2,712 research outputs found
Melting and Rippling Phenomenan in Two Dimensional Crystals with localized bonding
We calculate Root Mean Square (RMS) deviations from equilibrium for atoms in
a two dimensional crystal with local (e.g. covalent) bonding between close
neighbors. Large scale Monte Carlo calculations are in good agreement with
analytical results obtained in the harmonic approximation. When motion is
restricted to the plane, we find a slow (logarithmic) increase in fluctuations
of the atoms about their equilibrium positions as the crystals are made larger
and larger. We take into account fluctuations perpendicular to the lattice
plane, manifest as undulating ripples, by examining dual layer systems with
coupling between the layers to impart local rigidly (i.e. as in sheets of
graphene made stiff by their finite thickness). Surprisingly, we find a rapid
divergence with increasing system size in the vertical mean square deviations,
independent of the strength of the interplanar coupling. We consider an
attractive coupling to a flat substrate, finding that even a weak attraction
significantly limits the amplitude and average wavelength of the ripples. We
verify our results are generic by examining a variety of distinct geometries,
obtaining the same phenomena in each case.Comment: 17 pages, 28 figure
Fragility and compressibility at the glass transition
Isothermal compressibilities and Brillouin sound velocities from the
literature allow to separate the compressibility at the glass transition into a
high-frequency vibrational and a low-frequency relaxational part. Their ratio
shows the linear fragility relation discovered by x-ray Brillouin scattering
[1], though the data bend away from the line at higher fragilities. Using the
concept of constrained degrees of freedom, one can show that the vibrational
part follows the fragility-independent Lindemann criterion; the fragility
dependence seems to stem from the relaxational part. The physical meaning of
this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,
Science 302, 849 (2003)Comment: 4 pages, 2 figures, 2 tables, 33 references. Slightly changed after
refereein
Plasticization and antiplasticization of polymer melts diluted by low molar mass species
An analysis of glass formation for polymer melts that are diluted by
structured molecular additives is derived by using the generalized entropy
theory, which involves a combination of the Adam-Gibbs model and the direct
computation of the configurational entropy based on a lattice model of polymer
melts that includes monomer structural effects. Antiplasticization is
accompanied by a "toughening" of the glass mixture relative to the pure
polymer, and this effect is found to occur when the diluents are small species
with strongly attractive interactions with the polymer matrix. Plasticization
leads to a decreased glass transition temperature T_g and a "softening" of the
fragile host polymer in the glass state. Plasticization is prompted by small
additives with weakly attractive interactions with the polymer matrix. The
shifts in T_g of polystyrene diluted by fully flexible short oligomers are
evaluated from the computations, along with the relative changes in the
isothermal compressibility at T_g to characterize the extent to which the
additives act as antiplasticizers or plasticizers. The theory predicts that a
decreased fragility can accompany both antiplasticization and plasticization of
the glass by molecular additives. The general reduction in the T_g and
fragility of polymers by these molecular additives is rationalized by analyzing
the influence of the diluent's properties (cohesive energy, chain length, and
stiffness) on glass formation in diluted polymer melts. The description of
glass formation at fixed temperature that is induced upon change the fluid
composition directly implies the Angell equation for the structural relaxation
time as function of the polymer concentration, and the computed "zero mobility
concentration" scales linearly with the inverse polymerization index N.Comment: 12 pages, 15 figure
Analysis of Dislocation Mechanism for Melting of Elements: Pressure Dependence
In the framework of melting as a dislocation-mediated phase transition we
derive an equation for the pressure dependence of the melting temperatures of
the elements valid up to pressures of order their ambient bulk moduli. Melting
curves are calculated for Al, Mg, Ni, Pb, the iron group (Fe, Ru, Os), the
chromium group (Cr, Mo, W), the copper group (Cu, Ag, Au), noble gases (Ne, Ar,
Kr, Xe, Rn), and six actinides (Am, Cm, Np, Pa, Th, U). These calculated
melting curves are in good agreement with existing data. We also discuss the
apparent equivalence of our melting relation and the Lindemann criterion, and
the lack of the rigorous proof of their equivalence. We show that the would-be
mathematical equivalence of both formulas must manifest itself in a new
relation between the Gr\"{u}neisen constant, bulk and shear moduli, and the
pressure derivative of the shear modulus.Comment: 19 pages, LaTeX, 9 eps figure
Physical Aspects of Axonemal Beating and Swimming
We discuss a two-dimensional model for the dynamics of axonemal deformations
driven by internally generated forces of molecular motors. Our model consists
of an elastic filament pair connected by active elements. We derive the dynamic
equations for this system in presence of internal forces. In the limit of small
deformations, a perturbative approach allows us to calculate filament shapes
and the tension profile. We demonstrate that periodic filament motion can be
generated via a self-organization of elastic filaments and molecular motors.
Oscillatory motion and the propagation of bending waves can occur for an
initially non-moving state via an instability termed Hopf bifurcation. Close to
this instability, the behavior of the system is shown to be independent of
microscopic details of the axoneme and the force-generating mechanism. The
oscillation frequency however does depend on properties of the molecular
motors. We calculate the oscillation frequency at the bifurcation point and
show that a large frequency range is accessible by varying the axonemal length
between 1 and 50m. We calculate the velocity of swimming of a flagellum
and discuss the effects of boundary conditions and externally applied forces on
the axonemal oscillations.Comment: 14 pages, 8 figures, REVTE
Nonlinear instability in flagellar dynamics: a notel modulation mechanism in sperm migration
Throughout biology, cells and organisms use flagella and cilia to propel fluid and achieve motility. The beating of these organelles, and the corresponding ability to sense, respond to and modulate this beat is central to many processes in health and disease. While the mechanics of flagellum–fluid interaction has been the subject of extensive mathematical studies, these models have been restricted to being geometrically linear or weakly nonlinear, despite the high curvatures observed physiologically. We study the effect of geometrical nonlinearity, focusing on the spermatozoon flagellum. For a wide range of physiologically relevant parameters, the nonlinear model predicts that flagellar compression by the internal forces initiates an effective buckling behaviour, leading to a symmetry-breaking bifurcation that causes profound and complicated changes in the waveform and swimming trajectory, as well as the breakdown of the linear theory. The emergent waveform also induces curved swimming in an otherwise symmetric system, with the swimming trajectory being sensitive to head shape—no signalling or asymmetric forces are required. We conclude that nonlinear models are essential in understanding the flagellar waveform in migratory human sperm; these models will also be invaluable in understanding motile flagella and cilia in other systems
Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory
We demonstrate the accuracy of the hypernetted chain closure and of the
mean-field approximation for the calculation of the fluid-state properties of
systems interacting by means of bounded and positive-definite pair potentials
with oscillating Fourier transforms. Subsequently, we prove the validity of a
bilinear, random-phase density functional for arbitrary inhomogeneous phases of
the same systems. On the basis of this functional, we calculate analytically
the freezing parameters of the latter. We demonstrate explicitly that the
stable crystals feature a lattice constant that is independent of density and
whose value is dictated by the position of the negative minimum of the Fourier
transform of the pair potential. This property is equivalent with the existence
of clusters, whose population scales proportionally to the density. We
establish that regardless of the form of the interaction potential and of the
location on the freezing line, all cluster crystals have a universal Lindemann
ratio L = 0.189 at freezing. We further make an explicit link between the
aforementioned density functional and the harmonic theory of crystals. This
allows us to establish an equivalence between the emergence of clusters and the
existence of negative Fourier components of the interaction potential. Finally,
we make a connection between the class of models at hand and the system of
infinite-dimensional hard spheres, when the limits of interaction steepness and
space dimension are both taken to infinity in a particularly described fashion.Comment: 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor
changes in structure of pape
Fine Carbohydrate Structure of Dietary Resistant Glucans Governs the Structure and Function of Human Gut Microbiota
Increased dietary fiber consumption has been shown to increase human gut microbial diversity, but the mechanisms driving this effect remain unclear. One possible explanation is that microbes are able to divide metabolic labor in consumption of complex carbohydrates, which are composed of diverse glycosidic linkages that require specific cognate enzymes for degradation. However, as naturally derived fibers vary in both sugar composition and linkage structure, it is challenging to separate out the impact of each of these variables. We hypothesized that fine differences in carbohydrate linkage structure would govern microbial community structure and function independently of variation in glycosyl residue composition. To test this hypothesis, we fermented commercially available soluble resistant glucans, which are uniformly composed of glucose linked in different structural arrangements, in vitro with fecal inocula from each of three individuals. We measured metabolic outputs (pH, gas, and short-chain fatty acid production) and community structure via 16S rRNA amplicon sequencing. We determined that community metabolic outputs from identical glucans were highly individual, emerging from divergent initial microbiome structures. However, specific operational taxonomic units (OTUs) responded similarly in growth responses across individuals’ microbiota, though in context-dependent ways; these data suggested that certain taxa were more efficient in competing for some structures than others. Together, these data support the hypothesis that variation in linkage structure, independent of sugar composition, governs compositional and functional responses of microbiota
Immersed nano-sized Al dispersoids in an Al matrix; effects on the structural and mechanical properties by Molecular Dynamics simulations
We used molecular dynamics simulations based on a potential model in analogy
to the Tight Binding scheme in the Second Moment Approximation to simulate the
effects of aluminum icosahedral grains (dispersoids) on the structure and the
mechanical properties of an aluminum matrix. First we validated our model by
calculating several thermodynamic properties referring to the bulk Al case and
we found good agreement with available experimental and theoretical data.
Afterwards, we simulated Al systems containing Al clusters of various sizes. We
found that the structure of the Al matrix is affected by the presence of the
dispersoids resulting in well ordered domains of different symmetries that were
identified using suitable Voronoi analysis. In addition, we found that the
increase of the grain size has negative effect on the mechanical properties of
the nanocomposite as manifested by the lowering of the calculated bulk moduli.
The obtained results are in line with available experimental data.Comment: 15 pages, 8 figures. Submitted to J. Phys: Condens. Matte
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