Publications
354 results found
Puvirajesinghe TM, Zhi ZL, Craster RV, et al., 2018, Tailoring drug release rates in hydrogel-based therapeutic delivery applications using graphene oxide, Journal of the Royal Society Interface, Vol: 15, ISSN: 1742-5662
Graphene oxide (GO) is increasingly used for controlling mass diffusion in hydrogel-based drug delivery applications. On the macro-scale, the density of GO in the hydrogel is a critical parameter for modulating drug release. Here, we investigate the diffusion of a peptide drug through a network of GO membranes and GO-embedded hydrogels, modelled as porous matrices resembling both laminated and 'house of cards' structures. Our experiments use a therapeutic peptide and show a tunable nonlinear dependence of the peptide concentration upon time. We establish models using numerical simulations with a diffusion equation accounting for the photo-thermal degradation of fluorophores and an effective percolation model to simulate the experimental data. The modelling yields an interpretation of the control of drug diffusion through GO membranes, which is extended to the diffusion of the peptide in GO-embedded agarose hydrogels. Varying the density of micron-sized GO flakes allows for fine control of the drug diffusion. We further show that both GO density and size influence the drug release rate. The ability to tune the density of hydrogel-like GO membranes to control drug release rates has exciting implications to offer guidelines for tailoring drug release rates in hydrogel-based therapeutic delivery applications.
Roux P, Rupin M, Lemoult F, et al., 2018, New Trends Toward Locally-Resonant Metamaterials at the Mesoscopic Scale, World Scientific Series in Nanoscience and Nanotechnology, Pages: 251-299
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Berte R, Picca FD, Poblet M, et al., 2018, Generation and detection of surface acoustic waves using single plasmonic nanoresonators
We show in this work that coherent phonons generated after the decay of optically-excited plasmons in isolated metallic nanoantennas, are transmitted through the substrate as surface acoustic waves (SAWs) which can be detected by other nanoantennas used as receptors and positioned at distances up to 3μm away from the source. Two color sub-ps pump-probe technique and numerical methods suggest wave speed and amplitude decay characteristic of Rayleigh waves, the former within 3.2% of the predicted for fused silica. It is also shown that the mechanical excitation of the receptors via SAW modulates the optical response of the probe transmission and that its spectral content shows that the detection is feasible, even when the vibrational modes of the receptor are detuned from those of the source.
Brûlé S, Enoch S, Guenneau S, et al., 2018, Seismic Metamaterials: Controlling Surface Rayleigh Waves Using Analogies with Electromagnetic Metamaterials, World Scientific Series in Nanoscience and Nanotechnology, Pages: 301-337
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- Citations: 6
Craster R, Antonakakis T, Guenneau S, 2018, Dynamic Homogenization of Acoustic and Elastic Metamaterials and Phononic Crystals, World Scientific Series in Nanoscience and Nanotechnology, Pages: 1-56
Craster R, Guenneau S, Maier SA, 2018, Preface by Main Editor, World Scientific Series in Nanoscience and Nanotechnology, Vol: 16, Pages: V-VI, ISSN: 2301-301X
Guenneau S, Brule S, Enoch S, et al., 2018, Some challenges regarding cloaking and earthquake protection, 12th International Congress on Artificial Materials for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 158-160
Schnitzer O, Craster RV, 2017, Bloch waves in an arbitrary two-dimensional lattice of subwavelength Dirichlet scatterers, SIAM Journal on Applied Mathematics, Vol: 77, Pages: 2119-2135, ISSN: 0036-1399
We study waves governed by the planar Helmholtz equation, propagating in aninfinite lattice of subwavelength Dirichlet scatterers, the periodicity beingcomparable to the wavelength. Applying the method of matched asymptoticexpansions, the scatterers are effectively replaced by asymptotic pointconstraints. The resulting coarse-grained Bloch-wave dispersion problem issolved by a generalised Fourier series, whose singular asymptotics in thevicinities of scatterers yield the dispersion relation governing modes that arestrongly perturbed from plane-wave solutions existing in the absence of thescatterers; there are also empty-lattice waves that are only weakly perturbed.Characterising the latter is useful in interpreting and potentially designingthe dispersion diagrams of such lattices. The method presented, that simplifiesand expands on Krynkin & McIver [Waves Random Complex, 19 347 2009], could beapplied in the future to study more sophisticated designs entailing resonantsubwavelength elements distributed over a lattice with periodicity on the orderof the operating wavelength.
Vanel AL, Schnitzer O, Craster RV, 2017, Asymptotic network models of subwavelength metamaterials formed by closely packed photonic and phononic crystals, Europhysics Letters: a letters journal exploring the frontiers of physics, Vol: 119, ISSN: 1286-4854
We demonstrate that photonic and phononic crystals consisting of closely spaced inclusions constitute a versatile class of subwavelength metamaterials. Intuitively, the voids and narrow gaps that characterise the crystal form an interconnected network of Helmholtz-like resonators. We use this intuition to argue that these continuous photonic (phononic) crystals are in fact asymptotically equivalent, at low frequencies, to discrete capacitor-inductor (mass-spring) networks whose lumped parameters we derive explicitly. The crystals are tantamount to metamaterials as their entire acoustic branch, or branches when the discrete analogue is polyatomic, is squeezed into a subwavelength regime where the ratio of wavelength to period scales like the ratio of period to gap width raised to the power $1/4$ ; at yet larger wavelengths we accordingly find a comparably large effective refractive index. The fully analytical dispersion relations predicted by the discrete models yield dispersion curves that agree with those from finite-element simulations of the continuous crystals. The insight gained from the network approach is used to show that, surprisingly, the continuum created by a closely packed hexagonal lattice of cylinders is represented by a discrete honeycomb lattice. The analogy is utilised to show that the hexagonal continuum lattice has a Dirac-point degeneracy that is lifted in a controlled manner by specifying the area of a symmetry-breaking defect.
Craster R, Guenneau S, Hutridurga H, et al., 2017, Regularized transformation optics for transient heat transfer, 2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 127-129
Colombi A, Craster R, Colquitt D, et al., 2017, Elastic wave control beyond band-gaps: shaping the flow of waves in plates and half-spaces with subwavelength resonant rods., Frontiers in Mechanical Engineering, Vol: 3, ISSN: 2297-3079
In metamaterial science, local resonance and hybridization are key phenomena strongly influencing the dispersion properties; the metasurface discussed in this article created by a cluster of resonators, subwavelength rods, atop an elastic surface being an exemplar with these features. On this metasurface, band-gaps, slow or fast waves, negative refraction, and dynamic anisotropy can all be observed by exploring frequencies and wavenumbers from the Floquet–Bloch problem and by using the Brillouin zone. These extreme characteristics, when appropriately engineered, can be used to design and control the propagation of elastic waves along the metasurface. For the exemplar we consider, two parameters are easily tuned: rod height and cluster periodicity. The height is directly related to the band-gap frequency and, hence, to the slow and fast waves, while the periodicity is related to the appearance of dynamic anisotropy. Playing with these two parameters generates a gallery of metasurface designs to control the propagation of both flexural waves in plates and surface Rayleigh waves for half-spaces. Scalability with respect to the frequency and wavelength of the governing physical laws allows the application of these concepts in very different fields and over a wide range of lengthscales.
Colombi A, Craster R, Clark M, et al., 2017, Slow waves, elastic rainbow and dynamic anisotropy with a cluster of resonant rods on an elastic halfspace, 2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 409-410
Colombi A, Roux P, Miniaci M, et al., 2017, The role of large scale computing behind the development of seismic (and elastic) metamaterials., 2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 406-408
Colombi A, Ageeva V, Smith RJ, et al., 2017, Enhanced sensing and conversion of ultrasonic Rayleigh waves by elastic metasurfaces, Scientific Reports, Vol: 7, ISSN: 2045-2322
Recent years have heralded the introduction of metasurfaces that advantageously combine the vision of sub- wavelength wave manipulation, with the design, fabrication and size advantages associated with surface excitation. An important topic within metasurfaces is the tailored rainbow trapping and selective spatial frequency separation of electromagnetic and acoustic waves using graded metasurfaces. This frequency dependent trapping and spatial frequency segregation has implications for energy concentrators and associated energy harvesting, sensing and wave filtering techniques. Different demonstrations of acoustic and electromagnetic rainbow devices have been performed, however not for deep elastic substrates that support both shear and compressional waves, together with surface Rayleigh waves; these allow not only for Rayleigh wave rainbow effects to exist but also for mode conversion from surface into shear waves. Here we demonstrate experimentally not only elastic Rayleigh wave rainbow trapping, by taking advantage of a stop-band for surface waves, but also selective mode conversion of surface Rayleigh waves to shear waves. These experiments performed at ultrasonic frequencies, in the range of 400-600 kHz, are complemented by time domain numerical simulations. The metasurfaces we design are not limited to guided ultrasonic waves and are a general phenomenon in elastic waves that can be translated across scales.
O'Neill J, Selsil O, Haslinger SG, et al., 2017, ACTIVE CLOAKING FOR FINITE CLUSTERS OF PINS IN KIRCHHOFF PLATES, SIAM Journal on Applied Mathematics, Vol: 77, Pages: 1115-1135, ISSN: 0036-1399
This paper considers active cloaking of a square array of evenly spaced pins in a Kirchhoff plate in the presence of flexural waves. Active sources, modeled as ideal point sources, are represented by the nonsingular Green's function for the two-dimensional biharmonic operator and have an arbitrary complex amplitude. These sources are distributed exterior to the cluster, and their complex amplitudes are found by solving an algebraic system of equations. This procedure ensures that selected multipole orders of the scattered field are successfully annulled. For frequencies in the zero-frequency stop band, we find that a small number of active sources located on a grid is sufficient for cloaking. For higher frequencies, we achieve efficient cloaking with the active sources positioned on a circle surrounding the cluster. We demonstrate the cloaking efficiency with several numerical illustrations, considering key frequencies from band diagrams and dispersion surfaces for a Kirchhoff plate pinned in a doubly periodic fashion.
Lefebvre G, Antonakakis T, Achaoui Y, et al., 2017, Unveiling extreme anisotropy in elastic structured media, Physical Review Letters, Vol: 118, ISSN: 0031-9007
Periodic structures can be engineered to exhibit unique properties observed at symmetry points, such as zero group velocity, Dirac cones, and saddle points; identifying these and the nature of the associated modes from a direct reading of the dispersion surfaces is not straightforward, especially in three dimensions or at high frequencies when several dispersion surfaces fold back in the Brillouin zone. A recently proposed asymptotic high-frequency homogenization theory is applied to a challenging time-domain experiment with elastic waves in a pinned metallic plate. The prediction of a narrow high-frequency spectral region where the effective medium tensor dramatically switches from positive definite to indefinite is confirmed experimentally; a small frequency shift of the pulse carrier results in two distinct types of highly anisotropic modes. The underlying effective equation mirrors this behavior with a change in form from elliptic to hyperbolic exemplifying the high degree of wave control available and the importance of a simple and effective predictive model.
Achaoui Y, Antonakakis T, Brule S, et al., 2017, Clamped seismic metamaterials: Ultra-low broad frequency stop-bands, New Journal of Physics, Vol: 9, ISSN: 1367-2630
The regularity of earthquakes, their destructive power, and the nuisance of ground vibration in urbanenvironments, all motivate designs of defence structures to lessen the impact of seismic and groundvibration waves on buildings. Low frequency waves, in the range 1–10 Hz for earthquakes and up to afew tens of Hz for vibrations generated by human activities, cause a large amount of damage, orinconvenience; depending on the geological conditions they can travel considerable distances andmay match the resonant fundamental frequency of buildings. The ultimate aim of any seismicmetamaterial, or any other seismic shield, is to protect over this entire range of frequencies; the longwavelengths involved, and low frequency, have meant this has been unachievable to date. Notably thisis scalable and the effects also hold for smaller devices in ultrasonics. There are three approaches toobtaining shielding effects: bragg scattering, locally resonant sub-wavelength inclusions and zerofrequencystop-band media. The former two have been explored, but the latter has not and isexamined here. Elastic flexural waves, applicable in the mechanical vibrations of thin elastic plates, canbe designed to have a broad zero-frequency stop-band using a periodic array of very small clampedcircles. Inspired by this experimental and theoretical observation, all be it in a situation far removedfrom seismic waves, we demonstrate that it is possible to achieve elastic surface (Rayleigh)wavereflectors at very large wavelengths in structured soils modelled as a fully elastic layer periodicallyclamped to bedrock. We identify zero frequency stop-bands that only exist in the limit of columns ofconcrete clamped at their base to the bedrock. In a realistic configuration of a sedimentary basin 15 mdeep we observe a zero frequency stop-band covering a broad frequency range of 0–30 Hz.
Shi F, Lowe M, Craster R, 2017, Diffusely scattered and transmitted elastic waves by random rough solid-solid interfaces using an elastodynamic Kirchhoff approximation, PHYSICAL REVIEW B, Vol: 95, ISSN: 2469-9950
Elastic waves scattered by random rough interfaces separating two distinct media play an important role in modeling phonon scattering and impact upon thermal transport models, and are also integral to ultrasonic inspection. We introduce theoretical formulas for the diffuse field of elastic waves scattered by, and transmitted across, random rough solid-solid interfaces using the elastodynamic Kirchhoff approximation. The new formulas are validated by comparison with numerical Monte Carlo simulations, for a wide range of roughness (rms σ≤λ/3, correlation length λ0≥ wavelength λ), demonstrating a significant improvement over the widely used small-perturbation approach, which is valid only for surfaces with small rms values. Physical analysis using the theoretical formulas derived here demonstrates that increasing the rms value leads to a considerable change of the scattering patterns for each mode. The roughness has different effects on the reflection and the transmission, with a strong dependence on the material properties. In the special case of a perfect match of the wave speed of the two solid media, the transmission is the same as the case for a flat interface. We pay particular attention to scattering in the specular direction, often used as an observable quantity, in terms of the roughness parameters, showing a peak at an intermediate value of rms; this rms value coincides with that predicted by the Rayleigh parameter.
Haslinger SG, Movchan NV, Movchan AB, et al., 2017, Controlling flexural waves in semi-infinite platonic crystals with resonator-type scatterers, Quarterly Journal of Mechanics and Applied Mathematics, Vol: 70, Pages: 216-247, ISSN: 1464-3855
We address the scattering and transmission of a plane flexural wave through a semi-infinite array of point scatterers/resonators, which take a variety of physically interesting forms. The mathematical model accounts for several classes of point defects, including mass-spring resonators attached to the top surface of the flexural plate and their limiting case of concentrated point masses. We also analyse the special case of resonators attached to opposite faces of the plate. The problem is reduced to a functional equation of the Wiener–Hopf type, whose kernel varies with the type of scatterer considered. A novel approach, which stems from the direct connection between the kernel function of the semi-infinite system and the quasi-periodic Green's functions for corresponding infinite systems, is used to identify special frequency regimes. We thereby demonstrate dynamically anisotropic wave effects in semi-infinite platonic crystals, with particular attention paid to designing systems that exhibit dynamic neutrality (perfect transmission) and localisation close to the structured interface.
Theodorakis PE, Muller EA, Craster RV, et al., 2017, Physical insights into the blood-brain barrier translocation mechanisms, Physical Biology, Vol: 14, ISSN: 1478-3975
The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood–brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.
Skelton EA, Craster RV, Colombi A, et al., 2017, Fluid-loaded metasurfaces
We consider wave propagation along fluid-loaded structures which take theform of an elastic plate augmented by an array of resonators forming ametasurface, that is, a surface structured with sub-wavelength resonators. Suchsurfaces have had considerable recent success for the control of wavepropagation in electromagnetism and acoustics, by combining the vision ofsub-wavelength wave manipulation, with the design, fabrication and sizeadvantages associated with surface excitation. We explore one aspect of recentinterest in this field: graded metasurfaces, but within the context offluid-loaded structures. Graded metasurfaces allow for selective spatial frequency separation and areoften referred to as exhibiting rainbow trapping. Experiments, and theory, havebeen developed for acoustic, electromagnetic, and even elastic, rainbow devicesbut this has not been approached for fluid-loaded structures that supportsurface waves coupled with the acoustic field in a bulk fluid. This surfacewave, coupled with the fluid, can be used to create an additional effect bydesigning a metasurface to mode convert from surface to bulk waves. Wedemonstrate that sub-wavelength control is possible and that one can createboth rainbow trapping and mode conversion phenomena for a fluid-loaded elasticplate model.
Maling B, Schnitzer O, Craster RV, 2017, Radiation from structured-ring resonators, SIAM Journal on Applied Mathematics, ISSN: 0036-1399
We investigate the scalar-wave resonances of systems composed of identicalNeumann-type inclusions arranged periodically around a circular ring. Drawingon natural similarities with the undamped Rayleigh-Bloch waves supported byinfinite linear arrays, we deduce asymptotically the exponentially smallradiative damping in the limit where the ring radius is large relative to theperiodicity. In our asymptotic approach, locally linear Rayleigh-Bloch wavesthat attenuate exponentially away from the ring are matched to a ring-scaleWKB-type wave field. The latter provides a descriptive physical picture of howthe mode energy is transferred via tunnelling to a circularevanescent-to-propagating transition region a finite distance away from thering, from where radiative grazing rays emanate to the far field. Excluding thezeroth-order standing-wave modes, the position of the transition circlebifurcates with respect to clockwise and anti-clockwise contributions,resulting in striking spiral wavefronts.
Dubrovina E, Craster RV, Papageorgiou DT, 2017, Two-layer electrified pressure-driven flow in topographically structured channels, Journal of Fluid Mechanics, Vol: 814, Pages: 222-248, ISSN: 0022-1120
The flow of two stratified viscous immiscible perfect dielectric fluids in a channel withtopographically structured walls is investigated. The flow is driven by a streamwisepressure gradient and an electric field across the channel gap. This problem isexplored in detail by deriving and studying a nonlinear evolution equation for theinterface valid for large-amplitude long waves in the Stokes flow regime. For flatwalls, the electrified flow is long-wave unstable with a critical cutoff wavenumberthat increases linearly with the magnitude of the applied voltage. In the nonlinearregime, it is found that the presence of pressure-driven flow prevents electrostaticallyinduced interface touchdown that has been observed previously – time-modulatednonlinear travelling waves emerge instead. When topography is present, linearly stableuniform flows become non-uniform spatially periodic steady states; a small-amplitudeasymptotic theory is carried out and compared with computations. In the linearlyunstable regime, intricate nonlinear structures emerge that depend, among otherthings, on the magnitude of the wall corrugations. For a low-amplitude sinusoidalboundary, time-modulated travelling waves are observed that are similar to thosefound for flat walls but are influenced by the geometry of the wall and slide over itwithout touching. The flow over a high-amplitude sinusoidal pattern is also examinedin detail and it is found that for sufficiently large voltages the interface evolves tolarge-amplitude waves that span the channel and are subharmonic relative to the wall.A type of ‘walking’ motion emerges that causes the lower fluid to wash through thetroughs and create strong vortices over the peaks of the lower boundary. Non-uniformsteady states induced by the topography are calculated numerically for moderate andlarge values of the flow rate, and their stability is analysed using Floquet theory.The effect of large flow rates is also considered asymptotically to fi
Colquitt DJ, Colombi A, Craster RV, et al., 2017, Seismic metasurfaces: sub-wavelength resonators and Rayleigh wave interaction, Journal of the Mechanics and Physics of Solids, Vol: 99, Pages: 379-393, ISSN: 0022-5096
We consider the canonical problem of an array of rods, which act as resonators, placed on an elastic substrate; the substrate being either a thin elastic plate or an elastic half-space. In both cases the flexural plate, or Rayleigh surface, waves in the substrate interact with the resonators to create interesting effects such as effective band-gaps for surface waves or filters that transform surface waves into bulk waves; these effects have parallels in the field of optics where such sub-wavelength resonators create metamaterials, and metasurfaces, in the bulk and at the surface respectively. Here we carefully analyse this canonical problem by extracting the dispersion relations analytically thereby examining the influence of both the flexural and compressional resonances on the propagating wave. For an array of resonators atop an elastic half-space we augment the analysis with numerical simulations. Amongst other effects, we demonstrate the striking effect of a dispersion curve that transitions from Rayleigh wave-like to shear wave-like behaviour and the resultant change in displacement from surface to bulk waves.
Balmforth NJ, Craster RV, Hewitt DR, et al., 2017, Viscoplastic boundary layers, Journal of Fluid Mechanics, Vol: 813, Pages: 929-954, ISSN: 1469-7645
In the limit of a large yield stress, or equivalently at the initiation of motion, viscoplasticflows can develop narrow boundary layers that provide either surfaces of failure betweenrigid plugs, the lubrication between plugged flow and a wall, or buffers for regions ofpredominantly plastic deformation. (Oldroyd 1947,Proc. Camb. Phil. Soc.43, 383 - 395)presented the first theoretical discussion of these viscoplastic boundary layers, offeringan asymptotic reduction of the governing equations and a discussion of some modelflow problems. However, the complicated nonlinear form of Oldroyd’s boundary-layerequations has evidently precluded further discussion of them. In the current paper,we revisit Oldroyd’s viscoplastic boundary-layer analysis and his canonical examples ofa jet-like intrusion and flow past a thin plate. We also consider flow down channelswith either sudden expansions or wavy walls. In all these examples, we verify thatviscoplastic boundary layers form as envisioned by Oldroyd. For each example, we extractthe dependence of the boundary-layer thickness and flow profiles on the dimensionlessyield-stress parameter (Bingham number). We find that, while Oldroyd’s boundary-layertheory applies to free viscoplastic shear layers, it does not apply when the boundarylayer is adjacent to a wall, as has been observed previously for two-dimensional flowaround circular obstructions. Instead, the boundary-layer thickness scales in a differentfashion with the Bingham number, as suggested by classical solutions for plane-parallelflows, lubrication theory and, for flow around a plate, by (Piau 2002,J. Non-NewtonianFluid Mech.102, 193 - 218); we rationalize this second scaling and provide an alternativeboundary-layer theory.
Hernando Quintanilla F, Lowe MJ, Craster RV, 2017, The symmetry and coupling properties of solutions in general anisotropic multilayer waveguides, Journal of the Acoustical Society of America, Vol: 141, ISSN: 0001-4966
Multilayered plate and shell structures play an important role in many engineering settings where, for instance, coated pipes are commonplace such as in the petrochemical, aerospace, and power generation industries. There are numerous demands, and indeed requirements, on nondestructive evaluation (NDE) to detect defects or to measure material properties using guided waves; to choose the most suitable inspection approach, it is essential to know the properties of the guided wave solutions for any given multilayered system and this requires dispersion curves computed reliably, robustly, and accurately. Here, the circumstances are elucidated, and possible layer combinations, under which guided wave solutions, in multilayered systems composed of generally anisotropic layers in flat and cylindrical geometries, have specific properties of coupling and parity; the partial wave decomposition of the wave field is utilised to unravel the behaviour. A classification into five families is introduced and the authors claim that this is the fundamental way to approach generally anisotropic waveguides. This coupling and parity provides information to be used in the design of more efficient and robust dispersion curve tracing algorithms. A critical benefit is that the analysis enables the separation of solutions into categories for which dispersion curves do not cross; this allows the curves to be calculated simply and without ambiguity.
Sasihithlu K, Pendry JB, Craster RV, 2017, Van der Waals Force Assisted Heat Transfer, Zeitschrift für Naturforschung - Section A Journal of Physical Sciences, Vol: 72, Pages: 181-188, ISSN: 0932-0784
Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap – through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro- and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange.
Uppal AS, Matar OK, Craster RV, 2017, Dynamics of spreading thixotropic droplets, Journal of Non-Newtonian Fluid Mechanics, Vol: 240, Pages: 1-14, ISSN: 1873-2631
The e ect of thixotropy on the two-dimensional spreading of a sessiledrop is modelled using lubrication theory. Thixotropy is incorporated by theinclusion of a structure parameter, , measuring structure build-up governedby an evolution equation linked to the droplet micromechanics. A number ofmodels are derived for coupled to the interface dynamics; these range frommodels that account for the cross-stream dependence of to simpler ones inwhich this dependence is prescribed through appropriate closures. Numericalsolution of the governing equations show that thixotropy has a profound e ecton the spreading characteristics; the long-time spreading dynamics, however,are shown to be independent of the initial structural state of the droplet. Wealso compare the predictions of the various models and determine the rangeof system parameters over which the simple models provide su ciently goodapproximations of the full, two-dimensional spreading dynamics.
O'Neill J, Selsil Ö, Haslinger SG, et al., 2017, Active cloaking for flexural waves in a pinned kirchhoff plate
Smith ER, Müller EA, Craster RV, et al., 2016, A Langevin model for fluctuating contact angle behaviour parametrised using molecular dynamics, Soft Matter, Vol: 12, Pages: 9604-9615, ISSN: 1744-6848
Molecular dynamics simulations are employed to develop a theoretical model to predict the fluid-solid contact angle as a function of wall-sliding speed incorporating thermal fluctuations. A liquid bridge between counter-sliding walls is studied, with liquid-vapour interface-tracking, to explore the impact of wall-sliding speed on contact angle. The behaviour of the macroscopic contact angle varies linearly over a range of capillary numbers beyond which the liquid bridge pinches off, a behaviour supported by experimental results. Nonetheless, the liquid bridge provides an ideal test case to study molecular scale thermal fluctuations, which are shown to be well described by Gaussian distributions. A Langevin model for contact angle is parametrised to incorporate the mean, fluctuation and auto-correlations over a range of sliding speeds and temperatures. The resulting equations can be used as a proxy for the fully-detailed molecular dynamics simulation allowing them to be integrated within a continuum-scale solver.
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