951 research outputs found
Acoustic Fresnel lenses with extraordinary transmission
We investigate numerically and experimentally highly efficient acoustic lenses based on the principle of extraordinary acoustic transmission. We study circular, flat lenses composed of perforated air channels. The geometry is similar to binary Fresnel lenses, and the lenses exploit several resonance mechanisms to enhance the transmission, such as Fabry–Perot resonances in the channels and cavity resonances on the lens surface. The proposed lenses are able to transmit up to 83% of the incident energy and generate sharp focusing with very high amplification (up to 16 dB experimentally). Furthermore, the resulting lenses are thinner than other designs providing similar performance, making them ideal candidates for application in acoustic imaging and medical diagnostics
Visco-thermal effects in acoustic metamaterials: from total transmission to total reflection and high absorption
We theoretically and experimentally investigate visco–thermal effects on the acoustic propagation through metamaterials consisting of rigid slabs with subwavelength slits embedded in air. We demonstrate that this unavoidable loss mechanism is not merely a refinement, but that it plays a dominant role in the actual acoustic response of the structure. Specifically, in the case of very narrow slits, the visco–thermal losses avoid completely the excitation of Fabry–Perot resonances, leading to 100% reflection. This is exactly opposite to the perfect transmission predicted in the idealised lossless case. Moreover, for a wide range of geometrical parameters, there exists an optimum slit width at which the energy dissipated in the structure can be as high as 50%. This work provides a clear evidence that visco–thermal effects are necessary to describe realistically the acoustic response of locally resonant metamaterials
Characterization of vertically aligned carbon nanotube forests grown on stainless steel surfaces
Vertically aligned carbon nanotube (CNT) forests are a particularly
interesting class of nanomaterials, because they combine multifunctional
properties, such as high energy absorption, compressive strength,
recoverability and super-hydrophobicity with light weight. These
characteristics make them suitable for application as coating, protective
layers and antifouling substrates for metallic pipelines and blades. Direct
growth of CNT forests on metals offers the possibility to transfer the tunable
CNT functionalities directly onto the desired substrates. Here, we focus on
characterizing the structure and mechanical properties, as well as wettability
and adhesion of CNT forests grown on different types of stainless steel. We
investigate the correlations between composition and morphology of the steel
substrates with the micro-structure of the CNTs, and reveal how the latter
ultimately controls the mechanical and wetting properties of the CNT forest.
Additionally, we study the influence of substrate morphology on the adhesion of
CNTs to their substrate. We highlight that the same structure-property
relationships govern the mechanical performance of CNT forests grown on steels
and on Si
Discovery of topological metamaterials by symmetry relaxation and smooth topological indicators
Robustness against small perturbations is a crucial feature of topological
properties. This robustness is both a source of theoretical interest and a
drive for technological applications, but presents a challenge when looking for
new topological systems: Small perturbations cannot be used to identify the
global direction of change in the topological indices. Here, we overcome this
limitation by breaking the symmetries protecting the topology. The introduction
of symmetry-breaking terms causes the topological indices to become
non-quantized variables, which are amenable to efficient design algorithms
based on gradient methods. We demonstrate this capability by designing discrete
and continuous phononic systems realizing conventional and higher-order
topological insulators
A mechanical autonomous stochastic heat engine
Stochastic heat engines are devices that generate work from random thermal motion using a small number of highly fluctuating degrees of freedom. Proposals for such devices have existed for more than a century and include the Maxwell demon and the Feynman ratchet. Only recently have they been demonstrated experimentally, using e.g., thermal cycles implemented in optical traps. However, the recent demonstrations of stochastic heat engines are nonautonomous, since they require an external control system that prescribes a heating and cooling cycle, and consume more energy than they produce. This Report presents a heat engine consisting of three coupled mechanical resonators (two ribbons and a cantilever) subject to a stochastic drive. The engine uses geometric nonlinearities in the resonating ribbons to autonomously convert a random excitation into a low-entropy, nonpassive oscillation of the cantilever. The engine presents the anomalous heat transport property of negative thermal conductivity, consisting in the ability to passively transfer energy from a cold reservoir to a hot reservoir
- …