Influence of fluorinated self-assembled monolayer on wetting dynamics during evaporation of refrigerant–oil mixture on metal surface

Reducing wettability of a metal surface is a promising method for enhancing boiling heat transfer of refrigerant–oil mixture on the metal. As fluorinated self-assembled monolayer (F-SAM) coating is effective for wettability reduction, its influence on wetting dynamics including meniscus shape, contact angle, contact line velocity and rising liquid height during evaporation of refrigerant–oil mixture on metal surface were experimentally investigated. The refrigerant–oil mixture was prepared by R141b and NM56, the oil mass fraction ranged from 0 to 10 wt%, and the surface roughness ranged from 0.028 to 1.166 µm. The results show that during evaporation of refrigerant–oil mixture, the presence of F-SAM changes the evaporation mode to be constant contact line velocity followed by both constant contact angle and contact line velocity, while decreases the rising liquid height. The results suggest that larger surface roughness and higher oil mass fraction are preferred when using F-SAM to reduce surface wettability.     

Stability of dimethyldioctadecylammonium bromide Langmuir-Blodgett films on mica in aqueous salt solutions—implications for surface force measurements

Langmuir-Blodgett (LB) monolayer films of dimethyldioctadecylammonium bromide (DDOA) on muscovite mica have been studied using Wilhelmy plate type wetting measurements, surface force measurements, atomic force microscopy (AFM), and Brewster angle microscopy (BAM) on insoluble monolayers of DDOA before deposition. In particular, the effect of exposure to aqueous KBr salt solutions was investigated. BAM shows a heterogeneous monolayer with small condensed domains of dendritic shape under conditions normally used for deposition. A stick-jump behaviour of the meniscus is seen during deposition, leading to a large-scale heterogeneity measurable in wetting studies. These also show breakdown and hydrophilization of the LB film at the three-phase contact line (meniscus) and when exposed to salt solutions of approximately 10⁻² M concentration. The advancing contact angle against water is approximately 105°. Surface force measurements show long-range attraction in water, but also a surface charge which depends on salt concentration, and breakdown when surfaces are brought into contact in high salt concentrations. AFM images of untreated films show small holes, and breakdown when exposed to salt solution, especially at the three-phase line. The LB film is judged to be less suitable as a model hydrophobic surface owing to its heterogeneity and instability in salt solution.     

A model for the quartz crystal microbalance frequency response to wetting characteristics of corrugated surfaces

We consider the effect of surface roughness, and its unique wetting behavior, on the response of a quartz crystal microbalance (QCM) resonator operating in contact with a fluid. The rough surface is modeled as sinusoidally corrugated particular to the case of a fixed relationship between amplitude and periodicity, as would arise from polishing with monodisperse spherical particles. The penetration of fluid into the troughs of the corrugations and the resulting meniscus are determined as a competition between surface tension and compression of the trapped gas. Liquid contained below the corrugation peaks, but above the gas/liquid meniscus, is trapped and behaves as an ideal mass layer, contributing a frequency shift that adds to that arising from liquid entrainment. This model allows QCM responses on rough surfaces to be described as a function of liquid properties and contact angle. This permits responses on hydrophobic surfaces to be understood in terms of incomplete surface wetting.     

Impact of Marangoni effects on the apparent contact angle — a numerical investigation

The influence of thermocapillary stress on the shape of the gas-liquid phase boundary is investigated numerically. We consider the case of a cold liquid meniscus at a heated solid wall in the absence of gravity. An “apparent contact angle” is defined geometrically and the deviation of this apparent contact angle from the prescribed static contact angle due to thermocapillary convection is studied. We observe an enlarged apparent contact angle compared to the isothermal case. Since a fixed static contact angle is used in the computations, we emphasize that this effect does not depend on the specific model of a dynamic contact angle.     

Optical measurement of contact angle of liquid helium on cesium

We have studied the wetting properties of helium-4 on a carefully prepared cesium substrate using an interferometric technique. Below the wetting temperature, which is 1.9 K in our experiment, we observe a striking hysteretic behaviour, which is due to the pinning of the contact line on defects of the substrate. We have measured the temperature dependence of the contact angle θ_a for an advancing meniscus. At low temperature, we find θ_a≈25°, in contrast with a previous result by Klier et al.     

The relation of steady evaporating drops fed by an influx and freely evaporating drops

We discuss a thin film evolution equation for a wetting evaporating liquid on a smooth solid substrate. The model is valid for slowly evaporating small sessile droplets when thermal effects are insignificant, while wettability and capillarity play a major role. The model is first employed to study steady evaporating drops that are fed locally through the substrate. An asymptotic analysis focuses on the precursor film and the transition region towards the bulk drop and a numerical continuation of steady drops determines their fully non-linear profiles. Following this, we study the time evolution of freely evaporating drops without influx for several initial drop shapes. As a result we find that drops initially spread if their initial contact angle is larger than the apparent contact angle of large steady evaporating drops with influx. Otherwise they recede right from the beginning.     

Axisymmetric viscous liquid oscillations in a cylindrical container

The axisymmetric natural damped frequencies (m=0) of a viscous liquid in a cylindrical container are obtained for slipping and anchored contact line at the container wall. The results may also be applied to viscous liquid in a micro gravity environment, if the contact angle of the contact line to the container wall is in the vicinity of π/2, indicating that the free liquid surface remains a plane surface in equilibrium. It was found that there exists, in contrast to frictionless liquid, a range where only aperiodic motion of the liquid is possible. This appears for small liquid height ratios h/a.     

Anisotropy in wetting of oriented sapphire surfaces by liquid Al–Cu alloys

The coexistence of a liquid with a solid and a gas phase causes a contact angle at the triple line and results in a certain work of adhesion. These properties were studied for liquid Al, Cu, and their alloys on single-crystalline sapphire surfaces with C(0001)-, A(11-20)-, and R(1-102)- orientation. Measurements were performed at 1100 °C and under $3 \cdot 10^4\, \hbox{Pa}$ Ar atmosphere in a sessile drop apparatus. There, the sample was heated and melted separately from the substrate within a drop dispenser. Only after the desired measurement conditions were reached, the liquid metal was released. Depending on the alloy composition, the wetting angle approached a constant value within few minutes after the contact of droplet and substrate was established: For pure Cu the contact angle increased to an equilibrium value of 116° ± 5°, which is identical for all the studied sapphire surfaces. For pure Al an anisotropy of the contact angle with regard to these surfaces is found: time evolution of the Al contact angle is only observed for wetting of C-surfaces. Wetting of A- and R-surfaces shows no pronounced time dependence. In these cases, a smaller contact angle of about 90° is observed. Wetting of the different sapphire surfaces by Al–Cu alloys corresponds qualitatively to their wetting by pure Al: again, only for C-surfaces a time-dependent increase of the contact angle is observed. On A- and R-surfaces wetting is not time-dependent and the contact angle increases with Cu content of the alloy.     

玻纤表面动态润湿行为

提出了采用线性回归处理分析玻纤与浸润液体动态润湿的新方法,结合高精度电子天平,表征了玻纤表面动态润湿性能。研究结果表明：在玻纤表面动态润湿过程中,随着润湿速度的增加,动态接触角有增大的趋势,玻纤与去离子水、乙二醇、760E环氧、CYD128环氧的接触角分别由66.04°、42.21°、51.31°、73.90°增加到69.05°、46.95°、74.58°、170.06°,玻纤表面可润湿性能下降。玻纤表面动态润湿过程中,黏度越大,随着润湿速度增加,可润湿性能下降越快,即玻纤与CYD128环氧体系的接触角下降96.16°,而与760E环氧树脂和乙二醇的接触角下降分别为23.27°和4.74°。基于新方法的玻纤表面动态润湿系统中,玻纤所受作用力随三相接触线移动速率和浸润液体黏度的增加而增大。     

An experimental investigation of modes of flow under conditions of evaporation of liquid on a vertical heating surface

An experimental investigation is performed of the effect of temperature head on the flow of evaporating film of liquid, defined by the wetting line or by ribs, on a vertical heating surface. The experiments are performed under conditions of evaporation of R11 Freon in a medium of own vapor on a vertical copper plate, including the presence of ribbing. The visualization of flow is performed. Analysis is made of the effect of the evaporation intensity in the neighborhood of liquid-vapor-wall contact line on the conditions of film discontinuity and on the pattern of resultant streamer flow. It is demonstrated that, rather than spreading, the liquid in the case of streamer flow on the heating surface contracts downstream even for a close-to-zero equilibrium wetting angle. This is due to intense evaporation of liquid in the region of liquid-vapor-wall contact line, where the liquid film exhibits a minimal thickness, to the variation of curvature of the interface in this region, and to the emergence of thermal contact angle. The dependence of thermal contact angle on temperature head is determined. Dynamic measurements are performed of the local thickness of flowing films of liquid using a capacitance meter, and spectral analysis is performed of waves which arise because of instability of film flow on the evaporating film surface.     

The dynamic contact angle in the presence of a non-isothermal boundary condition

Experiments have been performed to investigate thermal effects on the dynamic contact angle of an advancing meniscus. The situation is similar to the capillary transport of propellant along vanes in tanks of spacecrafts or the capillary rise of propellant along the hot tank wall entering the ballistic phase after the end of thrust. The results show that the dynamic contact angle depends on the ratio of viscous forces to capillary forces (Capillary number) as well as on the Marangoni forces due to surface tension gradients along the liquid interface caused by the non-isothermal boundary condition. The occurrence of a Marangoni flow causes an increase of the apparent dynamic contact angle. Experiments at isothermal conditions show good agreement with the dynamic contact angle model from Jiang et al. [1]. Thus we used his correlation as a basis for a dynamic contact angle model in the nonisothermal case. Our assumption is that the increase of the dynamic contact angle is caused by a shift of the static contact angle. We modified the model from Jiang in a way that we substitute the static isothermal contact angle by a shift contact angle. A relation has been determined for this shift contact angle in dependence on the thermocapillary Reynolds Marangoni number and the ratio of the Weber number and the thermocapillary Weber number. The knowledge of the behavior of the dynamic contact angle at non-isothermal conditions can be used to predict the flow behavior on a larger scale.     

Meniscus and viscous forces during separation of hydrophilic and hydrophobic smooth/rough surfaces with symmetric and asymmetric contact angles

Adhesive or repulsive forces contributed by both meniscus and viscous forces can be significant and become one of the main reliability issues when the contacting surfaces are ultra smooth, and the normal load is small, as is common for micro/nano devices. In this study, both meniscus and viscous forces during separation for smooth and rough hydrophilic and hydrophobic surfaces are studied. The effects of separation distance, initial meniscus height, separation time, contact angle and roughness are presented. Meniscus force decreases with an increase of separation distance, whereas the viscous force has an opposite trend. Both forces decrease with an increase of initial meniscus height. An increase of separation time, initial meniscus height or a decrease of contact angle leads to an increase of critical meniscus area at which both forces are equivalent. An increase in contact angle leads to a decrease of attractive meniscus force but an increase of repulsive meniscus force (attractive or repulsive dependent on hydrophilic or hydrophobic surface, respectively). Contact angle has a limited effect on the viscous force. For asymmetric contact angles, the magnitude of the meniscus force and the critical meniscus area are in between the values for the two angles. An increase in the number of surface asperities (roughness) leads to an increase of meniscus force; however, its effect on viscous force is trivial. A slightly attractive force is observed for the hydrophobic surface during the end stage of separation though the magnitude is small. The study provides a fundamental understanding of the physics of the separation process and it can be useful for control of the forces in nanotechnology applications.     

Meniscus and viscous forces during normal separation of liquid-mediated contacts

Menisci form between two solid surfaces with the presence of an ultra-thin liquid film. Meniscus and viscous forces contribute to an adhesive force when two surfaces are separated. The adhesive force can be very large and can result in high friction, stiction and possibly high wear. The situation may become more pronounced when the contacting surfaces are ultra-smooth and the normal load is small, as is common for micro-/nanodevices. In this study, equations for meniscus and viscous forces during separation of two flat surfaces, and a sphere and a flat surface, are developed, and the corresponding adhesive forces contributed by these two types of forces are examined. The geometric meniscus curvatures and break point are theoretically determined, and the role of meniscus and viscous forces is evaluated during separation. The influence of separation distance, liquid thickness, meniscus area, separation time, liquid properties and contact angles are analyzed. Critical meniscus areas at which transition in the dominance of meniscus to viscous forces occurs for different given conditions, i.e.?various initial liquid thicknesses, contact angles and designated separation time, are identified. The analysis provides a fundamental understanding of the physics of separation process, and insights into the relationships between meniscus and viscous forces. It is also valuable for the design of the interface for various devices.     

Wettability and interfacial reaction in SiC/Ni plus Ti system

Wettability and interfacial reactions in both SiC ceramic/pure Ni system and SiC ceramic/Ni plus Ti system were studied. The contact angle was determined by high temperature photography. The microstructure and composition of the interfacial region were analyzed. Under the applied experimental conditions, the contact angle of SiC/pure Ni system at 1350°C in vacuum is about 86°. The contact angle slightly decreases with the prolonging of time of the test at 1350°C. The interfacial reactions take place during the test. The reactions facilitate the wetting. The wettability of the system is improved by adding active element titanium into the metallic phase. The contact angle decreases with the increasing of content of the additive within the tested range. Moreover, this effect becomes more remarkable with the prolonging of reaction time at high temperatures. This can be explained by the fact that titanium accumulates at the wetting interface and facilitates the interfacial reactions, resulting in an increase of the driving force of wetting.     

Electric field singularity on the line of wetting of dielectric surface

The pattern of electric field singularity on the line of wetting of dielectric by conducting liquid is considered. It is known that the modulus of electric field in this case tends to infinity while the distance from the line of wetting tends to zero, i.e., the electric field has a singularity on the line of wetting. In the case of nonzero wetting angle, this singularity is integrable, with the total volume density of electric energy remaining finite. It is demonstrated that, if the surface is wetted by dielectric liquid, a critical wetting angle exists defined by the permittivity ratio of contacting dielectrics. When the wetting angle becomes less than critical, the electric field singularity disappears.     

Mn对熔融Zn-Mn合金与X80钢润湿行为的影响

研究了锌液中0.1%~0.5%(质量分数)Mn对X80钢表面润湿行为的影响。采用改良座滴法获得了450℃时Zn-Mn合金的接触角,通过SEM/EDS观察分析样品表面及界面的组织结构,研究了Zn-χMn(χ=0.1~0.5)合金与X80钢基板的润湿行为和界面反应。结果表明：锰元素对锌合金与钢基体间的润湿性反应起到正吸附作用。在450℃时,当熔体中的锰含量为0.1~0.5时,Zn-χMn合金与钢基间的润湿接触角从85°减小到62°。锌合金熔体/X80钢属于反应性润湿体系,生成的界面产物由FeZn₁₀(δ)、FeZn₁₃(ζ)和Fe₃Zn₁₀(Γ)/Fe₅Zn₂₁(Γ₁)相组成,润湿行为受锌合金界面反应影响。在铺展三相线前沿存在前驱膜,前驱膜的出现能够促进润湿。     

A new experiment for investigating evaporation and condensation of cryogenic propellants

Passive and active technologies have been used to control propellant boil-off, but the current state of understanding of cryogenic evaporation and condensation in microgravity is insufficient for designing large cryogenic depots critical to the long-term space exploration missions. One of the key factors limiting the ability to design such systems is the uncertainty in the accommodation coefficients (evaporation and condensation), which are inputs for kinetic modeling of phase change.A novel, combined experimental and computational approach is being used to determine the accommodation coefficients for liquid hydrogen and liquid methane. The experimental effort utilizes the Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland to image evaporation and condensation of hydrogenated propellants inside of metallic containers. The computational effort includes numerical solution of a model for phase change in the contact line and thin film regions as well as an CFD effort for determining the appropriate thermal boundary conditions for the numerical solution of the evaporating and condensing liquid. Using all three methods, there is the possibility of extracting the accommodation coefficients from the experimental observations. The experiments are the first known observation of a liquid hydrogen menisci condensing and evaporating inside aluminum and stainless steel cylinders. The experimental technique, complimentary computational thermal model and meniscus shape determination are reported. The computational thermal model has been shown to accurately track the transient thermal response of the test cells. The meniscus shape determination suggests the presence of a finite contact angle, albeit very small, between liquid hydrogen and aluminum oxide.     

Thermo-hydrodynamic transport phenomena in partially wetting liquid plugs moving inside micro-channels

Single-phase as well as two-phase fluid flows inside mini/micro-channels and capillary tubes are of practical importance in many miniaturized engineering systems. While several issues related to single-phase transport are fairly well understood, two-phase systems still pose challenges for engineering design. The presence of gas–liquid interfaces, dominance of surface forces, moving contact lines, wettability, dynamic contact angle hysteresis and flow in confined geometries are some of the unique features of two-phase systems, which manifest into complex transport phenomena. While Taylor plug/bubble flow is a fairly common flow pattern in several micro-fluidic devices operating at low Bond number, the ensuing transport characteristics are complex and still not fully discerned. This review paper aims at highlighting the nuances and features of a unit cell of a Taylor plug flow, especially focusing on partially wetting systems, which are more common in engineering applications. Emphasis is given to a ‘unit cell’ flow system consisting of an isolated liquid Taylor plug with adjacent gas phase, confined in a capillary tube. Such a seemingly simple flow condition poses considerable challenges for discerning and modelling local thermo-hydrodynamic transport coefficients. Relevant background information and fundamentals are carefully scrutinized while summarizing the state-of-the-art. The role of wettability and dissipation near the contact line is highlighted via available experimental and simulation results. Local momentum and heat transfer exchange processes during the motion of an isolated plug of partially wetting liquid moving inside a capillary tube are delineated.     

Wetting behaviour of SiC ceramics: Part II—Y2O3/Al2O3 and Sm2O3/Al2O3

Wettability is the most significant phenomenon in SiC liquid phase sintering. The wetting of Y₂O₃/Al₂O₃ and Sm₂O₃/Al₂O₃ on SiC was analysed by the “Sessil drop” method. The wetting of liquid on solid during liquid phase sintering is very important. The behaviour of the additive on the SiC plate was observed using an imaging system with a CCD camera, and the contact angle measurements were analysed by Qwin Leica software. The samples were cut transversally and characterized by scanning electron microscopy and X-ray spectrometry (SEM/EDS). The wetting was found to be strongly influenced by the temperature; the SiC/additive contact angle decreased with increasing temperature. The YA and SA additives presented low contact angle values, indicating their good wetting on SiC in the argon atmosphere. The contact angle could not be measured when the test was performed in the nitrogen atmosphere because bubbles formed in the liquid during the test. The best atmosphere for this sintering was found to be argon, which allows uniform spreading.