Undulation instability of lamellar phases under shear: A mechanism for onion formation?

We consider a lamellar phase of bilayer membranes held between two parallel plates and subject to a steady shear. Accounting for the coupling with the shear flow of the short wavelength undulation modes that are responsible for the membrane excess area, we argue that the flow generates an effective force which acts to reduce the excess area. From the viewpoint of the macroscopic lamellar whose geometric dimensions are fixed, this force translates into an effective lateral pressure. At low shear rates this pressure is balanced by the elastic restoring forces of the lamellar. Above a critical shear rate , where d is the interlayer distance and D is the gap spacing, the lamellar buckles into a harmonic shape modulation, and we predict its wavelength and amplitude . We show that our model is isomorphic to a dilative strain, which is known to induce a similar buckling (undulation) instability. Indeed, at threshold the wavelength is and is identical in both cases. Using a non-linear analysis, we discuss how the wavelength and amplitude vary with shear rate away from the threshold. For we find and . We then focus on the coupling of the buckling modulation itself with the flow, and obtain a criterion for the limit of its stability. Motivated by experiments of D. Roux and coworkers, we assume that at this limit of stability the lamellar breakups into “onion"-like, multilamellar, vesicles. The critical shear rate for the formation of onions is predicted to scale as . The scaling with d is consistent with available experimental data. Received 15 April 1998 and Received in final form 4 March 1999     

Effect of Re on stacking fault nucleation under shear strain in Ni by atomistic simulation

The effect of Re on stacking fault(SF) nucleation under shear strain in Ni is investigated using the climbing image nudged elastic band method with a Ni–Al–Re embedded-atom-method potential. A parameter(?Eb sf), the activation energy of SF nucleation under shear strain, is introduced to evaluate the effect of Re on SF nucleation under shear strain. Calculation results show that ?Eb sfdecreases with Re addition, which means that SF nucleation under shear strain in Ni may be enhanced by Re. The atomic structure observation shows that the decrease of ?Eb sfmay be due to the expansion of local structure around the Re atom when SF goes through the Re atom.     

Void growth via atomistic simulation: will the formation of shear loops still grow a void under different thermo-mechanical constraints?

Abstract

Molecular dynamics (MD) simulations under different mechanical and thermal constraints are carried out with a nanovoid embedded inside a single-crystal, face-centred-cubic copper. The dislocation emission angles measured from MD plots under 0.1 K, uniaxial-strain simulation are in line with the theoretical model. The dislocation density calculated from simulation is qualitatively consistent with the experimental measurement in terms of a saturation feature. The ‘relatively farthest-travelled’ atoms are employed to reflect the correlation between the dislocation structure and the void growth. At a smaller scale, the incomplete shear dislocation loops on the slip plane contribute to the local material transport. At a larger scale, the dislocation structures formed by those incomplete shear loops further facilitate the growth of nanovoid. Compared to the uniaxial-strain case, the void growth under the uniaxial-stress is very limited. The uniaxial-strain loading results in an octahedron void shape. The uniaxial-stress loading turns the nanovoid into a prolate ellipsoid along the loading direction. In the simulation, the largest specimen contains 12 million atoms and the lowest strain rate applied is 2 × 10⁶ s^?1. Under all the different thermomechanical constraints concerned, the formation of incomplete shear dislocation loops are found capable of growing the void.     

On the stability of the shear–free condition

The evolution equation for the shear is reobtained for a spherically symmetric anisotropic, viscous dissipative fluid distribution, which allows us to investigate conditions for the stability of the shear–free condition. The specific case of geodesic fluids is considered in detail, showing that the shear–free condition, in this particular case, may be unstable, the departure from the shear–free condition being controlled by the expansion scalar and a single scalar function defined in terms of the anisotropy of the pressure, the shear viscosity and the Weyl tensor or, alternatively, in terms of the anisotropy of the pressure, the dissipative variables and the energy density inhomogeneity.     

Ultrasonic field radiated by a normal shear wave probe

The field of horizontally polarized ultrasonic shear wave radiated by a 2/2 piezoelectric composite normal probe was studied. The strictly analytic field solutions for two different surface forces were deduced and the field distribution and directivity patterns were given by numerical calculations. The vertical radiated field of shear wave was obtained theoretically and compared with experiment. The experimental result of directivity pattern conformed the theoretical prediction, which provided the theoretical basis for the engineering application of the transducer.     

Visualizing shear stress in Görtler vortex flow

Shear stress distributions were obtained from velocity measurements in a concave surface boundary layer flow in the presence of Görtler vortices by means of a single hot-wire probe for several streamwise (x) locations. A set of vertical wires of 0.20 mm diameter were positioned at a distance of 10 mm upstream from the leading edge of a concave surface of radius of curvature R=1.0 m to pre-set the wavelength of the vortices so to obtain the most amplified wavelength Görtler vortices. Consequently, the wavelength of the vortices was set equal to the wire spacing and preserved downstream. In addition to the high shear regions near the wall, one positive peak at the head of the mushroom-like structures and two relatively weak negative peaks at the vicinity of the low-speed streaks are found in the iso-?u/?y contours. They are believed to be related to the formation of the inflectional point in the velocity profile across boundary layer. The occurrence of the inflection points in the spanwise distributions of streamwise velocity component u is associated with the appearance of the second peak of the ?u/?z shear near the boundary layer edge. The nonlinear effect of Görtler instability is to increase the wall shear stress, and further enhancement beyond the turbulent values is due to the presence of secondary instability.     

Collectivity under random two-body interactions

In this paper we study collective motion under random two-body interactions in the fermion dynamical symmetry Model (FDSM). It is found that a Hamiltonian with the SO(8) symmetry of the FDSM does not give generic vibration and rotation under random interactions while that with the SP(6) symmetry does.     

Tracking problem under a time-varying topology

This paper studies the multi-agent tracking problem of a third-order maneuvering target under uncertain communication environments. Each tracking agent is assumed to be a third-order system and can only use its own and neighbors＇ position, velocity, and acceleration information to design its control input. In this work, the uncertain communication environments are modelled by a finite number of constant Laplacian matrices together with their corresponding scheduling functions. Sufficient conditions for the existence of a tracking strategy have been expressed in terms of the solvability of linear matrix inequalities. Finally, a numerical example is employed to demonstrate the effectiveness of the proposed tracking strategy.     

Fragmentation dynamics within shear bands--a model for aging tectonic faults?

A numerical model for packing of fragmenting blocks in a shear band is introduced, and its dynamics is compared with that of a tectonic fault. The shear band undergoes a slow aging process in which the blocks are being grinded by the shear motion and the compression. The dynamics of the model have the same statistical characteristics as the seismic activity in faults. The characteristic magnitude distribution of earthquakes appears to result from frictional slips at small and medium magnitudes, and from fragmentation of blocks at the largest magnitudes. Aftershocks to large-magnitude earthquakes are local recombinations of the fragments before they reach a new quasi-static equilibrium. The aftershocks satisfy Omori's law. Local precursor activity at a few times the normal background level appears at a short time before a major earthquake. Seismic gaps appear as a natural consequence of the aging process of a fault. Explanation of the heat flux and principal stress direction anomalies at the faults both involve the value of fracture stress of the blocks in the gouge. The final form of a tectonic fault is predicted to involve a gouge dominated by fine-grained and rather rounded blocks so that it cannot withstand large shear stresses. Received 26 July 2000     

Control of gradient-driven instabilities using shear Alfvén beat waves

A new technique for manipulation and control of gradient-driven instabilities through nonlinear interaction with Alfvén waves in a laboratory plasma is presented. A narrow, field-aligned density depletion is created in the Large Plasma Device, resulting in coherent, unstable fluctuations on the periphery of the depletion. Two independent shear Alfvén waves are launched along the depletion at separate frequencies, creating a nonlinear beat-wave response at or near the frequency of the original instability. When the beat wave has sufficient amplitude, the original unstable mode is suppressed, leaving only the beat-wave response, generally at lower amplitude.     

Partially Diffusive Helium-Silica Compound under High Pressure

Helium is the second most abundant element in the universe, and together with silica, they are important components of giant planets. Exploring the reactivity and state of helium and silica under high pressure is crucial for understanding of the evolution and internal structure of giant planets. Here, using first-principles calculations and crystal structure predictions, we identify four stable phases of a helium-silica compound with seven/eight-coordinated silicon atoms at pressure of 600–4000 ...     

Laboratory observation of a nonlinear interaction between shear Alfvén waves

An experimental investigation of nonlinear interactions between shear Alfvén waves in a laboratory plasma is presented. Two Alfvén waves, generated by a resonant cavity, are observed to beat together, driving a pseudomode at the beat frequency. The pseudomode then scatters the Alfvén waves, generating a series of sidebands. The observed interaction is very strong, with the normalized amplitude of the driven pseudomode comparable to the normalized magnetic field amplitude (deltaB/B) of the interacting Alfvén waves.     

Can cosmic shear shed light on low cosmic microwave background multipoles?

The lowest multipole moments of the cosmic microwave background (CMB) are smaller than expected for a scale-invariant power spectrum. One possible explanation is a cutoff in the primordial power spectrum below a comoving scale of k(c) approximately equal to 5.0 x 10(-4) Mpc(-1). Such a cutoff would increase significantly the cross correlation between the large-angle CMB and cosmic-shear patterns. The cross correlation may be detectable at >2sigma which, combined with the low CMB moments, may tilt the balance between a 2sigma result and a firm detection of a large-scale power-spectrum cutoff. The cutoff also increases the large-angle cross correlation between the CMB and the low-redshift tracers of the mass distribution.     

Propagation of guided waves in a bonded plate:Antiplane shear modes

1 IntroductionAdhesive bonding has been used eXtensively for many years in aerospace and other highteclmology industries and hajs potentials for application to other areas of manufacturing. Itis attractive because it distributes stress over the entire bond area and thai avoids the stressconcelltrations[1]. The mechanical performance of a layered composite is mainly determinedby the interface properties[2]. Ultrasonic inspection of the adhesion state between layers ina composite structure has …     

Multitude of core-localized shear Alfvén waves in a high-temperature fusion plasma

Evidence is presented for a multitude of discrete frequency Alfvén waves in the core of magnetically confined high-temperature fusion plasmas. Multiple diagnostic instruments confirm wave excitation over a wide spatial range from the device size at the longest wavelengths down to the thermal ion Larmor radius. At the shortest scales, the poloidal wavelengths are comparable to the scale length of electrostatic drift wave turbulence. Theoretical analysis confirms a dominant interaction of the modes with particles in the thermal ion distribution traveling well below the Alfvén velocity.     

Investigation on method for measuring dynamic shear modulus of underwater acoustic structure materials

1IfltroductionSince6o's,withthedevelopmentoflargemolecularmaterialsandcompositematerials,structurematerialsinunderwateracousticapplicationshavemadeagreatchanges.Forex-ample,thepropertiesofsonardomesthatmadefromreinforcedpolymersinsteadofstainlesssteelhavebeenimprovedobviously.Inordertodesignrationally,itisnecessaryforengineerstofindouttheiracousticpropertiescompletely.Accordingtotheelastictheories,twokindsofwave:longitudinalwaveandshearwavewillbeexcitedontheboundaryoftwomediumswhilethesoundsar…     

Rotational stabilization of resistive wall modes by the shear alfvén resonance

It is found that resistive wall modes with a toroidal number n = 1 in tokamaks can be stabilized by plasma rotation at a low Mach number, with the rotation frequency being lower than the ion bounce frequency but larger than the ion and electron precession drift frequencies. The stabilization is the result of the shear-Alfvén resonance, since the thermal resonance effect is negligible in this rotation frequency range. This indicates that tokamaks can operate at normalized pressure values beyond the no-wall stability limit even for low values of plasma rotation, such as those expected in fusion reactor scale devices.     

Weibull analysis of microbond shear strength at sisal fibre–polyester resin interfaces

An analysis has been made of the tensile strength of sisal fibres and the interfacial adhesion between fibres and polyester resin droplets. Density and microscopy methods were used to determine the cross-sectional area of the sisal fibres. The average tensile strength of treated sisal fibres decreased by a modest amount following treatment with 0.06 M NaOH. However, this treatment resulted in a substantial increase in the interfacial shear strength at the sisal fibre to polyester resin interface. Weibull analysis has been used successfully to analyse variability in tensile strengths and interfacial shear strength using probability of failure plots. Scanning electron microscopy has revealed the shape of resin droplets on the surface of treated and untreated sisal fibres and contact angles are much lower for droplets on treated fibres. Damage to the surface of fibres has been examined following shear testing. Weibull analysis is an effective tool for characterising highly variable fibre properties and evaluating the level of adhesion between polymer resin and the fibre surface.     

Cholesteric–nematic transitions induced by a shear flow and a magnetic field

The untwisting of the helical structure of a cholesteric liquid crystal under the action of a magnetic field and a shear flow has been studied theoretically. Both factors can induce the cholesteric–nematic transition independently; however, the difference in the orienting actions of the magnetic field and the shear flow leads to competition between magnetic and hydrodynamic mechanisms of influence on the cholesteric liquid crystal. We have analyzed different orientations of the magnetic field relative to the direction of the flow in the shear plane. In a number of limiting cases, the analytic dependences are obtained for the pitch of the cholesteric helix deformed by the shear flow. The phase diagrams of the cholesteric–nematic transitions and the pitch of the cholesteric helix are calculated for different values of the magnetic field strength and the angle of orientation, the flow velocity gradient, and the reactive parameter. It is shown that the magnetic field stabilizes the orientation of the director in the shear flow and expands the boundaries of orientability of cholesterics. It has been established that the shear flow shifts the critical magnetic field strength of the transition. It is shown that a sequence of reentrant orientational cholesteric–nematic–cholesteric transitions can be induced by rotating the magnetic field in certain intervals of its strength and shear flow velocity gradients.