arXiv subjects

Karthick Subramani

Publications and source records attributed to Karthick Subramani.

7 recordsLinked to original sources

arXiv2026

Acoustofluidic Suppression of Rayleigh Taylor Instability and Fluid Mixing: Stabilization of Stratified Fluids in a Minichannel

Rayleigh-Taylor Instability (RTI) typically arises when a dense fluid is superimposed on a lighter fluid, where the desta- bilizing gravitational force acting on miscible fluids drives chaotic mixing. We theoretically present an acoustofluidic method utilizing standing bulk acoustic waves (BAW) to counteract RTI and suppress the mixing of fluids. To success- fully achieve this suppression, we demonstrate that two concurrent conditions are to be satisfied: the acoustic energy density (Eac) of the standing waves must exceed its critical threshold (Ecr), and the orientation of the acoustic waves must be perpendicular to the fluid-fluid interface. This acoustofluidic mechanism reduces the mixing index (MI) by up to an order of magnitude compared to the mixing induced solely by gravity. By analyzing the interplay between acoustic and gravitational forces, this study provides a comprehensive understanding of acoustically modulated mixing dynamics in minichannels.

physics.flu-dyn↗

arXiv2025

Ferroelectric Nematic Liquid Crystals as Charge Boosters for Triboelectric Nanogenerators

Driven by growing demand for clean energy, triboelectric nanogenerators (TENGs) have emerged as promising self-powered systems, yet achieving high charge density remains a critical challenge. In polymer dielectrics, triboelectricity can be further amplified by incorporating high-dielectric and polar materials for functional adaptability. Conventional dielectrics, including liquid crystals (LCs), offer limited improvement for triboelectrification, whereas the breakthrough ferroelectric nematic liquid crystals (NF-LCs), with giant spontaneous polarization and high dielectric constant, act as highly effective charge boosters. Here, we introduce NF-LC (DIO) as a functional filler in a PVDF-based TENG. Increasing DIO content progressively grows the electroactive phase and effective polarization in PVDF and defines a marked improvement in TENGs electrical performances through favourable dipolar alignment and strengthened charge-trap effects. The optimized composite film achieves an impressive open-circuit voltage of 1.1 kV, short-circuit current of 50 micro Amp, and power density of 110 W/m2 - seven times higher than pure PVDF. The device exhibits excellent charge-storage capability, powers over 500 LEDs without power management. This work establishes NF-LC-based TENGs as a new platform for high-performance self-powered energy harvesting, linking soft matter physics with applied energy technology.

physics.app-ph↗

arXiv2024

Suspension Dynamics of Droplets in Acoustic and Gravitational Fields

In the field of acoustic suspension or levitation of droplets against gravity, the application of Gorkov's acoustic radiation force for small particles (within the Rayleigh limit) or its extensions to larger ones (beyond the Rayleigh limit) is limited to predicting the suspension position of the droplet. Since this approach treats the droplet as a rigid particle, it fails to capture the fluid dynamics of the droplet and is also unsuitable for studying interfacial phenomena such as droplet deformation, splitting, or coalescence. In this work, we employ our recently developed acoustic body force in Eulerian form, which models the droplet as a fluid, to theoretically investigate the suspension dynamics of droplet subjected to standing waves through the interaction between acoustic, interfacial, and gravitational forces. Our theory predicts that when interfacial forces are dominant, the presence of positive and negative acoustic force regions within droplets exceeding the Rayleigh limit reduces the net acoustic force counteracting gravity. As a result, the suspension dynamics become highly dependent on droplet size, in contrast to droplets within the Rayleigh limit, where the dynamics remain size-independent. Thus, beyond the Rayleigh limit, as the droplet size to wavelength ratio increases, the critical acoustic energy density ($E_{cr}$) required to suspend the droplet initially rises sharply, which agrees with recent experimental results. After $E_{cr}$ reaches a local maximum at $d/\lambda \approx 0.65$, it exhibits a pattern of alternating decreases and increases, with each successive peak surpassing the previous one. Remarkably, our study reveals a size-dependent shifting of the suspension position between nodes and antinodes for droplets beyond the Rayleigh limit, whereas droplets within this limit maintain a consistent suspension position regardless of size.

physics.flu-dyn↗

arXiv2024

Droplets in Acoustic Fields: A Unified Theory from Migration to Splitting

We present a comprehensive theoretical framework governing the dynamics of droplets in acoustic fields, applicable to all droplet sizes, from the Rayleigh limit (D<<lambda) and beyond. Our theory elucidates the nature of acoustic forces and incorporates the effects of interfacial tension, enabling predictions of droplet migration, deformation, and splitting under suitable conditions. Importantly, we demonstrate droplet deformation and splitting through bulk acoustic wave (BAW) silicon-glass microscale experiments, which validate the proposed theory.

physics.flu-dyn↗

arXiv2023

Suspending droplets beyond the Rayleigh limit: The interplay of acoustic and gravity forces

In this work, we experimentally investigate the suspension behavior of droplets subjected to standing acoustic waves. We focus on the droplet sizes beyond the Rayleigh limit, i.e., when the droplet size is comparable to the wavelength of the acoustic wave. We show that an acoustic field can disrupt the uniform motion of aqueous droplets in oil and cause them to either suspend or settle, depending on the interplay between acoustic and gravity forces. Remarkably, in contrast to droplets within the Rayleigh limit, the critical acoustic power or minimum pressure amplitude required to suspend droplets beyond the Rayleigh limit is dependent on droplet size. As the droplet size increases, the critical acosutic power increases significantly. Building upon this understanding, a novel sorting method is proposed based on critical acoustic power.

physics.flu-dyn↗

arXiv2022

On the stability of inhomogeneous fluids under acoustic fields

In this work, we present the stability theory for inhomogeneous fluids subjected to standing acoustic fields. Starting from the first principles, the stability criterion is established for two fluids of different acoustic impedance separated by a plane interface. Through stability theory and numerical simulations we show that, in the presence of interfacial tension, the relocation of high-impedance fluid from anti-node to node occurs when the acoustic force overcomes interfacial tension force, which is in agreement with recent microchannel experiments. Furthermore, we establish an acoustic Bond number that characterizes stable (Bo_a < 1) and relocation (Bo_a > 1) regimes. Remarkably, it is found that the critical acoustic energy density required for relocation can be significantly reduced by increasing the channel height which could help design acoustofluidic microchannel devices that handle immiscible fluids.

physics.flu-dyn↗

arXiv2021

Theory of Nonlinear Acoustic Forces Acting on Inhomogeneous Fluids

Recently, the phenomena of streaming suppression and relocation of inhomogeneous miscible fluids under acoustic fields were explained using the hypothesis on mean Eulerian pressure. In this letter, we show that this hypothesis is unsound and any assumption on mean Eulerian pressure is needless. We present a theory of non-linear acoustics for inhomogeneous fluids from the first principles, which explains streaming suppression and acoustic relocation in both miscible and immiscible inhomogeneous fluids inside a microchannel. This theory predicts the relocation of higher impedance fluids to pressure nodes of the standing wave, which agrees with the recent experiments.

physics.flu-dyn↗