Aqueous Ferrofluids Research Articles

Particle correlations in concentrated aqueous ferrofluids upon dilution by small-angle X-ray scattering

Small-angle X-ray scattering is applied to study particle correlations in concentrated (up to 7.5 vol% magnetite) aqueous magnetic fluids with double-layer coating by either oleic or lauric acids. The structure of aggregates in these fluids is analyzed when diluting samples with the goal of finding out a concentration threshold at which the aggregates can be considered weakly interacting independent formations. For dilute systems, suitable models fitting experimental scattering curves are proposed, which take into account the polydispersity of both particles and aggregates. It is shown that particle correlations found in aggregates are dependent on the surfactant coating.

Field-induced interfacial instabilities in a two-phase ferrofluid flow

Forming a stable ferrofluid and non-magnetic interface in a microchannel is essential for biomedical applications with two miscible aqueous solutions. Here, we report the interface profile and its instability of aqueous ferrofluid and water co-flow in a microchannel under the influence of a magnetic field. The scaling law related to diffusion width and dimensionless Pe number is introduced by solving the momentum and diffusion equation and combining the experimental results. It is found that the dimensionless number of diffusion width δ/h ∝ X/Peα, when core flow concentration cf, residual magnetic flux density Br, sheath flow and core flow ratio Qw/Qf, and core flow velocity Qf are changed, α is 0.633–1.071, α ∝ Qβ, β = −0.059,0.147, 0.019 and −0.101, respectively. Altering the core flow velocity and magnetic field intensity can quickly impact the interface's stability by improving the transport efficiency by 32.9 % and counteracting the impacts of material diffusion by 51.8 %, respectively. Elevating the proportion of sheath flow to core flow and magnetic field will impede the diffusion of core flow. This will result in a drop of 11.9 % in the magnetic convection effect and a 20.8 % counteraction of material diffusion effects.

Coflowing aqueous and oil-based ferrofluid streams exposed to a magnetic field.

We report the transition between stream and droplet regimes in a coflow of an aqueous stream and oil-based ferrofluid. The transition between stream and droplet regimes is typically attained by controlling the capillary numbers (Ca) of the phases. Remarkably, we experimentally evidence a transition between the regimes by adjusting the exposure of the system to a magnetic field, with Ca fixed. We represent the various regimes: stable coflow, interface deformation, and droplet generation, in terms of the magnetic bond number (Bom) and the ratio of capillary numbers of the phases (Car). The different regimes are a consequence of the interplay of the magnetic, viscous, and interfacial tension forces, represented by the two dimensionless numbers. We explain the regimes in terms of the magnetic pinch-off (τmp) and advection (τa) time scales: for τmp ≫ τa a stable coflow is observed, for τmp ∼ τa interface deformation is observed, and for τmp ≪ τa droplet breakup is observed. We study the interface deformation and droplet size from experiments and predict the same from theoretical scaling. We find the interface deformation increases and the droplet size decreases with increases in Bom and Car. The present study may find applications in magnetic field-assisted on-demand droplet generation in microfluidics.

Modular Droplet‐Based Fluidics for Large Volume Libraries of Individual Multiparametric Codes in Lab‐On‐Chip Systems

AbstractDroplet‐based lab‐on‐a‐chip systems offer vast possibilities in the manipulation, guidance, tracking, and labeling of individual droplet‐based bioreactors. One of the targeted application scenarios is in drug discovery where millions of unique codes are required, which is out of reach for current technologies. Here, a concept for the realization of multiparametric codes, where information is stored in distinct physical and chemical parameters, is proposed and validated. Exemplarily, the focus is on the use of impedance and magnetic sensing by monitoring ionic concentration as well as magnetic content per droplet and droplet volume. Codes based on aqueous ferrofluid droplets are prepared using a tubing‐based millifluidic setup and consist of up to six droplets of different combinations of volumes and magnetic concentration. It is demonstrated that a droplet chain of three single droplets of different volumes with nine different magnetic nanoparticle concentrations accompanied with four different ionic concentrations per droplet offers up to 3 million unique codes. The developed fluidic platform can be readily extended to other types of sensors including optical ones to boost the coding capacity even further.

Rheology of Aqueous Ferrofluids: Transition from a Gel-Like Character to a Liquid Character in High Magnetic Fields

Rheological measurements under an applied magnetic field were used to investigate the changes to the internal structure and stability of an aqueous ferrofluid. The ferrofluid was prepared by dispersing 1.8 wt.% of maghemite nanoparticles with a size of d = 14 ± 3 nm and a saturation magnetization MS = 68 emu/g in water using citric acid as the surfactant. In this study, oscillatory tests were used to investigate the internal structural changes and the stability of ferrofluid under the influence of the magnetic field B. In a magnetic field of approximately 50 mT, the G′ became higher than the loss modulus G″ as the ferrofluid exhibited a gel-like character. However, at a magnetic field of approximately 200 mT, the character of the ferrofluid reverted to that of a liquid. The change in the character of the ferrofluid in this high magnetic field was associated with a gradual change from chain agglomerates to the energetically more favourable globular agglomerates, using a calculation based on a model described in a separate work. The globular agglomerates impeded the flow to a much lesser degree than the chains, causing a reduction in the viscosity. Further increase of the magnetic field resulted in sedimentation of agglomerates and loss of magneto-rheological effect.

Multifunctional aqueous ferrofluid stabilized by cellulose nanofibrils with long term stability

Due to the excellent flowability and shape adaptability, ferrofluids (or magnetic fluids, MF), offer potential in food and pharma, intelligent materials, fluid seals and devices, such as, robots, and remote controllers. Here, we stabilized a ferrofluid after coprecipitation of metal nanoparticles onto cellulose nanofibrils in aqueous suspension, obtaining a colloidal system with a magnetization value of 3.55 emu/mL. The MF was stable at pH > 7 and showed contactless magnetic direct driving or indirect guiding. Several use demonstrations were tested, for instance, as a flexible plug to separate, stir/mix or seal hydrophobic liquids. In addition, the MF exhibited gradients of relative density under the space-dependent magnetic field, which endowed sorting of plastic particles, according to their density. Our findings provide guidance for the design of multifunctional cellulose-stabilized aqueous MFs.

Study on structural, optical and magnetic properties of cobalt substituted magnetite nanoparticles for ferrofluid applications

Cobalt substituted magnetite nanoparticles having the composition CoxFe3−xO4 (0.0 ≤ x ≤ 0.4 in steps of 0.2) have been prepared using co-precipitation method. Various structural, microstructural, optical and magnetic characterizations have been carried out using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscope (FESEM), UV–Visible spectroscopy and Vibrating sample magnetometer (VSM) respectively. The spinel structure of the synthesized nanoparticles has been confirmed using XRD technique and Rietveld refinement techniques. Crystallite size calculated from Scherrer’s and Williamson – Hall (W-H) methods are in agreement and found to decrease from 15 nm to 10 nm with increase in cobalt substitution. The ultrafine nano crystallite is desirable for ferrofluid preparation. Lattice strain is obtained from the slope of the linear fit in the W-H plot. FTIR spectra reveals two absorption bands around 540–420 cm−1 which corresponds to the vibrations of the MO bonds at the tetrahedral and octahedral sites respectively, and is characteristic of the ferrite samples. The average particle size obtained from FESEM measurement is found to be in the range 15 nm to17 nm. UV–Visible spectroscopy shows maximum absorption around 380–400 nm. Optical band gap of the samples estimated using Tauc plot is found to be in the range 2.10 eV to 2.55 eV. The saturation magnetization of the samples is found to be in the range 57 emu/g to 60 emu/g. Stable aqueous ferrofluids of the nanoparticles have been obtained using tetramethyl ammonium hydroxide as the ionic surfactants.

Experimental assessment on convection heat transfer characteristics of aqueous magnetite ferrofluid in a rifled tube under a rotating magnetic field

This work is devoted to the experimental evaluation of the impact of a rotating magnetic field (RMF) on the thermal-hydraulic characteristics of laminar forced convection of aqueous magnetite ferrofluid through a rifled tube. The RMF consists of two small permanent magnets that revolve around the rifled tube. Considering the clockwise and counterclockwise directions for the rotation of each magnets, four different arrangements were considered for the RMF. The results were determined for different values of Reynolds number (Re), nanoparticle concentration (φ) and rifling pitch (P). Nusselt number (Nu), friction factor and Performance Evaluation Criterion (PEC) were considered as performance parameters. The analysis of the PEC parameter showed that from an overall hydrothermal point of view, the use of rifled tube and RMF is desirable. In the absence of RMF, the highest PEC was equal to 1.813, which occurred in the case of Re=2000, φ=0% and P=5 mm. In addition, among the cases with RMF, the maximum PEC was equal to 1.601, which belonged to the case of Re=2000, φ=2% and P=5 mm.

Experimental evaluation and development of predictive models for rheological behavior of aqueous Fe3O4 ferrofluid in the presence of an external magnetic field by introducing a novel grid optimization based-Kernel ridge regression supported by sensitivity analysis

In the present study, experiments are performed to determine the changes in the viscosity of water-Fe3O4 magnetic nanofluid (MNF) with shear rate, nanoparticle concentration and magnetic field (MF) induction. It was observed that as the shear rate elevates, the MNF viscosity first diminishes and then remains almost constant. Besides, the viscosity elevated with the application of the MF and its induction and also with increasing the concentration of nanoparticles. As another novelty of this research, a novel kernel based machine learning scheme namely, grid optimization based-kernel ridge regression (Grid-KRR) model was developed to accurate prediction of viscosity of water-Fe3O4 MNF based on volume fraction of nanoparticles, shear rate, and magnitude of external MF as input features. Besides, the Random forest (RF) and Gene expression programming (GEP) models were examined for validating the Grid-KRR model. The performance criteria demonstrated that the Grid-KRR outperformed the RF.

Colloidal Stability and Concentration Effects on Nanoparticle Heat Delivery for Magnetic Fluid Hyperthermia.

The heat produced by magnetic nanoparticles, when they are submitted to a time-varying magnetic field, has been used in many auspicious biotechnological applications. In the search for better performance in terms of the specific power absorption (SPA) index, researchers have studied the influence of the chemical composition, size and dispersion, shape, and exchange stiffness in morphochemical structures. Monodisperse assemblies of magnetic nanoparticles have been produced using elaborate synthetic procedures, where the product is generally dispersed in organic solvents. However, the colloidal stability of these rough dispersions has not received much attention in these studies, hampering experimental determination of the SPA. To investigate the influence of colloidal stability on the heating response of ferrofluids, we produced bimagnetic core@shell NPs chemically composed of a ZnMn mixed ferrite core covered by a maghemite shell. Aqueous ferrofluids were prepared with these samples using the electric double layer (EDL) as a strategy to maintain colloidal stability. By starting from a proper sample, ultrastable concentrated ferrofluids were achieved by both tuning the ion/counterion ratio and controlling the water content. As the colloidal stability mainly depends on the ion configuration on the surface of the magnetic nanoparticles, different levels of nanoparticle clustering are achieved by changing the ionic force and pH of the medium. Thus, the samples were submitted to two procedures of EDL destabilization, which involved dilution with an alkaline solution and a neutral pH viscous medium. The SPA results of all prepared ferrofluid samples show a reduction of up to half the efficiency of the standard sample when the ferrofluids are in a neutral pH or concentrated regime. Such results are explained in terms of magnetic dipolar interactions. Our results point to the importance of ferrofluid colloidal stability in a more reliable experimental determination of the NP heat generation performance.

Particle encapsulation in aqueous ferrofluid drops and sorting of particle-encapsulating drops from empty drops using a magnetic field.

Handling and manipulation of particle-encapsulating droplets (PED) have profound applications in biochemical assays. Herein we report encapsulation of microparticles in aqueous ferrofluid droplets in a primary continuous phase (CP) and sorting of PED from empty droplets (ED) at the interface of the CP in coflow with a second continuous phase using a magnetic field. We find that the encapsulation process results in a size contrast between the PED and ED that depends on the flow regime - squeezing, dripping, or jetting - which in turn is governed by the ratio of the discrete phase to the continuous phase capillary number, Car. The difference between the volume fractions of ferrofluid in the PED and ED, ΔαPED, is utilized for sorting, and is found to depend on the ratio of the capillary numbers, Car. The difference ΔαPED is found to be maximum in the jetting regime, suggesting that the jetting regime is most suitable for encapsulation and sorting. The sorting criterion is represented in terms of a parameter ξ, which is a function of the ratios of the magnetic force to the interfacial force experienced by the PED and ED. Our study revealed that sorting is possible for ξ < 0, which corresponds to ΔαPED > 0.25. The maximum sorting efficiency of our system is found to be ∼95% at a throughput of ∼100 drops per s.

Hierarchical Structure and Magnetic Behavior of Zn-Doped Magnetite Aqueous Ferrofluids Prepared from Natural Sand for Antibacterial Agents.

This study performs natural sand-based synthesis using the sonochemical route for preparing Zn-doped magnetite nanoparticles. The nanoparticles were dispersed in water as a carrier liquid to form Zn-doped magnetite aqueous ferrofluids. Structural data analysis indicated that the Zn-doped magnetite nanoparticles formed a nanosized spinel structure. With an increase in the Zn content, the lattice parameters of the Zn-doped magnetite nanoparticles tended to increase because Zn2+ has a larger ionic radius than those of Fe3+ and Fe2+. The existence of Zn-O and Fe-O functional groups in tetrahedral and octahedral sites were observed in the wavenumber range of 400-700 cm-1. The primary particles of the Zn-doped magnetite ferrofluids tended to construct chain-like structures with fractal dimensions of 1.2-1.9. The gas-like compression (GMC) plays as a better model than the Langevin theory to fit the saturation magnetization of the ferrofluids. The ferrofluids exhibited a superparamagnetic character, with their magnetization was contributed by aggregation. The Zn-doped magnetite ferrofluids exhibited excellent antibacterial activity against gram-positive and negative bacteria. It is suggested that the presence of the negatively charged surface and the nanoparticle size may contribute to the high antibacterial activity of Zn-doped magnetite ferrofluids and making them potentially suitable for advanced biomedical.

Temperature-dependent fractal structure of particle clusters in aqueous ferrofluids by small-angle scattering

The heating effect in the temperature interval of 20―70 °C on the aggregate structure in aqueous ferrofluids is analyzed basing on the data of small-angle X-ray (SAXS) and neutron (SANS) scattering. Samples prepared according to the same stabilization procedure (double-layer surfactant coating) but with different dispersed magnetic materials (magnetite and cobalt ferrite) show different temperature sensitivity. A temperature-induced reorganization from developed aggregates to elongated chain-like aggregates is revealed by observing changes in the scattering curves, which are considered in the frame of the fractal concept. In addition to the determination of fractal dimension, the accent is also given to the cut-off effect in the scattering curves induced by the finite aggregate size. The scattering model is chosen from the available approaches basing on the analysis of the dimension-dependent universal parameter of the average aggregate form-factor, which continuously changes with varying temperature. The particle polydispersity is taken into account. The contrast variation based on isotope (H/D) substitution in solvent is done in SANS experiments to conclude about the structural organization of magnetic particles and surfactant component in the systems.

Magnetic Sedimentation Velocities and Equilibria in Dilute Aqueous Ferrofluids.

Dilute ferrofluids have important applications in the separation of materials via magnetic levitation. However, dilute ferrofluids pose an additional challenge compared to concentrated ones. Migration of the magnetic nanoparticles toward a magnet is not well counteracted by a buildup of an osmotic pressure gradient, and consequently, homogeneity of the fluid is gradually lost. Here, we investigate this phenomenon by measuring and numerically modeling time-dependent concentration profiles in aqueous ferrofluids in the field of a neodymium magnet and at 10 T in a Bitter magnet. The numerical model incorporates magnetic, frictional, and osmotic forces on the particles and takes into account the polydispersity of the particles and the spatial dependence of the magnetic field. The magnetic sedimentation rate in our most stable ferrofluids can be understood in terms of the magnetophoresis of separate nanoparticles, a best-case scenario when it comes to applications.

Ferro-advection aided evaporation kinetics of ferrofluid droplets in magnetic field ambience

The present article discusses the physics and mechanics of evaporation of pendant, aqueous ferrofluid droplets, and modulation of the same by an external magnetic field. We show experimentally and by mathematical analysis that the presence of a horizontal magnetic field augments the evaporation rates of pendant ferrofluid droplets. First, we tackle the question of improved evaporation of the colloidal droplets compared to water and propose physical mechanisms to explain the same. Experiments show that the changes in evaporation rates aided by the magnetic field cannot be explained on the basis of changes in surface tension or based on classical diffusion driven evaporation models. Probing via particle image velocimetry shows that the internal advection kinetics of such droplets plays a direct role toward the augmented evaporation rates by modulating the associated Stefan flow. Infrared thermography reveals changes in thermal gradients within the droplet and evaluating the dynamic surface tension reveals the presence of solutal gradients within the droplet, both brought about by the external field. Based on the premise, a scaling analysis of the internal magneto-thermal and magneto-solutal ferroadvection behavior is presented. The model incorporates the role of the governing Hartmann number, the magneto-thermal Prandtl number, and the magneto-solutal Schmidt number. The analysis and stability maps reveal that the magneto-solutal ferroadvection is the more dominant mechanism, and the model is able to predict the internal advection velocities with accuracy. Furthermore, another scaling model to predict the modified Stefan flow is proposed and is found to accurately predict the improved evaporation rates.

Improvements in the Organic-Phase Hydrothermal Synthesis of Monodisperse M x Fe3-x O4 (M = Fe, Mg, Zn) Spinel Nanoferrites for Magnetic Fluid Hyperthermia Application.

In the quest for optimal heat dissipaters for magnetic fluid hyperthermia applications, monodisperse MxFe3–xO4 (M = Fe, Mg, Zn) spinel nanoferrites were successfully synthesized through a modified organic-phase hydrothermal route. The chemical composition effect on the size, crystallinity, saturation magnetization, magnetic anisotropy, and heating potential of prepared nanoferrites were assessed using transmission electron microscopy (TEM), dynamic light scattering, X-ray diffraction (XRD), thermogravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), atomic absorption spectroscopy (AAS), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometer (VSM) techniques. TEM revealed that a particle diameter between 6 and 14 nm could be controlled by varying the surfactant ratio and doping ions. EDS, AAS, XRD, and XPS confirmed the inclusion of Zn and Mg ions in the Fe3O4 structure. Magnetization studies via VSM revealed both the superparamagnetic nature of the nanoferrites and the dependence on substitution of the doped ions to the final magnetization. The broader zero-field cooling curve of Zn-doped Fe3O4 was related to their large size distribution. Finally, a maximum rising temperature (Tmax) of 66 °C was achieved for an aqueous ferrofluid of nondoped Fe3O4 nanoparticles after magnetic field activation for 12 min.

Colloidal Stability of Aqueous Ferrofluids at 10T.

Magnetic density separation is an emerging recycling technology by which several different waste materials—from plastic products, electronics, or other—can be sorted in a single continuous processing step. Larger-scale installations will require ferrofluids that remain stable at several teslas, high magnetic fields at which colloidal stability was not investigated before. Here we optically monitor the concentration profile of iron oxide nanoparticles in aqueous ferrofluids at a field of 10 T and a gradient of 100 T/m. The sedimentation velocities and equilibrium concentration profiles inform on maintenance or breakdown of colloidal stability, which depends on the concentration and magnetic coupling energy of the nanoparticles. Comparison with results obtained with a small neodymium magnet indicate that stability at moderate fields is predictive of stability at much higher fields, which facilitates the development of new ferrofluids dedicated to magnetic density separation.

Structural investigations of poly(ethylene glycol)-dodecylbenzenesulfonic acid complexes in aqueous solutions

Abstract The structures arising in mixed colloidal solutions of anionic surfactant dodecylbenzenesulfonic acid (DBSA) and neutral poly(ethylene glycol) (PEG) polymer in water for a wide range of concentrations and different molecular masses of the polymer were studied by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The micellization behavior of the system near the critical micelle concentration was characterized by fluorescence spectroscopy and measurements of the surface tension of the solutions. The assumption of the coexistence of free micelles, surfactant-polymer complexes in the form of a necklace structure, and polymer coils was confirmed by DLS and SANS. The typical sizes of micelles and complexes were estimated. It was found that there is an impact of PEG molecular mass on the ability of multi-micelle complex formation. The obtained results are important in particular for the PEGylation of magnetic nanoparticles in sterically stabilized aqueous ferrofluids.

White Magnetic Paper with Zero Remanence Based on Electrospun Cellulose Microfibers Doped with Iron Oxide Nanoparticles.

The preparation procedure of zero magnetic remanence superparamagnetic white paper by means of three-layer membrane configuration (sandwiched structure) is presented. The cellulose acetate fibrous membranes were prepared by electrospinning. The middle membrane layer was magnetically loaded by impregnation with an aqueous ferrofluid of 8 nm magnetic iron oxide nanoparticles colloidally stabilized with a double layer of oleic acid. The nanoparticles show zero magnetic remanence due to their very small diameters and their soft magnetic properties. Changing the ferrofluid magnetic nanoparticle volume fraction, white papers with zero magnetic remanence and tunable saturation magnetization in the range of 0.5–3.5 emu/g were prepared. The dark coloring of the paper owing to the presence of the black magnetite nanoparticles was concealed by the external layers of pristine white cellulose acetate electrospun fibrous membranes.

Effects of uniform magnetic induction on heat transfer performance of aqueous hybrid ferrofluid in a rectangular cavity

Utilization of hybrid nanoparticles to formulate nanofluids is one of the ways to improve nanofluids’ thermal and fluid properties. This work attempted to study the free convection heat transfer performance of aqueous Fe2O3-Al2O3 (75:25) nanofluids in a rectangular cavity under magnetic induction. The thermal properties of the aqueous hybrid ferrofluids (AHFs) were measured for volume concentrations of 0.05–0.3 vol% and temperatures of 20–40 °C. Also, the stability and morphology of AHFs were examined. On charging the AHF and base fluid samples into the cavity, the two opposite vertical walls were subjected to temperature gradients ranging from 20 to 35 °C to create buoyancy. Parameters such as Nuav, hav, Ra and Qav were determined for each AHF sample and the base fluid. The AHF with the highest heat transfer performance was exposed to magnetic induction to investigate its convective heat transfer. With Ra range of 1.65 × 108–3.80 × 108, the Nuav was observed to intensify with increasing Ra. Without the magnetic induction, the improvements in hav, Nuav, and Q̇av were observed for 0.05–0.2 vol% AHFs in comparison with the base fluid. Optimum heat transfer enhancement of 10.79% was achieved for 0.10 vol% HF at ΔT = 35 °C. By inducing magnetic field (118.4 G) vertically on the side wall of the cavity, a maximum enhancement of Nuav (4.91%) was achieved compared to the case without magnetic induction. Additionally, increasing the magnetic field strength (48.9–219.5 G) was noticed to further enhance heat transfer of 0.10 vol% AHF. Correlations were proposed for predicting viscosity, thermal conductivity and Nuav of AHF in a rectangular cavity. The use of hybrid nanofluid was revealed to have better heat transfer performance than mono-particle nanofluids. Conclusively, the heat transfer performance was observed to depend on φ, ΔT, AHF deployment, magnetic induction, strength and direction.