The contact line motion in liquid-liquid-solid systems involves a complex interplay of surfactants and interfacial forces, which is a critical issue not yet fully resolved. While Marangoni flow, interfacial tension reduction, and surfactant adsorption are all implicated, their respective roles across different time scales remain contentious. To elucidate these dynamics, we investigated the coalescence between a surfactant-laden pendant drop and a pure water sessile drop on a substrate in decane. Using a high-speed camera and drop shape analyzer (DSA 100), we systematically analyzed the process across milliseconds to minutes. Experiments were conducted by employing a series of surfactants: anionic sodium dodecylbenzenesulfonate, cationic cetyltrimethylammonium bromide, zwitterionic lauramidopropyl hydroxysulfobetaine, and nonionic polyoxyethylene (20) sorbitan monolaurate (TW 20), on both hydrophilic and hydrophobic substrates. The results reveal a timescale-dependent transition in the governing mechanisms of contact line motion. The millisecond-scale dynamics are initiated by the coalescence-induced capillary waves and Marangoni flow. Subsequently, contact line motion transitions to a long-term stage (seconds to minutes) driven by surfactant adsorption at the solid-liquid interface. The overall behavior of the contact line results from synergistic effects of surfactant properties, concentration, and substrate wettability. Our work clarifies the apparent complexity by establishing a time-resolved framework: the sustained contact line motion is predominantly controlled by surfactant adsorption on the solid substrate, with Marangoni effects playing negligible roles in the long-term stage.

A comprehensive methodological approach is used to study bubble dynamics under the downward-facing substrate including high-speed imaging and interferometry simultaneously with shadow technique and infrared thermometry. A transparent indium-tin-oxide heater deposited on a sapphire substrate is utilized. This approach allows bubble and dry-spot dynamics, bubble coalescence, and the formation and evaporation of the microlayer to be investigated. The fluorocarbon liquid FC-72 is used as the working fluid. The study revealed significant nonlinear bubble and dry-spot growth rates with increasing heating power. The motion of the bubble relative to the heated substrate facilitates the formation of a microlayer at the bubble's periphery. It was observed that bubbles moving at higher velocity along the substrate at the same heating power experienced faster growth, apparently due to an increase in the area of the microlayer. It is established that the coalescence of bubbles results in the formation of a microlayer under the resulting bubble. The dynamics of the microlayer after coalescence is studied using interferometry. A significant increase in the evaporation rate of the microlayer with increasing heating power is observed. The considered processes play an important role in the microgravity conditions where, due to the lack of buoyancy force, the bubble does not detach from the heater. The presented experimental results can be useful for a deep understanding of bubble dynamics under the heated downward-facing substrate and for the future model development.

The liquid film driven by a turbulent gas flow in a flat minichannel under conditions of drop entrainment is investigated using a two-dimensional numerical model which employs a coupled volume of fluid and level-set (CVOFLS) method for interface capturing and the large eddy simulation (LES) approach with the Smagorinsky model to resolve turbulence. The results confirm that the transition from large-scale ducts to minichannels leads to a significant suppression of fast ripple waves on the rear slopes of disturbance waves, consistent with previous findings. However, the use of LES enables the resolution of these small-amplitude waves, revealing their increased frequency in minichannels. The analysis of the Q-criterion for vortex cores demonstrates that the generation of both initial waves near the liquid inlet and ripple waves on disturbance waves is driven by small-scale eddies in the gas phase. Furthermore, the model predicts the existence of vortical flow within the liquid phase of the disturbance wave itself and near the liquid inlet, which promotes the formation of ripple waves and initial waves, respectively. Thus, a qualitative similarity was observed between the formation of initial waves near the liquid inlet and the generation of ripple waves on the disturbance wave.

A series of boiling experiments was conducted on a single artificial nucleation site aboard the International Space Station (ISS) using the Multiscale Boiling (RUBI) facility. These experiments aim to elucidate the mechanisms of bubble nucleation and growth during boiling under microgravity conditions, where the effects of gravity and natural convection are eliminated. This unique environment allows bubbles to grow to sizes unattainable under terrestrial conditions. The bubble dynamics were monitored using a side-view black-and-white camera and a bottom-view infrared camera observing through a transparent heated substrate. This study focuses on the results of a single-bubble pool-boiling experiment, with particular attention to the influence of varying levels of liquid subcooling. The experimental findings are supported by numerical simulations based on a previously developed model. Certain observed phenomena, such as a bubble avoiding collapse and then resuming its growth, were found to be hardly explainable without presuming the presence of non-condensable residuals, in spite of a careful degassation of the working FC-72 liquid. The model was modified accordingly to test such a picture of the phenomenon, which included the thermal Marangoni (thermocapillary) convection as a consequence of non-condensables.

Subcooled water flow boiling experiments were carried out in flat mini-and micro-channels under intense uniform and nonuniform heating for different heater surface morphology, including two-dimensional (graphene) and three-dimensional (porous medium) coatings. Heat transfer coefficient and critical heat flux data were obtained on 3 x 3 and 10 x 10 mm heaters in channels with a height range from 0.55 to 2 mm. In experiments on flow boiling, it was found that with an increase in the channel height in the range of 0.55 to 2 mm, the critical heat flux increases significantly. For a given fluid flow rate and channel height, the critical heat flux on the 3 x 3 mm2 heater exceeds the heat flux on the 10 x 10 mm2 heater. It was found that in the studied range of parameters the behavior of boiling curves is similar, both with and without graphene coating. In turn, porous coatings (compared with a bare heater) provide significant enhancement of heat transfer and increase the critical heat flux.

In this work, a model for direct numerical simulation of the boiling process on three-dimensional porous surfaces is proposed and tested. To solve this problem, the VOF method was applied using the CSF method to simulate surface tension. Images of formation of vapor bubbles and value of the heat transfer coefficient were obtained depending on substrate temperature, pore material, backfill geometry, and contact angle.

A 3D-modeling of the changes in the temperature field of the most important elements of the wiggler workstation vacuum windows of the Siberian Circular Photon Source SKIF synchrotron was carried out with a decrease in the pressure of cooling water and the window structure water flow rate, up to an emergency stop of the pumping station. It has been established that if the pumps are turned off, the maximum temperature in the diamond foil will rise above 372◦C, which can cause recrystallization of the diamond and rupture of the foil. At the same time, boiling will not occur on the minichannel walls. However, the temperature in the liquid metal will increase by 37◦ . This can significantly increase the diffusion of liquid metal components into the polycrystalline copper flange and diamond foil, which can also lead to rupture of the foil. Recommendations for preventing these accidents are presented.

The article is devoted to the synthesis of nanocarbon coatings and their application in systems with heat exchange between a wall and liquid or vapor, including phase transition; i.e. boiling and condensation. Using droplet image processing, the wettability of samples with vertically aligned carbon nano-tube arrays and graphene was investigated. The paper presents qualitative results for liquid behavior on the coating of a carbon nanotube "forest" obtained by visualizing the thermocapillary rupture of liquid films and vapor condensation on this coating. The main influence on the behavior of liquid drop-lets and films on nanotube arrays was apparently exerted by the air inside the coating, which provided hydrophobicity. However, air could be displaced from the coating by water as a result of annealing of the sample or condensation of vapors inside the coating, even with the original (nonannealed) sample, and hydrophobicity was lost. Also, using high-speed visualization (100,000 fps) of water flow boiling in a minichannel, a significantly higher number of vapor bubbles were recorded on graphene compared to bare copper, indicating its potential for heat transfer enhancement.

This paper proposes to use a methodology for measuring deformations of solids that has important advantage over similar methods of this class, such as Phase measuring deflectometry and digital image correlation (DIC) based deflectometry which are known to require complex and labor-intensive calibration schemes, sometimes involving additional reference surfaces. We propose to use the reflective (moon-glade) background oriented schlieren (BOS) technique for measuring relative deformations of mirror surfaces. This method employs cross-correlation analysis of two reflected images of the background dot pattern from the mirror surface, similar to DIC-based deflectometry. The difference lies in the procedure of deformation field recovery, which is performed by solving the Poisson equation with the corresponding boundary conditions. A key advantage of the approach is the trivial calibration, requiring resolution in the background dot pattern plane and the mirror surface plane. Relative deformations of a steel specular substrate, induced by heating with a copper rod at its center, were measured. The results indicate that the resolution of this approach is approximately 50 nm. Measurement reliability was further validated by comparing the data with a commercial confocal optical sensor of the Micro-Epsilon, showing good agreement.

This paper is devoted to the study of the dynamics of vapor bubbles during water flow boiling in a graphene-coated minichannel. Graphene was synthesized directly on the surface of the boiling surface. It was found that applying a graphene film to a copper surface does not lead to a significant change in wettability with water. During the experiments, a significantly larger number of bubbles were observed on the graphene-coated surface than on the surface without graphene.

The paper investigates a levitating array of microdroplets above a heated layer of distilled degassed water with an area of 40×40 mm2. The droplets are distributed unevenly, and most of them accumulate at the edges of the test section. The coalescence of droplets with the liquid layer is studied. It is shown that with increasing liquid layer temperature, the average diameter of the coalescing microdroplets increases.

The results of physical and mathematical modelling in the ANSYS software package of three-dimensional unsteady coupled heat and mass transfer in the previously developed diamond vacuum window (DVW) of the workstation of the Siberian Circular Photon Source under construction are presented. Previous simulations have convincingly shown: the possibility of using a mini-channel cooling system to implement reliable and proper temperature management of this optical device, the advantage of a mini-channel cooling system over a one channel system in terms of fulfilling the optical requirements for DVW and its design. Here are detailed calculations of unsteady three-dimensional flow in the mini-channels of the cooling system, which will prevent possible dramatic destruction of the workstation due to local overheating of the CVD-diamond foil and its recrystallization.

There is a lack of understanding of the physical mechanisms involved in the rupture of a liquid film on a solid surface. This is in part due to the large number of parameters of the liquid film—substrate system on which this complex process is dependent. Due to the thermocapillary effect, local heating can have a destabilizing effect on the stability of the film. In this work, using a numerical model verified by the experimental data, the influence of parameters not available for variation in the experiment has been studied. It was found that the formation of dry spots in a locally heated liquid film occurs only under the influence of thermocapillary forces. In turn, the expansion of the dry spots takes place under the influence of the capillary forces. Thus, until the dry spot appears, the rupture does not differ on either hydrophilic or superhydrophobic substrates, but the contact line velocity during dry spot growth is extremely sensitive to the contact angle.

Abstract A series of boiling experiments on a single artificial nucleation site has been carried out on the International Space Station (ISS) in the framework of the Multiscale Boiling (RUBI) setup installed in space in 2019. The aim of these unique experiments is to understand the bubble nucleation-growth mechanisms involved in heat transfer during boiling under well-controlled operating conditions without being masked/impeded by gravity and natural convection. The bubbles can grow to large sizes that are inaccessible in the presence of gravity. They are observed by a side-view black-and-white camera and by an infrared camera through a transparent heated substrate. In this paper, we present the results of a single-bubble pool-boiling experiment with an emphasis upon the role of the surrounding liquid subcooling, several values of which are investigated. The experimental results are complemented by numerical simulations using a previously developed model. However, the model is slightly modified to account in a simplest way for possible non-condensable residuals, without which we would have a hard time to explain certain observed behaviors.

Numerical solutions of the three-dimensional equations for Rayleigh-Benard convection in extremely thin layers of different liquids (water and Fluorinert Electronic Liquid FC-72 coolant) uniformly heated from below are presented. The relative size of the liquid layer Gamma = D/H varied from 8 to 400. The sizes of ordered Rayleigh-Benard convective cells were determined, which for water are about 0.42 mm with a layer diameter of 40 mm and a layer height of 0.4 mm, which is comparable with the minimum experimentally recorded transverse size of a monolayer of hovering micro droplets above an evaporating liquid layer in the atmosphere. The effect of temperature difference and liquid layer height on heat transfer was studied. The relative contribution of buoyancy forces and the thermo capillary effect was investigated. The temperature difference at which the transition to non-stationary flow regimes occurs was determined for water. Analytical approximations of the integral heat flux from the free surface of the liquid layer are presented.

To date, in the public domain there is a lot of experimental data on adiabatic two-phase flow in microchannels of various geometries (round, rectangular, square). However, microchannels with high aspect ratio (slit microchannels) remain poorly understood. In this article, an adiabatic two-phase flow in a slit microchannel with a height of 12.5 mu m and width of 10 mm (aspect ratio 1:800) has been experimentally investigated for the first time in the wide range of liquid and gas superficial velocities (from 0.0026 to 0.266 m/s for liquid and from 0.266 to 7.73 m/s for gas). The combination of photolithography, anisotropic etching and thermal anode bonding allow us to create flow cells with slit microchannel height of 12.5 mu m and width of 10 mm. Channel height and characteristics of channel surfaces (roughness and contact angles) have been measured using scanning electron microscopy (SEM) stylus profilometer, atomic force microscopy (AFM) and sessile drop technique respectively. For the first time, the new flow pattern and flow patterns transitions have been investigated and flow pattern maps for several liquids with various physical properties have been constructed. One of these maps has been compared with a map of a slit microchannel with large height. It is shown that the flow pattern maps for liquids that completely wet the mixing section can be generalized in coordinates liquid Capillary number Cal and gas Weber number Weg, which indicates the negligible effect of liquid inertia forces on the flow pattern boundaries. It has been shown that with an increase of surface tension, the region of the Jet flow pattern expands. It has been established that Saffman-Taylor instability in the mixing section and along the channel, instability caused by transverse pressure gradient and surface wettability are pattern-determined factors.

The present work is devoted to the study of heat fluxes from a heating surface into a liquid during single vapor bubble growth. The experiment was conducted as part of the RUBI (Reference mUltiscale Boiling Investigation) boiling project implemented on the International Space Station between 2019 and 2021. Since microgravity increases the spatial and temporal resolution of the problem, this made it possible to study in detail the processes of heat and mass transfer in the area of a three-phase contact line. The paper presents a developed algorithm for determining heat fluxes and the amount of heat transferred from various zones at different experimental parameters: from the contact line area, from the centre under the bubble and from the liquid-vapor interface. In particular, it is shown that it is in the area of the contact line that the maxima of heat fluxes are observed and that evaporation from this area makes a significant contribution to the growth of the bubble (about 50%).

The evolution of the flow structure of thermal gravity-capillary convection in thin cylindrical water layers with a free surface, heated from below, was numerically studied. The layer diameter ranges from 10 mm to 40 mm, and the height ranges from 0.1 mm to 5 mm. The influence of the layer height and temperature difference on heat transfer was investigated, and the boundaries for the transition to nonstationary flow regimes were determined. The relative contributions of buoyancy and thermocapillary effects were examined, and analytical dependencies for heat dissipation from the free surface as a function of temperature difference and layer height were constructed. The dimensions of the ordered Rayleigh-B & eacute;nard convective cells were determined to be approximately 0.42 mm for a diameter of 40 mm and a layer height of 400 mu m, which is comparable to the size of monolayer levitating microdroplets above an evaporating liquid layer in the atmosphere.