Crystal structure of indium and lead under confined geometry conditions

The crystal structure of lead and indium nanoparticles produced by the introduction of lead and indium into porous glasses with an average pore diameter of 7 nm at a high pressure is studied by X-ray diffraction. In contrast to the lead nanoparticles, the indium nanoparticles exhibit pronounced anomalous asymmetry of their elastic peaks, which can be explained by the fact that the tetragonal body-centered phase in the “core” of a particle transforms gradually into a face-centered cubic phase near the surface.     

Critical deflagration waves leading to detonation onset under different boundary conditions

High-speed turbulent critical deflagration waves before detonation onset in H2–air mixture propagated into a square cross section channel,which was assembled of optional rigid rough,rigid smooth,or flexible walls.The corresponding propagation characteristic and the influence of the wall boundaries on the propagation were investigated via high-speed shadowgraph and a high-frequency pressure sampling system.As a comprehensive supplement to the different walls effect investigation,the effect of porous absorbing walls on the detonation propagation was also investigated via smoke foils and the high-frequency pressure sampling system.Results are as follows.In the critical deflagration stage,the leading shock and the closely following turbulent flame front travel at a speed of nearly half the CJ detonation velocity.In the preheated zone,a zonary flame arises from the overlapping part of the boundary layer and the pressure waves,and then merges into the mainstream flame.Among these wall boundary conditions,the rigid rough wall plays a most positive role in the formation of the zonary flame and thus accelerates the transition of the deflagration to detonation(DDT),which is due to the boost of the boundary layer growth and the pressure wave reflection.Even though the flexible wall is not conducive to the pressure wave reflection,it brings out a faster boundary layer growth,which plays a more significant role in the zonary flame formation.Additionally,the porous absorbing wall absorbs the transverse wave and yields detonation decay and velocity deficit.After the absorbing wall,below some low initial pressure conditions,no re-initiation occurs and the deflagration propagates in critical deflagration for a relatively long distance.     

Dynamics of argon in confined geometry

The role of geometrical confinement on the dynamics of argon is studied. We have investigated ³⁶Argon adsorbed in nanoporous Gelsil glass by inelastic neutron scattering. By fractional filling the `dimensionality' of the system is tuned from a two-dimensional towards the bulk state. Ab-initio calculations of plane sheets of Ar atoms and of bulk Ar are compared to the experimental results. A shift of various phonon modes to lower energies with decreasing dimensionality is observed in the results of both methods.     

Spiral evolution in a confined geometry

Supported nanoscale lead crystallites with a step emerging from a noncentered screw dislocation on the circular top facet were prepared by rapid cooling from just above the melting temperature. STM observations of the top facet show a nonuniform rotation rate and shape of the spiral step as the crystallite relaxes. These features can be accurately modeled using curvature driven dynamics, as in classical models of spiral growth, with boundary conditions fixing the dislocation core and regions of the step lying along the outer facet edge.     

Distinctive fluctuations in a confined geometry

Spurred by recent theoretical predictions [Phys. Rev. E 69, 035102(R) (2004)10.1103/PhysRevE.69.035102; Surf. Sci. Lett. 598, L355 (2005)10.1016/j.susc.2005.09.023], we find experimentally using STM line scans that the fluctuations of the step bounding a facet exhibit scaling properties distinct from those of isolated steps or steps on vicinal surfaces. The correlation functions go as t0.15 +/- 0.03 decidedly different from the t0.26 +/- 0.02 behavior for fluctuations of isolated steps. From the exponents, we categorize the universality, confirming the prediction that the nonlinear term of the Kardar-Parisi-Zhang equation, long known to play a central role in nonequilibrium phenomena, can also arise from the curvature or potential-asymmetry contribution to the step free energy.     

Enhanced dispersion interaction in confined geometry

The dispersion interaction between two pointlike particles confined in a dielectric slab between two plates of another dielectric medium is studied within a continuum (Lifshitz) theory. The retarded (Casimir-Polder) interaction at large interparticle distances is found to be strongly enhanced as the mismatch between the dielectric permittivities of the two media is increased. The large-distance interaction is multiplied due to confinement by a factor of (33gamma(5/2) + 13gamma(-3/2))/46 at zero temperature, and by (5gamma2 + gamma(-2))/6 at finite temperature, gamma = epsilon(in)(0)/epsilon(out)(0) being the ratio between the static dielectric permittivities of the inner and outer media. This confinement-induced amplification of the dispersion interaction can reach several orders of magnitude.     

Mathematical modeling of the chemical inhibition of the detonation of hydrogen-air mixtures

A mathematical modeling of the chemical inhibition of the detonation of hydrogen-air mixtures is performed. It is demonstrated that a one-dimensional model of detonation based on a chain-branching mechanism of hydrogen combustion makes it possible to describe the main regularities of the effect of inhibitors on detonation. The calculation results, which are in good agreement with the available experimental data, show that inhibition causes a narrowing of the concentration limits of detonation and an increase in the critical diameter of detonation.     

Peculiarities of gallium crystallization in confined geometry

The freezing and melting phase transitions for gallium embedded into a porous glass with a pore size of about 8 nm were studied using acoustic, NMR, and x-ray techniques. It was shown that the broadened solidification and melting transitions upon deep cooling up to complete freezing at 165 K were due to the formation of β-Ga within pores. The offset of confined β-Ga melting was lowered by about 21 K compared to the bulk β-Ga melting point. Both melting and freezing in pores were irreversible. The fulfillment of some special thermal conditions led to gallium crystallization into other modifications. The role of heterogeneous crystallization in freezing of confined gallium is discussed.     

Classical Coulomb fluids in a confined geometry

It has already been argued that a classical (three-dimensional) Coulomb fluid confined between two parallel walls exhibits ideal gas features when the distance between the walls becomes small; this is confirmed in the present paper. Two-dimensional models of Coulomb fluids (with a logarithmic interaction), confined in a strip, are also studied. These models do not become ideal gases in the narrow strip limit. The correlation functions are also studied. There is a special temperature at which exact results are obtained. At that temperature, the two-dimensional, two-component plasma (two-dimensional Coulomb gas), which is a conductor when unconfined, becomes a dielectric as soon as it is confined in a strip of noninfinite width. This can be understood as a displacement of the Kosterlitz-Thouless transition by the confinement.     

Recovery and suppression of the detonation of hydrogen-air mixtures at an obstacle with orifices

A numerical simulation of the interaction of detonation waves with an obstacle having orifices and an analysis of the results were performed. The calculations were conducted using the 3D GasDynamicTool code for a model gas with parameters of detonation corresponding to a hydrogen-air stoichiometric mixture under normal conditions. Within the framework of the assumptions made, it was shown that, upon interaction with a perforated partition, a detonation wave experiences disintegration accompanied by the formation of unsteady jets of detonation products, with each one being preceded by a shock wave. The simulations demonstrated that the reinitiation of detonation downstream from the partition is determined by the dynamics of the ignition caused by the interaction between the converging shock waves formed ahead of the jets outflowing from neighboring orifices.     

First-passage time distributions for subdiffusion in confined geometry

In this Letter, we derive a relationship between the moments of the first-passage time for a random walk and the first-passage time density for subdiffusive processes modeled by continuous-time random walks. In particular, we show that the exact long-time behavior of the density depends only on the mean first-passage time of the corresponding normal diffusive process. In addition, we give explicit evaluations of the first-passage time distribution for general three-dimensional bounded domains. These results are relevant to systems involving anomalous diffusion in confinements.     

Dynamics and phase behaviour of materials in confined geometry

Phase transitions and dynamics of the liquid crystal MBBA [N-( p-methoxybenzylidene)-p-n-butylaniline] microconfined within porous glasses of average pore diameters: 82 and 337 Å were investigated by ¹H Nuclear Magnetic Resonance (NMR). Confinement is found to favour the depression of phase transition temperatures below the corresponding transitions of bulk MBBA. Confinement induces further effects for the molecular dynamics related to the phase investigated. The correlation times of the molecular motions were obtained and the results are discussed by making comparison with the bulk analysis.     

Magnetic susceptibility of noninteracting fermions in a confined geometry

A model system of an ideal gas of neutral fermions in a confined geometry of different symmetry and size is theoretically examined. The behavior of these systems is found to exhibit qualitatively new features such as the oscillations in magnetic susceptibility with changing geometry size and particle density, indicating that the geometric confinement substantially affects the thermodynamic properties of the system.     

Properties of the Ising magnet confined in a corner geometry

The properties of Ising square lattices with nearest neighbor ferromagnetic exchange confined in a corner geometry, are studied by means of Monte Carlo simulations. Free boundary conditions at which boundary magnetic fields ±h are applied, i.e., at the two boundary rows ending at the lower left corner a field +h acts, while at the two boundary rows ending at the upper right corner a field −h acts. For temperatures T less than the critical temperature T_c of the bulk, this boundary condition leads to the formation of two domains with opposite orientation of the magnetization direction, separated by an interface which for T larger than the filling transition temperature T_f(h) runs from the upper left corner to the lower right corner, while for T<T_f(h) this interface is localized either close to the lower left corner or close to the upper right corner. It is shown that for T=T_f(h) the magnetization profile m(z) in the z-direction normal to the interface simply is linear and the interfacial width scales as w∝L, while for T>T_f(h) it scales as . The distribution P(?) of the interface position ? (measured along the z-direction from the corners) decays exponentially for T<T_f(h) from either corner, is essentially flat for T=T_f(h), and is a Gaussian centered at the middle of the diagonal for T>T_f(h). Unlike the findings for critical wetting in the thin film geometry of the Ising model, the Monte Carlo results for corner wetting are in very good agreement with the theoretical predictions.     

Critical condition of inner cylinder radius for sustaining rotating detonation waves in rotating detonation engine thruster

We describe the critical condition necessary for the inner cylinder radius of a rotating detonation engine (RDE) used for in-space rocket propulsion to sustain adequate thruster performance. Using gaseous C₂H₄ and O₂ as the propellant, we measured thrust and impulse of the RDE experimentally, varying in the inner cylinder radius r_i from 31 mm (typical annular configuration) to 0 (no-inner-cylinder configuration), while keeping the outer cylinder radius (r_o = 39 mm) and propellant injector position (r_inj = 35 mm) constant. In the experiments, we also performed high-speed imaging of self-luminescence in the combustion chamber and engine plume. In the case of relatively large inner cylinder radii (r_i = 23 and 31 mm), rotating detonation waves in the combustion chamber attached to the inner cylinder surface, whereas for relatively small inner cylinder radii (r_i = 0, 9, and 15 mm), rotating detonation waves were observed to detach from the inner cylinder surface. In these small inner radii cases, strong chemical luminescence was observed in the plume, probably due to the existence of soot. On the other hand, for cases where r_i = 15, 23, and 31 mm, the specific impulses were greater than 80% of the ideal value at correct expansion. Meanwhile, for cases r_i = 0 and 9 mm, the specific impulses were below 80% of the ideal expansion value. This was considered to be due to the imperfect detonation combustion (deflagration combustion) observed in small inner cylinder radius cases. Our results suggest that in our experimental conditions, r_i = 15 mm was close to the critical condition for sustaining rotating detonation in a suitable state for efficient thrust generation. This condition in the inner cylinder radius corresponds to a condition in the reduced unburned layer height of 4.5–6.5.     

Bilayer membrane in confined geometry: Interlayer slide and entropic repulsion

We derive the free energy functional of a bilayer lipid membrane from the first principles of elasticity theory. The model explicitly includes position-dependent mutual slide of monolayers and bending deformation. Our free energy functional of a liquid-crystal membrane allows for incompressibility of the membrane and vanishing of the in-plane shear modulus and obeys reflectional and rotational symmetries of the flat bilayer. Interlayer slide at the midplane of the membrane results in local difference of surface densities of the monolayers. The slide amplitude directly enters the free energy via the strain tensor. For small bending deformations, the ratio between the bending modulus and the area compression coefficient, K _b /K _A , is proportional to the square of monolayer thickness h. Using the functional, we perform self-consistent calculation of the entropic potential acting on a bilayer between parallel confining walls separated by distance 2d. We find that at the minimum of the confining potential, the temperature-dependent curvature α ∝ T ² /K _b d⁴ is enhanced four times for a bilayer with slide as compared with a unit bilayer. We also calculate viscous modes of a bilayer membrane between confining walls. Pure bending of the membrane is investigated, which is decoupled from area dilation at small amplitudes. Three sources of viscous dissipation are considered: water and membrane viscosities and interlayer drag. The dispersion relation gives two branches ω_{1, 2} (q).