Ferrofluid Layer Research Articles

A numerical study of rupture of a ferrofluid interlayer in a sandwiched fluid system

Surface rupture in ferrofluid layers is a special case of the well-known Rosensweig instability, which can be triggered by applying a strong magnetic field. This study investigates the rupture dynamics in a ferrofluid interlayer sandwiched between two non-magnetic fluids, influenced by a non-homogenous vertical magnetic field. Simulations are performed using a generalized conservative phase-field lattice Boltzmann method for the flow field and interface with a coupled solution of Maxwell's equations for the evolution of magnetic field. The numerical results demonstrate the complete rupture process of ferrofluid layers. In most cases, the ferrofluid layer ruptures into two parts, while under certain conditions, such as a thinner interlayer or high magnetic field intensity, daughter droplets appear at the meniscus. A parametric analysis involving Weber number (We) and dimensionless magnetic parameter (Nm) elucidates the connection between different rupture conditions, such as a deformed interlayer without rupture, rupture with two semi-spindle shaped domains, and rupture with droplets. Additionally, a phase diagram illustrating the various rupture regions is also provided.

Drag reduction utilizing a wall-attached ferrofluid film in turbulent channel flow

This study explores the application of a wall-attached ferrofluid film to decrease skin-friction drag in turbulent channel flow. We conduct experiments using water as a working fluid in a turbulent channel flow set-up, where one wall is coated with a ferrofluid layer held in place by external permanent magnets. Depending on the flow conditions, the interface between the two fluids is observed to form unstable travelling waves. While ferrofluid coating has been previously employed in laminar and moderately turbulent flows (Reynolds number $Re<4000$ ) to reduce drag by creating a slip condition at the fluid interface, its effectiveness in fully developed turbulent conditions, particularly when the interface exhibits instability, remains uncertain. Our primary objective is to assess the effectiveness of ferrofluid coating in reducing turbulent drag with particular focus on scenarios when the ferrofluid layer forms unstable waves. To achieve this, we measure flow velocity using two-dimensional particle tracking velocimetry (2D-PTV), and the interface contour between the fluids is determined using an interface tracking algorithm. Our results reveal the significant potential of ferrofluid coating for drag reduction, even in scenarios where the interface between the surrounding fluid and ferrofluid exhibits instability, with observed drag reduction rates up to 95 %. In particular, waves with an amplitude significantly smaller than a viscous length scale positively contribute to drag reduction, while larger waves are detrimental, because of induced turbulent fluctuations. However, for the latter case, slip outcompetes the extra turbulence so that drag is still reduced.

Effect of viscosity variation with temperature on convective instability in a hot dusty ferrofluid layer with permeable boundaries

As the subject under consideration has numerous industrial, biological, and environmental applications, so the prime objective of this article is to investigate the onset of thermal convection in a dusty ferrofluid layer under the effect of change in viscosity due to temperature in the presence of permeable boundaries. The linear stability analysis is employed to acquire the eigenvalue problem and then the eigenvalue problem is solved for the stationary convection mode using a single-term Galerkin method. The numerical values of critical wave number and critical Rayleigh number are calculated using MATLAB software, and the results are graphically displayed. The investigation shows that the permeability of the boundaries affects the size of the convection cells that form at the beginning of thermal convection. Further, it is found that the parameters related to temperature-dependent viscosity and permeable boundaries have a stabilizing effect on the system, while magnetic and dust particle parameters destabilize the system.

Saturated Porous Ferroconvection in a Ferrofluid Layer with Viscosity as a Function of Magnetic Field: Focus on Convective Boundary Condition

The present work aims to examine the influence of magnetic field dependent (MFD) viscosity on the onset of ferroconvection (FC) in a horizontal porous layer saturated with a quiescent ferrofluid (FF) and subjected to a uniform vertical magnetic field. It is assumed that the porous boundaries at the bottom and top are rigid-paramagnetic. The thermal conditions consist of a constant heat flux at the lower surface and a convective boundary condition at the upper surface, encompassing fixed temperature and uniform heat flux cases. The application of the Galerkin technique to the resulting eigenvalue problem reveals that the stability region expands as the porous parameter, Biot number, MFD viscosity parameter and magnetic susceptibility increase in magnitude. Conversely, the stability region contracts as the magnetic number and non-linearity of magnetization increase. Furthermore, it is noted that under uniform heat flux boundary conditions, the criterion for the initiation of ferroconvection remains unaffected by the non-linearity of fluid magnetization.

Directional Manipulation of Drops and Solids on a Magneto-Responsive Slippery Surface.

The cloaking of the droplet and solid spheres by a thin ferrofluid layer forms a ferrofluid-wetting ridge, enabling the magnet-assisted directional manipulation of droplets and solid spheres on the magneto-responsive slippery surface. Understanding the interplay of various forces governing motion unravels the manipulation mechanism. The transportation characteristics of droplets and solid spheres on such surfaces enable their controlled manipulation in multiple applications. Here, we prepare magneto-responsive slippery surfaces by using superhydrophobic coatings on glass slides, creating a porous network and impregnating them with ferrofluid. Using a permanent magnet (and its translation) in the proximity of these surfaces, we manipulate the motion of liquid drops and solid spheres. Upon dispensing the droplet on the magneto-responsive slippery surface, the droplet is cloaked by a thin ferrofluid layer and forms a ferrofluid wetting ridge. Incorporating the magnetic field creates a magnetic-nanoparticle-rich zone surrounding the ferrofluid ridge. Thereafter, the motion of the magnet gives rise to the movement of the droplet. We found that the interplay of the magnetic force and viscous drag leads to the magnetic manipulation of droplets in a controlled fashion up to a certain magnet speed. Increasing the magnet speed further results in the uncontrolled motion of the droplet, where the droplet cannot follow the magnet trajectory. Moreover, we delineate multifunctional droplet manipulations, such as trapping, pendant droplet manipulation, coalescence, and microchemical reactions, which have wide engineering applications.

Investigation of thermomagnetic gravitational convection and energy distribution in a vertical layer of ferrofluid

Investigation of thermomagnetic gravitational convection and energy distribution in a vertical layer of ferrofluid

The Disintegration of a Floating Ferrofluid Layer into an Ordered Drop System in a Vertical Magnetic Field

The Disintegration of a Floating Ferrofluid Layer into an Ordered Drop System in a Vertical Magnetic Field

Magnetically Induced Flows in Thrombosed Channels with a Ferrofluid Layer

We proposed a theoretical model and a method for its approximate analysis for flows induced by a uniform rotating magnetic field in a channel filled with a nonmagnetic fluid with a ferrofluid layer injected into it. One end of the channel is assumed to be blocked (thrombosed). This study is aimed at the development of the scientific basis of the magnetically induced intensification of drug transport in blocked blood vessels.

Magnetically Induced Flows in Thrombosed Channels with a Ferrofluid Layer

Magnetically Induced Flows in Thrombosed Channels with a Ferrofluid Layer

Thermoconvective instability in a ferrofluid saturated porous layer

The Forchheimer-extended Brinkman’s Darcy-flow model was used to investigate the initiation of ferroconvection in a flat porous layer while accounting for effective viscosity. The rigid ferromagnetic, rigid paramagnetic and stress-free isothermal boundary conditions are the three categories. The eigenvalue issue can be properly addressed for stress-free boundaries; the Galerkin approach is utilized to find the critical stability constraints quantitatively for other barriers. It was discovered that the boundary types had a strong influence on the system’s stabilization. Ferromagnetic boundaries are less preferred than paramagnetic boundaries in control of convection. The dependence of many physical limitations on the linear stability of the system is intentionally given, and it is demonstrated that increasing the value of the viscosity ratio delays the beginning of convection.

Oscillatory convection in viscoelastic ferrofluid layer: Linear and non-linear stability analyses

The problem of ferroconvection in a viscoelastic fluid layer is studied with the aim to investigate oscillatory motions. In this article, a stability analysis for both linear and nonlinear systems is carried out. In the linear stability analysis, the expressions for steady and oscillatory Rayleigh numbers are obtained and the effects of magnetic as well as viscoelastic parameters on the onset of viscoelastic ferromagnetic convection are investigated numerically. From the analysis, we found that the magnetic number (M_1), the stress relaxation time (lambda_1) and the nonlinearity of magnetization (M_3) have destabilizing influences on the onset of ferroconvection, whereas the strain retardation time (lambda_2) has stabilizing influence. In the weakly nonlinear stability analysis, the formula for heat transfer rate in terms of the Nusselt number is derived for oscillatory convection. From the analyses, we found that for increasing values of the magnetic number, stress relaxation time and nonlinearity of magnetization, the heat transfer rate rises, whereas it decreases for larger values of the strain retardation time. Moreover, the pitchfork bifurcation analysis yields that in order to reach the stable positions, the value of amplitude increases as the stress relaxation time increases, whereas a reverse trend is observed for the strain retardation time.

Linear and weakly non-linear stability analysis of oscillatory convection in rotating ferrofluid layer

In the present paper, the problem of ferroconvection in the presence of uniform vertical rotation is considered to investigate the linear and weakly non-linear oscillatory stability. Two-dimensional convective roll instability is discussed with finite-amplitude disturbances. For linear stability, as a first-order problem, the expressions for Rayleigh numbers for stationary and oscillatory convection are derived and the effects of Coriolis force and magnetic parameters on the onset of ferromagnetic convection are studied, numerically. In weakly nonlinear oscillatory analysis, the second-order and third-order stability problems are discussed and the complex Ginzburg-Landau equation describing the amplitude of convection cell in rotating ferrofluid is derived and consequently, the expression for Nusselt number representing the heat transfer rate is obtained. From the present analysis, we observed that the rotation has the usual stabilizing effect on the linear stability in ferroconvection, however, the magnetic number (M1) and the measure of nonlinearity of magnetization (M3) both have a destabilizing effect on the onset of linear instability. Also, we found that for non-linear convection, the heat transfer rate (the Nusselt number) increases with increasing values of Taylor number, magnetic number, and the measure of nonlinearity of magnetization.

Effects of Magnetic Fields, Coupled Stefan Blowing and Thermodiffusion on Ferrofluid Transport Phenomena

The paramagnetic feature of ferrofluid allows it to be utilised in electronic devices and improvise fluid circulation in transformer windings. Hence, the present article aims to conduct the numerical study of ferrofluid boundary layer flow along with the Stefan blowing, velocity and thermal slip, and Soret effects within the stagnation region over a stretching/shrinking surface. The governing equations were solved numerically using the bvp4c function in the MATLAB computing package. Based on the results, a stronger magnetic field of ferrofluid was needed to identify the numerical solutions past the shrinking surface, while the Stefan blowing diminished the solution’s availability. More than one solution is acquired for some specific values of the shrinking parameter, and the stability analysis validated that only one solution is reliable and stable.

Comment on the paper “Effect of temperature-dependent viscosity on the onset of Bénard–Marangoni ferroconvection in a ferrofluid saturated porous layer, C. E. Nanjundappa, B. Savitha, B. Arpitha Raju, I. S. Shivakumara, Acta Mechanica 225 (2014) 835–850”

Comment on the paper “Effect of temperature-dependent viscosity on the onset of Bénard–Marangoni ferroconvection in a ferrofluid saturated porous layer, C. E. Nanjundappa, B. Savitha, B. Arpitha Raju, I. S. Shivakumara, Acta Mechanica 225 (2014) 835–850”

Effect of Rosensweig instability in a ferrofluid layer on reflection loss of a high-frequency electromagnetic wave

A ferrofluid layer separates into numerous subscale crests, which is referred to as Rosensweig instability, whose shape and size depend on the field condition and the composition of the ferrofluid. A ferrofluid consisting of nanoscale magnetite particles is also used as an electromagnetic (EM) wave absorption and reflection material. For this study, oil-based and mixture ferrofluid layers that split into various shapes of crests in the presence of an external magnetic field are used to form a protruding structure to reflect and scatter the EM wave and decrease EM radiation energy. For an identical field strength, a mixture ferrofluid layer splits into more crests than an oil-based ferrofluid. A mixture crest shows a less uniform size and shape than the oil-based one. A high-power green laser light is used as a visual EM wave emitting to a crest, which has varying tip angles, and to demonstrate the reflection and scattering. The reflection loss increases as the field strength is increased to create a crest of a smaller tip angle. The reflection loss of an EM wave is significantly affected by the transmitting position on a crest and the shape of a crest. Inter-reflection arises if an EM wave is repeatedly reflected on the surfaces of crests, which contributes to a significant reflection loss. An EM wave incident at an angle of 45° on a crest resulting in a larger area of the inter-reflection zone without specular reflection in a trough gives the most significant reflection loss.

NUMERICAL STUDY OF THE SHIELDING PROPERTIES OF A FERROFLUID TAKING INTO ACCOUNT MAGNITOPHORESIS AND PARTICLE INTERACTION

Shielding properties of a cylindrical thick-walled ferrofluid layer that protects against externally applied uniform magnetic fields are numerically investigated. We take into account the diffusion of magnetic nanoparticles in the ferrofluid with magnetic dipole-dipole, steric and hydrodynamic interactions between particles. Permeability of the ferrofluid is considered to be dependent on the magnetic-field strength and the particle concentration. A combined method of finite differences and boundary elements is applied to solve a nonlinear transmission problem of magnetostatics in the whole space, augmented by nonlinear algebraic equations based on the mass transfer equation for magnetic nanoparticles in ferrofluids. Numerical experiments revealed that the diffusion of particles has negligible influence on the shielding properties at weak and strong intensities of the applied magnetic field when comparing with the results of computations for a uniform particle distribution.

The Onset of Buoyancy and Surface Tension Driven Convection in a Ferrofluid Layer by Influence of General Boundary Conditions

This paper investigated the buoyancy and surface tension-driven ferro-thermal-convection (FTC) in a ferrofluid (FF) layer due to influence of general boundary conditions. The lower surface is rigid with insulating to temperature perturbations, while the upper surface is stress-free and subjected to general thermal boundary condition. The numerically Galerkin technique (GT) and analytically regular perturbation technique (RPT) are applied for solving the problem of eigenvalue. It is analyzed that increasing Biot number, decreases the magnetic and Marangoni number is to postponement the onset. Additionally, magnetization nonlinearity parameter has no effect on FTC in the non-existence of Biot number. The results under the limiting cases are found to be in good agreement with those available in the literature.

Effect of anisotropy pores on thermohaline convective instability in micropolar ferrofluid

This work deals with the thermohaline convective instability on horizontal micropolar ferrofluid layer in anisotropic porous effect subjected to a transverse uniform magnetic field. The linear stability analysis is used to omit the non-linear terms on governing equations, normal mode analysis is taken to the study and free boundary conditions are applied. The critical magnetic thermal Rayleigh number N SC is calculated analytically for sufficient large values of M 1 The principle of exchange of stabilities is satisfied for micropolar ferrofluid in an anisotropic porous medium without N 1, N ’ 3, N ’ 5 and τ. The sufficient conditions for the non-existent of overstability are also examined. Moreover, in this work, we tried to investigate the anisotropy effect on porous medium on the system. The parameters N 1and N 5 ’ dominate the system and the large porous medium is taken into account.

Waves on a Free Surface of Ferrofluid Layer, Laying on a Liquid Substrate

Waves on the free surface of a magnetic fluid located on a liquid substrate were studied experimentally. The wave motion of the surface was induced by a homogeneous oscillating magnetic field orthogonal to the layer. In this case two types of waves can be formed on the surface of a magnetic fluid: a standing wave of the same frequency as the alternating magnetic field, and a standing wave, independent of the field frequency. The paper reviews the main theoretical and experimental studies of wave instability of such systems. The stability of a two-layered liquid system in an alternating vertical magnetic field is investigated. For efficient processing of the experimental results, the optical part of the experimental setup was modified. An algorithm has been developed for processing the profiles of the magnetic fluid surface obtained during the experiment, which helps to determine the length of the generated waves.

Stability of a ferrofluid layer on a liquid substrate

The stability of a horizontal magnetic fluid layer located on a liquid substrate in the alternating magnetic field orthogonal to the surface is experimentally investigated. Kerosene-based magnetite liquid stabilized with oleic acid (ferrofluid) and perfluorooctane (to create a liquid substrate) were chosen as working fluids. The presence of a free surface and interface in the magnetic fluid layer determines the influence of spatial characteristics of the system, such as the diameter of the cell and the thickness of the liquid layer, on the resonant frequency. The stability of the ferrofluid layer in an orthogonal stationary magnetic field is investigated, and the dependence of the critical field strength on the layer thickness is obtained. The dependences of intensity amplitude on the alternating magnetic field frequency are depicted for the ferrofluid layers of various initial thicknesses. The obtained stability curves show that the low frequency field (1–4 Hz) destabilizes the system, while the high frequency one (from 4 Hz and above) has a stabilizing effect.