doctoral thesis
Aljaž Draškovič-Bračun (Author), Daniel Svenšek (Mentor), Matej Praprotnik (Co-mentor)

Abstract

We explore the dynamic properties and potential applications of ultrasonic metafluids, specifically focusing on suspensions of discrete microscopic oscillators. Contrary to a widespread assumption about metamaterials, and as already established in the field of metafluids, the metafluid concept need not be based on position periodicity or correlation of the suspended micro-oscillators, and in this case not even on ideally designed micro-oscillators. Due to their discrete nature, the oscillators, which could potentially be realized by objects such as macromolecules or even artificially microfabricated (microprinted) entities, exhibit a unique resonant behavior that differs from conventional continuum objects. The research investigates how the inclusion of these discrete structures affects the effective compressibility and density of the medium, key parameters that determine the propagation of sound waves. By developing a detailed model, that reveals the operating principles of a metafluid based on these oscillators, we first investigate the interaction between fluid-coupled eigenmodes and the resulting acoustic response using a simple oscillator prototype. This analysis reveals the conditions under which both effective moduli can attain negative values, resulting in unconventional wave propagation properties typical of metamaterials due to the negative phase velocity. We then advance towards realistic structures featuring a high density of low-frequency “floppy modes”—resonances that occur in loosely connected microstructures and are a distinctive characteristic of discrete oscillators. We also provide a brief overview of scattering theory, the most common tool for the theoretical study of continuous resonant structures in soft metamaterials, and compare it with our model description. Finally, the theoretical framework is supported by molecular simulations, which validate the predicted acoustic behavior. We first present an enhanced version of the conventional analysis method that allows to extract eigenmodes that affect the dynamic density from thermal trajectories—an outcome not achievable with the traditional approach. In our simulations, we employ a hybrid technique combining molecular dynamics for the micro-oscillator with Lattice-Boltzmann for the solvent. To address the challenge of simulating a fluid with accurately represented density, compressibility and viscosity— properties central to this study — within feasible computational times, we introduce a novel concept of a fictitious fluid. The fictitious fluid retains the dynamic effects of the oscillator interaction while significantly reducing computational intensity. This concept is not strictly limited to the study of metamaterials and, with some adaptations, can easily be applied to any study of the interaction of dynamic objects with a fluid.

Keywords

acoustic metamaterials;metafluids;discrete oscillators;dynamic density;dynamic compressibility;floppy modes;molecular dynamics;fictious fluids;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UL FMF - Faculty of Mathematics and Physics
Publisher: [A. Draškovič-Bračun]
UDC: 534:620.1(043.3)
COBISS: 229425667 Link will open in a new window
Views: 93
Downloads: 16
Average score: 0 (0 votes)
Metadata: JSON JSON-RDF JSON-LD TURTLE N-TRIPLES XML RDFA MICRODATA DC-XML DC-RDF RDF

Other data

Secondary language: Slovenian
Secondary title: Modeliranje ultrazvočnih metatekočin
Secondary abstract: Raziskujemo dinamične lastnosti in potencialne možnosti uporabe ultrazvočnih metatekočin, pri čemer se posebej osredotočimo na suspenzije diskretnih mikroskopskih oscilatorjev. V nasprotju z razširjenim prepričanjem o metamaterialih in kot je že uveljavljeno na področju metatekočin pokažemo, da za metamateriale nista nujni niti periodična razporeditev oz. prostorska koreliranost gradnikov niti njihova natančno osnovana zgradba. Predlagani oscilatorji, ki bi jih bilo mogoče realizirati z objekti, kot so makromolekule ali celo umetno mikroproizvedeni (mikrotiskani) objekti, imajo zaradi svoje diskretne narave edinstven resonančni spekter, ki jih razlikuje od konvencionalnih kontinuumskih objektov. V disertaciji pokažemo, kako se ob prisotnosti teh diskretnih struktur spremenita efektivna stisljivost in gostota medija, ključna parametra, ki določata propagacijo zvočnih valov. Z razvojem modela, ki razkriva načela delovanja metatekočine, najprej z uporabo preprostega prototipa oscilatorja preučimo interakcijo med lastnimi načini, povezanimi s tekočino, in posledičnim akustičnim odzivom. S tem razkrijemo pogoje, pod katerimi lahko oba efektivna parametra dosežeta negativne vrednosti, kar povzroči za metamateriale značilne nekonvencionalne lastnosti širjenja valov. Nato se pomaknemo proti realističnim strukturam, ki vsebujejo visoko gostoto nizkofrekvenčnih “floppy načinov”—resonanc, ki se pojavijo v ohlapno povezanih mikrostrukturah in so značilna lastnost diskretnih oscilatorjev. Prav tako podajamo kratek pregled teorije sipanja, najpogosteje uporabljene metode za teoretično proučevanje kontinuumskih resonančnih struktur v mehki snovi, in jo primerjamo z našim modelom. V nadaljevanju teoretični model podpremo z molekularnimi simulacijami, ki potrjujejo napovedani akustični odziv. Najprej predstavimo izboljšano različico konvencionalne metode analize za prepoznavanje lastnih načinov. Ta omogoča prepoznavanje načinov, ki vplivajo na dinamično gostoto iz toplotnih trajektorij — rezultat, ki s tradicionalnim pristopom ni dosegljiv. V naših simulacijah uporabljamo hibridno metodo, ki združuje molekulsko dinamiko za mikrooscilator z Lattice-Boltzmannovo metodozatekočino. Pri tem se srečamo z izzivom simuliranja tekočine z natančno reprezentirano gostoto, stisljivostjo in viskoznostjo — lastnostmi, ki so ključne za to raziskavo — v okviru izvedljivih računskih časov, zaradi česar uvedemo nov koncept fiktivne tekočine, ki ohranja dinamične učinke interakcije oscilatorja, medtem ko znatno zmanjša računsko zahtevnost. Ta koncept ni strogo omejen na proučevanje metamaterialov in se lahko z določenimi prilagoditvami zlahka uporabi v katerikoli brezdimenzijski raziskavi interakcije dinamičnih objektov s tekočino.
Secondary keywords: akustični metamateriali;akustične metatekočine;metatekočine;diskretni oscilatorji;dinamična gostota;dinamična stisljivost;floppy načini;molekulska dinamika;fiktivne tekočine;Metamateriali;Disertacije;Modeliranje;
Type (COBISS): Doctoral dissertation
Study programme: 0
Thesis comment: Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages: 125 str.
ID: 26206904