If redox reactions occur on the surface of a bimetallic soft actuator, creating a current loop, a force will act on it in a nonuniform magnetic field. The nature of this force is due to the Lorentz force acting on moving charges in a magnetic field. Due to the creation of a current loop, the actuator acquires additional magnetic properties and is essentially a chemical magnet. Here, we demonstrate for the first time an experimental method for measuring the chemical magnetism force acting on a deformable actuator. This method is based on comparing the actuator's deformation under the action of chemical magnetism and a gravitational force of a given magnitude. It has been shown that the chemical magnetism force depends on the type and concentration of the electrolyte, as well as the surface area of the actuator. Both deformable and rotating actuators can operate using the phenomenon of chemical magnetism. In this study, a new type of rotating actuator is proposed. The actuator rotates if the directions of the magnetic moment and magnetic field vectors do not coincide. The rotation speed of such actuators depends on the electrolyte concentration and the type of metals used. Such actuators can directly convert the chemical energy of the fuel into the kinetic energy of a rotating rotor. Rotary actuators do not require external energy sources to operate. The results obtained in this study can serve as the basis for creating a new type of sensor, where the deformation or rotation of sensitive elements will depend on the rate of redox reactions.

Soft actuators have a number of important advantages over traditional rigid robots due to their flexibility. Here we demonstrate the production method and the operating features of untethered soft actuators based on a new class of materials - photo-magnetic materials. The essence of the photo-magnetism phenomenon is that if photocatalytic reactions occur on the actuator surface under the influence of light and a current loop arises, then a force will act on it in a non-uniform magnetic field. The nature of this force is in no way related to the magnetic properties of the material from which the actuator is made, but is determined by the action of the Lorentz force on moving charges in a magnetic field. In fact, a photocatalytic cell is located on the surface of the soft actuator, with the help of which the conversion of light energy into electrical energy is carried out. The soft actuator is a polymer film on the surface of which there are electrodes made of metal and ZnO semiconductor. When exposed to light and a magnetic field in a hydrogen peroxide solution, such actuators can deform and the degree of their deformation depends on the concentration, temperature and pH of the solution. Based on such actuators, sensors can be created to detect the composition of the environment.

An active suspension can be used as a heat transfer fluid. An active suspension contains micro-/nanoparticles that can move independently by using energy from the surrounding environment. The mobility of the particles enhances mixing in the fluid, which can contribute to intensified heat transfer. Here, we present the results of an experimental study of transient heat transfer on a horizontal wall using an active bubble fluid as a heat transfer fluid and analyze the effect of gas bubbles formed during photocatalytic reactions on the heat transfer rate. To create this fluid, TiO2 nanoparticles are dispersed in an aqueous H2O2 solution. When the suspension is irradiated with UV light, photocatalytic reactions cause nanoparticle clusters to become mobile, forming O2 bubbles in the liquid. The results of the study show that the active bubble fluid has an ambiguous effect on heat transfer intensity. This is because, on the one hand, mobile cluster contribute to intensified heat transfer by mixing the liquid; on the other hand, the reduced thermal conductivity of the coolant - due to the presence of gas bubbles - may actually decrease heat transfer. The pH of the solution has a complex effect on heat transfer, and the nature of this effect depends on the temperature of the liquid. The results of the study show that the presence of surfactant in the solution helps to reduce the intensity of heat exchange. The results obtained in this study may contribute to the development of a new class of coolants based on active fluids.

Active suspensions are a promising type of coolant. When exposed to light, photocatalytic reactions occur on the surface of the suspension particles, resulting in the formation of gas bubbles. These mobile bubbles enhance heat transfer. However, if catalyst particles are also applied to the cooled surface, then gas bubbles will also form on the wall, which can further enhance heat transfer. Here, we demonstrate for the first time a method for cooling a catalytic wall using an active suspension as a coolant. The catalytic wall is a copper foil coated with TiO2 nanoparticles. A suspension of TiO2 nanoparticles in an H2O2 solution is used as an active coolant. The results of the study show that the highest heat transfer intensity occurs when an active suspension and a catalytic wall are used simultaneously. This is due to more uniform mixing of the liquid near the wall due to moving bubbles. When using a catalytic surface and a passive coolant, the heat transfer intensity is the lowest. Intermediate results are demonstrated when an active suspension is used as a coolant and the cooled surface does not have catalytic properties.

An active fluid can be used as a heat transfer fluid (HTF). The active fluid contains particles that can move independently by using energy from the surrounding environment. Here, we present the first experimental study of transient heat transfer on a vertical wall using an active bubble fluid as HTF. This type of HTF can be obtained by dispersing TiO2 and/or ZnO in H2O2 solution and irradiating the suspension with UV light. Photocatalytic reactions on the surface of nanoparticle clusters result in the formation of O2 bubbles. The clusters can move in the liquid via diffusiophoresis. The mobile clusters enhance heat transfer on the vertical wall. However, mobile bubbles can have an ambiguous effect on heat transfer. This is because, due to the lower thermal conductivity of gas compared to liquid, they reduce heat transfer by conduction. However, due to their mobility, the bubbles enhance convective heat transfer. The study results show that with increasing H2O2 concentration in the solution from 5% to 37% and decreasing solution temperature from 29 degrees C to 15 degrees C, the wall temperature decreases. The results obtained in this study may contribute to the development of a new class of heat transfer fluids based on active fluids.

To create soft actuators that can deform under the action of the Lorentz force in a magnetic field, it is necessary to use external sources of electrical energy, which inevitably entails the use of connecting wires or a battery. This study describes a method of creating untethered soft actuators that are capable of deforming under the action of the Lorentz force in a magnetic field and for the operation of such actuators no external energy source is required. To create a current on the surface of the actuator, chemical magnets are used here, the simplest version of which is a bimetallic plate that is in an electrolyte solution. During redox reactions a current loop appears on the surface of the bimetallic plate, which is affected by the Lorentz force in a magnetic field. Under the action of this force, soft actuators can be deformed in a magnetic field, and the degree of their deformation depends on the rate of redox reactions.

Microfluidic technology can be used to generate multi-component droplets. If the multi-component droplets contain ferrofluid, the droplet generation process can be controlled using a magnetic field. Here, we demonstrate for the first time an active method for generating two-component magnetic droplets using microfluidics with a flow-focusing configuration under the influence of a transverse magnetic field. The two-component droplet consists of a non-magnetic fluid core surrounded by a magnetic shell. The results of the study show that using a magnetic field allows for significantly increased flexibility in controlling the structure of two-component droplets. When using a magnetic field, the volume of the droplet's magnetic shell depends not only on the volumetric water flow rate in the chip, V2 ∼ Q1−1, but also on the magnetic field gradient, V2 ∼ ∇H−1. At low water flow rates in the chip, disturbances that develop on the jet surface under the action of the magnetic force play a key role in ferrofluid dispersion. However, with increasing water flow rate, the influence of the magnetic force degenerates. The results obtained in this work open up new possibilities for the creation of soft magnetic robots that can be used to move cargo.

Hypothesis: Emulsion Janus droplets containing magnetic nanoparticles can be used as soft magnetic robots. If the densities of the immiscible liquids comprising the Janus droplets differ significantly, the Janus droplets acquire a non-spherical shape in a gravitational field. Due to the anisotropic structure, such droplets may possess unique properties necessary for creating "smart liquids." Experiments: VDL 100 oil and fluocarbon oil Fluorinert FC-40, whose densities differ significantly, are used to create non-spherical magnetic Janus droplets. The VDL 100 oil contains paramagnetic nanoparticles Fe3O4. Using a microfluidic chip, an emulsion oil/oil//water containing spherical monodisperse droplets was obtained. When exposed to a gravitational and magnetic field for several days, the spherical droplets transform into non-spherical quasi-stable Janus droplets. Findings: Due to the difference in density between hydrocarbon oil and fluocarbon oil, spherical binary emulsion droplets in a gravity field, under the influence of the buoyancy force, acquire a dumbbell-shaped form. Under the combined action of a gravity and magnetic field, the droplets acquire a barrel-shaped form. Such droplets can be used as soft robots for carrying loads and for mixing liquids in microreactors. Emulsions with barrel-shaped droplets can be used to create "smart liquids" whose light transmittance can be varied using a magnetic field.

The effective viscosity of an active suspension depends on the mobility of microswimmers. In addition, there is an inverse effect of the rheological properties of the liquid on the motion of the microswimmers themselves, so the analysis of the effect of mobile microswimmers on the effective viscosity of suspensions based on non-Newtonian fluids is extremely complex. Here, we analyze the effect of the mobile unicellular micro-organism Paramecium caudatum on the effective viscosity of an aqueous solution of hydroxyethylcellulose. The results of the study show that the effective viscosity of the suspension has a maximum depending on the concentration of mobile microswimmers in the suspension. The ambiguous effect of microswimmers on the effective viscosity of the suspension is due to the competition between two competing factors. On the one hand, the motion of microswimmers contributes to the perturbation of the shear rate field in the liquid and thereby leads to additional energy dissipation and, as a consequence, to an increase in the effective viscosity. On the other hand, due to the fact that microswimmers move in a polymer solution, which is characterized by a shear-thinning property, their mobility contributes to a decrease in the effective viscosity of the suspension.

Magnetic fields can be used to control the process of magnetic fluid dispersion in microfluidics. Here we demonstrate a method for generating magnetic fluid droplets of a given size and shape in a microfluidic chip with a flow-focusing configuration under the influence of a transverse magnetic field. The results of the study show that a magnetic field can have a significant impact on ferrofluid dispersion. At low volumetric flow rates of the continuous phase, magnetic force plays a primary role in ferrofluid dispersion due to disturbances on the magnetic fluid surface. Furthermore, the droplet diameter decreases with increasing magnetic field Dd ∼ H-1/3. At high volumetric flow rates of the continuous phase, shear stress plays a primary role in ferrofluid dispersion. In this case, the droplet diameter depends on the continuous phase velocity according to the law Dd ∼ u-1. The magnetic field and the volumetric flow rate of the continuous phase affect not only the liquid dispersion process but also the coalescence and deformation of droplets. The pressure gradient in the channel and the magnetic field contribute to droplet deformation. The thickness of the deformed droplet decreases with increasing volumetric flow rate according to the law l ∼ Q-1. The non-uniformity of the velocity field distribution over the channel thickness is responsible for droplet coalescence. The magnetic field, on the contrary, prevents droplet coalescence. These findings open up new possibilities for generating soft magnetic robots of a given size and shape.

Motors are devices that are capable of converting various forms of energy into kinetic energy of their motion. Surface forces can play an important role in the movement of motors. One type of surface force is the force of chemical magnetism. If a loop with current occurs during redox reactions on the surface of a bimetallic swimmer, then a force acts on it in a non-uniform magnetic field. The nature of this force is in no way related to the magnetic properties of the material from which the swimmer is made but is determined by the action of the Lorentz force on moving charges in a magnetic field. Here we demonstrate various methods for controlling the speed and orientation of bimetallic swimmers that move under chemical magnetism force. The results of the study show that depending on the mutual direction of the magnetic force and the force of chemical magnetism chemical reactions can contribute to both acceleration and deceleration of the swimmer in a non-uniform magnetic field. By changing the concentration and type of electrolyte, the type of metals from which the swimmer is made, and the pH of the solution, it is possible to control the orientation and speed of the swimmer. These results open up new possibilities for the direct conversion of the chemical energy of the "fuel" into the kinetic energy of moving motors.

The task of developing new nanomaterials for effective water purification is currently extremely urgent. Here we demonstrate a method of synthesis of highly efficient, magnetic catalysts based on manganese, iron, cobalt and zinc oxides using the plasma-arc method. Due to the large number of contact zones between different types of semiconductor materials, such photocatalysts are extremely efficient. By changing the pressure in the synthesis chamber in the range from 5 Torr to 50 Torr, as well as the annealing temperature of nanoparticles in a muffle furnace in the range from 150 degrees & Scy; to 800 degrees & Scy;, the properties of the catalysts can be changed over a wide range of values. It has been shown that the catalyst Mn3O4@Fe2O3@CoO@ZnO@graphite is the most effective for the decomposition of methylene blue in the presence of hydrogen peroxide and UV exposure. This catalyst was synthesized at pressure 5 Torr and annealing temperature 600 degrees & Scy;. This catalyst has magnetic properties, so it can easily be separated from the system using a magnetic field. Due to the presence of a porous carbon layer on the outer surface of catalytic nanoparticles, their properties do not change during storage. In a solution of hydrogen peroxide, nanoparticles form microscopic clusters that are capable of moving under the influence of selfdiffusiophoresis, as well as magnetic force in a non-uniform magnetic field. Due to the mobility of the catalyst, mass transfer processes during wastewater treatment can be significantly intensified.

An active emulsion consists of self-propelled droplets that are dispersed in another immiscible liquid. Under certain conditions, droplet clusters may form in such a system. This study analyzes the process of cluster formation in an octane-in-water emulsion activated by ammonia. The movement of droplets in the emulsion is due to the emergence of the Marangoni flow on the surface of the droplets. It has been found that clusters are formed when the average droplet velocity is less than a certain critical value. The existence of a critical velocity is due to the fact that the rate of droplet attachment to a cluster is proportional to their velocity and the rate of droplet detachment from a cluster is proportional to the square of the droplet velocity. At supercritical droplet velocities, the rate of droplet detachment from a cluster exceeds the rate of their attachment. The critical velocity depends on the droplet density in the emulsion and increases as the droplet density decreases.

Absorption of light by a substance does not change its magnetic properties. However, if redox reactions occur on the surface of a material when irradiated with light and a current loop is formed, it turns into a magnet. This study reports a method for producing a new type of material-photo-magnets, which are capable of changing their magnetic properties when exposed to light. The simplest photo-magnet is a bimetallic plate made of two dissimilar metals, one part of which is coated with a semiconductor material-zinc oxide-and it is immersed in a solution of hydrogen peroxide. When exposed to light, holes and electrons are formed in the semiconductor, which take part in redox reactions during the decomposition of hydrogen peroxide. Since a current loop is formed in this case, the photo-magnet becomes a source of a magnetic field. In addition, any loop with current in a non-uniform magnetic field is affected by a force whose nature is determined by the action of the Lorentz force on moving charges. Therefore, on the basis of photo-magnets, it is possible to create motors that will move in a non-uniform magnetic field when irradiated with light.

Although at present, passive liquids are the primary choice for coolants, active liquids can also be used to enhance heat transfer. Here, we demonstrate for the first time a method for intensifying heat transfer on a heat-releasing surface by using an active bubble liquid as a coolant. To obtain an active bubble liquid, TiO2 nanoparticles, which agglomerate into microscopic clusters, are dispersed in an aqueous solution of H2O2. When exposed to ultraviolet light, photocatalytic reactions occur on the surface of the clusters, which are accompanied by the formation of bubbles. Clusters with bubbles adsorbed on their surface move chaotically in the liquid under the action of diffusiophoresis. By mixing different layers of liquid, heat exchange on the heat-releasing surface can be intensified. The results of the study show that with increase in the concentration of H2O2 in an aqueous solution, the intensity of heat transfer increases. The liquid temperature has an ambiguous effect on the temperature of the heat-releasing wall. The study's results show that there is an optimal liquid temperature corresponding to the highest heat transfer rate. If the liquid temperature is below this optimal temperature, the heat transfer rate is limited by the velocity of the active bubbles. However, at high liquid temperatures, mobile bubbles mix relatively hot layers in the liquid, which limits heat exchange at the surface. The results obtained here can serve as a basis for developing a new class of coolants based on active liquids.

One of the most effective methods for cooling overheated surfaces is drip irrigation. If the surface temperature exceeds the Leidenfrost temperature, then a vapor film is formed between the droplet and the surface, which leads not only to a decrease in heat transfer intensity but also causes droplet mobility. For a number of applications, the mobility of droplets is an undesirable phenomenon, so the analysis of the factors responsible for their movement is a relevant task. Here we analyze the movement mechanism of the Leidenfrost droplets with variations in the composition and volume of the droplets. The data obtained show that the droplet speed increases with an increase in the droplet volume. However, smaller droplets change direction of motion more often than larger droplets. To substantiate the experimental data, a hypothesis is proposed, according to which the mechanism of movement of Leidenfrost droplets is caused by the reactive force that arises due to the evaporation of liquid. A Leidenfrost droplet changes the direction of its movement due to the deformation of its surface under the influence of gravity and capillary force. To substantiate the experimental data a simple phenomenological model is proposed.

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The motion of a single active droplet and a swarm of droplets in a dense emulsion can differ significantly, which is due to the interaction of the droplets with each other. It has been found that with a decrease in the velocity of active droplets, their motion in a dense emulsion becomes more spatially correlated, and the size of clusters, in which the velocities of the droplets are close, increases. During diffusion motion, active droplets spend most of their time confined in cages and move significant distances after cage rearrangements. With an increase in the average velocity of active droplets in the emulsion, the residence time of the droplets within the cage decreases according to the law similar to u(-2). In this case, the mean square displacement of the isolated droplet turns out to be proportional to similar to t(3/2). The deviation of the diffusion law of a droplet from the Brownian law is due to the existence of a repulsive force between them.