Modelling of ultrasonic metafluidsDraškovič-Bračun, Aljaž (Avtor) Svenšek, Daniel (Mentor) Praprotnik, Matej (Komentor) Acoustic metamaterialsmetafluidsdiscrete oscillatorsdynamic densitydynamic compressibilityfloppy modeshybrid molecular dynamics simulationsfictitious fluidenhanced quasiharmonic analysis.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.20252025-04-12 08:15:21Doktorsko delo/naloga168420VisID: 150153COBISS_ID: 229425667sl