Formation Of Thin Liquid Film Research Articles

Enhanced contact flexibility from nanoparticles in capillary suspensions

Hypothesis: Sample-spanning particle networks are used to induce structure and a yield stress, necessary for 3D printing of porous ceramics and paints. In capillary suspensions, a small quantity of immiscible secondary fluid is incorporated into a suspension. By further adding nanoparticles with a range of hydrophobicities, the structure of the bridges and microparticle-microparticle contacts is expected to be modified, resulting in a tunable yield stress and shear moduli. Moreover, the compressibility of these samples, important in many processing and application steps, is expected to be sensitive to these changes.Experiment: The nanoparticle hydrophobicity was altered and their position relative to the microparticles and the bridges was examined using confocal microscopy where the correlation between bridge size and network structure was observed. A step-wise uniaxial compression test on the confocal was conducted to monitor the microparticle movement and structural changes between capillary suspension networks with and without nanoparticles.Findings: Our observation suggests that nanoparticles induce the formation of thin liquid films on the surface of the microparticles, mitigating contact line pinning and promoting internal liquid exchange. Additionally, nanoparticles at microparticle contact regions further diminish Hertzian contact, enhancing the capacity for rearrangement. These effects enhance microparticle movement, narrowing the bridge size distribution.

Parametric study of water spray cooling on enhanced relatively large surfaces

Spray cooling as an effective cooling technique is widely used in practice. Compared to small heating surfaces, the significantly increase of surface would pose a great challenge to the significant enhancement of spray cooling. Specially, single nozzle spray cooling suffers from non-uniform liquid thin film on a relatively large heating surface, severely inhibiting the formation of efficient thin film evaporation. In this study, two enhanced surfaces such as straight fin surface and cubic fin surface with a relatively large heating surface of 4 cm2 were designed for single nozzle spray cooling. A parametric study was conducted with aims to evaluate the effects of flow rate, spray distance and enhanced surfaces on the formation of thin liquid film and then the performance of spray cooling. Visualization was presented to observe the development of liquid film to elucidate the enhanced mechanism of spray cooling. In this work, spraying height is varying from 20 to 40 mm. Coolant flow rate is ranging from 70 to 100 ml/min. The experimental results show the distribution of liquid film plays an important role in spray cooling, especiallyin the single-phase regime. In two-phase regime, nucleate boiling is significantly boosted on the engineered surfaced. For instance, the cubic fin surface leads to an enhancement of HTC of nearly 24 % compared to that of flat surface.

Janus film evaporator with improved light-trapping and gradient interfacial hydrophilicity toward sustainable solar-driven desalination and purification

Solar-driven desalination and purification offers a promising approach to water treatment, while the clean water yields under natural sunlight can be further increased by developing well-designed evaporators. Herein, a TiN/GO//GO Janus film was fabricated to realize the synergetic effect of effective photothermal conversion and water transportation. The top TiN/GO hybrid layer possesses high solar absorption and thermal conversion efficiency, while the bottom GO layer promotes expeditious water transport. Moreover, the film maintains gradient interfacial hydrophilicity due to the reservation of oxygen-functional groups, which facilitate the formation of thin liquid film at water-film interface and increase heat utilization efficiency. Therefore, the Janus film is demonstrated to possess high evaporation rate of 1.70 kg m−2 h−1 for various non-conventional water resources under 1 solar illumination, which is comparable or better than most of the film evaporators. The excellent durability is demonstrated in a wide acid–base range (pH 2–11) and salinity range (0–250‰) over 30 cycles. The outdoor device with Janus film exhibits daily water yield of 7.44 kg m−2, indicating a potential application in efficient solar-driven desalination and non-conventional water treatment.

Heat transfer enhancement in nucleate boiling on micropillar-arrayed surfaces with time-varying wettability

• A conceptual design of textured surfaces with time-varying wettability is proposed. • The design increases the bubble departure frequency and suppresses dry spots. • The design enhances interface evaporation but deteriorates microlayer evaporation. • The heat transfer rate is enhanced by 38.5% as compared with constant wettability. Microstructured surfaces with spatial arrangements of hydrophilic and hydrophobic regions have been widely studied in boiling heat transfer enhancement. In this study, a conceptual design of microstructured surfaces with time-varying wettability is proposed to regulate bubble dynamics and thus enhance the heat transfer performance in the nucleate boiling regime. The effectiveness of the design is examined numerically for single-bubble heat transfer on a micropillar-arrayed silicon surface using a comprehensive three-dimensional model. The intrinsic contact angle is initially set as 48°, it is subsequently reduced to 20° in the bubble growth (0.152 and 0.4 ms) or departure stage (0.60 and 0.80 ms). The simulations show that two mechanisms contribute to heat transfer enhancement. First, the time-varying wettability decreases the adhesion force on the bubble exerted by the surface, thereby increasing the bubble departure frequency. Second, it also increases the local curvatures of the bubble, leading to the formation of thin liquid films between the bubble and micropillar walls. The liquid films suppress dry spots and hence enhance the heat removal of the bubble from the surface. The simulations also show that the reduction in the intrinsic contact angle enhances the evaporation on the liquid–vapor interface but deteriorates the evaporation in the microlayer. Because the former dominates the heat transfer process of the bubble on the micropillar-arrayed surface, the time-varying wettability increases the total evaporation rate. In the meantime, the time-varying wettability reduces the time proportion of the departure stage, which shortens the weakened heat transfer period in a bubble life-cycle and thus enhances the average heat transfer rate. As compared the case with constant wettability, the average heat transfer rate for the cases with time-varying wettability increases by 38.5%, 23.1%, 19.2%, and 15.4%, respectively, when the intrinsic contact angle is reduced at 0.152, 0.40, 0.60, and 0.80 ms, indicating that earlier wettability modulation gives rise to more significant heat transfer enhancement. The optimal time for wettability modulation is exactly the same as the time at which the total evaporation rate begins to decline.

On the extension of Bretherton theory for thin liquid films formed around elongated bubbles

The formation of thin liquid films around an elongated bubble moving in a capillary is pertinent to many applications. However, development of a theoretical model for the film thickness has been a challenge for several decades. The prominent theory characterizing the film thickness was developed by Bretherton. This theory relates the liquid film thickness in axisymmetric capillaries to the flow capillary number (Ca) for values of up to 0.003. Modified forms of the Bretherton theory have been presented for Ca values as high as 2. However, the validity of these models has not been rigorously examined. In addition, the validity of the Bretherton model itself in non-axisymmetric cross section capillaries remains uncharted. The objective of this paper is to determine whether the Bretherton relation can be extended to a broader range of Ca values and non-axisymmetric cross section channels, wherein the film thickness is not uniform along the channel circumference. A series of experiments are conducted on a set of fluids and different channel sizes and cross-sectional geometries to produce a wide range of viscous, surface tension, and inertial forces. The results show that when inertial forces are significant, and modified Bretherton models fail to predict the film thickness at Ca well below 2. The experimental results also show that depending on fluid properties, another key requirement of Bretherton solution of Landau–Levich equation may not be met. In rectangular cross section channels, the difference in surface tension forces along the longer and shorter channel axes, results in great deviation from the existing lubrication-based theories. This deviation greatly expands with the increase in the channel aspect ratio.

CO2 absorption performance in a rotating disk reactor using DBU-glycerol as solvent

Abstract Gas–liquid mass transfer of rotating disk reactor was studied in CO2 absorption using 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)-glycerol solution as solvent. Effects of the rotating disk structure and various operation parameters on the CO2 absorption rate and CO2 removal efficiency were investigated. The rotating disk with optimal holes is conducive to mass transfer of CO2 and the formation of thin liquid film at the opening increases the gas–liquid contact area. With the increase of rotating speed, the liquid flow pattern on the rotating disk surface changes from thin film flow to separated streams and creates extra liquid lines attached to the rim of the disk, which leads to a very complicated change on the CO2 absorption rate and CO2 removal efficiency. The overall gas-phase mass transfer coefficient increases 138% as the rotating speed increasing from 250 to 1400 r·min−1. Increasing temperature from 298 to 338 K can enhance the CO2 absorption rate due to lowering the viscosity of the solvent. The rate-determined step for the absorption is focused on the gas side. The rotating disk reactor can effectively enhance the absorption of CO2 with viscous DBU-glycerol solvents.

Development of a 3D-printed modified Scheludko-cell: Potential application for adsorption and thin liquid film study

A systematic study on the formation of thin liquid films (TLFs) of n-dodecane, as an intervening liquid phase in aqueous surfactant solution, was performed using a modified 3D-printed Scheludko-cell (MSC). The formation and stability of TLFs are electrically detected using the current density-voltage (J-Ψ) characteristics of the system, and it is confirmed by in-situ visualization of TLFs. The conductivity of the TLFs is measured using the J-Ψ curve and resistors in a series model of the system. The induced conductivity of the film is measured for different concentrations of potassium chloride (KCl) and sodium dodecyl sulfate (SDS) aqueous solutions. We monitored an increase in TLF conductivity due to the accumulation of inorganic hydrophilic ions like (K+ or Cl−) and adsorption of amphiphilic anion of dodecyl sulfate to the interface. The higher rate of increase in the TLF conductivity in the case of SDS is attributed to the ability of amphiphilic ions to pass through a so-called unscreened potential in the vicinity of oil-aqueous solution interface leading to an unscreenable potential at the boundary of the TLF. The predicted adsorption time-scale of surfactants using the MSC cell in ionic liquid (IL) phase, at O/W interface, is found to be in good agreement with the dynamic interfacial tension test results.

Numerical prediction of thin liquid film near the solid wall for hydraulic cavitating flow in microchannel by a multiphase lattice Boltzmann model

Based on a multiphase lattice Boltzmann (LB) model integrating body forces such as inter-particle interaction force and fluid-solid interaction force, a numerical prediction of thin liquid film near the solid wall for hydraulic cavitating flow in microchannel is conducted in the present work. The validity of model is tested by means of Laplace law and experimental result. On this basis, a full thin liquid film near the solid wall is successfully predicted, which proves that the single-component multiphase LB model integrated with various interaction forces is a good solution to this type of problem like hydraulic cavitating flow in microchannel. The further simulation indicates that the fluid-solid interaction strength (gs) has a significant effect on the formation of thin liquid film. A critical value (gs = −3.5) of fluid-solid interaction strength is found, and thin liquid film fails to be predicted at gs > −3.5. A simple simulation on the contact angle (θ) of droplet on the solid surface is conducted to analyze the reason for this phenomenon. It is found that a full thin liquid film could be successfully predicted only when the solid surface is fully wetted by liquid, i.e., θ = 0° corresponding to gs ≤ −3.5. The current research provides a potential way for future investigation on heat transfer accompanied by hydraulic cavitating flow.

Numerical Model of Liquid Film Formation and Breakup in Last Stage of a Low-Pressure Steam Turbine

Formation of thin liquid films on steam turbine airfoils, particularly in last stages of low-pressure (LP) steam turbines, and their breakup into coarse droplets is of paramount importance to assess erosion of last stage rotor blades given by the impact of those droplets. An approach for this problem is presented in this paper: this includes deposition of liquid water mass and momentum, film mass and momentum conservation, trailing edge breakup and droplets Lagrangian tracking accounting for inertia and drag. The use of thickness-averaged two-dimensional (2D) equations in local body-fitted coordinates, derived from Navier–Stokes equations, makes the approach suitable for arbitrary curved blades and integration with three-dimensional (3D) computational fluid dynamics (CFD) simulations. The model is implemented in the in-house solver MULTI3, which uses Reynolds-averaged Navier–Stokes equations κ – ω model and steam tables for the steam phase and was previously modified to run on multi-GPU architecture. The method is applied to the last stage of a steam turbine in full and part load operating conditions to validate the model by comparison with time-averaged data from experiments conducted in the same conditions. Droplets impact pattern on rotor blades is also predicted and shown.

Beyond Wenzel and Cassie-Baxter: second-order effects on the wetting of rough surfaces.

The Wenzel and Cassie-Baxter models are almost exclusively used to explain the contact angle dependence of the structure of rough and patterned solid surfaces. However, these two classical models do not always accurately predict the wetting properties of surfaces since they fail to capture the effect of many interactions occurring during wetting, including, for example, the effect of the disjoining pressure and of crystal microstructure, grains, and defects. We call such effects the second-order effects and present here a model showing how the disjoining pressure isotherm can affect wettability due to the formation of thin liquid films. We measure water contact angles on pairs of metallic surfaces with nominally the same Wenzel roughness obtained by abrasion and by chemical etching. These two methods of surface roughening result in different rough surface structure, thus leading to different values of the contact angle, which cannot be captured by the Wenzel- and Cassie-type models. The chemical and physical changes that occur on the stainless steel and aluminum alloy surfaces as a result of intergranular corrosion, along with selective intermetallic dissolution, lead to a surface roughness generated on the nano- and microscales.

Obtaining calcium carbonate in a multiphase system by the use of new rotating disc precipitation reactor

Rotating disc reactor (RDR) was constructed to conduct gas–liquid–solid reactions with controlled reagent transfer from gaseous to liquid phase. The concept is based on continuous formation of thin liquid films at a surface of rotating discs where the mass transfer proceed in diffusion–convective way.

Asymptotic analysis of the formation of thin liquid film in spin coating

Unsteady thin liquid film flow of non-uniform thickness on a rotating disk is analyzed by asymptotic methods. Short- and long-time-scale solutions for the transient film profile near the rotating axis are independently derived as a function of space and time. Analyses were performed for a case in which the initial film thickness is even in radial distance and the peripheral effects of the liquid film are assumed to be negligible. The result reveals the effects of the gravitational and surface tension forces, coupled with inertial force, on the film planarization and thinning process.

Thin liquid films

The concept of the effective interface potential as a framework for describing the formation of thin liquid films in thermal equilibrium is described both for one-component and binary fluids. Heterogeneous substrates give rise to an interesting morphology of such films. The onset of critical fluctuations within films leads to anomalies such as critical adsorption. Attention is paid to thin film helium films close to their superfluid transition which represent a realization of the Casimir effect induced by critical fluctuations. New aspects of the structure of liquid-vapor interfaces of simple and dipolar fluids are discussed. The use of x-rays and neutrons under the condition of grazing incidence allows one to infer quantitative informations about the structural properties of thin films.

シリカゲル担持硫酸塩を触媒とする選択的反応およびシリカゲル担持硝酸塩を用いた酸化反応 固体表面での有機合成

Metallic sulfates supported on silica gel efficiently catalyze dehydration and transesterification of alcohols. Symmetrical primary and secondary diols from C2 to C16 have been monoprotected in high selectivity by acylation and THP-ether formation by these catalysts in ester/alkane and DHP/alkane mixtures, respectively. The selectivity of these reactions depends upon the composition and the volume of the solvent systems. The selectivity can be explained by (1) preferential adsorption of diols to monoesters or monoethers and (2) the formation of thin liquid films of diols on the surface of the catalysts caused by the limited dissolution of the diols in the solvents. Metallic nitrates supported on silica gel efficiently oxidized secondary and primary benzyl and allyl alcohols and ethers to the corresponding ketones and aldehydes under mild conditions. Silica gel is essential for the efficient oxidation and Cu (NO3)2 and Zn (NO3)2 are suitable for practical use. Mechanistic study suggests that NO2 is an active species. Alcohols and ethers are readily oxidized by NO2 to give the corresponding aldehydes or ketones in the presence of silica gel.

Highly selective monoetherification of symmetrical diols catalysed by metallic sulfate supported on silica gel

Several symmetrical primary and secondary diols from C2 to C16 have been protected in high selectivity by tetrahydropyranyl ether formation catalysed by metallic sulfates supported on silica gel. This selective etherification is simple and practical. The selectivity of monoether formation depends upon the composition and the volume of solvents. The selectivity can be explained by preferential adsorption of diols to monoethers and the formation of thin liquid films of diols on the surface of catalysts caused by the limited dissolution of the diols in DHP–hexane mixtures.

Melting of CuIn solid solutions at small superheating by droplet formation and liquid film migration

The melting of homogenized, polycrystalline α-CuIn solid solutions was studied by Differential Thermal Analysis and metallographic methods. At small superheating melt forms by two mechanisms. The formation of homogeneously distributed droplets in the interior of the grains and the subsequent diffusion of the excess indium to these nuclei dominates, if the solid solution is heated up rapidly (≈ 20 K s −1) up to 50 K above its solidus temperature. At small heating rates of ≈ 8.3·10 −1 K s −1, homogenization takes place by the formation of thin liquid films at the grain boundaries and their subsequent migration. The moving liquid films accumulate the excess indium from the dissolved supersaturated solid solution and leave a solid solution of equilibrium concentration in their wake. From the correspondence between the experimental migration velocities and calculated values it is concluded, that coherency strains in the solid adjacent to the liquid film provide the driving force for the second mechanism.