Negative Viscosity Research Articles

Non-Newtonian behaviors of ferrofluid Couette–Poiseuille flows in time-varying magnetic fields

We analyze the fully developed Couette–Poiseuille flows of ferrofluids between two parallel flat walls subject to three types of time-varying magnetic fields. In these scenarios, ferrofluids exhibit diverse non-Newtonian characteristics such as distinct flow velocity distribution, apparent viscosity and shear stress compared to ordinary Couette–Poiseuille flows. The influence of spin viscosity is explored first through the solution of the governing equations with zero and non-zero spin viscosities. It shows that although the value of the spin viscosity is very small, its inviscid limit would have great influence over the velocity and spin velocity distributions. The assumption of zero spin viscosity leads to an exaggerated non-Newtonian behavior induced by time-varying magnetic fields in the ferrofluid Couette–Poiseuille flows. Then the solutions of equations with non-zero spin viscosity are utilized to delve into non-Newtonian behaviors of ferrofluid Couette–Poiseuille flow under the application of the three time-varying magnetic fields. The results indicate that negative rotational viscosity will occur if the dimensionless frequency lies in the range 1–10, which is a distinguishing feature compared with Newtonian flows. At this point, non-Newtonian flow induced by magnetic field arises, although this effect is very tiny. Within the same frequency range, reversed tangential stress appears in strong uniform alternating magnetic fields. The minimum negative rotational viscosity may arrive at up to 20 % of the intrinsic viscosity in the rotating magnetic field when the magnetization relaxation time is 4 ms.

Optimization of heat transfer properties in micropolar ternary (Al2O3 + TiO2 + SiO2/water) nanofluid with porosity and magnetic field effects

Decades of study provide substantial evidence supporting that nanofluids possess improved heat transmission capabilities. Optimal qualities of nanoliquids may be achieved by manipulating the volume concentration of the nanoparticles. Nevertheless, this method has constraints due to the balance between negative and positive viscosity. A recent advancement has been produced in creating and thoroughly examining a particular form of hydraulic fluid that incorporates three solid nanoparticles scattered inside an existing fluid. This improvement aims to tackle this limitation. This study investigates the characteristics of a micro-rotational incompressible micropolar ternary hybrid nanofluid in a porous media. It considers the influence of magnetohydrodynamics, Darcy-Forchheimer, and linear thermal radiations on a stretched surface. The nanofluid consists mostly of water, which is blended with titanium oxide, aluminum oxide, and silicon oxide nanoparticles. Using dimensionless similarity transformations, the collection of higher-order non-dimensional partial differential equations is converted into higher-order ordinary differential equations. The BVP4C technique in MATLAB software is used to solve these equations and assess the graphical and tabular outcomes. The graphical analysis examines the influence of various physical characteristics. The heat transfer of a ternary hybrid nanofluid is significantly affected by many factors, including the Forchheimer coefficient, Prandtl number, thermal radiation parameter, and magnetic field parameters, as seen. The research demonstrates that an increase in the magnetic field and Darcy-Forchheimer parameters reduces linear momentum. Conversely, increased porosity, magnetic field, and radiation parameters increase the temperature profile.

New experimental model to evaluate the effect of negative pressure wound therapy and viscosity exudates in foam dressings using confocal microscopy.

Negative pressure wound therapy is currently one of the most popular treatment approaches that provide a series of benefits to facilitate healing, including increased local blood perfusion with reduced localized oedema and control of wound exudate. The porous foam dressing is a critical element in the application of this therapy and its choice is based on its ability to manage exudate. Industry standards often employ aqueous solutions devoid of proteins to assess dressing performance. However, such standardized tests fail to capture the intricate dynamics of real wounds, oversimplifying the evaluation process. This study aims to evaluate the technical characteristics of two different commercial polyurethane foam dressings during negative pressure wound therapy. We introduce an innovative experimental model designed to evaluate the effects of this therapy on foam dressings in the presence of viscous exudates. Our findings reveal a proportional increase in dressing fibre occupancy as pressure intensifies, leading to a reduction in dressing pore size. The tests underscore the pressure system's diminished efficacy in fluid extraction with increasing fluid viscosity. Our discussion points to the need of establishing standardized guidelines for foam dressing selection based on pore size and the necessity of incorporating real biological exudates into industrial standards.

Scaling regimes of the one-dimensional phase turbulence in the deterministic complex Ginzburg-Landau equation.

We consider the one-dimensional deterministic complex Ginzburg-Landau equationin the regime of phase turbulence, where the order parameter displays a defect-free chaotic phase dynamics, which maps to the Kuramoto-Sivashinsky equation, characterized by negative viscosity and a modulational instability at linear level. In this regime, the dynamical behavior of the large wavelength modes is captured by the Kardar-Parisi-Zhang (KPZ) universality class, determining their universal scaling and their statistical properties. These modes exhibit the characteristic KPZ superdiffusive scaling with the dynamical critical exponent z=3/2. We present numerical evidence of the existence of an additional scale-invariant regime, with the dynamical exponent z=1, emerging at scales which are intermediate between the microscopic ones, intrinsic to the modulational instability, and the macroscopic ones. We argue that this new scaling regime belongs to the universality class corresponding to the inviscid limit of the KPZ equation.

Countergradient turbulent transport in a plume with a crossflow

AbstractDirect numerical simulation of a turbulent forced buoyant plume in a crossflow is performed at a source Reynolds number $${\text{ Re }}_0=1000$$ Re 0 = 1000 , Richardson number $$\mathrm{{Ri}}_0=1$$ Ri 0 = 1 , Prandtl number $$\mathrm{{Pr}}=1$$ Pr = 1 and source-to-crossflow velocity ratio $$R_0=1$$ R 0 = 1 . The instantaneous and temporally averaged flow fields are assessed in detail, providing an overview of the flow dynamics. The velocity, temperature and pressure fields are used together with enstrophy fields to describe qualitatively the evolution of the plume as it is swept downstream by the crossflow, and the mechanisms involved in its evolution are outlined. The plume trajectory is determined quantitatively in a number of ways, and it is shown that the central streamline and the centre of buoyancy of the plume differ significantly—as with jets in crossflow, the central streamline is seen to follow the top of the plume, whereas the centre of buoyancy, by definition, describes the plume as a whole. We then investigate the turbulence properties inside the plume; in particular the eddy viscosity and diffusivity are presented, which are significant parameters in turbulence modelling. Assessment of turbulence production demonstrates the presence of regions where turbulence kinetic energy is redistributed to the kinetic energy of the mean flow, implying a negative eddy viscosity within certain regions of the domain. Similarly, the observation that the buoyancy flux and buoyancy gradient are anti-parallel in specific regions of the flow implies a negative eddy diffusivity in said regions, which must be realised in models of such flows in order to capture the countergradient transport of thermal properties. A characteristic eddy viscosity and diffusivity are presented, and shown to be approximately constant in the fully developed regime, resulting in a constant characteristic turbulent Prandtl number, in turn signifying self-similarity.

Improving emulsion stabilizing capacity of sodium caseinate by colloidal lignin particles near the isoelectric point

Sodium caseinate (Cas) is an attractive candidate for food emulsion formulation due to its surface-active properties as well as the ability to prevent the oxidation of encapsulation oils. However, the emulsions underwent destabilization near its isoelectric point (pI, around pH 4.8) due to the lack of enough electronic repulsion. To overcome the drawback, colloidal lignin particles (CLPs) fabricated from dietary fiber with superior negative repulsion and abundant surface active sites will be inspiring as the patch of Cas-based emulsion. This study systemically explored the modified effect of varied CLPs concentrations in the Cas-based emulsions prepared at pH 4.8. Due to hydrophobic attractions and hydrogen bonds between protein and CLPs, the CLPs significantly increased the negative charge and viscosity of composites while reduced surface hydrophobicity of Cas. Thus, in contrast to Cas, Cas/CLPs complex can fabricate a more elastic oil-water interfacial film, accompanying with a reduced droplet size (52–136 μm). Despite the inhibition of Cas adsorption at interfacial film, Cas/CLPs complex increase the stability of emulsion mainly by increasing electrostatic repulsion, enhancing film elasticity, and thickening continuous phase. Emulsions fabricated with Cas/CLPs complex effectively protected polyunsaturated fatty acids from deterioration, with the lowest levels of hydroperoxide and malondialdehyde observed in Cas/1.00 %CLPs emulsion. This study uncovered the facts behind the non-covalent interactions of Cas and lignin and proposed a novel strategy to stabilize Cas-based emulsions in acidic conditions.

К теории спиральной турбулентности немагнитного астрофизического диска. Образование крупномасштабных вихревых структур

A closed system of three-dimensional hydrodynamic equations of the mean motion scale is presented for modeling spiral turbulence in a rotating astrophysical disk. The diffusion equations for the averaged vortex and the integral vortex spirality transport equation are derived. A general concept of the emergence of energy-consuming mezoscale coherent vortex structures in a thermodynamically open subsystem of turbulent chaos, associated with the realization of the inverse cascade of kine-tic energy in mirror-nonsymmetric disk turbulence, is formulated. It is shown that negative viscosity in the rotating disk three-dimensional system is apparently a manifestation of cascade processes in spiral turbulence, when an inverse energy transfer from small vortices to larger ones is realized. It is also shown that a relatively long decay of turbulence in the disk is associated with the absence of reflective symmetry of the anisotropic field of turbulent velocities relative to its equatorial plane. The work is of a review character, made with the aim of improving new models of astrophysical nonmagnetic disks for which the effects of spiral turbulence play a determining role.

Microrheology of gemini surfactants at interfaces and in solutions in the dilute and semidilute regimes.

Gemini surfactants are ideal systems to study a wide range of rheological behaviours in soft matter, showing fascinating analogies with living polymers and polyelectrolytes. By only changing the concentration, the shear viscosity can vary by 7 orders of magnitude in the bulk when transitioning through the semidilute regime. In order to elucidate on the intrinsic shear viscosity profile at the interface in soft matter systems manifesting various concentration regimes and morphological transitions, we performed microrheology and adsorption experiments under a wide range of experimental conditions. The surface shear viscosity has been characterized by passive microrheology, tracking Brownian particles trapped at the air-solution interface, under particle wetting conditions precisely characterized by interferometry. We observe that a steep increase in bulk shear viscosity as a function of the concentration does not translate at the interface, which may show a negative surface shear viscosity. By comparing macrorheology and microrheology, we measure significant differences both at the interface and in the bulk in the semidilute regime, where wormlike micelles start to entangle. The disparity in rheological measurements can be attributed to notable depletion effects near both the air-solution and particle-solution interfaces.

Volumetric Behaviour of Binary Mixture of Dual Purpose Kerosene (DPK) and Automotive Gas Oil (AGO) at 303K

The study of volume changes in binary mixtures is attracting considerable interest from researchers due to the degree of molecular interaction between the mixed solvent molecules and their importance in many chemical industries and engineering disciplines. The experimental values of densities (ρ) and viscosities (η) of pure and binary mixtures of dual-purpose kerosene (DPK) and automotive gas oil (AGO) were obtained over the whole range of mixture composition at a temperature of 303 K. The experimental data obtained were employed to determine the excess molar volume (V_m^E), excess viscosity (VE), and excess Gibb’s free energy of activation (ΔGE) based on the molecular interactions arising due to the structural effect of the mixture. The measured viscosity values were correlated with Grunberg and Nissan, Kendel and Monroe, and Frenkel’s semi-empirical models to evaluate the best fit for the system. The results acquired were explained according to their structural differences, strength, and nature of intermolecular interactions in the binary mixture. The experimental result revealed that the binary mixture of DPK and AGO shows positive excess molar volume and negative excess viscosity. Weak, specific interactions were responsible for the dispersion force in the binary mixtures.

On the oscillations in a nonextensive complex plasma by improved differential transformation method: An application to a damped Duffing equation

In this study, the nonlinear damping oscillations in a complex non-Maxwellian plasma are investigated. For this purpose, the set of fluid equations of the present plasma model is reduced to the Burger-modified Korteweg De Vries equation (BmKdV) equation using a reductive perturbation technique. Using the traveling wave transformation, the BmKdV equation can be reduced to a damped Duffing equation. The numerical solutions to the damped Duffing equation are obtained using multistage differential transformation method (MsDTM). Also, we compared the obtained results to the semi-analytical approximations using the Padé differential transformation (PDTM) method and numerical solution, by the 4th-order Rung Kutta (RK4) method and analytical solution by He’s frequency method. The impact of relevant plasma parameters, namely, negative dust concentrations and ion kinematic viscosity on the profile of dust ion-acoustic oscillations are examined. The suggested mathematical approaches can help many authors for explaining the mystery of their laboratory results. Moreover, the suggested numerical method can be applied for solving higher order nonlinearity oscillations for a long domain.

Application to the World Ocean of the Theory of Transformation of a Mesoscale Vortex into a Submesoscale Vortex Thread When the Vortex Elongates by an Inhomogeneous Barotropic Flow

This paper is a continuation of the authors’ research [5]. We apply some aspects of the theory about strong horizontal stretching of large-scale mesoscale eddies in the World ocean based on ‘the ellipsoidal eddies’ theory to the ocean conditions. Two areas are considered, characterized by increased eddy manifestation. They are the Lofoten Basin in the Norwegian Sea and the Agulhas Current area. We also discuss the condition of the Atlantic Ocean and the World Ocean. The main purpose of this paper is to check the feasibility of the physical conditions for unbounded eddy elongation in the real ocean and estimate the fraction of mesoscale ocean eddies that are elongating into filaments and redistributing energy from the mesoscale to the submesoscale. We estimate the share of eddies in the World Ocean that are strongly elongated. We propose the maps of the geographical location of unlimited eddy elongation zones and give the interpretation of the obtained results. We also describe the effect of a decrease in the vortices’ self-energy due to the tensile action of the background flow. We suppose that it is a possible mechanism for energy transfer from the vortex to the flow and a manifestation of the negative viscosity effect.

Scale-to-scale energy transfer in rarefaction-driven Rayleigh–Taylor instability-induced transitional mixing

The rarefaction-driven Rayleigh–Taylor instability-induced mixing flow is numerically investigated via large eddy simulation. Prior analyses of interfacial diffusion are conducted to clarify the scale-to-scale transfer of kinetic energy during the laminar-to-turbulent transition. The statistical characteristics, including subgrid-scale (SGS) turbulent kinetic energy and SGS stresses, are outlined and highlight the mechanical production as well as pressure-related effects. Further inspection reveals that the relative intensity of SGS backscatter is somewhat noticeable, particularly for the transition onset, and the large-scale pressure-dilatation work is regulated through volumetric compression and expansion. Joint probability density function and the conditional averaging approaches both manifest that SGS backscatter is extremely associated with properties of the surrounding flow expansion induced by quadrupolar vortex structures. Furthermore, investigations on the effects of SGS backscatter on eddy viscosity are performed, and a regime classification, illustrating the relationship between various energy conversion modes and signs of the eddy viscosity, is provided. It is found that there is a significantly strong correlation between SGS backscatter and negative eddy viscosity; meanwhile, the volumetric compression and expansion tend to modulate the scale-to-scale energy transfer throughout the transitional process.

Oscillating rheological behavior of Turbatrix aceti nematodes

We present an experimental investigation of the rheological aspects of collective motion by the swimming Turbatrix aceti nematodes. We discover that these nematodes can significantly change the rheological properties of the suspension due to their body oscillations and form synchronized waves, which produce strong fluid flows. The strength of the collective state changes the shape of the interface where they swim in synchronization. We unravel that the effective viscosity of the nematode suspension at higher shear rates shows steady viscous behavior with time, where no significant effect of nematode activity is observed. For the first time, we have reported that at low shear rates, the activity effect is significant enough to generate oscillating viscous effects. In addition, we also measured the influence of the nematode concentration on suspension viscosity. This work opens a new way for understanding the rheological aspects of active matter under low and high shear rates. We illustrate these dynamics by showing that the force generated by these nematodes is sufficient to change the suspension rheology. The various aspects of nematodes, especially their large size and ease of culturing, make them a good model organism for experimental investigation as active fibers with oscillations. The oscillating behavior regulates the interfacial phenomenon and produces oscillatory rheological dynamics at low shear rates. The results of our work can be utilized to further study the novel metamaterials with negative viscosity, which have applications in healthcare and energy systems.

Stabilization of ginger essential oil Pickering emulsions by pineapple cellulose nanocrystals

Pickering emulsions (PE) are systems made up of two incompatible fluids, these are stabilized by solid organic or inorganic particles located on their interface. Cellulose nanocrystals (CNCs) are sustainable and biocompatible value-added naturally occurring biomolecules which are being investigated as PE stabilizers in the cosmetic, food, and pharmaceutical industries. The objective of this research was to investigate the efficacy of pineapple cellulose nanocrystals as stabilizers for a ginger essential oil-in-water Pickering emulsion. Anionic pineapple cellulose nanocrystals were prepared by acid hydrolysis. Ginger essential oil-in-water emulsions were prepared by ultrasonication. Pineapple CNC produced stable Pickering emulsions with surface average droplet size of 4.3 μm–6.2 μm, high negative zeta potential, high viscosity, and high adsorption at the interface. Pickering emulsions by ultrasonication were stable against droplet coalescence, phase separation, and droplet flocculation for at least 8 weeks at 25 °C or 40 °C at various droplet sizes. The emulsion droplet size and volume density (droplet size distribution) were evaluated by varying the particle concentration (CNC 0.25 g/100 ml or 0.50 g/100 ml) and/or oil fraction (10–20 g/100 ml). At constant oil fraction, the emulsion viscosity increased as the nanocrystal concentration increased. The cellulose nanocrystal-stabilized ginger oil-Pickering emulsions exhibited shear-thinning characteristics of a pseudo-plastic fluid. Pineapple nanocellulose crystal -stabilized ginger oil-Pickering emulsions exhibited high stability with a creaming index of zero. CNC was found to be an effective Pickering stabilizer for oil-in-water emulsions in various food applications.

Redistribution of Energy during Horizontal Stretching of Ocean Vortices by Barotropic Currents

The paper proposes a study of the transformation of the physical properties of mesoscale vortices during their strong elongation by horizontal barotropic currents. It is shown that when the core is pulled out, the kinetic and available potential energies of the vortex individually, as well as their sum (the total mechanical energy of the vortex) decreases, and the vortex itself degrades in all physical parameters. The decrease in the energy of the ensemble of vortices when they are pulled out by the background flow is interpreted as a manifestation of the reverse energy cascade property or, in older terminology, the phenomenon of negative viscosity.

Emergence of negative viscosities and colored noise under current-driven Ehrenfest molecular dynamics

Molecules in molecular junctions are subject to current-induced forces that can break chemical bonds, induce reactions, destabilize molecular geometry, and halt the operation of the junction. Theories behind current-driven molecular dynamics simulations rely on a perturbative time-scale separation within the system with subsequent use of nonequilibrium Green's functions (NEGF) to compute conservative, non-conservative, and stochastic forces exerted by electrons on nuclear degrees of freedom. We analyze the effectiveness of this approximation, paying particular attention to the phenomenon of negative viscosities. The perturbative approximation is directly compared to the nonequilibrium Ehrenfest approach. We introduce a novel time-stepping approach to calculate the forces present in the Ehrenfest method via exact integration of the equations of motion for the nonequilibrium Green's functions, which does not necessitate a time-scale separation within the system and provides an exact description for the corresponding classical dynamics. We observe that negative viscosities are not artifacts of a perturbative treatment but also emerge in Ehrenfest dynamics. However, the effects of negative viscosity have the possibility of being overwhelmed by the predominantly positive dissipation due to the higher-order forces unaccounted for by the perturbative approach. Additionally, we assess the validity of the white-noise approximation for the stochastic forces, finding that it is justifiable in the presence of a clear time-scale separation and is more applicable when the current-carrying molecular orbital is moved outside of the voltage window. Finally, we demonstrate the method for molecular junction models consisting of one and two classical degrees of freedom.

Non-ideal instabilities in sinusoidal shear flows with a streamwise magnetic field

We investigate the linear stability of a sinusoidal shear flow with an initially uniform streamwise magnetic field in the framework of incompressible magnetohydrodynamics (MHD) with finite resistivity and viscosity. This flow is known to be unstable to the Kelvin–Helmholtz instability in the hydrodynamic case. The same is true in ideal MHD, where dissipation is neglected, provided the magnetic field strength does not exceed a critical threshold beyond which magnetic tension stabilizes the flow. Here, we demonstrate that including viscosity and resistivity introduces two new modes of instability. One of these modes, which we refer to as an Alfvénic Dubrulle–Frisch instability, exists for any non-zero magnetic field strength as long as the magnetic Prandtl number ${{{Pm}}} < 1$ . We present a reduced model for this instability that reveals its excitation mechanism to be the negative eddy viscosity of periodic shear flows described by Dubrulle & Frisch (Phys. Rev. A, vol. 43, 1991, pp. 5355–5364). Finally, we demonstrate numerically that this mode saturates in a quasi-stationary state dominated by counter-propagating solitons.

Dimits transition in three-dimensional ion-temperature-gradient turbulence

We extend our previous work on the two-dimensional (2-D) Dimits transition in ion-scale turbulence (Ivanov et al., J. Plasma Phys., vol. 86, 2020, 855860502) to include variations along the magnetic field. We consider a three-field fluid model for the perturbations of electrostatic potential, ion temperature, and ion parallel flow in a constant-magnetic-curvature geometry without magnetic shear. It is derived in the cold-ion, long-wavelength asymptotic limit of the gyrokinetic theory. Just as in the 2-D model, a low-transport (Dimits) regime exists and is found to be dominated by a quasistatic staircase-like arrangement of strong zonal flows and zonal temperature. This zonal staircase is formed and maintained by a negative turbulent viscosity for the zonal flows. Unlike the 2-D model, the three-dimensional (3-D) one does not suffer from an unphysical blow up beyond the Dimits threshold where the staircase becomes nonlinearly unstable. Instead, a well-defined finite-amplitude saturated state is established. This qualitative difference between the 2-D and 3-D models is due to the appearance of small-scale ‘parasitic’ modes that exist only if we allow perturbations to vary along the magnetic field lines. These modes extract energy from the large-scale perturbations and provide an effective enhancement of large-scale thermal diffusion, thus aiding the energy transfer from large injection scales to small dissipative ones. We show that in our model, the parasitic modes always favour a zonal-flow-dominated state. In fact, a Dimits state with a zonal staircase is achieved regardless of the strength of the linear drive, provided the system is sufficiently extended along the magnetic field and sufficient parallel resolution is provided.

A study of dense suspensions climbing against gravity

Dense suspensions have previously been shown to produce a range of anomalous and gravity-defying behaviors when subjected to strong vibrations in the direction of gravity. These behaviors have previously been interpreted via analogies to inverted pendulums and ratchets, language that implies an emergent solid-like structure within the fluid. It is therefore tempting to link these flow instabilities to shear jamming (SJ), but this is too restrictive since the instabilities can also be observed in systems that shear thicken but do not shear jam. As an alternative perspective, we re-frame earlier ideas about “racheting” as a “negative viscosity” effect, in which the cycle-averaged motion of a vibrated fluid is oriented opposite to the direction implied by the cycle-averaged stresses. Using ideas from the Wyart and Cates modeling framework, we predict that such a “negative viscosity” can be achieved in shear flows driven by oscillating stress with both square and sinusoidal wave forms. We extend this same modeling approach to study falling films in a vibrating gravitational field, where we similarly find it is possible to attain an overall flow opposite the direction of gravity. Preliminary experimental findings are also provided in support of the modeling work. • Vertically vibrated dense suspensions can climb against gravity. • We explain this phenomena in terms of an “effective” negative viscosity. • Mechanism is explored in detail using simple modeling tools. • Preliminary experiments reveal climbing on vertically oriented surfaces. • Experimental observations are consistent with model predictions.

High-order compact finite volume schemes for solving the Reynolds averaged Navier-Stokes equations on the unstructured mixed grids with a large aspect ratio

In this paper, high-order compact finite volume schemes on the unstructured grids based on the variational reconstruction are developed to solve the Reynolds averaged Navier-Stokes equations closed by the Spalart-Allmaras one-equation turbulence model. Encouraging progress is made in addressing the following two challenging problems: reducing the numerical errors on the large aspect ratio grids and avoiding the negative turbulent viscosity associated with the high-order methods. On grids with large aspect ratios, a three-step procedure is designed to optimize the functional parameters of variational reconstruction. In addition, an exponential decay procedure is proposed to cure the negative turbulent viscosity problem of the Spalart-Allmaras model. The exponential decay procedure has the advantage of being able to be used with any spatial discretization method and with the implicit temporal discretization. Numerical tests show significant benefits of the high-order schemes in predicting the skin frictions, capturing some important flow structures, and achieving grid-independent solutions. The numerical tests also show that the proposed schemes are sufficiently robust for practical applications.