Film Spreading Research Articles

Dynamic Behaviors of Droplet Impacts in Dropwise Condensation on Horizontal Tube Bundles.

Dropwise condensation offers substantial heat transfer advantages over filmwise condensation, enhancing the industrial condenser efficiency and reducing energy losses. However, the dynamics of condensate droplets on horizontal tube bundles remains complex and insufficiently studied. This paper presents a detailed investigation of the impact of dynamic behaviors of condensate droplets by numerical simulation using the Volume of Fluid model. The droplet Weber number (Wed) between horizontal tube bundles determines the characteristics of droplet impact behaviors. This study examines the effects of tube pitch and contact angle on Wed and its distribution of detached droplets, revealing the transition process of the Wed-dominant factors. Based on visualization results, three dynamic behaviors of condensate droplets impacting the lower tube are identified: attachment, rebound, and breakup. The dominant behaviors of multiple droplets impacting the lower tube under various conditions are clarified through the Wed distribution, leading to the establishment of a droplet impact regime map. Breakup behavior results in a larger proportion of the renewal area, with a maximum exceeding 88.80%, enhancing the surface flushing effect. However, it is essential to mitigate liquid film spreading to prevent the onset of near-FWC. Rebound behavior, characterized by minimal contact times down to 15.40 ms, make it more suitable for tubes that can perform quick self-renew. Finally, based on the characteristics of droplet impact behaviors, guidelines for regulating droplet behavior under different conditions are proposed. The findings provide critical insights for the design of novel dropwise condensers.

Spreading and leveling of finite thin films with elastic interfaces: An eigenfunction expansion approach

Abstract The flow of a thin viscous liquid layer under an elastic film arises in natural processes, such as magmatic intrusions between rock strata, and industrial applications, such as coating surfaces with cured polymeric films. We study the linear dynamics of small perturbations to the equilibrium state of the film in a closed trough (finite film). Specifically, we are interested in the spreading (early-time) and leveling (late-time) dynamics as the film adjusts to equilibrium, starting from different initial perturbations. We consider both smooth and non-smooth spatially symmetric and localized initial conditions (perturbations). We find the exact series solutions for the film height, using the sixth-order complete orthonormal eigenfunctions associated with the posed initial-boundary-value problem derived in our previous work [Papanicolaou N C and Christov I C 2023 J. Phys.: Conf. Ser. 2675 012016]. We show that the evolution of the perturbations begins with the spreading of the localized perturbations, followed by their mutual interaction as they spread, and finally, interactions with the confining lateral boundaries of the domain as the perturbations level. In particular, we highlight how the leading eigenvalues of the problem determine the scalings of certain figures of merit with time.

Multifunctional Ti3C2Tx/Fe3O4/glass fiber paper composites for flexible microwave absorption materials with ultralow filling ratio

The preparation of multifunctional microwave absorbing materials with the characteristics of stronger absorption, wider bandwidth, lighter weight, higher strength and thinner thickness is an urgent research direction. Due to the difficulty of controllable distribution of absorbers, excellent microwave absorption performance is usually accompanied by large absorber amount and thickness, which can result in microwave absorption composite materials lack flexibility, mechanical properties, and adaptability to the environment. In this study, Ti3C2Tx/Fe3O4/glass fiber paper composite was prepared by vacuum-assisted filtration. The controllable pore structure constructed by glass fiber enables uniform distribution of MXene/Fe3O4 and effectively spreading of MXene monolayer film, enhancing conductivity loss and interface loss. By controlling the diameter of glass fiber, the pore size and the distribution of absorber can be regulated, thereby achieving a minimum reflection loss of up to −57.2 dB at an ultra-low filling ratio of 1.66 wt%. With the addition amount of 1.90 wt%, the effective absorption bandwidth can cover the whole X-band. The ultra-low specific reflection loss exceeds most of fabric materials and MXene-based absorbers. Meanwhile, the composite material exhibited excellent mechanical properties with a tensile strength of 3.054 kN/m and a burst resistance of 290 kPa. In addition, the adhesion between the filler and the glass fiber was increased by adding acrylic resin-based aqueous adhesive, and the surface grafting functional group (-F) made the composite material superhydrophobic. This work provides a new strategy to synthesize multifunctional absorbing material with low filling ratio and strong environmental practicality.

A general model for spin coating on a non-axisymmetric curved substrate

We derive a generalised asymptotic model for the flow of a thin fluid film over an arbitrarily parameterised non-axisymmetric curved substrate surface based on the lubrication approximation. In addition to surface tension, gravity and centrifugal force, our model incorporates the effects of the Coriolis force and disjoining pressure, together with a non-uniform initial condition, which have not been widely considered in existing literature. We use this model to investigate the impact of the Coriolis force and fingering instability on the spreading of a non-axisymmetric spin-coated film at a range of substrate angular velocities, first on a flat substrate, and then on parabolic cylinder- and saddle-shaped curved substrates. We show that, on flat substrates, the Coriolis force has a negligible impact at low angular velocities, and at high angular velocities results in a small deflection of fingers formed at the contact line against the direction of substrate rotation. On curved substrates, we demonstrate that, as the angular velocity is increased, spin-coated films transition from being dominated by gravitational drainage with no fingering to spreading and fingering in the direction with the greatest component of centrifugal force tangent to the substrate surface. For both curved substrates and all angular velocities considered, we show that the film thickness and total wetted substrate area remain similar over time to those on a flat substrate, with the key difference being the shape of the spreading droplet.

Study on oil film spreading characteristics on the jet lubricating tooth surface of aviation herringbone gear

Purpose During the oil lubrication process of aviation herringbone gears, lubricating oil is injected into and spreads around the tooth surface. Its spreading characteristics directly affect the lubrication state and transmission efficiency of gears. This study aims to investigate the effect of changing the injection parameters and gear operating conditions on the oil film deposition during injection lubrication. Design/methodology/approach The computational fluid dynamics method was used to establish the computational domain for the simulation and optimization using an orthogonal test. The oil film spreading and thickness of the tooth surface were calculated with the change in gear speed, fuel injection distance and fuel injection speed. Optimized oil injection lubrication parameters were obtained, and design ideas were provided for the oil injection lubrication analysis of aviation herringbone gears. Findings Under different operating conditions, such as varying the injection speed, gear rotational speed and injection distance, there were differences in the distribution of the lubricating oil film formed on the tooth surface. The thickness of the oil film decreases as the distance from the injection port increases. The thickness of the oil film deposition can be increased by increasing the injection velocity and reducing the gear rotation speed. The injection distance had a relatively small effect on the spreading thickness of the deposited oil film, affecting only the dynamic spreading process of oil film deposition. Furthermore, the simulation results are compared with the analytical calculation results, and finally the experimental verification is carried out. Originality/value Oil spray lubrication characteristics are important for achieving precise gear lubrication designs in the aerospace field. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2023-0250/

Anisotropic piezoresistive response of 3D-printed pressure sensor based on ABS/MWCNT nanocomposite

Nanocomposites based on carbon nanotubes (CNTs) are suitable for sensors, due to matrix ability to incorporate nanotube properties. Thus, we developed a low-cost, nanostructured poly(acrylonitrile-butadiene-styrene) (ABS) polymer piezoresistive sensor produced by additive manufacturing. For this, solution layers of acetone, dimethylformamide and CNTs functionalized with carboxylic acid were pulverized on an ABS substrate using an aerograph. Electrical characterization revealed an anisotropic piezoresistive response of the material, induced by the printing lines direction. Field Emission Gun-Scanning Electron Microscopy showed the nanostructured film spreading after five layers of CNTs as well as the random entanglement of nanotubes on parallel and perpendicular 3D-printed ABS substrates. Raman spectroscopy indicated compression and p-type doping of CNTs in interaction with the polymer, as seen mainly by the blueshift of the G and 2D subbands. The results show that the material is promising for pressure sensors, with potential applications in robotic haptic feedback systems.

Flowerlike Spreading of Micellar Films during Emulsion Drop Evaporation.

We investigated the film spreading during the evaporation of submillimeter oil-in-water emulsion droplets on a solid surface, and observed a novel phenomenon where the film follows a two-layer spreading. In combination with the instability at the film front, the spreading front acquires a flowerlike pattern. The emergence of the two-layer structure is attributed to micelles within the oil film that yield an oscillating disjoining pressure. By considering both the slipping condition and the disjoining pressure, a scaling analysis is carried out that agrees well with the observed film spreading dynamics. The film spreading follows Tanner's law initially, while it becomes faster at a later stage, where the film radius follows r∼t^{1/2} for weak slip and r∼t^{3/8} for strong slip conditions.

Coating flow of a liquid film with colloidal particles on a vertical fiber

A thin liquid film of colloidal suspension is considered which is spreading down on a vertical cylinder under the effect of gravity. A precursor film model is employed at the three-phase contact line to relieve the stress singularity. The curvature pressure leads to the formation of a capillary ridge at the contact-line. Bulk and surface colloids are assumed to be present in the liquid film. The interfacial pattern is governed by the film evolution equation, obtained by simplifying mass and momentum balance equations within the lubrication assumption, while rapid vertical diffusion is assumed for the advection–diffusion equation of the bulk concentration. Fluid viscosity and diffusivity are considered to be functions of the particle volume fraction. The effects of the bulk and surface colloids on the spreading film dynamics are systematically studied. The presence of bulk colloids leads to the thinning of the capillary ridge for smaller inlet bulk colloid concentrations. On the other hand, a larger inlet bulk concentration leads to the formation of a secondary advancing front behind the capillary ridge in the film profile. A reduced contact point velocity is observed with an increase in the upstream bulk concentration. Surface concentration is found to result in a hump-like structure behind the capillary ridge in the upstream direction due to solutal Marangoni stress. The Marangoni stress also hinders the surface-tension-driven spreading, resulting in a smaller contact line velocity. Reducing the Bond number results in unstable film profiles, which lead to a wave-like structure in the region with no bulk concentration gradients.

Spreading and engulfment of a viscoelastic film onto a Newtonian droplet

Spreading and engulfment of a viscoelastic film onto a Newtonian droplet

Enhanced propagation of free films with fast spread-out phenomena under the influence of megahertz surface acoustic waves

In this study, a novel approach for enhancing the rapid spreading of free liquid film on lithium niobate (LiNbO3) substrate under megahertz (MHz) surface acoustic wave (SAW) excitation is presented by treating it with surfactants. Through the design of a specific interdigital transducer structure, it was discovered that exciting the SAW at a frequency of 32.3 MHz can achieve optimal spreading performance for water droplets on the surface of surfactant-treated LiNbO3 substrate. The maximum average velocity reaches 1.76 mm/s at position P2 = 1250 μm in the water film front, and the stable film spreading speed shows a 204.9% increase compared to the existing research. Simultaneously, through the investigation of the spreading experiment phenomenon of silicone oil and de-ionized water droplets at varying frequencies, we have discovered the dynamic mechanism of “reverse phase” propagation in liquid film for the first time. This entails that the advancing edge of the wetting film demonstrates a spreading motion law that is opposite to the traditional spreading phenomena, with the spreading velocity in the central exceeding that on both sides. Our research demonstrates that this microfluidic device developed by SAWs enhances the spreading efficiency of the free films, enabling rapid expansion of the target liquid to form a high-surface area film layer. This advancement holds promise for overcoming the limitations of low sensitivity and short response time in the field of rapid pathological diagnosis in contemporary medicine.

A novel coaxial air-R134a spray cooling for heat transfer enhancement of laser dermatology

Efficient cooling is essential in laser dermatology to prevent thermal damage, but current methods are often hindered by thermal barrier film deposition. This study presents coaxial air-R134a spray cooling for heat transfer enhancement and deposited-film mitigation. Surface temperature was measured using a TFTC thermocouple, while heat flux was estimated through the improved Duhamel theorem. High-speed camera visualization with the Mie-scattering technique captured the spray and film behavior. The results showed uniform and effective cooling via transient surface heat transfer and film behaviors. The boiling curve identified three distinct cooling phases: transition boiling, nucleate boiling, and single-phase film deposition. Airflow adjustments not only enhance the heat transfer coefficient but also control spray dynamics, droplet rebounding, and film spreading. Self-organizing maps revealed that medium-to-high nozzle spacing (y/D) reduces surface temperature and increases the heat transfer coefficient. Film resistance time is minimized by either high airflow and low y/D or low airflow and high y/D. For critical heat flux, low y/D and airflow are optimal. A y/D = 32 and 60 L/min airflow provided appropriate operating conditions. However, airflow increased frost deposition during off-duty, suggesting moisture-free air in the future.

Aerodynamically Levitated Droplets as Small-Scale Chemical Reactors and Liquid Microprinters.

A thin liquid film spread over the inner surface of a rapidly rotating vial creates an aerodynamic cushion on which one or multiple droplets of various liquids can levitate stably for days or even weeks. These levitating droplets can serve as wall-less ("airware") chemical reactors that can be merged without touching-by remote impulses-to initiate reactions or sequences of reactions at scales down to hundreds of nanomoles. Moreover, under external electric fields, the droplets can act as the world's smallest chemical printers, shedding regular trains of pL or even fL microdrops. In one modality, the levitating droplets operate as completely wireless aliquoting/titrating systems delivering pg quantities of reagents into the liquid in the rotating vial; in another modality, they print microdroplet arrays onto target surfaces. The "airware", levitated reactors are inexpensive to set up, remarkably stable to external disturbances and, for printing applications, require operating voltages much lower than in electrospray, electrowetting, or ink jet systems.

Aerodynamically Levitated Droplets as Small‐Scale Chemical Reactors and Liquid Microprinters

AbstractA thin liquid film spread over the inner surface of a rapidly rotating vial creates an aerodynamic cushion on which one or multiple droplets of various liquids can levitate stably for days or even weeks. These levitating droplets can serve as wall‐less (“airware”) chemical reactors that can be merged without touching—by remote impulses—to initiate reactions or sequences of reactions at scales down to hundreds of nanomoles. Moreover, under external electric fields, the droplets can act as the world's smallest chemical printers, shedding regular trains of pL or even fL microdrops. In one modality, the levitating droplets operate as completely wireless aliquoting/titrating systems delivering pg quantities of reagents into the liquid in the rotating vial; in another modality, they print microdroplet arrays onto target surfaces. The “airware”, levitated reactors are inexpensive to set up, remarkably stable to external disturbances and, for printing applications, require operating voltages much lower than in electrospray, electrowetting, or ink jet systems.

(Invited) Modification of Dielectric Properties of Film Dielectric Materials by Solid Solution System

Compound semiconductor based power devices such as SiC and GaN require high temperature operating passive devices such as a capacitor. SiC-based active devices can operate above 250 °C. The development of high-temperature operational passives is urgently needed to advance to the module and system level. In the case of capacitors, currently available capacitors can only operate efficiently up to 175 °C. Therefore, a thin film capacitor that can operate at high temperatures and be monolithically integrated near the active device should be developed. In this talk, I will present our achievements in the development of high-temperature operating dielectric film materials using the solid solution system. One topic is A2B2O7 pyrochlore ferroelectrics. The ferroelectric materials with the pyrochlore structure such as Sr2Nb2O7 exhibited the high dielectric constant above 100 at high and low temperature ranges. However, due to the crystallographic direction dependence of the Curie-Weiss temperature, the temperature stability of the dielectric constant from room temperature to 300 °C was not available. In order to obtain the high temperature stability of dielectric constant from room temperature to 300 °C with high dielectric constant over 100, the pyrochlore ferroelectric solid solution thin films were investigated by employing the combinatorial synthesis. Composition spread thin films of solid solution of two or three kinds of materials were deposited on substrates by physical vapor deposition technique, which provided the systematic physical properties. High-throughput combinatorial synthesis method is effective in the rapid optimization of the correlation between the crystallization and the dielectric property change behavior. For the composition spread solid solution film, the Sr- or La-containing A2B2O7 pyrochlore ferroelectrics were selected from the viewpoints of ion radios and crystallographic direction dependence of the Curie-Weiss temperature. The composition spread films were formed on a Pt/Ti/SiO2/Si substrate by combinatorial RF sputtering with stoichiometric composition targets. The X-ray fluorescence spectrum of the composition-spread film showed a linear variation of the metal composition. After deposition, the samples were annealed at different temperatures under oxygen atmosphere to reduce oxygen vacancies and improve crystallinity. The crystallization of the composition-spread solid solution thin film was confirmed by X-ray diffraction patterns, indicating the composition dependence of the crystallographic orientation. Some compositions showed high dielectric constant over 100 and high temperature stability from room temperature to 300 °C. In the presentation, the correlation between film composition, orientation and dielectric constant will be discussed in detail. In the presentation, we also introduce our achievement of the BaTiO3 based relaxor ferroelectrics and other dielectric materials development using the solid solution system. For the relaxor ferroelectrics, among the BaTiO3 based relaxor ferroelectrics, we have selected x[BaTiO3]-(1-x)[Bi(Mg2/3Nb1/3)O3] . The permittivity of 400 and the stability <8% from room temperature to 400 °C were obtained, which are promising as high-temperature dielectric medium.

Simulation-based research on enhancing lubricity: investigating wettability and textured surfaces in alkane lubricants under boundary lubrication conditions.

Changing the wettability and surface texturing have a significant impact on lubrication. In this study, the researchers used the molecular dynamics (MD) method to investigate how adjusting the interaction between alkanes and the wall affects oil film morphology and frictional properties under boundary lubrication. The findings revealed that the bearing capacity was influenced by both the morphology of the oil film and the strength of solid-liquid adsorption. In cases where the walls had weak wettability, the alkanes formed clusters to effectively separate the walls, while in cases where the walls had strong wettability, the oil film spread and formed a strong adsorption film. The super oleophilic textured surface could enhance the oil film adsorption capacity and replenish the oil film to the friction area in time, and the super oleophobic smooth surface could further reduce the friction coefficient. Therefore, a composite surface consisting of a super oleophilic textured surface and a super oleophobic smooth surface can be designed to enhance the bearing capacity of the oil film and reduce friction.

Film cooling performance evaluation of bi-directional diffusion hole with compound-angle on turbine blade: Study on spanwise width

A bi-directional diffusion hole configuration, which expands along the flow direction and blade tip with a compound-angle, is proposed based on an actual turbine blade in the present study. The turbulence model was adjusted to enhance simulation accuracy by correcting the turbulent viscosity coefficient, significantly improving the prediction of jet diffusion in mixing areas. The film cooling effectiveness distribution was studied through pressure-sensitive paint (PSP) experimental measurements. The effect of the hole outlet spanwise width on the bi-directional diffusion hole film cooling performance is characterized by a significant influence, non-monotonicity and weak coupling. The optimal hole outlet spanwise width of bi-directional diffusion holes is explored. The dimensionless temperature distribution and vortex structure of the fluid in the hole outlet section reveal the intrinsic mechanism of the effect of the hole outlet spanwise width on the film cooling characteristics. The numerical simulation results and experimental validation results show that the optimal hole outlet spanwise width of the bi-directional diffusion hole is approximately 4.3D with the goal of achieving the optimal level of film cooling effectiveness. Considering the flow blending situation, the optimal hole outlet spanwise width is determined by balancing the increase in film cooling effectiveness caused by the coolant film spreading covering mode and the decrease in film cooling effectiveness caused by mainstream intrusion. Therefore, the engineering applicability of the optimal hole outlet spanwise width as a design criterion for bi-directional diffusion holes of the turbine blade is demonstrated.

Effect of liquid properties on film behavior on the surface of an airfoil in a high-speed flow and subsequent atomization from the trailing edge

This study examines the behavior of ethanol or water films on the surface of a NACA 0012 airfoil placed in a high-speed air flow. New water film results complement previous measurements of time averaged ligament lengths and droplet sizes generated with the same airfoil. In addition, new data are obtained for ethanol. Air velocities up to 175 m/s are used with liquid flow rates between 1.4 cm2/s and 2.6 cm2/s. The liquid was introduced onto one side of the airfoil via 26 holes, each measuring 0.5 mm in diameter and spaced 1 mm apart. These holes are positioned 35 mm downstream from the leading edge and 65 mm upstream from the trailing edge. Film behavior on the vane is captured with front-lit high-speed video. The accumulation of liquid and breakup of ligaments are captured with backlit highspeed video. Phase Doppler interferometry is used to measure droplet size and simultaneous axial and pitch wise velocity 70 mm downstream of the trailing edge. Finally, laser diffraction is used to measure line averaged droplet sizes. The results indicate notable differences between ethanol and water films. The ethanol film spreads more extensively across the width of airfoil which is attributed due to its lower surface tension. The maximum ligament length for both liquids decrease with higher air velocity and increase with the liquid flow rate. A correlation for maximum ligament length is developed and captures the effect of liquid properties, flow rate, and air velocity. Furthermore, the variation in liquid volume flow rate did not impact droplet size. However, for a given air velocity, the Sauter Mean Diameter is found to depend on surface tension. On the dependency of droplet size on the Weber number, ethanol produced smaller droplet sizes at low values of Weber number. However, at relatively high Weber numbers, the droplet sizes for both water and ethanol were similar. This suggests that droplet size is influenced by factors besides surface tension at low Weber numbers but not at higher Weber numbers. In addition, the Nukiyama-Tanasawa model to fit the volume distribution curves is simplified by fixing p and q terms. Simplification makes the model easier to understand and implement.

Dimethyl sulfoxide dispersion of aramid nanofibers: An excellent medium for exfoliating high-quality boron nitride nanosheets towards heat spreading films with high thermal conductivity

Herein, we used dimethyl sulfoxide (DMSO) dispersion of aramid nanofibers (ANFs) as medium to exfoliate boron nitride nanosheets (BNNSs) from their bulk materials via a ball-milling route. The obtained BNNSs displayed large lateral sizes and were very clean without any contaminants. When milling under a high energy condition, many small-area nanosheets were appeared but the sample produced with high ANF concentration could still contain more large-size BNNSs. It was concluded that the DMSO is a good exfoliating medium for BNNSs due to the suitable viscosity, molecular weight, and polarity, while the introduction of ANFs can further improve the exfoliated sizes of BNNSs because of the buffering to impacting forces of milling balls. The ANF/BNNS composite exfoliated under high energy ball-milling and high ANF concentration gained the highest in-plane thermal conductivity of 27.23 W m−1 K−1 since the densified microstructure and the high dispersity, large lateral size, and high horizontal orientation of BNNSs.

Dynamic behaviors of shear-thinning droplet impacting on a spherical particle surface

Non-Newtonian droplet impact is common in nature and industry. The viscosity of non-Newtonian liquids changes with shear stress, leading to different impact behaviors compared to Newtonian liquids. This study prepared shear-thinning solutions by adding HPMC polymer into deionized water and investigated its impact dynamics on hydrophobic spherical surface. The evolution patterns of droplet spreading factor, film thickness, and dynamic contact angle are investigated under different impact conditions. The results show that addition of polymers increases the energy dissipation during impact and suppresses droplet rebound from spherical surface compared to Newtonian droplet with similar viscosity. The droplet-impacting process is distinguished into three phases: (I) initial deformation, (II) inertia-dominated, and (III) viscosity-dominated phases. The temporal evolution of film thickness in Phases I and II is fitted by simple equations. In addition, an empirical formula that integrates Weber and Reynold numbers is proposed to predict the maximum spreading factor of HPMC droplets.

Molecular Dynamics Simulation of Dual Nanodroplet Impacts on a Cylindrical Surface.

Droplet impact behavior is ubiquitous in various fields. However, the dynamics and spreading mechanisms of micro- and nanoscale droplet impact on curved surfaces, particularly in the case of multiple droplets, have yet to be fully elucidated. In this study, molecular dynamics (MD) methods are employed to investigate the dynamic evolution of double nanodroplet impact on a nano cylindrical wall. The effects of droplet spacing, initial impact velocity, and wall wettability on droplet impact characteristics are analyzed. The results demonstrate that there are five impact modes of nanoscale double-droplet impacts with nanocylinders: spreading-partial wrapping-splitting-complete detachment (SPSC), spreading-complete wrapping-complete attachment (SCC), spreading-partial wrapping-complete attachment (SPC), spreading-partial wrapping-partial attachment (SPP), and spreading-partial wrapping-fragmentation-partial attachment (SPFP). The droplet spacing has an insignificant effect on the impact modes but affects the droplets' spreading shape in both the axial and radial directions. The initial velocity and wall wettability have significant impacts on the droplet impact modes and liquid film spreading characteristics. As the initial velocity increases, the liquid film's radial and axial spreading distances gradually increase. Under hydrophobic conditions, the spreading of the droplet is dominant in the radial direction, while under hydrophilic conditions, the spreading is dominant in the axial direction. Properly reducing the droplet spacing, increasing the impact velocity, and enhancing the wall hydrophobicity can promote detaching the droplet from the cylindrical wall.