Wettability Inversion of Aluminum-Magnesium Alloy Surfaces

The paper presents the experimental results on the use of low-temperature heating to reduce time of wetting inversion (from superhydrophilicity to hydrophobicity) of aluminum-magnesium alloy surfaces textured by laser radiation. Stable growth of the contact angle to 137.3-144.2° after heating surfaces (wettability properties deteriorate) was recorded. Wetting inversion from superhydrophilicity to hydrophobicity occurs in 2-3 hours of low-temperature heating of textured samples. The wettability inversion time depends on the type of texture. A significant increase in carbon content of elemental composition of the near-surface layer of samples after their low-temperature heating was registered

Wettability inversion of aluminum-magnesium alloy surfaces

The paper presents the experimental results on the use of low-temperature heating to reduce time of wetting inversion (from superhydrophilicity to hydrophobicity) of aluminummagnesium alloy surfaces textured by laser radiation. Stable growth of the contact angle to 137.3-144.2° after heating surfaces (wettability properties deteriorate) was recorded. Wetting inversion from superhydrophilicity to hydrophobicity occurs in 2-3 hours of low-temperature heating of textured samples. The wettability inversion time depends on the type of texture. A significant increase in carbon content of elemental composition of the near-surface layer of samples after their low-temperature heating was registered

Mechanism of Contact Line Movement of a Droplet Spreading Over a Solid Surface

The contact line movement, advancing and receding, of the distilled water droplet was studied on the polished and laser patterned aluminum surfaces. The obtained dependences of the dynamic contact angles on the contact line speed were fitted by the combined theory consisting of molecular-kinetic and hydrodynamic theories. Results obtained on laser patterned surface are found to fit better to the theory in comparison with polished surface. The assumption was made that this result is connected with the low contact angle hysteresis on the patterned surface compared to the polished one. It means that combined theory does not consider the contact angle hysteresis

Measurement of free surface energy of laser treated aluminum substrate

The change in the surface energy of the aluminum substrate after laser treatment has been determined experimentally. Surface energy is determined using the Advance KRUSS software. The laser treatment of the surface is found to increase its free surface energy. It changes the ratio of the polar and the dispersion components. Analysis of the crystalline planes of aluminum surface after laser treatment was provided by XRD X-ray diffractometry using a Shimadzu XRD 7000S diffractometer

Experimental study of evaporation of distilled water and 10% NaCl and СaCl[2] aqueous salt solutions droplets under their free falling on a heated surface

The paper presents the experimental results of evaporation of distilled water and 10% aqueous salt solutions of NaCl and CaCl[2] droplets under their free falling on a heated surface. It is proved that it is more expedient to conduct the experimental research in this field according to classical multifactorial experiment. Laser treatment of surfaces is found to increase the evaporation rate and to biases the point of boiling crisis in the region of lower surface temperatures. In this case, in the conditions of boiling crisis the frequency of contact of a droplet with a heated surface will decrease

Evaporation of Promising Fire Extinguishing Agent Droplets

Woodland fires are a major issue worldwide. The aviation method of extinguishing forest fires is one of the main ones. However, the use of the latter is carried out according to the results of experiments, bench or flight tests in the absence of models that adequately describe the mechanisms for suppressing the combustion of forest combustible materials with specific fire extinguishing compositions (solutions, emulsions or suspensions). Therefore, the task of studying the evaporation and interaction of single drops with the surfaces of combustible materials is relevant and practically significant in the field of fire hazards. The paper presents the experimental research findings on the evaporation of specialized composition droplets for extinguishing fires under different heat supply schemes. The compositions under study included a group of widely used fire extinguishing agents: water; flame retardant FR-Les (5% solution); FR-Les (20% solution); bentonite slurry (1%); bentonite slurry (5%); bischofite solution (5%); bischofite solution (10%); fire extinguishing agent OS-5 (5% solution); fire extinguishing agent OS-5 (10% solution); fire extinguishing agent OS-5 (15% solution); foaming agent emulsion (1%); foaming agent emulsion (5%); fire retardant (5% solution). Specialized composition droplets were heated using conductive, convective and radiant heating. Empirical coefficients were obtained

Influence of the Metal Surface Texture on the Possibility of Controlling the Phase Transition of Water Droplets in the Single-Phase Regime

We experimentally studied the influence of the texture of copper and steel surfaces on the possibility of controlling the phase transition of water droplets in the single-phase regime. The texture of metals was formed by polishing and grinding, which corresponded to the finishing treatment of heat transfer surfaces in cooling systems for energy-saturated equipment. The samples were studied by microscopy and profilometry. The texture was estimated by three-dimensional roughness parameters. It was found that, with a 2–2.5-fold increase in roughness, the wetting of copper deteriorates (the contact angle increases from 66° to 93°), whereas the wetting of steel improves (the contact angle decreases from 89° to 71°). It was experimentally proven that, among the two main factors that affect the spreading diameter (wetting and roughness), wetting is the most significant. A hypothesis was formulated regarding the reason for the increase in the contact angle of 7–10° and the drop in the decrease rate of the contact diameter during the transition from the pinning to the mixed stage of droplet evaporation. It was found that an increase in the surface area of 0.1% leads to an increase in the total droplet evaporation rate of 4–6.5%

Interaction of dropsof fire-extinguishing compositions with fragments of combustible materials

Актуальность. Наиболее распространенным огнетушащим составом для локализации и ликвидации возгораний природного и техногенного характера является вода. Практически ежегодное увеличение площадей возгораний бореальной зоны, а также удаленности очагов возгорания от водоисточников обуславливает необходимость повышения ресурсоэффективности тушения. Основной задачей при тушении техногенных возгораний является снижение расхода огнетушащих средств, что позволит снизить риск чрезмерного заливания водой помещений и сократить материальные затраты на восстановление объектов. Для этих целей целесообразно совершенствовать существующие и разрабатывать новые огнетушащие составы. Недостаточная изученность процессов, происходящих при взаимодействии капель различных типичных огнетушащих жидкостей с поверхностью горючих материалов, и высокая научно-практическая значимость подобных исследований обуславливает актуальность. Цель: установление закономерностей смачивания, растекания и испарения капель огнетушащих составов по поверхностям горючих материалов при кондуктивном нагреве. Объект: огнетушащие составы (вода; растворы антипирена, ФР-Лес, бишофита; суспензии бентонита; эмульсии пенообразователя) различной концентрации. Методы. Исследование процессов смачивания и испарения капель огнетушащих составов проводились методом «неподвижной» капли (размещенной на твердой поверхности с помощью электронного дозатора). Для определения геометрических характеристик капель применялась теневая оптическая система, полученные изображения обрабатывались с помощью специального программного обеспечения методами «тангенциальный 1» и «Юнга-Лапласа». Результаты. Установлены основные закономерности смачивания и испарения капель огнетушащих составов на поверхностях горючих материалов (листьев березы, древесины, ламинированной древесно-стружечной плите, линолеуме и поливинилхлориде). Показано, что при подавлении пиролиза горючих материалов в глубине слоя более эффективными будут составы с меньшим поверхностным натяжением, проникающие в более глубокие слои горючего материала. В таких условиях охлаждаются нагретые до высоких температур фрагменты материалов и подавляется процесс генерации горючего - газообразных продуктов термического разложения. Установлено, что доминирующим механизмом подавления горения и пиролиза при тушении горючих материалов (а именно при использовании антипиренов, бишофита, бентонита, ФР-Лес) является формирование защитного слоя на поверхности элементов веществ и материалов, а не поглощение теплоты при испарении растворителя. Зависимости средних по времени скоростей испарения капель огнетушащих составов от температуры нагрева поверхности описаны экспоненциальной кривой WE=a∙Tb.The relevance. The most common fire extinguishing composition for the localization and elimination of natural and man-made fires is water. Almost annual increase in the areas of fires in the boreal zone, as well as the remoteness of fires from water sources, necessitates an increase in the resource efficiency of extinguishing. The main task in extinguishing man-made fires is to reduce the consumption of fire extinguishing agents, which will reduce the risk of excessive water flooding of the premises and material costs for the restoration of facilities. For these purposes, it is advisable to improve existing and develop new fire extinguishing compositions. The insufficient knowledge of the processes occurring during the interaction of drops of various typical fire extinguishing liquids with the surface of combustible materials, and the high scientific and practical significance of such studies determines the relevance. Purpose: to establish the patterns of wetting, spreading and evaporation of drops of fire extinguishing compositions on the surfaces of combustible materials during conductive heating. Object: fire extinguishing compositions (water; flame retardant solutions, FR-Les, bischofite; bentonite suspensions; foam concentrate emulsions) of various concentrations. Methods. The study of wetting and evaporation of drops of fire extinguishing compositions was carried out by the method of a «fixed» drop (placed on a solid surface using an electronic dispenser). To determine the geometric characteristics of the drops, a shadow optical system was used, and the obtained images were processed using special software using the «tangential 1» and «Young-Laplace» methods. Results. The main patterns of wetting and evaporation of drops of fire extinguishing compositions on the surfaces of combustible materials (birch leaves, wood, laminated chipboard, linoleum and polyvinyl chloride) have been established. It is shown that in suppressing the pyrolysis of combustible materials in the depth of the layer, compositions with a lower surface tension, penetrating into deeper layers of combustible material, will be more effective. Under such conditions, fragments of materials heated to high temperatures are cooled, and generation of fuel - gaseous products of thermal decomposition - is suppressed. It was established that the dominant mechanism for suppressing combustion and pyrolysis when extinguishing combustible materials (namely, when using fire retardants, bischofite, bentonite, FR-Les) is the formation of a protective layer on the surface of elements of substances and materials, and not heat absorption during solvent evaporation. The dependences of the time-average evaporation rates of droplets of fire-extinguishing compositions on the surface heating temperature are approximated by the exponential curve WE=a∙Tb