Paolo Celli

Tunable directivity in metamaterials with reconfigurable cell symmetry

We introduce a strategy to attain reconfigurable, highly focused, subwavelength wave patterns in cellular metamaterials via electromechanical tuning of their microstructures. The metamaterial cells feature a population of auxiliary microstructural elements instrumented with piezoelectric patches connected to negative capacitance shunting circuits. By tuning the circuital characteristics of selected subsets of shunts, we relax the symmetry of the cell material property landscape, thus affecting the global directivity and enabling a plethora of wave manipulation capabilities. The acoustic reconfiguration is decoupled from other mechanical functions and is carried out without affecting the shape or the static properties of the host cellular structure.

Low-frequency spatial wave manipulation via phononic crystals with relaxed cell symmetry

Phononic crystals enjoy unique wave manipulation capabilities enabled by their periodic topologies. On one hand, they feature frequency-dependent directivity, which allows directional propagation of selected modes even at low frequencies. However, the stellar nature of the propagation patterns and the inability to induce single-beam focusing represent significant limitations of this functionality. On the other hand, one can realize waveguides by defecting the periodic structure of a crystal operating in bandgap mode along some desired path. Waveguides of this type are only activated in the relatively high and narrow frequency bands corresponding to total bandgaps, which limits their potential technological applications. In this work, we introduce a class of phononic crystals with relaxed cell symmetry and we exploit symmetry relaxation of a population of auxiliary microstructural elements to achieve spatial manipulation of elastic waves at very low frequencies, in the range of existence of the acoustic mo...

Manipulating waves with LEGO® bricks: A versatile experimental platform for metamaterial architectures

In this letter, we discuss a versatile, fully reconfigurable experimental platform for the investigation of phononic phenomena in metamaterial architectures. The approach revolves around the use of 3D laser vibrometry to reconstruct global and local wavefield features in specimens obtained through simple arrangements of LEGO® bricks on a thin baseplate. The agility by which it is possible to reconfigure the brick patterns into a nearly endless spectrum of topologies makes this an effective approach for rapid experimental proof of concept, as well as a powerful didactic tool, in the arena of phononic crystals and metamaterials engineering. We use our platform to provide a compelling visual illustration of important spatial wave manipulation effects (waveguiding and seismic isolation), and to elucidate fundamental dichotomies between Bragg-based and locally resonant bandgap mechanisms.

Wave control through soft microstructural curling: Bandgap shifting, reconfigurable anisotropy and switchable chirality

In this work, we discuss and numerically validate a strategy to attain reversible macroscopic changes in the wave propagation characteristics of cellular metamaterials with soft microstructures. The proposed cellular architecture is characterized by unit cells featuring auxiliary populations of symmetrically-distributed smart cantilevers stemming from the nodal locations. Through an external stimulus (the application of an electric field), we induce extreme, localized, reversible curling deformation of the cantilevers---a shape modification which does not affect the overall shape, stiffness and load bearing capability of the structure. By carefully engineering the spatial pattern of straight (non activated) and curled (activated) cantilevers, we can induce several profound modifications of the phononic characteristics of the structure: generation and/or shifting of total and partial bandgaps, cell symmetry relaxation (which implies reconfigurable wave beaming), and chirality switching. While in this work we discuss the specific case of composite cantilevers with a PDMS core and active layers of electrostrictive terpolymer P(VDF-TrFE-CTFE), the strategy can be extended to other smart materials (such as dielectric elastomers or shape-memory polymers).

Pathway towards Programmable Wave Anisotropy in Cellular Metamaterials

In this work, we provide a proof-of-concept experimental demonstration of the wave control capabilities of cellular metamaterials endowed with populations of tunable electromechanical resonators. Each independently tunable resonator comprises a piezoelectric patch and a resistor-inductor shunt, and its resonant frequency can be seamlessly re-programmed without interfering with the cellular structures default properties. We show that, by strategically placing the resonators in the lattice domain and by deliberately activating only selected subsets of them, chosen to conform to the directional features of the beamed wave response, it is possible to override the inherent wave anisotropy of the cellular medium. The outcome is the establishment of tunable spatial patterns of energy distillation resulting in a non-symmetric correction of the wavefields.

A disorder-based strategy for tunable, broadband wave attenuation

One of the most daunting limitations of phononic crystals and acoustic/elastic metamaterials is their passivity: a given configuration is bound to display its phononic properties only around its design point, i.e., working at some pre-determined operating conditions. In the past decade, this shortcoming has inspired the design of phononic media with tunable wave characteristics; noteworthy results have been obtained through a family of methodologies involving shunted piezoelectric elements. Shunting a piezoelectric element means connecting it to a passive electric circuit; tunability stems from the ability to modify the effective mechanical properties of the piezoelectric medium by modifying the circuit characteristics. One of the most popular shunting circuits is the resistor-inductor, which allows the patch-and-shunt system to behave as an electromechanical resonator. A common motif among the works employing shunted piezos for phononic control is periodicity: the patches are typically periodically placed in the domain and the circuits are identically tuned. The objective of this work is to demonstrate that the wave attenuation performance of structures with shunted piezoelectric patches can be improved by leveraging notions of organized disorder. Based on the idea of rainbow trapping broadband wave attenuation obtained by tuning an array of resonators at distinct neighboring frequencies we design and test an electromechanical waveguide structure capable of attenuating waves over broad frequency ranges. In order to emphasize the fact that periodicity is not a binding requirement when working with RL shunts (which induce locally resonant bandgaps), we report on the performance of random arrangements of patches. In an attempt to demonstrate the tunability attribute of our strategy, we take advantage of the reconfigurability of the circuits to show how a single waveguide can attenuate both waves and vibrations over different frequency ranges.

Cellular phononic crystals with piezoelectric shunts for tunable directivity

Phononic crystals (PCs) are periodic media known for their spectral and spatial wave manipulation capabilities, among which we recall their stop-band filtering behavior, due to the formation of phononic bandgaps, and the spatial directivity, i.e., the inherent ability to produce directional wave patterns. In general, the anisotropic wave propagation patterns of PCs are characterized by multiple equipotent directions of wave beaming, a characteristic which prevents the effective de-energization of arbitrarily selected regions of the PC domain. In this work we discuss a few enhancements of the directivity of lattice-like PCs, obtained through the introduction of shunted piezoelectric inclusions. The lattice links of each unit cell are instrumented with piezoelectric patches, each connected to a separate negative capacitance circuit. By properly choosing the shunting parameters for selected subsets of patches, we can generate peculiar anisotropic stiffness landscapes and reconfigure the elastic wave patterns accordingly.