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With the broader use of ferrofluid in industry and the sciences, ferrofluid research has produced many new branches. To obtain systematic analytical results and to help researchers gain insight into the dynamics of ferrofluid research, in this review, we first track and summarize the ferrofluid research over the past 15 years to assay how ferrofluid can contribute to modern technology. We have got several important research directions (basic physics research, medical applications, and engineering applications) using keywords as an entry point by visualizing the cluster analysis. Visualization to highlight important schools and organizations. By analyzing the number of papers and citations at a given institution, we obtained a list of influential institutions and schools and indicated the current research directions of these institutions. This list greatly helps researchers to understand the authoritative papers as well as the latest results. In particular, we draw connections between publication year and novel engineering applications and show how they impact future development—the increasing use of ferrofluids in microfluidic environments and the increasing study of their soft and reshapable properties. Next, we focus on ferrofluid applications in six engineering fields and summarize the development history and research status. Finally, by analyzing the current status of each application, we summarize two challenges, that is to strengthen the research on the micromechanism of ferrofluid and to prepare a ferrofluid with better performance. The originality of this study is that it uses visualization to describe the current state of research and development patterns instead of thousands of papers, providing valuable reference information for existing researchers and practitioners.

Ferrofluids are the colloidal suspensions of magnetic nanoparticles and base fluids. It has several medical applications like drug targeting, cell separation, magnetic resonance imaging, etc. The dispersion of more than one magnetic nanoparticle into the convectional fluid is called hybrid nanofluid. It has various technological applications like damping, dynamic sealing, heat dissipation, etc. Due to the immense applications of the ferrofluids, in this analysis, we examined the liquid film flow and heat transfer of hybrid ferrofluid in the attendance of radiation and irregular heat source/sink. We considered the magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) nanoparticles and suspended them into water–ethylene glycol (EG) mixture (50–50%). A mathematical model is developed for the present investigation and solved numerically after applying the suitable similarity transformations. The impact of various governing non-dimensional parameters on the momentum, energy fields, and local Nusselt number of ferro- and hybrid ferrofluids is studied with the aid of graphical illustrations. It is perceived that the rate of heat transfer is higher in hybrid ferrofluid than that of ferrofluid.

Ferrofluids are a type of nanometer-scale functional material with fluidity and superparamagnetism. They are composed of ferromagnetic particles, surfactants, and base liquids. The main characteristics of ferrofluids include magnetization, the magnetoviscous effect, and levitation characteristics. There are many mature commercial ferrofluid damping applications based on these characteristics that are widely used in numerous fields. Furthermore, some ferrofluid damping studies such as those related to vibration energy harvesters and biomedical devices are still in the laboratory stage. This review paper summarizes typical ferrofluid dampers and energy harvesting systems from the 1960s to the present, including ferrofluid viscous dampers, ferrofluid inertia dampers, tuned magnetic fluid dampers (TMFDs), and vibration energy harvesters. In particular, it focuses on TMFDs and vibration energy harvesters because they have been the hottest research topics in the ferrofluid damping field in recent years. This review also proposes a novel magnetic fluid damper that achieves energy conversion and improves the efficiency of vibration attenuation. Finally, we discuss the potential challenges and development of ferrofluid damping in future research.

Magnetically actuated miniature soft robots are capable of programmable deformations for multimodal locomotion and manipulation functions, potentially enabling direct access to currently unreachable or difficult-to-access regions inside the human body for minimally invasive medical operations. However, magnetic miniature soft robots are so far mostly based on elastomers, where their limited deformability prevents them from navigating inside clustered and very constrained environments, such as squeezing through narrow crevices much smaller than the robot size. Moreover, their functionalities are currently restricted by their predesigned shapes, which is challenging to be reconfigured in situ in enclosed spaces. Here, we report a method to actuate and control ferrofluid droplets as shape-programmable magnetic miniature soft robots, which can navigate in two dimensions through narrow channels much smaller than their sizes thanks to their liquid properties. By controlling the external magnetic fields spatiotemporally, these droplet robots can also be reconfigured to exhibit multiple functionalities, including on-demand splitting and merging for delivering liquid cargos and morphing into different shapes for efficient and versatile manipulation of delicate objects. In addition, a single-droplet robot can be controlled to split into multiple subdroplets and complete cooperative tasks, such as working as a programmable fluidic-mixing device for addressable and sequential mixing of different liquids. Due to their extreme deformability, in situ reconfigurability and cooperative behavior, the proposed ferrofluid droplet robots could open up a wide range of unprecedented functionalities for lab/organ-on-a-chip, fluidics, bioengineering, and medical device applications.

In this paper, the effect of different parameters on Fe 3 O 4 /water nanofluid preparation characteristics is investigated experimentally. Two criteria, stability and magnetism, are employed to characterize prepared ferrofluids. Dynamic light scattering methods (DLS) distribution and transmission electron microscopy (TEM) images are applied in nanoparticle size investigation. Two-step preparation method is used to prepare the ferrofluid samples. Zeta potential and vibrating sample magnetometer (VSM) methods are used to study the stability and magnetism characteristics of prepared ferrofluid samples, respectively. The effect of six parameters (surfactant material, surfactant mass, heater stirring speed, heater stirring time, pH, initial sonication time and final sonication time) with three levels and one parameter (surfactant material) with six levels on the stability and magnetization is considered. The Taguchi method is applied in design of experiments, and 18 samples are prepared. The results show that the effective parameters on the stability of the prepared ferrofluid as their importance are: surfactant material, pH number, initial sonication time, surfactant mass, final sonication time, heater stirring speed and heater stirring time, respectively. According to magnetization viewpoint, the order of importance for effective parameters is: surfactant material, surfactant mass, pH number, final sonication time, heater stirring time, initial sonication time and heater stirring speed, respectively.

Solid ferromagnetic materials are rigid in shape and cannot be reconfigured. Ferrofluids, although reconfigurable, are paramagnetic at room temperature and lose their magnetization when the applied magnetic field is removed. Here, we show a reversible paramagnetic-to-ferromagnetic transformation of ferrofluid droplets by the jamming of a monolayer of magnetic nanoparticles assembled at the water-oil interface. These ferromagnetic liquid droplets exhibit a finite coercivity and remanent magnetization. They can be easily reconfigured into different shapes while preserving themagnetic properties of solid ferromagnets with classic north-south dipole interactions. Their translational and rotational motions can be actuated remotely and precisely by an external magnetic field, inspiring studies on active matter, energy-dissipative assemblies, and programmable liquid constructs.

Developing adaptive materials with geometries that change in response to external stimuli provides fundamental insights into the links between the physical forces involved and the resultant morphologies and creates a foundation for technologically relevant dynamic systems 1 , 2 . In particular, reconfigurable surface topography as a means to control interfacial properties 3 has recently been explored using responsive gels 4 , shape-memory polymers 5 , liquid crystals 6 – 8 and hybrid composites 9 – 14 , including magnetically active slippery surfaces 12 – 14 . However, these designs exhibit a limited range of topographical changes and thus a restricted scope of function. Here we introduce a hierarchical magneto-responsive composite surface, made by infiltrating a ferrofluid into a microstructured matrix (termed ferrofluid-containing liquid-infused porous surfaces, or FLIPS). We demonstrate various topographical reconfigurations at multiple length scales and a broad range of associated emergent behaviours. An applied magnetic-field gradient induces the movement of magnetic nanoparticles suspended in the ferrofluid, which leads to microscale flow of the ferrofluid first above and then within the microstructured surface. This redistribution changes the initially smooth surface of the ferrofluid (which is immobilized by the porous matrix through capillary forces) into various multiscale hierarchical topographies shaped by the size, arrangement and orientation of the confining microstructures in the magnetic field. We analyse the spatial and temporal dynamics of these reconfigurations theoretically and experimentally as a function of the balance between capillary and magnetic pressures 15 – 19 and of the geometric anisotropy of the FLIPS system. Several interesting functions at three different length scales are demonstrated: self-assembly of colloidal particles at the micrometre scale; regulated flow of liquid droplets at the millimetre scale; and switchable adhesion and friction, liquid pumping and removal of biofilms at the centimetre scale. We envision that FLIPS could be used as part of integrated control systems for the manipulation and transport of matter, thermal management, microfluidics and fouling-release materials. By infusing a ferrofluid into a microstructured matrix and applying a magnetic field, dynamic, multiscale topographical reconfigurations emerge, enabling functions such as colloidal self-assembly, switchable adhesion and friction, and biofilm removal.

Recently, the suspension of hybrid nanoparticles in conventional fluids has been investigated as a technique for improving the thermophysical properties of nanofluids. The dearth of documentation on the trio influence of volume concentration, base fluid, and temperature on the electrical conductivity and viscosity of hybrid alumina–ferrofluids [Al2O3–Fe2O3 (25:75 mass%)] has led to this study. The effective viscosity and electrical conductivity of the deionized water (DW)-based and ethylene glycol (EG)–DW-based (50:50 vol%) hybrid alumina–ferrofluids were measured at temperatures of 20–50 °C and volume concentrations of 0.05–0.75%. Based on the importance of soft computing methods to engineers, adaptive neuro-fuzzy inference system (ANFIS) and artificial neural network (ANN) were used for predicting the relative viscosity and electrical conductivity of the two types of hybrid ferrofluids. The measured data for viscosity and electrical conductivity were used in the modeling. Model performances were evaluated using the root mean squared error index. Viscosity was enhanced by 3.23–43.64% and 2.79–49.38%, while electrical conductivity was increased by 163.37–1692.16% and 717.14–7618.89% for the DW- and EG–DIW-based hybrid ferrofluids, respectively, compared with the respective base fluids. Increasing volume concentration augmented the viscosity and electrical conductivity of all the hybrid alumina–ferrofluids, whereas a rise in temperature enhanced their electrical conductivity and detracted the viscosity. DW-based hybrid alumina–ferrofluid was observed to have a lower viscosity and higher electrical conductivity than the EG–DW-based counterpart. The results showed that the optimum ANN and ANFIS models have a maximum error of less than 4.5% and 3.9% for relative viscosity and electrical conductivity, respectively, which were lower than those proposed using regression analysis. With the hybrid alumina–ferrofluids possessing a lower viscosity relative to single-particle ferrofluids, they are recommended for engineering application.

Self-assembly is a process in which interacting bodies are autonomously driven into ordered structures. Static structures such as crystals often form through simple energy minimization, whereas dynamic ones require continuous energy input to grow and sustain. Dynamic systems are ubiquitous in nature and biology but have proven challenging to understand and engineer. Here, we bridge the gap from static to dynamic self-assembly by introducing a model system based on ferrofluid droplets on superhydrophobic surfaces. The droplets self-assemble under a static external magnetic field into simple patterns that can be switched to complicated dynamic dissipative structures by applying a time-varying magnetic field. The transition between the static and dynamic patterns involves kinetic trapping and shows complexity that can be directly visualized.

Magnetite nanoparticles with different surface coverages are of great interest for many applications due to their intrinsic magnetic properties, nanometer size, and definite surface morphology. Magnetite nanoparticles are widely used for different medical-biological applications while their usage in optics is not as widespread. In recent years, nanomagnetite suspensions, so-called magnetic ferrofluids, are applied in optics due to their magneto-optical properties. This review gives an overview of nanomagnetite synthesis and its properties. In addition, the preparation and application of magnetic nanofluids in optics, nanophotonics, and magnetic imaging are described.