The article presents the results of experimental studying the hysteresis of water contact angles at crystalline surfaces. Receding and advancing contact angles and their hysteresis at hydrophilic and hydrophobic substrates with different surface structures (silica, mica, and calcite) have been studied under the action of an external pressure applied to the three-phase contact line of a sessile air bubble. Hysteresis of the contact angles has been observed on hydrophobized samples of silica and mica. The anchoring of the three-phase contact line (pinning) facilitates a change in the contact angles under the applied external pressure. When the three-phase contact line moves along the hydrophilic surfaces of silica and mica no marked changes in the advancing and receding contact angles are observed, and the hysteresis is actually absent. The stable pinning observed on the polished surface of a calcite crystal brought in contact with water leads to the appearance of contact angle hysteresis and hydrophilization of the calcite surface, with the hydrophilization being related to structural displacements in the crystal lattice.

The spontaneous growth of sessile air bubbles with sizes of 1 mm and below on a solid surface has been experimentally studied in ethanol and its aqueous solutions under constant external conditions (atmospheric pressure and a temperature of 20°С). A noticeable growth of bubble volume has been observed for at least 2 h in ethanol solutions beginning from an alcohol concentration of 50%. The time dependences of bubble volume have different patterns in ethanol solutions and undiluted alcohol.

A relatively new term of colloid science, “pinning,” characterizes the phenomenon when a drop or bubble, spreading over a solid surface, suddenly stops its movement at its base. Usually pinning is observed with an increase in the drop or bubble volume by pumping substances into them. In this paper, small but macroscopic air bubbles were studied at the water–silicon interface, and their volume increased in a non-contact way by increasing temperature. The observed phenomenon can be called temperature pinning. The experiments were carried out both on the natural hydrophilic surface of silicon wafers and on their surfaces artificially hydrophobized to various degrees in the temperature range of 20–75°C. In all cases, temperature pinning was observed in the initial temperature range from 20°C, but was also observed at other temperatures. The general conclusion is that temperature pinning is common and has various manifestations.

The goal of this work is the experimental verification of a recently formulated thermodynamic theory of the temperature dependence of sessile bubble contact angle [Rusanov, A.I., Colloid J. , 2020, vol. 82, p. 303]. In particular, the possibilities of an increase in small contact angles and a decrease in large ones (larger than 90°) are tested. The experiments are carried out with a bubble at a water–silicon interface using two types of polished silicon plates: clean plates with a water contact angle of 12° and plates with a hydrophobized surface and the water contact angle of 104° within a temperature range 20–80°C. In both cases, the experiments have confirmed the theory. In addition, a temperature pinning effect (anchoring of the three-phase contact line) has been revealed for the sessile bubble.

Although usual pressures have typically a weak effect on the properties of condensed phases and their surface layers, a parameter has been found in the surface physical chemistry—a contact angle at a three-phase contact line—that is rather sensitive to hydrostatic pressure. Experiments with an air bubble adhered to a solid surface immersed in water have shown that an increase in the hydrostatic pressure by less than two times causes a growth of the contact angle by more than 10°, if the angle is markedly smaller than 90°. Therewith, the three-phase contact line remains immobile, and only the liquid−gas interface changes its orientation. If the angle (no matter, acute or obtuse) is close to 90°, the three-phase contact line acquires mobility as an alternative way to reach an equilibrium . A thermodynamic theory has been developed on the basis of the generalized Young equation to explain these phenomena. It has been shown that, when the three-phase contact line is fixed, a growth of the pressure in a liquid always leads to a rise in the contact angle.

Исследовано коррозионное поведение сложнолегированного сплава в растворе 3М НСl при статической деформации переменного знака. Результаты, полученные методом кулонометрической регистрации продуктов коррозии в начальный период взаимодействия металла со средой, обеспечили количественную оценку специфики растворения сплава. Полученные результаты позволили объемным методом в сочетании с методом кулонометрической регистрации продуктов коррозии разработать методологию селективной оценки коррозионного поведения многофазных сплавов в условиях агрессивной среды REFERENCES Berenshtein G. V., Dyachenko A. M., Rusanov A. I. Mekhanohimicheskij effekt rastvoreniya [Mechanochemical effect of dissolution] Report Academy of Sciences of the USSR, 1988, v. 298 (6), pp. 1402–1404. (in Russ.) Rusanov A. I., Uriev N. В., Eryukin P. V., Movchan T. G., Esipova N.E. Effect of the strain sign in corrosion under stress. Mendeleev Commun., 2004, v. 14(2), рр. 58–59. https://doi.org/10.1070/mc2004v014n02abeh001875 Rusanov A. , Ur’ev N. B., Eryukin P. V., Movchan T. G., Esipova N. E. Otkrytie effekta znaka deformacii v yavleniyah korrozii pod napryazheniem [Discovery of the sign deformation effect in stress corrosion phenomena]. Report Academy of Sciences, 2004, v. 395(3), pp. 364-366. (in Russ.) Esipova N. E., Blinov E. B., Movchan T. G., Bannykh O. Corrosion Resistance of a Bent Plate from a High-Nitrogen Nonmagnetic 05Kh22AG15N8M2F Steel in Aggressive Media. Russian metallurgy (Metally), 2007(2), pp. 148-75. Movchan T. G., Esipova N. E., Eryukin P. V., Uryev N. B., Rusanov A. I. Mechanochemical effects in processes of corrosion of metals. Russian Journal of General Chemistry, 2005, v. 75(11), pp. 1681–1686. https://doi.org/10.1007/s11176-005-0491-8 Rusanov A. I. Termodinamicheskie osnovy mekhanohimii [Thermodynamic principles of mechanochemistry]. Saint Petersburg, Nauka Publ., 2006, 221 p. (in Russ.) GOST 9.908-85. Metals and alloys. Interstate standard. (in Russ.) Kuzmak A. E., Kozheurov A. V., Efi menko L. A., Ilyukhin V. I. Kulonometricheskaya ocenka korrozii okoloshovnoj zony svarnogo shva pri deformacionnom starenii [Coulometric corrosion assessment of the heat-affected weld zone during strain aging]. Korroziya: Materialy, Zashchita, 2009(1), pp. 43–46. (in Russ.) Kuzmak A. E., Kozheurov A. V. Kulonometricheskaya ocenka skorosti korrozii uglerodistoj stali [Coulometric corrosion rate assessment for carbon steel]. Zashchita Metallov, 2004, v. 40(3), pp. 315–320. (in Russ.) Kuzmak A. E., Kozheurov A. V., Marin A. V. Coulometric evaluation of infl uence of the welding technology of 12Kh18N10T steel on welded zone corrosion. Korroziya: Materialy, Zashchita, 2005(1), pp. 43-46. (in Russ.)

A set of equations describing the evolution of the volume and composition of evaporating sessile binary droplets consisting of infinitely miscible liquids has been employed to consider the evaporation dynamics of sessile droplets of aqueous solutions of sulfuric acid, 1-propanol, and ethanol in the atmosphere of humid air. It has been shown that experimentally observed features of droplet evolution may be explained with allowance for the nonideality of a solution in a droplet and the influence of thermal effects. In particular, the numerically obtained values of parameters at which the realization of this or that scenario is observed for the evolution of droplets of aqueous 1-propanol and ethanol solutions agree with the corresponding experimental data.

Experimental data have been obtained on the complete diffusion evaporation of a sessile microdroplet of an aqueous 1-propanol solution on a hydrophobized polished quartz substrate in air at atmospheric pressure and room temperature. During the evaporation of the sessile droplet, time dependences have been determined for its key thermodynamic and geometric parameters, i.e., the contact angle, base surface area, and volume. It has been revealed that the character of time variations in the contact angle depends on the initial alcohol concentration in the droplet and air humidity. At a high alcohol concentration and a low air humidity, the droplet contact angle monotonically decreases throughout the evaporation process. The contact angle of a solution droplet with a prevailing content of water varies in several stages. In this case, the monotonic reduction in the contact angle is, at a certain moment, replaced by a stage of its growth. The comparison of the maximum contact angle of an evaporating droplet with the contact angle of a sessile droplet of pure water enables one to determine the amount of alcohol in a studied droplet by the end of this stage. The residual alcohol amount governs the subsequent evolution of the droplet up to its complete evaporation.

Diffusion evaporation of a sessile binary droplet in an atmosphere of a noncondensable carrier gas has been considered. For a droplet consisting of two infinitely miscible liquids, a relation between the current values of solution concentration and volume of the droplet has been derived in an explicit form under the ideal solution approximation. It has been shown that the volume of a sessile binary droplet may, as well as the volume of a free binary droplet, vary nonmonotonically with time. The evaporation of a droplet of an aqueous sulfuric-acid solution has been considered in detail taking into account the nonideality of the solution. Time variations in the volume, base area, and contact angle have been experimentally measured for the sessile droplet of an aqueous sulfuric-acid solution on a hydrophobized substrate. The experimental data obtained at different initial humidities of water-vapor and droplet-solution concentrations have been analyzed within the theory of the stationary isothermal diffusion evaporation of a sessile binary droplet.

Three consecutive stages of evaporation of a sessile water microdroplet are studied both theoretically and experimentally under the conditions of contact angle hysteresis. The influence of thermal effects on the dynamics of droplet evaporation is quantitatively investigated on the basis of the obtained experimental results. The features of droplet evolution are analyzed at the final stage, when both the contact angle and the radius of the droplet base decrease with time. It is shown that evaporation at this stage also occurs in a steady-state regime, but the average droplet temperature approaches the ambient temperature.

The state of the surface of organic-inorganic coatings derived by sol-gel technology on a steel surface and their effect on the reduction of the air friction losses of a mini-turbogenerator rotor were studied. Two hypotheses about the effect of the surface condition on the reduction of the air friction losses—by improving the surface smoothness and by creating a superhydrophobic surface—were suggested and tested. Smooth epoxy-titanate coatings including those doped with a fluorine-containing organosilicon polymer and superhydrophobic surfaces based on hydrolyzed methyltriethoxysilane with an aerosol additive were tested on a model mini-turbogenerator test setup. It was established that the smooth coatings reduce the total friction losses of the mini-turbogenertor by ∼5%, while the superhydrophobic coatings reduce them by ∼7%.

Three compositions based on epoxide-siloxane, epoxide-titanate, modified epoxide-titanate sols have been synthesized by the sol-gel method, and the morphology of surface and the hydrophobicity of coatings obtained on their basis has been studied. According to the results of the studies, the compositions of the coatings have been chosen for the conduction of tests on a rotor of high-rpm miniturbogenerator, with the aim to determine the effect of aerodynamic losses.