Dynamic Contact Angle Research Articles

Dynamic Wetting of Ionic Liquid Drops on Hydrophobic Microstructures.

Ionic liquids (ILs)─salts in a liquid state─play a crucial role in various applications, such as green solvents for chemical synthesis and catalysis, lubricants, especially for micro- and nanoelectromechanical systems, and electrolytes in solar cells. These applications critically rely on unique or tunable bulk properties of ionic liquids, such as viscosity, density, and surface tension. Furthermore, their interactions with different solid surfaces of various roughness and structures may uphold other promising applications, such as combustion, cooling, and coating. However, only a few systematic studies of IL wetting and interactions with solid surfaces exist. Here, we experimentally and theoretically investigate the dynamic wetting and contact angles (CA) of water and three kinds of ionic liquid droplets on hydrophobic microstructures of surface roughness (r = 2.61) and packing fraction (ϕ = 0.47) formed by micropillars arranged in a periodic pattern. The results show that, except for water, higher-viscosity ionic liquids have greater advancing and receding contact angles with increasing contact line velocity. Water drops initially form a gas-trapping, CB wetting state, whereas all three ionic liquid drops are in a Wenzel wetting state, where liquids penetrate and completely wet the microstructures. We find that an existing model comparing the global surface energies between a CB and a Wenzel state agrees well with the observed wetting states. In addition, a molecular dynamic model well predicts the experimental data and is used to explain the observed dynamic wetting for the ILs and superhydrophobic substrate. Our results further show that energy dissipation occurs more significantly in the three-phase contact line region than in the liquid bulk.

Polyacrylic Acid Copolymers as Adhesion‐Adapted Model Materials for Cleaning Tests

AbstractFor cleaning tests, which are important in many industrial processes, there are no test contaminations with which geometry‐independent homogeneous films can be produced. Polymer films can fill this gap when they are able to mimic the properties of reference contaminants. For this purpose, acrylic acid (AA) copolymers are prepared by conventional atom transfer radical copolymerization and subsequent acidolysis of the tert‐butyl acrylate (tBA) prepolymers. In this way polarity and adhesion can be adjusted by the amount of carboxy groups in the polymer. Dynamic contact angle measurements show that the advancing contact angle of acrylic acid/methyl(meth)acrylate statistical copolymer layers increase with decreasing carboxy group content. Cleaning experiments show accordingly that the amount of residual polymer of coatings made from these polymers is dependent on the absolute number of adhesive carboxy groups in the polymer. Therefore, for an adaption of the polymer layers to hydrophilic reference contaminants it is necessary to incorporate additional hydrophilic monomer components into the polymers, which lead to lower contact angles but also stronger adhesion of the layers. Hence, it is shown that the chosen polymer concept is delivering the leverages to adjust the properties of appropriate test contaminants.

Understanding the impact dynamics of droplets on superhydrophobic surface

The dynamics of droplets impacting superhydrophobic surfaces is investigated using experiments and numerical simulations. The superhydrophobic surface is fabricated by growing the carbon nanotube forest on stainless steel mesh in order to perform experiments. To numerically model the surface, Kistler dynamic contact angle model is used. Depending on different physical properties such as drop Weber number, the inclination of the surface and Ohnesorge number, the impact outcomes are categorized into symmetric bouncing, asymmetric bouncing, breakup, and tail impingement. It is observed that with the increase in the inclination angle of the surface, the contact time of droplets on the surface decreases drastically. The energy budget of impacting drops is analyzed for the entire evolution cycle to delineate the process of drop deformation and its bouncing. It is observed that the droplets bouncing off the inclined surface carried more kinetic energy compared to those of their counterparts on horizontal surfaces. To address the bouncing phenomena, co-efficient of restitution of the drop is also explored. The inclination of the substrate is found to increase the co-efficient of restitution of the drop, which is directly related to the energy budget of the bouncing drop. Furthermore, the force exerted by a drop on such surfaces is also analyzed, showing two peaks, making superhydrophobic surfaces more prone to damage in comparison to hydrophilic surfaces, which showed only a single peak. It is observed that the magnitudes of the peak forces are smaller for oblique impact as compared to horizontal impact.

Effect of warm mix additives and hydrated lime on viscosity and bonding–debonding behaviour of RET and PPA modified asphalt binder with aggregates

Higher energy requirement for asphaltic layer construction with asphalt binder containing reactive elastomeric terpolymer (RET) and poly phosphoric acid (PPA) can be addressed to a greater extent with the addition of warm mix asphalt (WMA) additives. However, moisture damage related problem is one of the concerns with such addition, which further aggravates with an increase in ageing level. The addition of lime may help in addressing moisture damage related problems in such cases. Previous research works show that based on the interaction between asphalt binder and particular WMA additive, the viscosity may undesirably increase also. So far, the influence of different WMA additives (wax, chemical surfactant (CS) and water-based (WB)) on the workability related properties of asphalt binder containing RET and PPA is not understood. It is also not clear if such warm mix additives with/without lime can help in improving long-term bonding with different aggregate types or not. Therefore, this research work aimed at identifying the appropriate WMA additive (with/without lime) which can help in (a) addressing the workability related problem and (b) improving long-term bonding with different aggregate types. To achieve this, different types of WMA additives (with/without lime) were added to RET and PPA modified binder. Viscosity-based measurement and chemical analysis using Fourier transform infrared spectroscopy were performed to understand workability and underlying chemical changes. Results indicated that although wax and CS-based warm mix additives can help in reducing energy demand, the addition of WB warm mix additive will have a negative impact from the workability perspective. Likewise, dynamic contact angle for different binder combinations was measured to evaluate the surface free energy components, which further helped to examine the long-term bonding with different aggregate types. Among different WMA additive types, the use of wax-based WMA additive was found to be most effective in improving compatibility with different aggregate types. Also, irrespective of the binder combination, the addition of lime improved the long-term bonding with considered aggregate types.

Controlling post-impact dynamics of ferrofluid droplets with magnetic field

Preventing droplets from splashing after impacting a thin layer or liquid pool is crucial in reducing cross-contamination, the spread of pathogens, and rough surfaces in three-dimensional (3D) printing. In this article, we demonstrate that an external magnetic field can be useful to actively control the post-impact dynamics of ferrofluid droplets. A simplified lattice Boltzmann method (SLBM) is applied to simulate the flow field with lower computational cost. For the magnetic field, a self-correcting procedure is coupled with SLBM by setting a permanent magnet of desired magnetic field strength at any location of the computational domain. In this article, four different phenomena are simulated, including static contact angle, dynamic contact angle, splashing droplet on a thin layer, and falling droplet into a pool same liquid. From the first two examples, it is deduced that a vertical non-uniform magnetic field not only controls the spreading diameter and apex height but also the puffy shape appearing at droplet laterals. In examples three and four, even more intricate crown structures and wave propagation are successfully controlled with the help of a permanent magnet. It is also discovered that a magnetic field introduced at an optimal starting moment improves control and speeds up the whole procedure.

Hydrophobic and Hydrophilic Solid-Fluid Interaction

We propose a novel solid-fluid coupling method to capture the subtle hydrophobic and hydrophilic interactions between liquid, solid, and air at their multi-phase junctions. The key component of our approach is a Lagrangian model that tackles the coupling, evolution, and equilibrium of dynamic contact lines evolving on the interface between surface-tension fluid and deformable objects. This contact-line model captures an ensemble of small-scale geometric and physical processes, including dynamic waterfront tracking, local momentum transfer and force balance, and interfacial tension calculation. On top of this contact-line model, we further developed a mesh-based level set method to evolve the three-phase T-junction on a deformable solid surface. Our dynamic contact-line model, in conjunction with its monolithic coupling system, unifies the simulation of various hydrophobic and hydrophilic solid-fluid-interaction phenomena and enables a broad range of challenging small-scale elastocapillary phenomena that were previously difficult or impractical to solve, such as the elastocapillary origami and self-assembly, dynamic contact angles of drops, capillary adhesion, as well as wetting and splashing on vibrating surfaces.

Thermally driven Marangoni effects on the spreading dynamics of droplets

The influence of the thermally-induced Marangoni stresses on the falling and spreading dynamics of a droplet surrounded by another liquid phase is numerically studied. A water droplet is released in a three-dimensional (3D) domain filled with oil. The droplet descends, comes at an apparent resting position above an oil film close to the bottom surface, and eventually wets the solid surface when the underneath film ruptures. A linear vertical temperature gradient is applied in the solution domain (with the bottom surface as the cold side) which imposes a surface tension gradient across the oil-water interface. The Marangoni source term is added to the momentum equation coupling the momentum and the energy equations. It is assumed that the dynamic contact angle changes according to the Kistler relation during the spreading. The Volume of Fluid (VOF) method is used to capture the interface between the phases. The solver is validated for both the falling and spreading phases. During the buoyancy-driven falling regime, the droplet retains its spherical shape at the low Reynolds number O(1) and finite Bond number O(0.001). It is revealed that the spreading rate of the droplet is a decreasing function of Marangoni number (Ma). Unlike the isothermal systems, where the bottom side of the droplet becomes slightly flattened at the resting stage, the Marangoni stress imposes an upward force on the droplet which elongates the shape of droplet in the temperature gradient direction. Consequently, the ultimate spreading radius at the equilibrium state is smaller at larger Ma. The slow rate of spreading and also the small wetted area at large temperature gradients adversely affect the heat transfer rate from the liquid to the cold plate where the local convective heat transfer coefficient and the average Nusselt number (Nu) are decreasing functions of the Marangoni stress.

Self-climbing of a low surface tension droplet on a vertical conical surface

The anti-gravity self-climbing behavior of an axisymmetric barrel-shaped droplet with low surface tension on a vertical conical surface was investigated. Parabolic equations, circular equations and linear equations were proposed to build the analytic expressions of the two-dimensional gas-liquid interface, the final advancing-/receding-end positions were solved numerically. The results showed that the apparent dynamic contact angles of the droplets became greater with the increase of volume, and were larger than the Young’s contact angle. The realizable highest self-climbing height of the droplet was increased monotonically with the decrease of the surface tension. The deposited pre-wetting film on the conical surface reduced the contact line pinning force of the droplet, which could dramatically facilitate the self-climbing of the droplet when the contact line pinning force played a more important role in the resistance than the gravity. Compared to the circle-shaped and cone-shaped gas-liquid interface of the droplet at the final self-climbing position, the droplet thickness and the self-climbing height of the droplet in a parabolic shape were much closer to that of experiments, and the relative differences between the theoretical results and the experimental results were lower than 20%. Moreover, the surface free energy of the droplet calculated based on a parabolic shape was close to that form SE simulations. The findings in this work help to manipulate the self-climbing behavior of low surface tension droplets in various potential applications.

Phase-field simulations of droplet impact on superhydrophobic surfaces

Although the impingement of droplets on superhydrophobic surfaces has been extensively studied, the critical role of the air is typically ignored, leading to some paradoxical conclusions. In this paper, a phase-field model with dynamic contact angles is implemented to simulate the impingement of droplets on a superhydrophobic surface. The simulation results agree well with the experimental data. The droplet's impact on the surface is observed and divided into four categories: contactless bouncing, wet bouncing, dry-out bouncing and non-bouncing. The role of air was emphasised by analysing pressure distribution and velocity field corresponding to the droplet evolution, revealing the mechanism behind the droplet impact. A non-monotonically varying trend of the non-dimensional maximum wetting diameter is observed by increasing Ohnesorge number (Oh). The value of dimensionless viscous extension first increased and then decreased with increasing impact number, and this counter-intuitively outcome has not been reported previously. Non-dimensional contact time of a bouncing droplet decreases rapidly with increasing Weber number (We), but approaches a plateau value if We is greater than 1. Additionally, for liquids with different viscosities, the non-dimensional contact time of bouncing droplets is independent of Oh. Moreover, regime maps of the dynamic behaviours of a droplet impact are developed to present an intuitive interpretation of different droplet fates.

Hydroxyapatite/TPU/PLA nanocomposites: Morphological, dynamic-mechanical, and thermal study

Abstract Nanocomposites based on thermoplastic polyurethane (TPU) and poly lactic acid (PLA) with different weight ratios of 10:90 and 30:70 were prepared by solution method. Hydroxyapatite nanoparticles (n-HA) were used to enhance the physical and mechanical properties of the alloys. To prepare the nanocomposites, the percentages of n-HA varied between 1% and 5%. Different tests were used to investigate the properties of these nanocomposites. Scanning electron microscopy (SEM) analysis, which was used to study the morphology of the blends, showed that there were rough morphologies in between materials, and the results of the mapping test showed that the dispersion of nanoparticles in the polymer matrix was almost good. TGA thermal degradation test showed that the presence of TPU to some extent can affect the thermal stability properties, and with the increase in this material, the thermal properties are strengthened. The crystalline behavior of the samples showed that the presence of TPU and n-HA nanoparticles had negative effects on the crystalline properties. The study of viscoelastic behaviors showed that the presence of TPU enhances the viscous behavior in the sample and decreases the glass transition temperature, while the presence of nanoparticles increases the elastic properties and glass transition temperature. Tensile test showed that the presence of n-HA has a greater effect on the mechanical properties. Dynamic contact angle analysis using water and dimethylformamide (DMF) solvent showed that the existing TPU and n-HA led to major changes in the interaction surface of scaffolds. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis showed that increasing the amounts of TPU and HA increases cell viability. SEM cell interactions analysis showed that the surfaces of PLA90TPU10H5 and PLA70TPU30H5 samples are very good for the preparation of bone tissue scaffolds. Our findings indicated that the addition of n-HA into PLA/TPU blends could impart new features to the PLA matrix as a promising candidate for bone, cartilage, and tendon tissue engineering.

Dynamic wetting characteristics during droplet formation in a microfluidic T-junction

• Dynamic wetting characteristics during droplet formation in a microfluidic T-junction are investigated. • The three-phase contact line dynamics are influenced by the channel wettability and other flow properties. • The dynamic contact angle increases with an increase in Capillary number, static contact angle, and dispersed phase viscosity. • In contrast it decreases with an increase in flow rate ratio and slip length. Droplet formation in a microfluidic T-junction is dominated by the interfacial force arising between the two fluids. The three-phase contact line at the channel wall is affected by the surface energy or the wettability of the channel wall and other hydrodynamic forces. The dynamics of a three-phase contact line can be evaluated in terms of dynamic contact angle, which a moving contact line makes with a wall. The present work investigates the influence of wall hydrophobicity and fluid properties on the three-phase contact angle during droplet formation in a microfluidic T-junction. Experimental and numerical investigations are performed to study the effects of flow properties on the dynamic wetting of the channel during the droplet formation process. A level-set-based numerical model is employed to predict the moving contact line for different operating parameters and wettabilities of channel walls. Further, a simplified analytical model is developed to express the influence of flow parameters on the dynamic wetting behavior of the wall. The interface is more stressed out for a low viscous dispersed fluid, whereas the interface is less affected by a high viscous dispersed phase fluid. During the transient droplet stage, the dynamic contact angle increases with time with the increase in channel wall hydrophobicity. The dynamic contact angle increases with time for a higher static contact angle, and the variation decreases at a lower static contact angle. At the steady-state, the dynamic contact angle increases with the static contact angle and dispersed phase viscosity. In contrast, it decreases with an increase in the flow rate ratio and the slip length. The droplet with a higher length tends to increase the wetting line in the channel wall. The wettability of the channel wall influences the droplet interface along the lateral direction resulting in corner leakage flow, which further affects the dynamic contact angle.

Design, preparation, and performance characterization of low temperature environment-resistant composite resin matrix in rocket fuel

Cryogenic technology is a vital part of our society, not only in our lives, but also in the field of cutting-edge technology. The research and application of cryogenic technology is involved in many important projects in many countries and even in the world, such as aviation, aerospace, energy, transportation, and medicine. With its high specific strength and high specific modulus, carbon fiber reinforced resin matrix composite materials have gradually become the key material for aerospace vehicles, with significant advantages in reducing structural weight and improving structural efficiency. However, in the ultra-low temperature environment such as liquid hydrogen and liquid oxygen, the overall structure of carbon fiber reinforced resin matrix composites is severely tested by the environment, and it is extremely important to evaluate the mechanical properties of the composites under ultra-low temperature due to the difference of their material structure and properties from those of traditional materials, and the difference of thermal expansion coefficients between the reinforcing material carbon fiber and the resin matrix in rocket fuel. In this paper, the epoxy resin-based composite system was prepared by modifying TDE-85 epoxy resin with low-viscosity cyanate resin through molecular structure design, which is suitable for ultra-low temperature environment. The surface tension and dynamic contact angle of the modified epoxy resin are better than those of the pure epoxy resin, and it can form a good infiltration with carbon fiber and the interfacial properties of the composite are excellent. Secondly, the modified epoxy resin-based composite unidirectional plate was prepared by wet winding molding process, and the low-temperature mechanical property test specimens were prepared according to the relevant test standards. The mechanical properties were tested at −196℃, −150℃, −90℃ and room temperature 25℃ to obtain the low-temperature mechanical properties of the composite system, which provided the basis for the design of the composite system in the ultra-low temperature environment. Finally, the microstructure of the epoxy matrix composites was characterized by SEM method after the different temperature tests. The material structure, morphology, and composition were characterized by field emission scanning electron microscopy (FESEM, M400 FEI) with energy dispersive X-ray spectroscopy (EDS). In this paper, TDE-85 epoxy resin was modified with low viscosity cyanate resin to produce a modified epoxy resin suitable for ultra-low temperature environment, and the process properties of the epoxy resin were characterized. The surface tension of the modified epoxy resin was 43.405 mN/m, which was significantly lower than that of the pure epoxy resin at room temperature of 48.814 mN/m. Therefore, the modified epoxy resin had better flowability and required less time to wet the fibers during the molding process, resulting in higher molding efficiency.

Modeling of self-driven directional movement of underwater oil droplets on bio-inspired nano-coated 3D-printed conical models

The contamination of water with oil from industrial processes has become a worldwide problem that requires the development of efficient, reliable and economical separation methods. Inspired by droplet movements on spider silk, we present in this contribution a model of the separation of oil and water that is based on the effect that liquid droplets move on cone-shaped structures, driven by capillary forces. We extend existing models to barrel-shaped oil droplets that move, completely surrounded by water, along suitably shaped filaments and derive the forces acting on the droplets from first principles of physics. By bringing together the results of the theoretical model and the corresponding experiments, the underlying mechanism of the directional movement of droplets in another liquid medium is thoroughly investigated and modeled. The resulting model predicts the dependence of the droplet motion on time and on “system defining” parameters, such as droplet size, viscosity, and dynamic contact angles formed between droplet and filament surface. We expect this work to pave the road for the design and preparation of bionic structures in the field of droplet transport phenomena, which is of great interest in various applications.

Improved contact angle measurement in multiphase lattice Boltzmann

Contact angle is an essential parameter to characterize substrate wettability. The measurement of contact angle in experiment and simulation is a complex and time-consuming task. In this paper, an improved method of measuring contact angle in multiphase lattice Boltzmann simulations is proposed, which can accurately obtain the real-time contact angle at a low temperature and larger density ratio. The three-phase contact point is determined by an extrapolation, and its position is not affected by the local deformation of flow field in the three-phase contact region. A series of simulations confirms that the present method has high accuracy and gird-independence. The contact angle keeps an excellent linear relationship with the chemical potential of the surface, so that it is very convenient to specify the wettability of a surface. The real-time contact angle measurement enables us to obtain the dynamic contact angle hysteresis on chemically heterogeneous surface, while the mechanical analyses can be effectively implemented at the moving contact line.

Organosilicon-Based Plasma Nanocoating on Crust Leather for Water-Repellent Footwear.

In this study, functional nanocoatings for water-repellent footwear leather materials were investigated by chemical plasma polymerisation by implanting and depositing the organosilicon compound hexamethyldisiloxane (HMDSO) using a low-pressure plasma system. To this end, the effect of monomers on leather plasma deposition time was evaluated and both the resulting plasma polymers and the deposited leather samples were characterised using different experimental techniques, such as: Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). In addition, leather samples were tested by standard tests for color change, water resistance, surface wetting resistance and dynamic water contact angle (DWCA). The resulting polysiloxane polymers exhibited hydrophobic properties on leather. Furthermore, these chemical surface modifications created on the substrate can produce water repellent effects without altering the visual leather appearance and physical properties. Both plasma coating treatments and nanocoatings with developed water-repellency properties can be considered as a more sustainable, automated and less polluting alternative to chemical conventional processing that can be introduced into product-finishing processes in the footwear industry.

Quantifying the dynamic spreading of a molten sand droplet using multiphase mesoscopic simulations

Molten sand droplets deposited on gas turbine engine components cause severe damage. To better understand the deposition process, we study the wetting dynamics of a highly viscous molten sand droplet on a smooth substrate. A universal contact line spreading, independent of the droplet size, is observed with the characteristic inertial and viscous spreading regimes. It is also shown that the dynamic contact angle is in excellent agreement with Cox's theory.

Effect of inertia on the dynamic contact angle in oscillating menisci

The contact angle between a gas–liquid interface and a solid surface is a function of the dynamic conditions of the contact line. Classic steady correlations link the contact angle to the contact line velocity. However, it is unclear whether they hold in the presence of inertia and the case of perfect wetting fluids. We analyze by means of experiments the shape of a liquid interface and the corresponding contact angle in accelerating conditions for two different fluids, that is, HFE7200 (perfect wetting) and demineralized water. The setup consists of a U-shaped quasi-capillary tube in which the liquid column oscillates in response to a pressure step on one of the two sides. We obtained the evolution of the interface shape from high-speed back-light visualization, fit interface models to the experimental data to estimate the contributions of all the governing forces, and perform measurements of the dynamic contact angle. We propose a new model to account for the impact of the interface acceleration on its shape, and we discuss the impact on the measurement of the transient contact angle. The new model allows us to perform dynamic contact angle measurements below 15°, which is challenging to obtain with traditional techniques. We show for the first time a dynamic characterization of the wetting behavior of HFE7200, and we compare the results with traditional hydrodynamic models.

Analytical, experimental, and numerical study of capillary rise dynamics from inertial to viscous flow

Fundamental understanding of capillary rise dynamics and precise evaluation of imbibition processes should be considered in many natural and industrial phenomena. In the presence of the capillary rise dynamics, it is obligatory to know the dominant forces. The assumptions considered to solve the capillary rise motion usually neglect specific forces, which limit the reliability of the derived solutions. In the present study, the dominant forces and regimes involved in the initial moments of the capillary rise imbibition process in a tube were investigated analytically, experimentally, and numerically. Analytical solutions available in the literature were discussed, and then, their validity was verified by comparing them to experimental observations and numerical results. Comparing the capillary rise behavior at the initial stages revealed significant differences between the theoretical models and the numerical lattice Boltzmann method. This behavior is attributed to simplify assumptions and ignore the entrance effect, dynamic contact angle, and the inertial term in the theoretical model. By removing these assumptions in numerical formulations, closer results to the experimental records were observed. In the present study, for the first time, capillary rise dynamics were divided into five steps: (1) a transition regime with h ∼ t2, (2) purely inertial (stage one) with h ∼ t, (3) viscous-inertial or crossover (stage two) with h ∼ log10(t), (4) purely viscous (stage three) with h ∼ t1/2, and (5) gravitational-viscous with constant h. It was known that stage one was purely dominated by the inertial forces, then the influence of viscosity increased (viscous-inertial flow), and finally, the effect of inertia faded and the flow became purely viscous and approached the Lucas–Washburn law.

A new insight into dynamic contact angle measurements: Implementation of smooth splines method

The contact angle (CA) of a drop is a common quantitative measurement that describes a surface’s wettability. However, manual selection of the drop’s solid–liquid–vapor contact points (CPs) by the user introduces substantial human error to the final measured CA. Hence, in this article we introduce a novel method for CA measurement that uses a smoothing spline function to simultaneously cancel probable image noise, find the solid–liquid contact point, and calculate the CA at the contact point. The accuracy of this technique is tested through measurement of a series of synthetic drop images with known CAs ranging from 5°to 175°, which are generated using a new mathematical approach and include a reflection area to accurately model real experiments. In these tests, the smoothing splines approach is shown to be extremely accurate in measuring both the CP position and the CA, with negligible error (R2=0.9999). Then, for added validation, the smoothing splines approach is tested on four real surfaces of different wettability: glass, single crystal silicon, polished polytetrafluoroethylene, and patterned superhydrophobic polytetrafluoroethylene. When tested on real surfaces, the smoothing splines measurement approach provides reliable contact angle data for all four different surfaces tested, while also reducing human error. Overall, our proposed measurement approach for the CA is shown to be more accurate than traditional measurement approaches such as the commonly used polynomial fitting approach, and the more recently introduced mask approach, which also includes automatic CP detection.

Investigation of bubble structure in a microchannel under microgravity conditions: Effects of discontinuous wettability with dynamic contact angle

A thorough understanding of the dynamics of bubble/liquid two-phase flows under microgravity conditions is essential for achieving efficient heat transfer between spacecraft and space stations. This paper examined the influence of the discontinuous wettability of a microchannel's wall on bubble structure movement under microgravity conditions. The volume-of-fluid methodology involving a dynamic contact angle code was employed by considering wall-adhesion equations to predict the form of the bubble/liquid interface at and near the wall. The flow field's governing equations were numerically solved utilizing the finite volume method. Initially, the results were compared to previously reported data for validation. Subsequently, the effects of discontinuous wettability with different configurations on the structure of bubbles were investigated; new insights were obtained into the size and shape of generated bubbles, as well as the slug length as a function of the wall wettability. The results showed that for a uniformly hydrophilic surface, the bubble had a concave shape under microgravity conditions, while for a uniformly superhydrophobic surface, the bubble had a convex shape. A microchannel wall surface with discontinuous wettability led to the generation of an asymmetric bubble, a variation in slug length, changes in the bubble's length and shape, and a continuous phase structure. Furthermore, variations in the wall wettability caused the flow upstream to vary and affected all four stages of bubble generation, namely lag, filling, necking, and the starting point of detachment.