Requisites on viscoelasticity for exceptional points in passive elastodynamic metamaterials

TL;DR

This paper demonstrates how passive viscoelastic materials with nearly constant loss tangent over a broad frequency range can be used to create exceptional points in elastodynamic metamaterials, avoiding complex gain mechanisms.

Contribution

It introduces strategies to achieve exceptional points in passive elastodynamic systems using viscoelastic materials with specific frequency-dependent loss properties.

Findings

EPs emerge when the loss tangent remains nearly constant across operational frequencies.

Materials like PDMS and polyurethane exhibit KVFD behavior suitable for EP creation.

Passive systems can realize EPs without complex gain-loss engineering.

Abstract

The recent progress of non-Hermitian physics and the notion of exceptional point (EP) degeneracies in elastodynamics has led to the development of novel metamaterials for the control of elastic wave propagation, hypersensitive sensors, and actuators. The emergence of EPs in a Parity-Time symmetric system relies on judiciously engineered balanced gain and loss mechanisms. Creating gain requires complex circuits and amplification mechanisms, making engineering applications challenging. Here, we report strategies to achieve EPs in passive non-Hermitian elastodynamic systems with differential loss derived from viscoelastic materials. We compare different viscoelastic material models and show that the EP emerges only when the frequency-dependent loss-tangent of the viscoelastic material remains nearly constant in the frequency range of operation. Such type of loss tangent occurs in materials that undergo stress-relaxation over a broad spectrum of relaxation times, for example, materials that follow the Kelvin-Voigt fractional derivative (KVFD) model. Using dynamic mechanical analysis, we show that a few common viscoelastic elastomers such as Polydimethylsiloxane (PDMS) and polyurethane rubber follow the KVFD behavior such that the loss tangent becomes almost constant after a particular frequency. The material models we present and the demonstration of the potential of a widely available material system in creating EPs pave the way for developing non-Hermitian metamaterials with hypersensitivity to perturbations or enhanced emissivity.