Magnetic fluids were historically the first active nano-dispersion material. Despite over half a century of research, interest in these nano-objects continues to grow every year. This is due to the impressive development of nanotechnology, the synthesis of nanoscale structures, and surface-active systems. The unique combination of fluidity and magnetic response allows magnetic fluids to be used in engineering devices and biomedical applications. In this review, experimental results and fundamental theoretical approaches are systematized to predict the micro- and macroscopic behavior of magnetic fluid systems under different external influences. The article serves as working material for both experienced scientists in the field of magnetic fluids and novice specialists who are just beginning to investigate this topic.

This work proposes a new technique for creating active bubbles and droplets with a non-magnetic core and a coating formed by a magnetic fluid. The procedure consists of the injection of a non-magnetic phase into a magnetic one that is supported by the presence of an inhomogeneous magnetic field from the source, which combines an annular magnet and an electromagnet. We explored various modes leading to different active bubbles and drops as well as the influence of the magnetic field on the size, velocity, and acceleration of the formed active droplets. It is shown that active bubbles change their trajectory under the action of a constant magnetic field and also disintegrate under the action of a pulsed one. This provides a new mechanism for controlling the absorption of droplets and bubbles using a magnetic field. Therefore, these results can be applied to create droplet-based microfluidics systems, in which an inhomogeneous magnetic field can be used for focusing droplet and bubble flows in a magnetic fluid.

We present an experimental system aimed at mapping the magnetic field created by a ring magnet in the working region of a single-magnet magnetic levitation system. The key idea is based on the photo registration of a transparent air-filled cavity formed in a thin magnetic fluid layer due to balancing the gas pressure and the pressure emerging due to magnetic forces. As a result, the contour of the magnetic field lines in 3D space is visible. It was shown that the theoretical analysis, numerical simulations, and experimental results are consistent.

The microfluidics of magnetic fluids is gaining popularity due to the possibility of the non-contact control of liquid composite systems using a magnetic field. The dynamics of non-magnetic droplets and gas bubbles in magnetic fluids were investigated for various configurations of magnetic fields, coatings, and channel geometries, as well as the rate of component supply and their physical properties. Optimal regimes for forming droplet and bubble flows were determined. The mechanism for non-contact control of the size of droplets and bubbles using a magnetic field is proposed in this article. The dependences of the sizes of non-magnetic inclusions in magnetic liquids on the continuous phase flow rate and the displacement of magnets were obtained. The obtained dependences of the volume of non-magnetic inclusions on the flow rate of the continuous phase follow the classic dependences. Changing the size of air bubbles can be achieved by shifting the magnet from −5 mm to +2 mm. The ratio of the maximum and minimum breakaway inclusion varies from 5 to 2 depending on the flow rates of the continuous phase. The range of changing the size of oil droplets with the displacement of magnets is from 1.1 to 1.51. These studies show how, with the help of various mechanisms of influence on microfluidic flows, it is possible to control the size of bubbles and droplets forming in microchannels. The obtained data can be applied for controlled microfluidic dosing and counting devices.

The formation of droplets in microchannels (droplet microfluidics) has a large number of applications, such as in micro-dosing and gas meters. This paper considers the dynamics of direct and inverse emulsions based on water, polydimethylsiloxane, and synthetic and mineral oil in microfluidic chips based on two technologies: glass–parafilm–glass sandwich structures and removable scaffold in a silicone compound. It is shown that wettability, roughness and chip wall material; channel thickness; magnetic fluid flow rate; and magnetic field strength affect the size of emulsion droplets formed in a microfluidic chip. The addition of another mechanism for regulating the hydrodynamics of emulsions using a magnetic field opens up new possibilities for the development of promising devices.