A new method for determining the interfacial tension of a magnetic fluid (MF) is proposed on the basis of deformation of a MF drop lying on a liquid substrate and subjected to a vertical uniform magnetic field. The results show that the drop elongates in the direction of the field with an increase of its intensity. As soon as the field strength reaches a certain value, the interface and the free surface of the drop become unstable, which causes the peaks of different height to form. It has been found that the ratio of the corresponding critical values of magnetic field intensity is determined by the ratio of surface tension at the interface to that on the free boundary of the drop. Surface and interfacial tension of liquids used in the experiment were measured with the help of tensiometer by the ring detachment method to verify the experimental data. The presented results on the ferrofluid interface tension measurements can be of interest for the specialists in the field of ferrohydrodynamics. The magnetic field causes the drop to elongate till the peak instability. The critical values of the field strength respond to the ferrofluid initial magnetic susceptibility. The ratio of the critical magnetic field values is determined by the ratio of the interfacial tension.

This paper presents a study of the ponderomotive force acting on a solid non-magnetic sphere immersed in a magnetic fluid cylindrical container magnetized by an external homogeneous magnetic field. The problem has been studied experimentally, numerically, and analytically. The analytical study was carried out in the framework of the inductive approach, which made it possible to take into account the demagnetizing fields generated by both the magnetic fluid and the non-magnetic body. All methods of investigation used in this work showed the same non-standard force behavior with two extrema. The existence of the non-monotonous force is explained by the competition between two mechanisms: the inhomogeneous demagnetizing field inside the cylindrical container and the interaction of the solid body dipole with its own mirror image. The results of numerical simulations are in quantitative agreement with the experimental data, and the analytical results are in qualitative agreement with the experimental data.

The influence of a magnetic field on the pattern of stationary waves formed on the surface of a magnetic fluid (ferrofluid) when an obstacle moves has been studied both theoretically and experimentally. It is found that a vertical magnetic field narrows the cone of stationary waves and increases their amplitude. In the wake region, the peaks of the Rosensweig instability appear in a magnetic field that is smaller than the critical field that determines this instability occurrence. A horizontal magnetic field parallel to the obstacle velocity expands the cone of waves but reduces their amplitude up to the suppression of stationary waves. A horizontal field perpendicular to the obstacle velocity also expands the cone of waves and stabilizes their amplitude.

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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.