Thin Liquid Film Research Articles

Vortex Fluidic Mediated Synthesis of Enhanced Hydrogen Producing Magnetic Gold

While bulk gold is well known to be diamagnetic, there is growing experimental and theoretical work supporting the formation of nano gold with unconventional magnetic properties. However, access to such magnetic gold nanoparticles at scale is limited. It is established that magnetic gold particles are readily accessible when exposing aqueous solutions of auric acid (H[AuCl4]) to UV irradiation (λ = 254 nm) under high shear in a vortex fluidic device (VFD), as a photo‐contact electrification process. Thin films of liquid in the VFD down to ≈200 μm thick are generated in a tilted rapidly rotating angled glass tube with induced mechanical energy imparted under high shear, which when exposed to UV, reduces Au3+ to elemental gold without the need for adding reducing agents, unlike in the conventional synthesis of nano gold particles. The use of magnetic force microscopy (MFM) is reported to show that VFD‐generated 2D gold sheets have magnetic gold nanoparticles embedded in them, with the material electron paramagnetic resonance active. A report is made on theoretical insights into the origin of the magnetism and that the material shows a dramatic enhancement of catalytic activity in the hydrogen generation reaction relative to using traditionally produced gold nanoparticles of comparable size.

Thermodynamics of an oblique droplet impinging on stationary water film

The flow and heat transfer of the oblique impact of a droplet on a stationary liquid film with various dimensionless thicknesses (01.0–0.5) are investigated experimentally and numerically. A superhydrophobic guideway is used to create the oblique impact of a droplet, which causes subsequent asymmetric crown structure and splashing. The thermal level set method is employed to capture the deformation and heat transfer of warm droplets' oblique impact on a cold liquid film. A parameter study of the effect of Weber number, oblique angle, and liquid film thickness on geometrical characteristics and wall heat flux is carried out. The results show that in the downstream direction, during the crown rising period, the radius is independent of the normal Weber number but increases for a larger tangential Weber number and a thinner liquid film. The maximum downstream crown height increases with an increase in the Weber number and exhibits a non-monotonic trend with the liquid film thickness. The heat transfer rate between the liquid film and surface decreases with larger oblique angles and thicker liquid films while having a poor dependence on the Weber number. In addition, the critical oblique angles for prompting splashing at different liquid film thicknesses are presented. Finally, modified thermodynamics models and splashing thresholds for the liquid film are developed to further enhance the understanding of aircraft icing.

In-situ Holographic Characterization of Thin Liquid film Thickness Evolution during Boundary-Constrained Motion

In-situ Holographic Characterization of Thin Liquid film Thickness Evolution during Boundary-Constrained Motion

Nonlinear Rayleigh–Taylor instability in an accelerated soap film

Rupture and fragmentation of thin liquid films are of great importance in manufacturing, agriculture, disease transmission, and other fields. An individual rupture event typically results from the nonlinear growth of small perturbations excited by the classical Rayleigh–Taylor mechanism and opposed by surface tension on the film interfaces. We study an initially uniform liquid film accelerated in the direction perpendicular to its interfaces, constructing a new reduced description of the flow in the limit of large surface tension consisting of three coupled (nonlinear) partial differential equations. This system is linearly unstable to disturbances of small wavenumber; we derive a simplified asymptotic description to gain analytical understanding of the nonlinear development until rupture. An initial phase of exponential film thinning leads to the accumulation of liquid in a central bulge. At a well-defined time, a localised pinch-off mechanism, driven by a mismatch in the interfacial curvature at the edge of the bulge, results in a self-similar thinning that breaks the film in finite time. Across a wide parameter range, the resultant droplet formed from the central bulge contains a significant fraction of the initial film volume.

Spontaneous Bubble Growth Inside High-Saturation-Vapor-Pressure and High-Air-Solubility Liquids and Emulsion Droplets.

Spontaneous bubble growths in liquids are usually triggered by rapid changes in pressure or temperature that can lead to liquid gas supersaturation. Here, we report alternative scenarios of the spontaneous growths of bubbles inside a high-saturation-vapor-pressure and high-air-solubility perfluorocarbon liquid (PP1) that were observed under ambient quiescent conditions. First, we investigate spontaneous bubble growth inside the single PP1 phase, which was left to evaporate freely. The bubble growth is explained by the difference in the PP1 vapor pressure inside the bubble and that above the freely evaporating PP1 interface. Next, we study the bubble growth inside the liquid PP1 covered with a layer of a second air-saturated immiscible liquid: low-air-solubility water or higher-air-solubility ethanol. In both cases, the bubble growth rates were accelerated, indicating mass transfer of air from the water or ethanol phases to the PP1 phase. The bubble growth rates significantly increase for bubbles trapped at the PP1-water or PP1-ethanol interfaces due to faster air diffusion through the thin PP1 liquid films separating the bubbles from the upper phases. Finally, we consider the case of bubbles inside millimeter-sized PP1 emulsion droplets in water or ethanol. The bubble growth inside the droplet leads to an increase in the PP1 droplet's effective buoyancy and to the detachment of the droplets from the substrate. The observed bubble growth rate in the case of emulsion droplets was much faster for PP1 droplets in ethanol than for PP1 droplets in water (minutes vs hours). The underlying physical mechanism of the increase of bubble volumes is the spontaneous mass transfer of both air and PP1 vapor to the bubbles produced by a colloidal diffusion pump effect.

Direct detection method for cusp thickness in wavy thin-film flow using an optical waveguide film

The wave crest (cusp) of the disturbance wave in thin liquid film flow is an important factor contributing to heat/mass transfer, e.g., fuel rods in boiling water reactors, stator/rotor blades in steam turbines, and cleaning/drying wafer processes in semiconductor manufacturing. We developed a new technique for directly detecting the thickness of wave cusps using array-based sensing with an optical waveguide film (OWF). This technique, based on geometrical optics assumptions, simultaneously obtains information on liquid films’ thickness and their interfacial shape, i.e., whether or not the local interface is convex upward. We first performed a pseudo-film flow measurement using a metal specimen to confirm the basic principle. According to the results, a meaningful signal indicating the wave-cusp passage, along with a thickness signal, was detected simultaneously. The OWF signal processing for cusp thickness detection was newly established based on this fact. We then applied this technique to a wavy liquid film flow formed on a flat plate in the entry region. A series of experiments were performed over a wide range of air speeds (jG = 20–70 m/s). As a result, the cusp thicknesses of relatively large waves on the wavy interface were successfully extracted from the OWF output signal. Further, the major thickness variables (i.e., base film thickness, median film thickness, and cusp thickness) were compared with those of conventional thickness estimation methods, which showed reasonable agreement. This paper provides a framework for wavy thin-film flow measurements via OWF that is specialized for directly detecting local thickness profiles.Graphical abstract

Linear stability analysis of a thin liquid film on a horizontal wall under quasi-periodic oscillation

We carry out a linear stability analysis of the flow of a thin layer of Newtonian fluid with a deformable free surface bounded at the bottom by a horizontal wall subjected to quasi-periodic oscillation in its own plane. Or's model (J. Fluid Mech., vol. 335, 1997, pp. 213–232), using a periodic oscillation, is extended to the configuration where oscillation has two incommensurate frequencies, $\omega _1$ and $\omega _2$ , with an irrational ratio $\omega ={\omega _2}/{\omega _1}$ . Using the long-wave expansion, we derive the asymptotic function involved in the long-wave instability criterion while taking into account the frequency ratio. It turns out that the maximum of this asymptotic function, as well as the frequency parameter at which long-wave instabilities occur, depend strongly on the frequency ratio. For arbitrary wavenumbers, the equations governing the problem under consideration are solved in space using Chebyshev's spectral collocation method, while the temporal resolution is performed using Floquet theory, knowing that an irrational number can be approximated by a rational number. For a large frequency ratio and for a velocity amplitude ratio equal to unity, we obtain, as in Or's work (J. Fluid Mech., vol. 335, 1997, pp. 213–232) considering the same frequency parameter interval, an alternation between the U shape and oblique shape referring respectively to instabilities of long wavelength and finite wavelength appearing in the diagram representing Reynolds number as a function of frequency parameter. By decreasing the frequency ratio towards $1/\sqrt {37}$ , the three initial U-shaped and three oblique instabilities merge into a single U-shaped and a single oblique instability. This merging phenomenon also occurs when the ratio of the amplitudes of the superimposed velocities, linked to the introduction of the second frequency, increases from small values to unity. For a fixed frequency parameter, the effect of frequency ratio and velocity amplitude ratio on the marginal stability curves in terms of Reynolds number versus wavenumber is also investigated, focusing on the appearance of long wavelength instability and finite wavelength instability.

Room-Temperature Vortex Fluidic Flow Chemistry Synthesis of Full Color Carbon Dots.

Carbon dots (CDs) as a new class of photoluminescent zero-dimension carbon nanoparticles have attracted significant research interests owing to their extraordinary opto-electro-properties and biocompatibility. So far, almost all syntheses of CDs require either heat treatment or exertion of high energy fields. Herein, a scalable room-temperature vortex fluidic method is introduced to the CDs synthesis using the angled vortex fluidic device (VFD). By judicious selection of the solvent, typical CDs precursor of phenylenediamine has been converted into high crystalline CDs through VFD processing. The VFD-synthesized CDs cover the full color spectrum from blue to red with the highest quantum yield of 45.6%. The synthesis shows that the dynamic thin liquid film generated by VFD spun at high rotational speed (7 - 9 k RPM) is able to induce cycloaddition reactions. The new method for CD synthesis is facile, occurring under ambient conditions in the VFD, potentially offering industrial scaling up of production of full color carbon dots.

A New Capillary and Adsorption‒Force Model Predicting Hydraulic Conductivity of Soil During Freeze‒thaw Processes

AbstractUnderstanding the change in soil hydraulic conductivity with temperature is key to predicting groundwater flow and solute transport in cold regions. The most commonly used models for hydraulic conductivity during freeze‒thaw cycles only consider the flow of capillary water in the soil and neglect water flowing along thin films around the particle surface. This paper proposed a new hydraulic conductivity model of frozen soil via the Clausius–Clapeyron equation based on an unsaturated soil hydraulic conductivity model over the entire moisture range using an analogy between freeze‒thaw and dry‒wet processes in soils. The new model used a single equation to describe the conductivity behaviors resulting from both capillary and adsorption forces, thus accounting for the effect of both capillary water and thin liquid film around soil. By comparison with other existing models, the results demonstrated that the new model is applicable to various types of soils and that the predicted hydraulic conductivity is in the highest agreement with the observed data, while reducing the root mean square error by 38.9% compared to the van Genuchten–Mualem model. Finally, our new model was validated with thermal–hydrological benchmark problem and laboratory experiment result. The benchmark results indicated that the advective heat transfer was more significant, and the phase change was completed earlier when considering both capillary and adsorption forces than when only considering capillary forces. Furthermore, the coupled flow–heat model with the new hydraulic conductivity expression replicated well the results from a laboratory column experiment.

Thin free-standing liquid films manipulation: device design to turn on/off gravity in flow regimes for thickness map control and for material structuring.

The manipulation and control of free-standing liquid film drainage dynamics is of paramount importance in many technological fields and related products, ranging from liquid lenses to liquid foams and 2D structures. In this context, we theoretically design and introduce a device where we can reversibly drive flow regime switch between viscous-capillary and viscous-gravity in a thin free-standing liquid film by altering its shape, allowing us to manipulate and stabilize the film thickness over time. The device, which mainly consists of a syringe pump, a pressure transducer, and a 3D-printed cylinder, is coupled with a digital holography setup to measure, in real time, the evolution of the local film thickness map, revealing characteristic features of viscous-capillary and viscous-gravity driven drainage regimes. By using polyvinyl alcohol/water concentrated solutions, we are also able to produce viscoelastic membranes after manipulation and water evaporation, which presents manipulation history-dependent geometrical properties. Furthermore, using a system composed of carboxymethyl cellulose, water, and rod-like zinc oxide nanoparticles, we show a clear effect of film manipulation on particle rearrangement. We believe this device could represent a starting point for the development of a useful and practical tool to study thin liquid film dynamics and to produce novel (patterned) 2D structures for the numerous scientific and technical fields where they are of use.

Electrodeposition of Ga-Sn thin liquid films from a deep eutectic solvent for CO2 reduction

Electrodeposition of Ga-Sn thin liquid films from a deep eutectic solvent for CO2 reduction

Modeling the structure and relaxation in glycerol-silica nanocomposites.

The relationship between the dynamics and structure of amorphous thin films and nanocomposites near their glass transition is an important problem in soft-matter physics. Here, we develop a simple theoretical approach to describe the density profile and the α-relaxation time of a glycerol-silica nanocomposite (S. Cheng et al., J. Chem. Phys., 2015, 143, 194704). We begin by applying the Derjaguin approximation, where we replace the curved surface of the particle with the planar one; thus, modeling the nanocomposite is reduced to that of a confined thin film. Subsequently, by employing the molecular dynamics (MD) simulation data of Cheng et al., we approximate the density profile of a supported liquid thin film as a stationary solution of a fourth-order partial differential equation (PDE). We then construct an appropriate density functional, from which the density profile emerges through the minimization of free energy. Our final assumption is that of a consistent, temperature-independent scaled density profile, ensuring that the free volume throughout the entire nanocomposite increases with temperature in a smooth, monotonic fashion. Considering the established relationship between glycerol relaxation time and temperature, we can employ Doolittle-type analysis ("naïve" free-volume model), to calculate the relaxation time based on temperature and film thickness. We then convert the film thickness into the interparticle distance and subsequently the filler volume fraction for the nanocomposites and compare our model predictions with experimental data, resulting in a good agreement. The proposed approach can be easily extended to other nanocomposite and film systems.

The Effect of the Film Thickness, Cooling Rate, and Solvent Evaporation on the Formation of L-Menthol Ring-Banded Spherulites

Periodic pattern formation is a prominent phenomenon in chemical, physical, and geochemical systems. This phenomenon can arise from various processes, such as the reaction and mass transport of chemical species, solidification, or solvent evaporation. We investigated the formation of ring-banded spherulites of l-menthol using a thin liquid film in a Petri dish. We found that the film thickness and cooling rate strongly influence the generation of crystallization patterns. We performed two-dimensional numerical simulations using the Cahn–Hilliard model to support the experimentally observed trend on the dependence of the layer thickness on the periodicity of the generated macroscopic patterns. In a specific scenario, we observed the formation of rings consisting of needle-like crystals on the cover of the Petri dish. This phenomenon was due to the evaporation of the menthol and its subsequent crystallization. In addition to these findings, we created crystallization patterns by solvent evaporation (using tert-butyl alcohol, methyl alcohol, and acetone).

Morphological dynamics of thin liquid films formed by bubbles oscillating near a free surface: Experimental and numerical investigation.

Bubbles oscillating near a free surface are common across numerous systems. Thin liquid films (TLFs) formed between an oscillating bubble and a free surface can exhibit distinct morphological features influenced by interfacial properties, evaporation, and deformation history. We hypothesize that a continuous film presence throughout oscillation results in a wimple morphology, whereas intermittent film presence leads to a dimple formation. Additionally, we propose that the thickness of a wimpled film depends solely on the capillary number Ca, whether the bubble oscillates toward and away from the free surface or is pressed against it in two consecutive motions. Using dynamic film interferometry, we investigate the morphology and drainage of TLFs formed with BSA and SDS surfactants. Controlled bubble oscillations allow us to test various deformation histories, and a lubrication theory-based model aids in interpreting the experimental findings. Experiments confirm that film morphology is heavily influenced by deformation history, with continuous film presence throughout the entire deformation process leading to wimple formation. A modified lubrication-theory-based scaling captures the universal behavior of film thickness as a function of the capillary number across different morphologies. Our model aligns well with observed transitions between wimple and dimple shapes, offering valuable insights for the design of nanostructured surfaces.

Calibration-free thickness and temperature measurement of oil films using broadband near-infrared absorption spectroscopy

Abstract This paper assesses broadband absorption spectroscopy for calibration-free, single-ended thickness and temperature measurements of thin liquid films with a free interface using spectrally resolved intensity measurements. The spectroscopic properties of the liquid in question, here an oil lubricant, are characterized using a Fourier transform infrared spectrometer. This data forms the basis for a measurement model that is used to infer the thickness and temperature of films wetting metal surfaces based on broadband spectral transmission measurements. The technique is demonstrated by inferring the properties of static films of known properties as well as free films on a rotating metal disk.

Air/water interfacial properties and thin film drainage dynamics of compositionally diverse wheat flour water extracts

The role of water-extractable (WE) wheat flour constituents and their interactions in determining the stability of wheat-based foam-type foods remain largely unexplored. Additionally, the contribution of drainage dynamics of the thin liquid films (TLFs) between gas bubbles to food foam stabilization is often overlooked. We here investigated the air/water (A/W) interfacial, TLF drainage, and foaming properties of compositionally diverse wheat flour aqueous extracts. Such extracts were prepared and then either used as such or modified by dialyzing out (i) low molecular mass constituents or (ii) both low molecular mass constituents and enzymatically hydrolyzed carbohydrates. This approach resulted in extracts with gradually increasing protein contents and distinct carbohydrate compositions. Wheat extract constituents created highly elastic A/W interfaces, regardless of the type of extract modification, suggesting a significant contribution from WE wheat proteins. TLFs stabilized by wheat extracts from which low molecular mass constituents were removed had significantly greater stability. This was ascribed to an increased disjoining pressure caused by steric repulsive forces, which in turn were attributed to the interaction between high molecular mass arabinoxylan in the bulk and adsorbed proteins at the A/W interfaces. All extracts showed similarly good foaming properties, implying a dominant role for WE wheat proteins in determining foaming. The strongly elastic A/W interfaces formed by WE wheat proteins likely reduced liquid drainage to such extent that coalescence and disproportionation were largely delayed. The findings of this study could facilitate further investigations on the possibility of tuning protein-AX interactions toward improved foam stability.

Transient behavior of thermocapillary convection in thin liquid film exposed to step laser heating

Temporally developing thermocapillary convection induced by step laser heating of a thin liquid film has been studied numerically. Computations are performed using the commercial software STAR CCM+ version 2022.1. The liquid film of silicone oil (high Prandtl number fluid) is 60 mm in diameter and 3 mm in thickness. Flow characteristics related to surface velocity and surface temperature have been studied. Validation of the computations is achieved for the surface velocity, the velocity along thickness and the surface temperature through comparison with PIV and IR camera measurements. The laser-beam with a carbon dioxide gas laser (10.4 μm in wavelength) is used for heating. It is found that the temporally developing profile of surface velocity shows two local velocity peaks (uS1 and uS2) at two radial locations (rS1 and rS2) respectively. The first peak, uS1, appearing due to the steep temperature gradient generated by laser-beam heating and its radial position, rS1, do not change noticeably with time. On the other hand, the second peak, uS2, travels radially outwards with decreasing magnitude in a self-propelling manner until its radial position, rS2, approaches an asymptotic maximum. Detailed analysis of the coupling among radial temperature gradient, local pressure variation and local convective acceleration near the second peak reveals that hydrothermal mechanisms are responsible for self-propelling travel of uS2. The transient behaviors of both primary and secondary velocity peaks are found to depend on the fluid viscosity and the laser-beam settings

Theory of Gas Purification by Liquid Absorber in Small Rotating Channels with Application to the Patented Rotational Absorber Device

A new design for absorbing vapour-phase impurities from gases is presented. It consists of small channels packed in a rotating vertical cylinder. Gas flows through the channels adjacent to a thin film of absorber liquid. The liquid film is pressed to the radially outward side of each channel by the centrifugal force and flows downwards by gravity. Formulae are presented which describe the concentration distributions of gaseous impurities subject to absorption in gas and liquid. Results include expressions for laminar and turbulent diffusion coefficients to be used in mass balance equations. The role of rotation is quantified including the effect on wavy motion and enhanced diffusion in the liquid layer. Application in design is indicated for the case of separation of the greenhouse gas CO2 from flue gases of fossil fuel combustion processes. At other equal dimensions, the height of the Rotational Absorber Device is calculated to be 25 times shorter than the enormous heights of conventional tray and packed columns.

Mean field control of droplet dynamics with high-order finite-element computations

Liquid droplet dynamics are widely used in biological and engineering applications, which contain complex interfacial instabilities and pattern formation such as droplet merging, splitting and transport. This paper studies a class of mean field control formulations for these droplet dynamics, which can be used to control and manipulate droplets in applications. We first formulate the droplet dynamics as gradient flows of free energies in modified optimal transport metrics with nonlinear mobilities. We then design an optimal control problem for these gradient flows. As an example, a lubrication equation for a thin volatile liquid film laden with an active suspension is developed, with control achieved through its activity field. Lastly, we apply the primal–dual hybrid gradient algorithm with high-order finite-element methods to simulate the proposed mean field control problems. Numerical examples, including droplet formation, bead-up/spreading, transport, and merging/splitting on a two-dimensional spatial domain, demonstrate the effectiveness of the proposed mean field control mechanism.

Nanoscale explosive boiling characteristics of thin liquid film over nano-porous substrates from molecular dynamics study

In this study, phase change characteristics of thin liquid argon film over nano-porous substrates has been investigated through Molecular Dynamics Simulation (MDS). Nano-porous substrate having hexagonal shaped nano-pores with variable pore size, height, number, and surface wettability has been considered. Geometric variation of the nano-porous substrate is characterized in terms of two parameters such as the void ratio and the height to arm thickness. The wettability variation is represented by the wall-fluid interaction strength relative to that of fluid-fluid interaction namely, hydrophilic, hydrophobic and superhydrophobic. Present study reveals that the occurrence of explosive boiling has been found to depend both on the surface topology as well as surface wetting condition. For hydrophilic and hydrophobic nano-porous substrates, explosive boiling phenomena have been observed while for superhydrophobic cases diffusive evaporation has been noted. Phase change characteristics have been analyzed in terms of transient atomic distribution and Mean Squared Displacement (MSD), system temperature, wall heat flux, evaporation rate and onset of explosive boiling designating the separation of liquid film from the wall. The phase change mode for hydrophilic and hydrophobic nano-porous substrates changes from sustained explosive boiling to pseudo explosive boiling with the decrease of void ratio and increase in height. Furthermore, it has been observed that nano-porous substrates with lower void ratio offer improved thermal performance, as designated by higher wall heat fluxes along with other transport parameters. In this regard, increasing the height of the nano-porous substrate has been found to have even much stronger effect. Finally, the underlying mechanisms behind better thermal performance of nano-porous substrates have been studied by analyzing early-stage bubble nucleation and evolution inside of the nanopores.