arXiv subjects

Bavand Keshavarz

Publications and source records attributed to Bavand Keshavarz.

7 recordsLinked to original sources

arXiv2025

Exploratory Study of Chaotic Behavior in Walking Droplets

The interaction of 'walking droplets' and capillary waves in a weakly subcritical Faraday wave experiment has been studied as a hydrodynamic analog to Bohmian quantum mechanics (see "Hydrodynamic Quantum Analogs", J. Bush and A. Oza, Rep. Prog. Physics (2021)). We report here experimental results of walking droplets interacting with supercritical Faraday waves with dimensionless acceleration of approximately 7.7, where the onset of Faraday instability occurs at dimensionless acceleration 6.3, in flat bath topography. Our working fluid is silicone oil with a kinematic viscosity of 20 cst that is placed as a 4.2 mm horizontal liquid layer in an intermediate-aspect-ratio circular bath with a radius to Faraday wavelength ratio of 5.8. We also use different 3D-printed subsurfaces that act as slit structures with local oil depth of 0.7 mm. We confirm expected behavior for walking droplets in the supercritical Faraday regime, such as erratic trajectories, droplets clustering together due to capillary effects, and spontaneous drop creation. We note a special case of walking-droplet behavior when the bath only partially displays Faraday waves. We discuss the influence of the lateral boundaries and slits on droplet trajectory in this chaotic regime and compare the measured trajectories found here to those single and double slit experiments previously studied in the subcritical Faraday regime.

physics.flu-dyn↗

arXiv2022

The hidden hierarchical nature of soft particulate gels

Soft particulate gels include materials we can eat, squeeze, or 3D print. From foods to bio-inks to cement hydrates, these gels are composed of a small amount of particulate matter (proteins, polymers, colloidal particles, or agglomerates of various origins) embedded in a continuous fluid phase. The solid components assemble to form a porous matrix, providing rigidity and control of the mechanical response, despite being the minority constituent. The rheological response and gel elasticity are direct functions of the particle volume fraction $ϕ$: however, the diverse range of different functional dependencies reported experimentally has, to date, challenged efforts to identify general scaling laws. Here we reveal a hidden hierarchical organization of fractal elements that controls the viscoelastic spectrum, and which is associated with the spatial heterogeneity of the solid matrix topology. The fractal elements form the foundations of a viscoelastic master curve, which we construct using large-scale 3D microscopic simulations of model gels, and can be described by a recursive rheological ladder model over a range of particle volume fractions and gelation rates. The hierarchy of the fractal elements provides the missing general framework required to predict the gel elasticity and the viscoelastic response of these ubiquitous complex materials.

cond-mat.soft↗

arXiv2018

Computing the linear viscoelastic properties of soft gels using an Optimally Windowed Chirp protocol

We use molecular dynamics simulations of a model three-dimensional particulate gel, to investigate the linear viscoelastic response. The numerical simulations are combined with a novel test protocol (the optimally- windowed chirp or OWCh), in which a continuous exponentially-varying frequency sweep windowed by a tapered cosine function is applied. The mechanical response of the gel is then analyzed in the Fourier domain. We show that i) OWCh leads to an accurate computation of the full frequency spectrum at a rate significantly faster than with the traditional discrete frequency sweeps, and with a reasonably high signal-to-noise ratio, and ii) the bulk viscoelastic response of the microscopic model can be described in terms of a simple mesoscopic constitutive model. The simulated gel response is in fact well described by a mechanical model corresponding to a fractional Kelvin-Voigt model with a single Scott-Blair (or springpot) element and a spring in parallel. By varying the viscous damping and the particle mass used in the microscopic simulations over a wide range of values, we demonstrate the existence of a single master curve for the frequency dependence of the viscoelastic response of the gel that is fully predicted by the constitutive model. By developing a fast and robust protocol for evaluating the linear viscoelastic spectrum of these soft solids, we open the path towards novel multiscale insight into the rheological response for such complex materials.

cond-mat.soft↗

arXiv2018

Time-Resolved Mechanical Spectroscopy of Soft Materials via Optimally Windowed Chirps

The ability to measure the bulk dynamic behavior of soft materials with combined time- and frequency-resolution is instrumental for improving our fundamental understanding of connections between the microstructural dynamics and the macroscopic mechanical response. Current state-of-the-art techniques are often limited by a compromise between resolution in the time and frequency domain, mainly due to the use of elementary input signals that have not been designed for fast time-evolving systems such as materials undergoing gelation, curing or self-healing. In this work, we develop an optimized and robust excitation signal for time-resolved mechanical spectroscopy through the introduction of joint frequency- and amplitude-modulated exponential chirps. Inspired by the biosonar signals of bats and dolphins, we optimize the signal profile to maximize the signal-to-noise ratio while minimizing spectral leakage with a carefully-designed modulation of the envelope of the chirp. A combined experimental and numerical investigation reveals that there exists an optimal range of window profiles that minimizes the error with respect to standard single frequency sweep methods. The minimum error is set by the noise floor of the instrument, suggesting that the accuracy of an optimally windowed chirp signal is directly comparable to that achievable with a standard frequency sweep, while the acquisition time can be reduced by up to two orders of magnitude, for comparable spectral content. Finally, we demonstrate the ability of this optimized signal to provide time- and frequency-resolved rheometric data by studying the fast gelation process of an acid-induced protein gel. The use of optimally windowed chirps enables a robust rheological characterization of a wide range of soft materials undergoing rapid mutation and has the potential to become an invaluable tool for researchers across different disciplines.

cond-mat.soft↗

arXiv2017

Nonlinear viscoelasticity and generalized failure criterion for polymer gels

Polymer gels behave as soft viscoelastic solids and exhibit a generic nonlinear mechanical response characterized by pronounced stiffening prior to irreversible failure, most often through macroscopic fractures. Here, we aim at capturing the latter scenario for a protein gel using a nonlinear integral constitutive equation built upon ($i$) the linear viscoelastic response of the gel, here well described by a power-law relaxation modulus, and ($ii$) the nonlinear viscoelastic properties of the gel, encoded into a "damping function". Such formalism predicts quantitatively the gel mechanical response to a shear start-up experiment, up to the onset of macroscopic failure. Moreover, as the gel failure involves the irreversible growth of macroscopic cracks, we couple the latter stress response with Bailey's durability criterion for brittle solids in order to predict the critical values of the stress $σ_c$ and strain $γ_c$ at the failure point, and how they scale with the applied shear rate. The excellent agreement between theory and experiments suggests that the crack growth in this soft viscoelastic gel is a Markovian process, and that Baileys' criterion extends well beyond hard materials such as metals, glasses, or minerals.

cond-mat.soft↗

arXiv2013

When Viscous Jets Collide; Liquid Chains, Threads, Webs, Fishbones and Balloons

In this fluid dynamics video prepared for the APS-DFD Gallery of Fluid Motion we study the collision of two identical viscous jets at different jet speeds and collision eccentricities. The dynamics of the jet motion are slowed down by orders of magnitude using a synchronized strobe effect coupled with precise timing control of the perturbation frequency imposed on one of the jets. Our results show that different shapes and morphologies appear as we change the collision eccentricity. At low jet speeds ($We_j = 45$) viscous threads and filaments are formed as the jets begin to impinge on each other. As the propagation axis of one of the jets (jet A) is moved closer to the center of the other stream (jet B) they exert a torque on each other. Because of the resulting swirl, jet A rotates around the axis of jet B and finally breaks into droplets through the action of capillary forces; the pinch off of droplets is similar to a stream of balloons which are released in the air. At zero eccentricity the two jets unite into one jet at low speed and the unified jet breaks up after undergoing Rayleigh-Plateau instability. For higher jet velocities ($We_j=125$) different morphologies develop; at high eccentricities of impact (0.95), threads will appear; triangular sheets or webs form at eccentricities close to 0.9 and as one jet moves closer to the center of the other one first partial/incomplete fishbones appear and eventually periodically-dancing fishbones develop.

physics.flu-dyn↗

arXiv2012

Kelvin-Helmholtz and Buckling Instabilities for a Viscoelastic Liquid

In this fluid dynamics video prepared for the APS-DFD Gallery of Fluid Motion we study the Kelvin-Helmholtz instability for both Newtonian and viscoelastic jets. The nonlinear dynamics of the jet motion are slowed down by orders of magnitude using a synchronized strobe effect coupled with precise timing control of perturbation frequencies. Our results show that at high wave-numbers the imposed perturbations initially grow linearly with time and the jet axis remains straight while the Kelvin-Helmholtz wave amplitude grows and rolls up into bags that encapsulate the central jet within themselves. At low wave-numbers (long wave-lengths) the jet axis buckles under the action of viscous stresses and a coupling between the Kelvin-Helmholtz instability and bending of the jet leads to new concertina or chevron modes which grow with time as they move downstream. Addition of viscoelasticity to the jet leads to the pronounced inhibition of the Kelvin-Helmholtz instability as the jet perturbation amplitude grows and large elongational stresses in the fluid become important. For long waves, the initially-relaxed viscoelastic jet first buckles in a manner similar to the Newtonian solvent but once again the viscoelastic effects suppress the instability growth as they are convected downstream.

physics.flu-dyn↗