Nanofluids, suspensions of nanoparticles, are increasingly being used in various industrial [1, 2] and medical applications .
Most of the industrial applications result from increased thermal conductivity, which was reported for the first time in the second half of the 90th twentieth century. Since then the announcement of the initial results of the measurement of thermal conductivity of these materials, researchers had been studying them very intensively [4–9]. A large number of papers on thermal conductivity of these materials have resulted in the formation of theoretical models of this issue [10–12].
Medical applications are possible thanks to the antibacterial behavior of certain types of nanoparticles [13, 14]. The issue of using nanofluids was then reduced to produce and use as a drug nanosuspension. In case of this type of application of nanofluids, not the thermal conductivity but the rheological properties of suspension are the most important factors.
Thermal conductivity of nanofluids depends on nanoparticle properties including material type, shape , size , aggregation , concentration, and type of base fluid. This parameters have also an influence on rheological behavior of nanofluids [18, 19].
Unfortunately, at the moment, there does not exist a coherent theoretical model of the rheological properties of nanofluids. There are works of Einstein  and many other scientists who have theoretically studied the viscosity of the suspension [21, 22]; but because of the unique properties of nanoparticles, these models cannot always be used to describe the nanofluids. Mackay et al.  presented non-Einstein-like decrease in viscosity of nanofluids caused by nanoscale effects.
There are a variety of methods of preparation of dry nanoparticles [24–26] since there is easy access to these materials and ability to use them in the production of nanofluids which will result in the further dynamic development of this field. As the base liquid, water [18, 27, 28], ethylene glycol [7, 29], diethylene glycol [30, 31], and ethyl alcohol [32, 33] are used.
Viscosity of liquid depends not only on the temperature and shear rate, but also on the pressure. Though the viscosity of the fluid decreases with increasing temperature, it generally increases with increasing pressure. The pressure exerted on the fluid causes the approach of the particles towards each other and the increase of the intermolecular interactions; therefore, the viscosity of the fluid rises. An increase of the viscosity is higher for the fluids with a more composite structure because it impedes the movement of the particles under pressure. Thus, the scale of the viscosity increase of the liquid with the pressure depends on the type of fluid. The use of low pressure causes a slight increase in the viscosity. Whereas this increment is significant at higher pressure, influence of the pressure on viscosity is almost directly proportional to the pressure from the atmospheric pressure up to 100 MPa. The enhancement of the pressure to about 100 MPa doubles the value of the viscosity of most of the organic liquids . However, in the area of high pressure, the dependence of the viscosity on the pressure is not directly proportional.
In the case of lubricating oils, the viscosity under the pressure of 1,000 MPa may increase even up to 107 times, in comparison to the viscosity under atmospheric pressure . At pressure ranks of several thousands of MPa, the impact of the intermolecular repulsion is visible, and thus, a curve of increment of viscosity with increasing pressure asymptotically approaches to a constant value .
The exception is the impact of the pressure on the viscosity of water and aqueous solutions. With the increase of the pressure to about 100 MPa and over a temperature to about 30°C, the viscosity of water decreases. The viscosity of water increases until from the pressures reaching a value of above 100 MPa and 30°C. Schmelzer et al.  measured the viscosity of water in the pressure range of 0 to 100 MPa and at the temperature range of 0°C to 25°C. This experiment confirmed the unique properties of water viscosity.
Consideration of the viscosity of various types of liquids depending on the pressure is not only a theoretical issue, but has a large practical importance. Exact knowledge of the viscosity of water at various pressures is important in the interpretation of the impact of pressure on the heat transfer in the aqueous solutions, flow problems, and also on the electrical conductance of aqueous electrolytes [37, 38].
Horne and Johnson  measured the effect of hydrostatic pressure on the viscosity of pure water in the pressure and temperature ranges of 1 to 2,000 kg/cm3 and 2°C to 20°C, respectively, with a rolling ball type of viscometer. Using the same kind of viscometer, Stanley and Baten  measured the viscosity of water at pressures of 0 to 1,406 kg/cm3 and over a temperature range of 2°C to 30°C. In turn, Först et al.  presented experimental data for the viscosity of water at high pressures of up to 700 MPa in the temperature range of −13°C to 20°C with two different types of viscometers.
Whereas, Grimes et al.  showed experimental data on the viscosity of aqueous KCl solutions over the pressure range of 0 to 30 MPa and the temperature range of 25°C to 150°C using the oscillating-disk viscometer. The change of viscosity with pressure is of particular relevance in the field of lubrication.
On the other hand, the knowledge on viscosity of hydrocarbon mixtures under high pressure is also significant in the petrochemical industry. Oliveira and Wakeham  measured the viscosity of five different liquid hydrocarbons at pressures of up to 250 MPa in the temperature range of 303 to 384 K with a vibrating-wire viscometer.
Further, in the study of dynamic properties of ions or solvent particles at high pressures, the viscosity measurements of electrolyte solutions are important. The high-pressure viscosity is also relevant for many processes involving polymer solutions. From the other side, viscosity measurements under high pressures are also needed to estimate the diffusion rate of the particles in a fluid.
Thermophysical properties of nanofluids are also studied by others researchers. Pastoriza-Gallego et al. [18, 44] examined the volumetric behaviour and the viscosity of CuO and Al2O3 in water nanofluids. Experimental density measurements of CuO-water nanofluids were performed at the pressure range from atmospheric pressure to 45 MPa, and the temperature range of 283.15 to 323.15 K, with a 10-K step. In turn, density measurements of Al2O3-water nanofluids were executed at an atmospheric pressure of 25 MPa, and the temperatures of 283.15, 298.15, and 313.15 K. Additionally, the viscosity measurements at atmospheric pressure were carried out.
Cabaleiro et al.  also experimentally determined the influence of pressure on the density of TiO2-ethylene glycol nanofluids. It was found that the impact of particle size on density is slight, but it may not be ignored. On the other side, the variations in viscosity are significant thus must be taken into consideration for any practical application. For this reason, examination on the influence of pressure on viscosity of nanofluids may have great practical importance.
Electrorheology is a field of science which studies liquids, whose viscosity changes reversibly and continuously under the influence of an electric field. Therefore, the viscosity of electrorheological fluids changes under the impact of an applied voltage. The electrorheological fluid is a suspension of particles in a base fluid, and for this reason, the simplest explanation for the viscosity increase is to assume that under the influence of an electric field, the particles connect to each other to form an ordered chain, whose direction is consistent with the direction of the force field. It increases the flow resistance of the liquid phase.
Effect of increased viscosity is proportional to the electric field intensity. That phenomenon is reversible - after the resolution of the electric field, the liquid returns to its initial properties. The effect of ‘curing liquid’ under the influence of an electric field is also called the Winslow effect, after the name of the American inventor Willis Winslow who was the first researcher of this phenomenon, and published an article about it in 1949 . ‘Winslow liquids’ were based on oil, which contained a suspension of starch, lime, gypsum, silicon dioxide, or carbon.
The current understanding of the microscopic phenomena is that it is believed to control the electrorheological effects, and the models used to describe macroscopic behavior is presented in the review of Parthasarathy and Klingenberg . Additionally, Hao  described the physical backgrounds behind phenomenon of electrorheological fluids.
Due to their unique properties, electrorheological liquids are used as working fluids in various types of machinery and vehicles, including active vibration damping devices, shock absorbers, clutches, electrically controlled valves, and in aerospace applications. In order to increase machine efficiency and speed of movement of vehicles, newer and newer technologies are being looked for.
Currently, one of the important directions of work on improving the performance of machines and vehicle components of hydraulic subassembly is to improve the working fluid. The tests are carried out to search for new types of these liquids, such as fluids, whose properties can be altered by external influences. Therefore, works on the working fluids, whose viscosity can be varied continuously and reversibly by the electric field have a large perspective. This allows the control of devices with these liquids in a very simple way.
The main qualities of electrorheological fluids are their high yield stress and enhanced viscosity under an applied electric fields. Therefore, it is worth to study the electrorheological properties of various suspensions in order to seek out possible industrial applications wherein the suspensions of nanoparticles in the base fluid deserve particular attention.
Sheng and Wen  explored the interaction between nanoparticles and an electric field from the electrorheological point of view. The yield stress is one of the critical design parameters in a device containing the electrorheological liquid and has attracted substantial attention both theoretically and experimentally. Farajian et al.  theoretically investigated the yield stress in carbon nanotube suspensions under an electric field. On the other hand, Raykar et al.  reported the electrorheological properties of low-concentration Fe2O3 nanofluids prepared in ethylene glycol under the less influence of electric fields while Yin and Zhao  presented the recent researchers on electrorheology of various nanofiber-based suspensions, including inorganic, organic, and inorganic/organic composite nanofibers.
Viscosity of the electrorheological fluids depends primarily on the shear rate, electric field strength, and also the temperature. An important issue which could not be neglected in the course of the examination of suspension is the problem of ensuring the stability of the dispersion of the particles and their protection against agglomeration and sedimentation . The long-term sedimentation causes loss of the electrorheological phenomenon despite the presence of the stimulating electric field.
Prekas et al.  reported the effect of temperature and surfactant concentration on the stability of electrorheology fluid prepared from zeolite particles and silicone oil.
Nanofluids may have many important applications in the industry and thus should be carefully studied, both in terms of occurrence of the electrorheological effects as well as other rheological properties. Therefore, further properties of MgAl2O4-diethylene glycol nanofluid were investigated and presented in the hereby paper.