145 research outputs found
Theory of water and charged liquid bridges
The phenomena of liquid bridge formation due to an applied electric field is
investigated. A new solution for the charged catenary is presented which allows
to determine the static and dynamical stability conditions where charged liquid
bridges are possible. The creeping height, the bridge radius and length as well
as the shape of the bridge is calculated showing an asymmetric profile in
agreement with observations. The flow profile is calculated from the Navier
Stokes equation leading to a mean velocity which combines charge transport with
neutral mass flow and which describes recent experiments on water bridges.Comment: 10 pages 12 figures, misprints corrected, assumptions more
transparen
Elasticity-driven Nanoscale Texturing in Complex Electronic Materials
Finescale probes of many complex electronic materials have revealed a
non-uniform nanoworld of sign-varying textures in strain, charge and
magnetization, forming meandering ribbons, stripe segments or droplets. We
introduce and simulate a Ginzburg-Landau model for a structural transition,
with strains coupling to charge and magnetization. Charge doping acts as a
local stress that deforms surrounding unit cells without generating defects.
This seemingly innocuous constraint of elastic `compatibility', in fact induces
crucial anisotropic long-range forces of unit-cell discrete symmetry, that
interweave opposite-sign competing strains to produce polaronic elasto-magnetic
textures in the composite variables. Simulations with random local doping below
the solid-solid transformation temperature reveal rich multiscale texturing
from induced elastic fields: nanoscale phase separation, mesoscale intrinsic
inhomogeneities, textural cross-coupling to external stress and magnetic field,
and temperature-dependent percolation. We describe how this composite textured
polaron concept can be valuable for doped manganites, cuprates and other
complex electronic materials.Comment: Preprin
Three-Dimensional Elastic Compatibility: Twinning in Martensites
We show how the St.Venant compatibility relations for strain in three
dimensions lead to twinning for the cubic to tetragonal transition in
martensitic materials within a Ginzburg-Landau model in terms of the six
components of the symmetric strain tensor. The compatibility constraints
generate an anisotropic long-range interaction in the order parameter
(deviatoric strain) components. In contrast to two dimensions, the free energy
is characterized by a "landscape" of competing metastable states. We find a
variety of textures, which result from the elastic frustration due to the
effects of compatibility. Our results are also applicable to structural phase
transitions in improper ferroelastics such as ferroelectrics and
magnetoelastics, where strain acts as a secondary order parameter
Effects of Laser Source Parameters on the Generation of Narrow Band and Directed Laser Ultrasound
The successful application of laser techniques for ultrasonic testing depends on the efficient coupling of optical energy into elastic energy so that laser probe detection sensitivity may be maximized. Through optimization of the laser source which is used to generate ultrasonic waves, the overall performance of laser ultrasonic systems may be enhanced by improving the efficiency with which optical energy is converted to elastic energy. This optimization depends primarily on the source laser wavelength which governs the physical interaction of the optical energy with the material of interest. For a given laser source wavelength, several techniques have been demonstrated which modify the laser source to enhance the detectability of laser ultrasonic waves and include the repetitively pulsed laser source [1,2], or temporal array, and the phased array laser source [3],or phased array. These techniques directly address the wave detectability issue by controlling the amplitude and/or the frequency content of the laser ultrasonic wave. Even though the overall conversion efficiency of optical energy to elastic energy is not improved primarily by repetitive pulsing or phasing laser arrays, the detectability of a given laser ultrasonic wave may be enhanced beyond that obtained using a single laser source
Current without bias and diode effect in shuttling transport of nanoshafts
A row of parallely ordered and coupled molecular nanoshafts is shown to
develop a shuttling transport of charges at finite temperature. The appearance
of a cu rrent without applying an external bias voltage is reported as well as
a natura l diode effect allowing unidirectional charge transport along one
field directi on while blocking the opposite direction. The zero-bias voltage
current appears above a threshold of initial thermal and/or dislocation energy
Defect-induced incompatibility of elastic strains: dislocations within the Landau theory of martensitic phase transformations
In dislocation-free martensites the components of the elastic strain tensor
are constrained by the Saint-Venant compatibility condition which guarantees
continuity of the body during external loading. However, in dislocated
materials the plastic part of the distortion tensor introduces a displacement
mismatch that is removed by elastic relaxation. The elastic strains are then no
longer compatible in the sense of the Saint-Venant law and the ensuing
incompatibility tensor is shown to be proportional to the gradients of the Nye
dislocation density tensor. We demonstrate that the presence of this
incompatibility gives rise to an additional long-range contribution in the
inhomogeneous part of the Landau energy functional and to the corresponding
stress fields. Competition amongst the local and long-range interactions
results in frustration in the evolving order parameter (elastic) texture. We
show how the Peach-Koehler forces and stress fields for any distribution of
dislocations in arbitrarily anisotropic media can be calculated and employed in
a Fokker-Planck dynamics for the dislocation density. This approach represents
a self-consistent scheme that yields the evolutions of both the order parameter
field and the continuous dislocation density. We illustrate our method by
studying the effects of dislocations on microstructure, particularly twinned
domain walls, in an Fe-Pd alloy undergoing a martensitic transformation.Comment: 24 pages, submitted to Phys. Rev. B (changes from v1 include mainly
incorporation of discrete slip systems; densities of crystal dislocations are
now tracked explicitly
Thermoelastic Waves in Microstructured Solids
Thermoelastic wave propagation suggests a coupling between elastic deformation and heat conduction in a body. Microstructure of the body influences the both processes. Since energy is conserved in elastic deformation and heat conduction is always dissipative, the generalization of classical elasticity theory and classical heat conduction is performed differently. It is shown in the paper that a hyperbolic evolution equation for microtemperature can be obtained in the framework of the dual internal variables approach keeping the parabolic equation for the macrotemperature. The microtemperature is considered as a macrotemperature fluctuation. Numerical simulations demonstrate the formation and propagation of thermoelastic waves in microstructured solids under thermal loading
Quantitative plane-resolved crystal growth and dissolution kinetics by coupling in situ optical microscopy and diffusion models : the case of salicylic acid in aqueous solution
The growth and dissolution kinetics of salicylic acid crystals are investigated in situ by focusing on individual microscale crystals. From a combination of optical microscopy and finite element method (FEM) modeling, it was possible to obtain a detailed quantitative picture of dissolution and growth dynamics for individual crystal faces. The approach uses real-time in situ growth and dissolution data (crystal size and shape as a function of time) to parametrize a FEM model incorporating surface kinetics and bulk to surface diffusion, from which concentration distributions and fluxes are obtained directly. It was found that the (001) face showed strong mass transport (diffusion) controlled behavior with an average surface concentration close to the solubility value during growth and dissolution over a wide range of bulk saturation levels. The (1Ì…10) and (110) faces exhibited mixed mass transport/surface controlled behavior, but with a strong diffusive component. As crystals became relatively large, they tended to exhibit peculiar hollow structures in the end (001) face, observed by interferometry and optical microscopy. Such features have been reported in a number of crystals, but there has not been a satisfactory explanation for their origin. The mass transport simulations indicate that there is a large difference in flux across the crystal surface, with high values at the edge of the (001) face compared to the center, and this flux has to be redistributed across the (001) surface. As the crystal grows, the redistribution process evidently can not be maintained so that the edges grow at the expense of the center, ultimately creating high index internal structures. At later times, we postulate that these high energy faces, starved of material from solution, dissolve and the extra flux of salicylic acid causes the voids to close
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