the reaction order (approaching zero) are commonly observed in char combustion experiments. These observations appear to conflict with porous catalyst theory as first expressed by Thiele, which suggests that the apparent reaction order must be greater than 0.5. In this work, we propose that this conflict may be resolved by considering the decrease in char reactivity with burnout due to ash effects, thermal annealing, or other phenomena. Specifically, the influence of ash dilution of the available surface area on the apparent reaction order is explored. Equations describing the ash dilution effect are combined with a model for particle burnout based on single-film nth-order Arrhenius char combustion and yield an analytical expression for the effective reaction order. When this expression is applied for experimental conditions reflecting combustion of individual pulverized coal particles in an entrained flow reactor, the apparent reaction order is shown to be lower than the inherent char matrix reaction order, even for negligible extents of char conversion. As char conversion proceeds and approaches completion, the apparent reaction order drops precipitously past zero to negative values. Conversely, the inclusion of the ash dilution model has little effect on the char conversion profile or char particle temperature until significant burnout has occurred. Taken together, these results suggest that the common experimental observation of low apparent reaction orders during char combustion is a consequence of the lack of explicit modeling of the decrease in char reactivity with burnout.

Plasma-controlled adatom delivery and (re)distribution: Enabling uninterrupted, low-temperature growth of ultralong vertically aligned single walled carbon nanotubes

Large-scale (∼109 atoms) numerical simulations reveal that plasma-controlled dynamic delivery and redistribution of carbon atoms between the substrate and nanotube surfaces enable the growth of ultralong single walled carbon nanotubes (SWCNTs) and explain the common experimental observation of slower growth at advanced stages. It is shown that the plasma-based processes feature up to two orders of magnitude higher growth rates than equivalent neutral-gas systems and are better suited for the SWCNT synthesis at low nanodevice friendly temperatures.

Evidence from numerical modelling for 3D spreading of [001] screw dislocations in Mg2SiO4 forsterite

Computer simulations have previously been used to derive the atomic scale properties of the cores of screw dislocations in Mg2SiO4 forsterite by direct calculation using parameterized potentials and via the Peierls–Nabarro model using density functional theory. We show that, for the [001] screw dislocation, the parameterized potentials reproduce key features of generalized stacking fault energies when compared to the density functional theory results, but that the predicted structure of the dislocation core differs between direct simulation and the Peierls–Nabarro model. The [001] screw dislocation is shown to exhibit a low-energy non-planar core. It is suggested that for this dislocation to move its core may need to change structure and form a high-energy planar structure similar to that derived from the Peierls–Nabarro model. This could lead to dislocation motion via an unlocking–locking mechanism and explain the common experimental observation of long straight screw dislocation segments in deformed olivine.

Calculation of Cyclodextrin Binding Affinities: Energy, Entropy, and Implications for Drug Design

The second generation Mining Minima method yields binding affinities accurate to within 0.8 kcal/mol for the associations of α-, β-, and γ-cyclodextrin with benzene, resorcinol, flurbiprofen, naproxen, and nabumetone. These calculations require hours to a day on a commodity computer. The calculations also indicate that the changes in configurational entropy upon binding oppose association by as much as 24 kcal/mol and result primarily from a narrowing of energy wells in the bound versus the free state, rather than from a drop in the number of distinct low-energy conformations on binding. Also, the configurational entropy is found to vary substantially among the bound conformations of a given cyclodextrin-guest complex. This result suggests that the configurational entropy must be accounted for to reliably rank docked conformations in both host-guest and ligand-protein complexes. In close analogy with the common experimental observation of entropy-enthalpy compensation, the computed entropy changes show a near-linear relationship with the changes in mean potential plus solvation energy.

Direct simulations of particle suspensions in a viscoelastic fluid in sliding bi-periodic frames

We present a new finite element scheme for direct simulation of inertialess particle suspensions in simple shear flows of Oldroyd-B fluids. The sliding bi-periodic frame concept of Lees & Edwards [J. Phys. C 5 (1972) 1921] has been combined with the DEVSS/DG finite element scheme, by introducing constraint equations along the domain boundary. The force-free, torque-free rigid body motion of a particle is described by the rigid-ring problem and implemented by Lagrangian multipliers only on the particle boundary, which allows general treatments for boundary-crossing particles. In our formulation, the bulk stress is obtained by simple boundary integrals of Lagrangian multipliers along the domain and particles. Concentrating on 2-D circular disk particles, we discuss the bulk rheology of suspensions as well as the micro-structural developments through the numerical examples of single-, two- and many-particle problems, which represent a large number of such systems in simple shear flow. We report the steady bulk viscosity and the first normal stress coefficient, from very dilute to highly concentrated systems. The results show shear-thickening behavior for both properties and the common experimental observation of the scaling of the first normal stress to the shear stress has been reproduced. Unlike Newtonian systems, two particles in an Oldroyd-B fluid result in kissing-tumbling-tumbling phenomena: they keep rotating around each other, when they are closely located. Many-particle problems reveal the occurrence of strong elongational flows between separating particles.

Theoretical studies on C-heteroatom bond formation via reductive elimination from group 10 M(PH3)2(CH3)(X) species (X = CH3, NH2, OH, SH) and the determination of metal-X bond strengths using density functional theory.

Density functional calculations have been used to investigate C-C, C-N and C-O bond forming reactions via reductive elimination from Group 10 cis-M(PH3)2(CH3)(X) species (X = C-I3, NH2, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH3 loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH3)2 and CH3-X follows the trend M = Pd < Pt < Ni. The reaction of the cis-M(PH3)2(CH3)(NH2) species is promoted by the formation of methylamine adducts. Non-planar transition states are located and the C-heteroatom bond forming processes are characterised by migration of CH3 onto the cis-heteroatom ligand. For a given ligand, X, activation energies follow the trend M = Ni < Pd < Pt. Formation of the three-coordinate M(PH3)(CH3)(X) species is promoted by a labilisation of the cis-PH3 ligand in the four-coordinate reactants when X = NH2 or OH. For the three-coordinate pathway the energy change for reductive elimination to give M(PH3) and CH3-X again follows the trend M = Pd < Pt < Ni and in all cases the initial product is an M(PH3)(XCH3) adduct. The three-coordinate transition states again involve migration of the CH3 ligand onto the cis-X ligand and for X = NH2 or OH activation energies follow the trend Ni > Pd < Pt. For a given metal activation energies in both the four- and three-coordinate pathways increase along the series CH3 < NH2 < OH. These trends in activation energy can be rationalised in terms of the strength of M-CH3/M-X bonding as long as the extent of geometrical distortion required to obtain the transition state geometry is taken into account. Further calculations on cis-Pd(PH3)2(CH3)(SH) suggest that the more common experimental observation of C(sp3)-S compared to C(sp3)-O reductive elimination arises from the greater kinetic accessibility of the former process rather than an intrinsic thermodynamic preference for C-S bond formation. By comparison, the calculations indicate that C(sp3)-N reductive elimination should be feasible from Ni and Pd systems. DF calculations are shown to reproduce the relative homolytic bond strengths determined experimentally for Pt-X bonds. In the cis-M(PH3)2(CH3)(X) systems the M-CH3 homolytic bond strength increases down the group while for M-NH2 and M-OH bonds the trend is M = N approximately equal to Pd < Pt. M-NH2 and M-OH are considerably stronger than M-CH3 bonds and the presence of a heteroatom ligand serves to weaken M-CH3 bonds even further.

Decrepitation and crack healing of fluid inclusions in San Carlos olivine

Fluid inclusions break, or decrepitate, when the fluid pressure exceeds the least principal lithostatic stress by a critical amount. After decrepitation, excess fluid pressure is relaxed, resulting in crack arrest; subsequently, crack healing may occur. Existing models of decrepitation do not adequately explain several experimentally observed phenomena. We developed a linear elastic fracture mechanics model to analyze new data on decrepitation and crack arrest in San Carlos olivine, compared the model with previous fluid inclusion investigations, and used it to interpret some natural decrepitation microstructures. The common experimental observation that smaller inclusions may sustain higher internal fluid pressures without decrepitating may be rationalized by assuming that flaws associated with the inclusion scale with the inclusion size. According to the model, the length of the crack formed by decrepitation depends on the lithostatic pressure at the initiation of cracking, the initial sizes of the flaw and the inclusion, and the critical stress intensity factor. Further experiments show that microcracks in San Carlos olivine heal within several days at 1280 to 1400°C; healing rates depend on the crack geometry, temperature, and chemistry of the buffering gas. The regression distance of the crack tip during healing can be related to time through a power law with exponent n = 0.6. Chemical changes which become apparent after extremely long heat‐treatments significantly affect the healing rates. Many of the inclusions in the San Carlos xenoliths stretched, decrepitated, and finally healed during uplift. The crack arrest model indicates that completely healed cracks had an initial fluid pressure of the order of 1 GPa. Using the crack arrest model and the healing kinetics, we estimate the ascent rate of these xenoliths to be between 0.001 and 0.1 m/s.

A study of the class II behaviour of rock

First, Class II behaviour of rock is discussed with a spring model. The model is characterized by non-uniform failure, which agrees qualitatively with common experimental observation, and shows not only class I but also class II behaviour depending on strength variation of springs.

Parametric amplification and frequency conversion in optical fibers

We find that the parametric four-photon gain for light pulses decreases for fibers longer than a characteristic length. This length is related to the common experimental observation that stimulated parametric emission is usually prominent only in short fibers while in long fibers stimulated Raman scattering dominates. Despite the fact that the actual process involves an intensity dependent bandwidth and broadening of the pump linewidth from self-phase modulation, it is possible to develop a simple expression for the characteristic length which requires only the initial pump linewidth and the low-power parametric bandwidth. This bandwidth can often be estimated from the pump wavelength and the measured frequency shift between the pump and the generated waves. Expressions for gain and amplification are derived from coupled wave equations and in the Appendixes it is shown that these are of the same form as the planewave equations, but modified by coupling coefficients called overlap integrals.

The relation between the surface electromyogram and muscular force.

1. Motor units in the first dorsal interosseus muscle of normal human subjects were recorded by needle electrodes, together with the surface electromyogram (e.m.g.). The wave form contributed by each motor unit to the surface e.m.g. was determined by signal averaging. 2. The peak-to-peak amplitude of the wave form contributed to the surface e.m.g. by a motor unit increased approximately as the square root of the threshold force at which the unit was recruited. The peak-to-peak duration of the wave form was independent of the threshold force. 3. Large and small motor units are uniformly distributed throughout this muscle, and the muscle fibres making up a motor unit may be widely dispersed. 4. The rectified surface e.m.g. was computed as a function of force, based on the sample of motor units recorded. The largest contribution of motor unit recruitment occurs at low force levels, while the contribution of increased firing rate becomes more important at higher force levels. 5. Possible bases for the common experimental observation that the mean rectified surface e.m.g. varies linearly with the force generated by a muscle are discussed. E.m.g. potentials and contractile responses may both sum non-linearly at moderate to high force levels, but in such a way that the rectified surface e.m.g. is still approximately linearly related to the force produced by the muscle.