Browsing by Subject "Rheology"

Now showing 1 - 8 of 8

Open Access
Chaotic behavior of gas bubble in non-Newtonian fluid: A numerical study
(2013) Behnia, S.; Mobadersani F.; Yahyavi, M.; Rezavand, A.
In the present paper, the nonlinear behavior of bubble growth under the excitation of an acoustic pressure pulse in non-Newtonian fluid domain has been investigated. Due to the importance of the bubble in the medical applications such as drug, protein or gene delivery, blood is assumed to be the reference fluid. Effects of viscoelasticity term, Deborah number, amplitude and frequency of the acoustic pulse are studied. We have studied the dynamic behavior of the radial response of bubble using Lyapunov exponent spectra, bifurcation diagrams, time series and phase diagram. A period-doubling bifurcation structure is predicted to occur for certain values of the effects of parameters. The results show that by increasing the elasticity of the fluid, the growth phenomenon will be unstable. On the other hand, when the frequency of the external pulse increases the bubble growth experiences more stable condition. It is shown that the results are in good agreement with the previous studies. © 2013 Springer Science+Business Media Dordrecht.
Open Access
Complex dynamics of sheared active particle suspensions
(2024-08) Bayram, Ayten Gülce
Active systems, whether natural or artificial, have a unique ability to extract energy from their surroundings, driving themselves out of equilibrium. This capability gives rise to a variety of non-equilibrium phenomena such as swarming and clustering, creating potential uses for new materials and technologies. Among these active systems, we are particularly interested in the complex dynamics and the rheological behaviors of active colloidal suspensions and chiral active polymer chains when they are exposed to shear flow. In this sense, active Brownian dynamics (ABP), one of the most common computational methods, is used to study the complex dynamics of these active systems. In addition, the Multiparticle Collision Dynamics (MPCD) method is chosen to simulate in a computationally efficient way how the solvent dynamics, especially the resulting hydrodynamic effects, around these active systems behave. Phase transitions and collective dynamics of active colloidal suspensions are fascinating topics in soft matter physics, particularly for out-of-equilibrium systems, which can lead to rich rheological behaviours in the presence of steady shear flow. The role of self-propulsion in the rheological response of a dense colloidal suspension is investigated by using particle-resolved Brownian dynamics simulations. First, the combined effect of activity and shear in the solid on the disordering transition of the suspension is analyzed. While both self-propulsion and shear destroy order and melt the system if critical values are exceeded, self-propulsion largely lowers the stress barrier needed to be overcome during the transition. We further explore the rheological response of the active sheared system once a steady state is reached. While passive suspensions show a solid-like behaviour, turning on particle motility fluidises the system. At low self-propulsion, the active suspension behaves in a steady state as a shear-thinning fluid. Increasing the self-propulsion changes the behaviour of the liquid from shear-thinning to shear-thickening. We attribute this to clustering in the sheared suspensions induced by motility. This new phenomenon of motility-induced shear thickening (MIST) can be used to tailor the rheological response of colloidal suspensions. Expanding the active Brownian dynamics simulation for particles of more complex shapes such as active polymers, we explore the complex formation of an active flexible polymer chain in linear shear flow in two spatial dimensions. The chiral head monomer is active and circling, while all other monomers are passive following both the motion of the head polymer and the shear flow. By the combination of activity, chirality and shear rate, a wealth of different states are found including the formation of a linear/complex folding and a spiralling state with both head-in and head-out morphologies. As the vorticity of the applied shear competes with the circling sense of the head, the chirality of the whole complex can be tuned by activity. Our results are relevant to characterize the response of living and artificial chiral active polymer chains to complex flow fields. These initial Brownian dynamics simulations of active polymer chains under shear flow did not account for hydrodynamic interactions (HI), which simplified the system to a dry environment influenced only by Gaussian-colored noises. Recognizing the potential impact of HI on the conformational and dynamical properties of these polymers, we advanced our study by incorporating shortly the hydrodynamic interactions using a hybrid Molecular Dynamics-Multiparticle Collision Dynamics (MD-MPCD) approach. In this way, we discuss the polymer dynamics and conformations for more realistic scenarios, observed in experiments. This hybrid approach is an improvement that captures the hydrodynamic effects of the solvent as well as thermal fluctuations. The swelling effect induced by HI is critical for these transitions, causing delayed conformational changes on the state diagram and preventing certain states already observed in the absence of HI. The results show that hydrodynamic interactions enhance linear folding and head-in spiraling states while suppressing complex folding and head-out spiraling. We could analyze simply the complex interplay between self-propulsion, shear, and hydrodynamics in active chiral polymer systems under shear flow, which will provide more realistic insights for future research and applications.
Open Access
Core/shell-structured, covalently bonded TiO2/poly(3,4-ethylenedioxythiophene) dispersions and their electrorheological response: The effect of anisotropy
(Royal Society of Chemistry, 2015) Erol, O.; Unal, H. I.
As a new electrorheological (ER) material, core/shell nanorods composed of a titania core and conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) shell were prepared via covalent bonding to achieve a thin polymer shell and make the interfacial interactions between the two components more impressive. The successful coating of PEDOT on the nanorod-TiO2 particles was confirmed by TEM analysis. The antisedimentation stability of the core/shell nanorod-TiO2/PEDOT particles was determined to be 100%. The ER properties of the materials were studied under controlled shear, oscillatory shear and creep tests. The dielectric spectra of the dispersions were obtained to further understand their ER responses and fitted with the Cole-Cole equation. The ER behavior of the dispersions was also observed using an optical microscope. The flow curves of these ER fluids were determined under various electric field strengths and their flow characteristics examined via a rheological equation using the Cho-Choi-Jhon (CCJ) model. In addition, the results were also compared with nanoparticle-TiO2/PEDOT. It was concluded that the conducting thin polymer shell and elongated structure of the hybrid material introduced a synergistic effect on the electric field induced polarizability and colloidal stability against sedimentation, which resulted in stronger ER activity, storage modulus and higher recovery after stress loadings when compared to nanoparticle-TiO2/PEDOT. © The Royal Society of Chemistry.
Open Access
Effect of magnetic field on the radial pulsations of a gas bubble in a non-Newtonian fluid
(Elsevier Ltd, 2015) Behnia, S.; Mobadersani F.; Yahyavi, M.; Rezavand, A.; Hoesinpour, N.; Ezzat, A.
Dynamics of acoustically driven bubbles' radial oscillations in viscoelastic fluids are known as complex and uncontrollable phenomenon indicative of highly active nonlinear as well as chaotic behavior. In the present paper, the effect of magnetic fields on the non-linear behavior of bubble growth under the excitation of an acoustic pressure pulse in non-Newtonian fluid domain has been investigated. The constitutive equation [Upper-Convective Maxwell (UCM)] was used for modeling the rheological behaviors of the fluid. Due to the importance of the bubble in the medical applications such as drug, protein or gene delivery, blood is assumed to be the reference fluid. It was found that the magnetic field parameter (B) can be used for controlling the nonlinear radial oscillations of a spherical, acoustically forced gas bubble in nonlinear viscoelastic media. The relevance and importance of this control method to biomedical ultrasound applications were highlighted. We have studied the dynamic behavior of the radial response of the bubble before and after applying the magnetic field using Lyapunov exponent spectra, bifurcation diagrams and time series. A period-doubling bifurcation structure was predicted to occur for certain values of the parameters effects. Results indicated its strong impact on reducing the chaotic radial oscillations to regular ones. © 2015 Elsevier Ltd. All rights reserved.
Open Access
Impedance-based viscoelastic flow cytometry
(WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Serhatlıoğlu, Murat; Asghari, Mohammad; Guler, M. T.; Elbuken, Çağlar
Elastic nature of the viscoelastic fluids induces lateral migration of particles into a single streamline and can be used by microfluidic based flow cytometry devices. In this study, we investigated focusing efficiency of polyethylene oxide based viscoelastic solutions at varying ionic concentration to demonstrate their use in impedimetric particle characterization systems. Rheological properties of the viscoelastic fluid and particle focusing performance are not affected by ionic concentration. We investigated the viscoelastic focusing dynamics using polystyrene (PS) beads and human red blood cells (RBCs) suspended in the viscoelastic fluid. Elasto‐inertial focusing of PS beads was achieved with the combination of inertial and viscoelastic effects. RBCs were aligned along the channel centerline in parachute shape which yielded consistent impedimetric signals. We compared our impedance‐based microfluidic flow cytometry results for RBCs and PS beads by analyzing particle transit time and peak amplitude at varying viscoelastic focusing conditions obtained at different flow rates. We showed that single orientation, single train focusing of nonspherical RBCs can be achieved with polyethylene oxide based viscoelastic solution that has been shown to be a good candidate as a carrier fluid for impedance cytometry.
Open Access
Interplay of gouge, fluid pressure and porosity in fault zones
(Elsevier, 2003-05) Tuncay, K.; Ozkan, G.; Ortoleva, P.
The objective of this study is to demonstrate how fault dynamics may naturally be placed in the context of incremental stress theory, rock textural evolution modeling and standard conservation laws. Casting the fault dynamics problem in this framework naturally introduces rock memory for failure, fluid pressure effects and the autonomous nature of fault evolution. Poroelasticity, nonlinear viscosity and gouge are combined in an incremental stress rheology approach to examine the effect of changes in particle size distribution on fluid pressure in fault zones. © 2003 Elsevier Science B.V. All rights reserved.
Open Access
Modeling the rheological properties of highly nano-filled polymers
(SAGE Publications Ltd, 2017) Kourki, Hajir; Mortezaei, Mehrzad; Famili, Mohammad Hossein Navid; Malekipirbazari, Milad
Organic and inorganic materials are usually added to polymers in order to achieve some benefits such as reducing the product cost, as well as achieving higher modulus and strength. Addition of these materials would change polymers’ behavior. Adding nano-materials to polymers on the other hand is a new challenge in the field of polymer composites where previous studies were unable to achieve good correlation with nano-composites at higher particle volume fractions. In this research, Yamamoto network theory is developed to investigate the behavior of highly nano-filled systems. For this purpose, five different types of sub-chain and two types of junctions are considered and the effect of particle size, concentration, and the model parameters in association with the behavior of the junctions are studied. Moreover, some experiments are performed on polystyrene filled with nano-silica at different particle size and concentration values in frequency mod in the linear region. At last, we compared the results of our final model with the experiments in order to evaluate its accuracy, which confirmed a very good agreement. © 2016, © The Author(s) 2016.
Open Access
Molecular rheology of unentangled polymer melts in nanochannels
(2024-08) Yıldırım, Ahmet Burak
We investigate the rheological properties of non-Newtonian melts of short polymer chains at the molecular level, with a focus on nanoscale confinement. Using a combination of all-atom molecular dynamics (MD) and coarse-grained simulations, we examine the behavior of oligomer melts with various topologies under non-equilibrium conditions. Our findings reveal significant deviations in the microscopic stress tensor under steady-state shear compared to predictions from continuum models, highlighting the limitations of the Oldroyd-B and generalized Phan-Thien–Tanner (gPTT) models in capturing nanoscale phenomena. We demonstrate that these deviations result in excess viscoelastic stress, which diminishes with decreasing confinement and vanishes in bulk systems. This excess stress is linked to the spatial orientation of carbon-carbon bonds near surfaces, where adsorbed chains form effective polymer brush-like layers. Additionally, we explore the effects of chain rigidity, surface-oligomer attraction, and molecular variables on rheological responses, providing a comprehensive understanding of the interplay between molecular-scale phenomena and macroscopic behavior in confined polymeric systems.