Study of the laws of internal friction in magnetic fluids with a strongly developed thixotropic nanostructure

. Due to their unique physical properties, magnetic fluids are promising for use in bearings, seals, sliding guides, and other devices of modern technology. Some restrictions on their use are imposed by the tendency of magnetic fluids to lose colloidal stability and structure formation in strong magnetic fields. Increasing the stability of a colloid by reducing the size of the dispersed particles of the magnetic fluid is limited by the Heisenberg uncertainty relation, on the condition of maintaining their ferromagnetic state. The search for ways to reduce internal friction in technical devices with magnetic fluids having a highly developed thixotropic nanostructure is important from a practical point of view. Using a device, simulating the operation of a magnetohydrostatic bearing, the rheological characteristics of a fluid nanostructured by a magnetic field, which is a colloidal system with a dispersed phase of magnetite particles (10 vol.%) and a dispersion medium of silicon organic fluid PESV-2, were studied. The dynamic viscosity of the magnetic fluid was about 0.05 Pa . s at 20°C. It has been established that the process of structuring a magnetic fluid in an external field can last hundreds of hours and depends mainly on the viscosity of the dispersion medium and the concentration of magnetite. It has been revealed that the motion of a cylinder with a terminal velocity begins only at shear stresses exceeding the limiting static stress and proceeds at a constant velocity. The breakdown of the structure begins after the shear stress exceeds the critical value. The critical stress is introduced to compare the strength of the structure of different fluids. The value of the critical stress was determined with an accuracy of up to 50 Pa by analyzing the curves of the change in the sliding speed with time. It has been established that the temperature dependence of the critical shear stress is very sharp and close to exponential.


Introduction
Magnetic fluids (MF) due to their unique physical properties are promising for use in new machines and mechanisms.However, this is hampered by disadvantages inherent to a greater or lesser extent in all magnetic fluids.It should be noted the tendency of magnetic fluids to lose colloidal stability, an insufficiently wide temperature range of operation, destruction upon contact with many liquid media, and much more.For example, the use of magnetic fluids in friction units and seals is hindered by their tendency to structure formation in strong magnetic fields [1][2][3][4][5][6][7].
An increase in the efficiency of magnetic fluid bearings and seals is associated with a decrease in the moment of internal friction forces during moving off from the stop, which can exceed the friction moment in the operating mode by several decimal orders.The reason for this phenomenon is the formation of a thixotropic reversible nanostructure of a magnetic fluid under the influence of a strong inhomogeneous magnetic field.Structuring of stable MF colloid can be avoided by reducing the size of dispersed particles.However, in this case, the problem of maintaining the high magnetic properties of the MF arises, since, in accordance with the Heisenberg uncertainty relation, the physical conditions for maintaining the ferromagnetic state are not satisfied when the particle size is less than 1 nm [8][9].
A magnetic fluid in a nanostructured state, as a rule, performs its functional tasks (for example, sealing bearings); however, the friction forces in such fluids are very high due to the magneto viscous effect [10].This significantly narrows field of application of bearings and seals, in which the MF, according to operating conditions, can be structured under the influence of a strong magnetic field.To develop technological and physicochemical approaches to reducing the friction moment during moving off from the stop, it is necessary to study the mechanism of internal friction in a structured magnetic fluid.
From a rheological point of view, friction in technical devices during start-up is determined by the ultimate shear stress (upper yield point).The existing theories of the initial flow stress of real magnetic fluids suggest that their transition from a quasi-solid to a fluid behavior mode is due to the destruction of chain or volumetric columnar aggregates, which often connect opposite boundaries of the area containing MF [11][12][13].In this case, it is usually assumed that the aggregates are rigidly linked to the boundary walls.However, sliding of aggregates on walls was often observed in experiments [14].
Therefore, determining the value of the initial flow stress, its nature, dependence on the properties and concentration of particles, as well as on the strength of the applied magnetic field, is important for the practical application of magnetic fluids.
The objective of the paper was to study the regularities of the process of internal friction in magnetic fluids with a highly developed thixotropic nanostructure in order to find the ways to reduce friction in magneto fluid technical devices.

Methodical issues of carrying out experimental studies
Studies of internal friction in a structured MF were carried out on the MT-3 device designed for tribotechnical testing of bearings and seals with a magnetic fluid.The device allows to determine the friction force in various tribounits: several designs of bearings and seals with nanodispersed magnetic fluids, depending on the value of the rotation speed or torque in the unit.It is also possible to determine the dependence of the rotation speed on the magnitude of the torque, which is quite informative.By the design, the device is close to a rotational viscometer with a rotating hollow cylinder.
The device was used to study the frictional characteristics of magnetohydrostatic bearings, a diagram of one of which is shown in Fig. 1.The principal design of an axial bearing includes a bearing support containing magnetic circuits 1, 2, a permanent magnet 3 and a bearing foot consisting of a non-magnetic thin-walled cylinder 4, closed on one side and inserted into the working gap filled with magnetic fluid 5.An inhomogeneous magnetic field in the gap is formed by the gear structure of the magnetic cores.The magnetic fluid seals the gas reservoir inside the dome-shaped foot 4. Therefore, the results obtained on the device characterize not only the properties of the bearing, but also the ones of the magnetic fluid seal.Magnetohydrostatic bearing works as follows.When an external axial load acts on the bearing, the shaft with cylinder 4 is axially displaced until an excess pressure is created in the volume inside it, sufficient to compensate for the external load.In addition, in the bearing, the magnetic fluid centers the shaft in the radial direction due to the effect of magnetic expulsion, which manifests itself when the gap between the housing and the hollow cylinder is uneven.
The load capacity and rigidity of the bearing depend on the breakdown pressure of the magnetic fluid seal and the type of working medium [15].In turn, the breakdown pressure is proportional to the number of separate stages of the magnetic fluid seal, of which there are three in the bearing shown in Fig. 1.
The device (Fig. 2) consists of a massive frame 1, on which a platform 2 is installed on ball-bearing guideways.The platform can be moved in mutually perpendicular directions with micrometer screws and positioned with an accuracy of 0.01 mm.The platform has a stand 3, which is fixed on the outer race of angular contact ball bearings 4 and can rotate around a vertical axis.The body of the tested bearing 5 (bearing thrust) is mounted on the stand coaxially with it.The foot 5 of the bearing is a cylinder made of magnetic material, inside of which a ring permanent magnet 6 is installed.The magnet is centered by a nonmagnetic T-shaped in section sleeve 7; two ring magnetic circuits 8 adjoin the magnet poles, one of which is made with a gap in relation to the body and has teeth of different shapes.A cylindrical foot 9 of the bearing is inserted into the gap, fixed on the shaft 10.The gap is filled with magnetic fluid 11 to seal the bearing.The shaft 10, together with the bearing foot, if necessary, can be centered using tapered clock-type bearings 12.The upper of the centering bearings 12 is located on the beam 13, which, for the convenience of operation on the device, can move along vertical guides, and the lower one is fixed in the housing of the bearing under test.Based on the results of the tests conducted, it was found that the friction moment that occurs in the centering bearings was less than 10 -6 N.m., which is many times less than the measured moment.
The stand rotation drive, together with the bearing foot, consists of a DC electric motor 14 with a centrifugal speed stabilizer and a reduction gearbox 15 that allows you to set 27 rotation speeds in a wide range.The rotation from the gearbox 15 to the stand 3 is transmitted using a V-belt drive.
When conducting the experiments to determine the dependence of the rotation speed on the magnitude of the torque, a torque is applied to the bearing shaft, created using flexible threads wound on the shaft and discrete sets of weights 16.To reduce the harmful effect on the shaft of the moment of forces rotating it around the horizontal axis, two threads are wound on it side by side diverging in different directions, at the ends of which loads of equal weight are always suspended.In this case, the speed of rotation of the shaft is determined noncontact, with the help of fixed reed switches, which were triggered by a rotating permanent magnet.The contact area of the MF with the surface of the cylinder 9, necessary for calculating the shear stresses, was determined from its trace on the surface of the cylinder.When operating on the device in the mode of a given rotation speed, the friction moment acting on the foot is fixed by the tensometric method with an accuracy of 0.5%.
The source of the magnetic field was KS-37 magnets made of SmCo5 alloy with dimensions ø37xø26x8 (mm).An inhomogeneous magnetic field in the working gap of the viscometer was formed using triangular teeth made on the pole tip.The teeth are obtained by means of turning 6 mm thick magnetic circuits with a cutter with a sharpening angle of 60 °.The distribution of the radial component of induction in the gap is shown in Fig. 3.The magnetic fluid of the MM-PES brand, which is a colloidal system with a dispersed phase of magnetite particles (10 vol.%) and a dispersion medium of organosilicon fluid PESV-2, was studied.The dynamic viscosity of the magnetic fluid was about 0.05 Pa .s at 20 °C.

Empirical data
To study the dynamics of destruction of the colloidal structure of the magnetic fluid, we experimentally determined the dependence of the linear velocity of the movement of the surface of cylinder 9 in Fig. 4 depending on the shear stress.Such a dependence in this case is more informative than the inverse dependence, since it allows one to trace in detail the dynamics of the destruction of the structure and the rheology of the processes preceding the destruction.The linear slip rate was chosen instead of the traditional shear rate because the second value during thixotropic transitions, and even more so under the conditions of creep and wall slip effects, preceding this transition, does not have a strict physical meaning [15].
The studies have shown that the movement of the cylinder with a finite speed begins only at shear stresses exceeding the so-called limiting static shear stress   and proceeds at a constant speed (Fig. 4).With a slight further increase in the shear stress, the velocity increases linearly, which is explained by creeping and wall slipping [15].  is determined by the simple extrapolation of the obtained dependence on the x-axis.In the considered speed range, structural changes in the MF do not occur.In the process of a further increase in stress after passing through a certain critical value of stress   , the equilibrium speed begins to increase nonlinearly as a result of the destruction of the structure.
Critical stress   was introduced to compare the structural strength of different fluids.The value of the critical stress was determined with an accuracy of up to 50 Pa by analyzing the curves of the change in the sliding speed with time.Compared to the ultimate shear stress, the critical stress has a clearer physical meaning and is uniquely determined.
Figure 5 shows the graphs of the dependence of the linear sliding speed on time for various values of shear stresses.If the shear stress does not exceed the critical one, then the speed does not change over time.When the shear stress exceeds τk slightly, the rotation speed first increases due to the destruction of bonds between weakly interacting particles in that region of the fluid where the area is smaller, and then, when a balance occurs between the processes of destruction and restoration of interparticle bonds, the speed stabilizes.A further increase in shear stress gradually leads to the destruction of the structure over the entire MF volume.The time spent on the destruction of the structure can vary by more than an order of magnitude, depending on the magnitude of the stress (Fig. 5).It should be noted that even for the colloidal systems under consideration, which differ in a wide range of interparticle interaction forces, the minimum structural fracture stress does not differ significantly from the critical one.
The value of the critical stress varies depending on the structuring time and temperature (Fig. 6).For fluids of the class under consideration, the formation of a thermodynamically stable structure can last hundreds of hours, but the process proceeds most intensively in the first hours after the MF is placed in the area.It should be noted that the temperature dependence of the critical stress is very sharp and close to exponential.

Conclusion
Using a device simulating the operation of a magnetohydrostatic bearing, such rheological characteristics of a liquid nanostructured by a magnetic field as the static limit of yield point and critical shear stress were studied.The process of structuring a magnetic fluid in an external field can last hundreds of hours and depends, in our experience, mainly on the viscosity of the dispersion medium and the concentration of magnetite.The destruction of the MF structure begins after the shear stress exceeds the critical value.At stresses below the critical, but above the limiting shear stress, the displacement of the measuring hollow cylinder occurs by overcoming the boundary friction of the MF on a solid surface.The determined, close to exponential course of the temperature dependence of the shear rate reflects the nature of the forces defining the internal friction in the magnetic fluid.

Fig. 2 .
Fig. 2. A device for determining the effective rheological properties of magnetic fluid and the frictional properties of bearings.

Fig. 3 .
Fig. 3. Radial component of the magnetic field induction in the gap opposite the magnetic pole N.