Thin Liquid Layer Research Articles

Measurement of the temperature profile during evaporation of water and ethanol

The temperature profiles across a liquid–gas layers at normal atmospheric conditions are measured for water and ethanol. A thin liquid layer is locally heated from the bottom and evaporates from the liquid–gas interface. Micro-thermocouple with the sensor element thickness of 3 μm is used for measurements. It is shown that the temperature profile has a different character for different liquids.

Interferometric analysis of laser-driven cylindrically focusing shock waves in a thin liquid layer.

Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression.

Effects of Gas Flow Rate on Supply of Reactive Oxygen Species Into a Target Through Liquid Layer in Cold Plasma Jet

Understanding the mechanisms for the supply of reactive oxygen species (ROS) into a target through a thin liquid layer by cold plasma jets is essential in biomedical applications. In this paper, impacts of gas flow rate on ROS supply into the liquid bottom were investigated using a KI-starch reagent that can be employed as an ROS indicator even in liquids. ROS distribution visualized on the regent at the liquid bottom was discussed from the stand point of liquid flows induced by the plasma jet irradiation. In the cases of lower gas flow rates, ROS were dominantly transported into the liquid bottom by the plasma-induced linear flow going in the depth direction, and were radially distributed at the bottom. On the other hand, in the cases of higher gas flow rates, ROS were dominantly transported by the supplied-gas-induced vortex flow spreading in the radial direction, and doughnut-shaped ROS distribution patterns were observed at the liquid bottom. The ROS supply into a target through a liquid layer markedly depended on the magnitude relationship between two flows; the vortex flow and the linear flow.

Effect of longitudinal electric field on capillary instability of a thin axisymmetric layer of liquid dielectric coating a dielectric fiber

The flow of a viscous dielectric liquid surrounded with a gas is investigated in the process of capillary disintegration of a thin axisymmetric liquid layer on an undeformable cylindrical dielectric fiber in a uniform electric field is investigated. An asymptotic analysis of the system of equations and hydrodynamic boundary conditions written with allowance for surface ponderomotive forces is carried out for the case when the average thickness of the layer is much smaller than the radius of the fiber cross section. The problem of the transition of the liquid configuration from the state of a stationary cylindrical layer to the hydrodynamic state in the form of a regular sequence of drops is formulated. In this formulation, a nonlinear parabolic equation that describes the evolution of the local thickness of the layer on the time interval to the instant of drop formation is derived. The effect of the key parameters on the capillary instability is analyzed based on the linearized version of the resultant equation and the linearized electrostatic problem of calculating the field perturbations.

Turbulent drag reduction in channel flow with viscosity stratified fluids

In this work we use Direct Numerical Simulation (DNS) to study the turbulent Poiseuille flow of two immiscible liquid layers inside a rectangular channel. A thin liquid layer (fluid 1) flows on top of a thick liquid layer (fluid 2), such that their thickness ratio is h1/h2=1/9. The two liquid layers have the same density but different viscosities (viscosity-stratified fluids). In particular, we consider three different values of the viscosity ratio λ=ν1/ν2: λ=1,λ=0.875 and λ=0.75. Numerical Simulations are based on a Phase Field method to describe the interaction between the two liquid layers. Although a small viscosity ratio is assumed, this physical setup aims at mimicking the situation where water (less viscous fluid) is used to favour the transport of oil (large viscous fluid) inside pipelines. Compared with the case of a single phase flow, the presence of a liquid-liquid interface produces a remarkable turbulence modulation inside the channel, since a significant proportion of the kinetic energy is subtracted from the mean flow and converted into work to deform the interface. This induces a strong turbulence reduction in the proximity of the interface and causes a substantial increase of the volume-flowrate. These effects become more pronounced with decreasing λ.

Ionic Liquids as the MOFs/Polymer Interfacial Binder for Efficient Membrane Separation.

Obtaining strong interfacial affinity between filler and polymer is critical to the preparation of mixed matrix membranes (MMMs) with high separation efficiency. However, it is still a challenge for micron-sized metal organic frameworks (MOFs) to achieve excellent compatibility and defect-free interface with polymer matrix. Thin layer of ionic liquid (IL) was immobilized on micron-sized HKUST-1 to eliminate the interfacial nonselective voids in MMMs with minimized free ionic liquid (IL) in polymer matrix, and then the obtained IL decorated HKUST-1 was incorporated into 4,4'-(hexafluoroisopropylidene)diphthalic anhydride-2,3,5,6-tetramethyl-1,3-phenyldiamine (6FDA-Durene) to fabricate MMMs. Acting as a filler/polymer interfacial binder, the favorable MOF/IL and IL/polymer interaction can facilitate the enhancement of MOF/polymer affinity. Compared to MMM with only HKUST-1 incorporation, MMM with IL decorated HKUST-1 succeeded in restricting the formation of nonselective interfacial voids, leading to an increment in CO2 selectivity. The IL decoration method can be an effective approach to eliminate interfacial voids in MMMs, extending the filler selection to a wide range of large-sized fillers.

Tunneling of the blocked wave in a circular hydraulic jump

The formation of a circular hydraulic jump in a thin liquid layer involves the creation of a horizon where the incoming wave (surface ripples) is blocked by the fast flowing fluid. That there is a jump at the horizon is due to the viscosity of the fluid which is not relevant for the horizon formation. By using a tunneling formalism developed for the study of the Hawking radiation from black holes, we explicitly show that there will be an exponentially small tunneling of the blocked wave across the horizons as anticipated in studies of “analog gravity”.

Surface tension and quasi-emulsion of cavitation bubble cloud

A quasi-emulsion phenomenon of cavitation structure in a thin liquid layer (the thin liquid layer is trapped between a radiating surface and a hard reflector) is investigated experimentally with high-speed photography. The transformation from cloud-in-water (c/w) emulsion to water-in-cloud (w/c) emulsion is related to the increase of cavitation bubble cloud. The acoustic field in the thin liquid layer is analyzed. It is found that the liquid region has higher acoustic pressure than the cloud region. The bubbles are pushed from liquid region to cloud region by the primary Bjerknes forces. The rate of change of CSF increased with the increase of CSF. The cavitation bubbles on the surface of cavitation cloud are attracted by the cavitation bubbles inside the cloud due to secondary Bjerknes forces. The existence of surface tension on the interface of liquid region and cloud region is proved. The formation mechanism of disc-shaped liquid region and cloud region are analysed by surface tension and incompressibility of cavitation bubble cloud.

Spontaneous Fabrication of Three-Dimensional Multiscale Fractal Structures Using Hele-Shaw Cell

Several biosystems such as leaf veins, respiratory system, blood circulation, and some plant xylem involving multiscale fractal topologies are being mimic for their inherent natural optimization. Three-dimensional fractal structures spanning multiple scales are difficult to fabricate. In this paper, we demonstrate a new method to fabricate structures spanning meso- and microscale in a relatively easy and inexpensive manner. A well-known Saffman–Taylor instability is exploited for the same in a lifted Hele-Shaw cell. In this cell, a thin layer of liquid is squeezed between two plates being lifted angularly leaving behind the fractal rearrangement of fluid which is proposed to be solidified later. We demonstrate and characterize fractal structures fabricated using two different fluids and corresponding methods of solidification. The first one is ceramic suspension in a photopolymer and another is polystyrene solution with photopolymerization and solvent vaporization as methods of solidification, respectively. The fabrication process is completed in period of a few seconds.

Air plasma treatment of liquid covered tissue: long timescale chemistry

Atmospheric pressure plasmas have shown great promise for the treatment of wounds and cancerous tumors. In these applications, the sample is usually covered by a thin layer of a biological liquid. The reactive oxygen and nitrogen species (RONS) generated by the plasma activate and are processed by the liquid before the plasma produced activation reaches the tissue. The synergy between the plasma and the liquid, including evaporation and the solvation of ions and neutrals, is critical to understanding the outcome of plasma treatment. The atmospheric pressure plasma sources used in these procedures are typically repetitively pulsed. The processes activated by the plasma sources have multiple timescales—from a few ns during the discharge pulse to many minutes for reactions in the liquid. In this paper we discuss results from a computational investigation of plasma–liquid interactions and liquid phase chemistry using a global model with the goal of addressing this large dynamic range in timescales. In modeling air plasmas produced by a dielectric barrier discharge over liquid covered tissue, 5000 voltage pulses were simulated, followed by 5 min of afterglow. Due to the accumulation of long-lived species such as ozone and NxOy, the gas phase dynamics of the 5000th discharge pulse are different from those of the first pulse, particularly with regards to the negative ions. The consequences of applied voltage, gas flow, pulse repetition frequency, and the presence of organic molecules in the liquid on the gas and liquid reactive species are discussed.

Desorption-Mediated Motion of Nanoparticles at the Liquid–Solid Interface

Nanoparticles (NPs) confined in thin layers of liquid within liquid cells used for in situ transmission electron microscopy (TEM) move very slowly, in contrast to free particles in bulk liquid. The reason is still poorly understood. Here, we tracked gold NPs moving in water at the liquid–solid interface with in situ TEM at rates of 100 frames per second. The recorded motion exhibited three key features: (1) it was made up of sustained sequences of “sticky” motion where NPs only moved a few nanometers each time; (2) sporadic long “flights” where the NPs traveled tens to hundreds of nanometers between frames; and (3) “flights” are accompanied by intermittent, fast pivoted rotations. Trajectory analysis shows that the displacements follow a truncated Levy distribution, pointing to desorption-mediated motion of NPs at the liquid–solid interface. We further associate pivoted rotations with a transient “weakly adsorbed” state between desorption and adsorption of NPs. The frequency of desorption was also control...

Fibre laser cutting stainless steel: Fluid dynamics and cut front morphology

In this paper the morphology of the laser cut front generated by fibre lasers was investigated by observation of the ‘frozen’ cut front, additionally high speed imaging (HSI) was employed to study the fluid dynamics on the cut front while cutting. During laser cutting the morphology and flow properties of the melt film on the cut front affect cut quality parameters such as cut edge roughness and dross (residual melt attached to the bottom of the cut edge). HSI observation of melt flow down a laser cutting front using standard cutting parameters is experimentally problematic because the cut front is narrow and surrounded by the kerf walls. To compensate for this, artificial parameters are usually chosen to obtain wide cut fronts which are unrepresentative of the actual industrial process. This paper presents a new experimental cutting geometry which permits HSI of the laser cut front using standard, commercial parameters. These results suggest that the cut front produced when cutting medium section (10mm thick) stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered by a thin layer of liquid. HSI observation and theoretical analysis reveal that under these conditions the humps move down the cut front at an average speed of approximately 0.4m/s while the covering liquid flows at an average speed of approximately 1.1m/s, with an average melt depth at the bottom of the cut zone of approximately 0.17mm.

Mathematical modeling of the instability of viscous fluid films

Nonlinear mathematical model of free surface fluid film is presents. Increment, frequency, phase velocity for thin layers of viscous liquids at low Reynolds numbers are calculated. The instability region is found. Optimal flow regimes of films of water and alcohol, corresponding to the maximum values of increment, are calculated.

Normal and oblique impacts between smooth spheres and liquid layers: Liquid bridge and restitution coefficient

Detailed understanding of individual collision mechanics of particles in the presence of liquid is crucial for modeling wet particle flows that are omnipresent in nature and various industries. Our early work (Ma et al., 2013) preliminarily characterized the oblique impacts between rough-surface sphere and liquid layers with wide-ranging impact parameters by means of restitution coefficients. The current paper deepens the early work by performing both normal and oblique impacts between smooth-surface collision bodies, focusing on the collision details such as liquid bridge configuration, liquid layer morphology as well as the restitution coefficients with the aid of improved experimental setup. Different from static liquid bridge or the dynamic bridge with constant liquid volume, the formation and development of impact-induced liquid bridge is greatly influenced by the inertia of surrounding liquid that could be represented by liquid Reynolds number. Moreover, liquid inertia is also found to affect the total kinetic energy reduction of spheres through changing the layer morphology, thus having to be considered during theoretical modeling. Liquid bridge force contributes a lot to the energy reduction of rebounding sphere in the normal direction, while has little effects in the tangential direction. For the oblique impacts of smooth spheres with thin liquid layers, no matter how viscous the liquid is, the tangential restitution coefficients are always maintained at higher values than dry impact for the lubrication effect exerted by liquid. The effect gradually weakens as the layer thickness increases. The lubrication effect is not observed for rough-surface impacts owing to the additional energy reduction caused by the physical interaction between surface asperities. Due to the significant role of layer thickness in the energy dissipation process, the liquid drag force arising during the impacts with considerable thickness has to be considered in the development of theoretical models for restitution coefficients.

Disjoining potential and grain boundary premelting in binary alloys

Many grain boundaries (GBs) in crystalline materials develop highly disordered, liquidlike structures at high temperatures. In alloys, this premelting effect can be fueled by solute segregation and can occur at lower temperatures than in single-component systems. A premelted GB can be modeled by a thin liquid layer located between two solid-liquid interfaces interacting by a disjoining potential. We propose a single analytical form of the disjoining potential describing repulsive, attractive, and intermediate interactions. The potential predicts a variety of premelting scenarios, including thin-to-thick phase transitions. The potential is verified by atomistic computer simulations of premelting in three different GBs in Cu-Ag alloys employing a Monte Carlo technique with an embedded atom potential. The disjoining potential has been extracted from the simulations by analyzing GB width fluctuations. The simulations confirm all shapes of the disjoining potential predicted by the analytical model. One of the GBs was found to switch back and forth between two (thin and thick) states, confirming the existence of thin-to-thick phase transformations in this system. The proposed disjoining potential also predicts the possibility of a cascade of thin-to-thick transitions caused by compositional oscillations (patterning) near solid-liquid interfaces.

Normal stress measurement in foams and emulsions in the presence of slip

Steady shear measurements on three yield-stress fluids, a foam and two types of emulsions, were carried out using a smooth torsional parallel-plate geometry, with different gap spacings to assess the effect of wall slip on the normal stress differences. The classical Mooney analysis gave slip-corrected flow curves that agreed with data from a roughened cone-and-plate for all three materials, despite the fact that the compatibility requirement between the flow curve and slip velocity functionalities was not satisfied, indicating that the method is robust for this geometry. All three materials exhibited finite normal stresses following yielding, with power-law exponents of approximately 0.4, as well as shear stresses that can be represented by the Herschel–Bulkley model. The systems showed complete slip for shear stresses below the yield stress, with the stress transmitted to the unyielded material through a thin liquid layer causing an elastic deformation that was measurable for the unyielded foam, which can be represented by the Mooney–Rivlin elastic constitutive equation. The unyielded foam exhibited large sample-to-sample variations in the apparent modulus for identical loading conditions.

Evaporative thermal resistance and its influence on microscopic bubble growth

Simulations of the formation of small steam bubbles indicate that the rate of growth of bubbles is very sensitive to the rate of evaporation of the micro-layer of liquid beneath the bubble. Such evaporation is rapid, and is modelled as being driven by the large heat flux through the thin liquid layer caused by the difference in temperature between the solid–liquid interface, and the saturation temperature in the interior of the bubble. However, application of this approach to recent experimental measurements of Jung and Kim generated anomalous results. In this paper we demonstrate that a model of the micro-layer heat flux that includes an allowance for the finite evaporative thermal resistance is able to eliminate these anomalies. This evaporative thermal resistance is a consequence of near-interface molecular dynamics, characterised by a quantity termed ‘evaporation coefficient’. Whilst in most engineering applications evaporative thermal resistance is small compared to conductive resistance, here, with the micro-layer thickness ranging from a few microns down to zero, it becomes of considerable importance. Selection of a molecular ‘evaporation coefficient’ to restore consistency to the anomalous measurements allows a plausible numerical value to be inferred. For the several times and multiple locations studied, a fairly consistent value of between 0.02 and 0.1 is indicated, (for saturated water in laboratory conditions), which itself is consistent with earlier literature values of this rather difficult quantity. It is shown that the evaporative resistance always represents a large fraction of the conductive resistance, and for important phases of the process dominates it. The need for inclusion of this phenomenon in the micro-layer models used in bubble analysis is clear.

Effect of medium range order on pulsed laser crystallization of amorphous germanium thin films

Sputter deposited amorphous Ge thin films had their nanostructure altered by irradiation with high-energy Ar+ ions. The change in the structure resulted in a reduction in medium range order (MRO) characterized using fluctuation electron microscopy. The pulsed laser crystallization kinetics of the as-deposited versus irradiated materials were investigated using the dynamic transmission electron microscope operated in the multi-frame movie mode. The propagation rate of the crystallization front for the irradiated material was lower; the changes were correlated to the MRO difference and formation of a thin liquid layer during crystallization.

Ultrasound-based measurement of liquid-layer thickness: A novel time-domain approach

Measuring the thickness of a thin liquid layer between two solid materials is important when the adequate separation of metallic parts by a lubricant film (e.g., in bearings or mechanical seals) is to be assessed. The challenge in using ultrasound-based systems for such measurements is that the signal from the liquid layer is a superposition of multiple reflections. We have developed an algorithm for reconstructing this superimposed signal in the time domain. By comparing simulated and measured signals, the time-of-flight of the ultrasonic pulse in a layer can be estimated. With the longitudinal sound velocity known, the layer thickness can then be calculated. In laboratory measurements, we validate successfully (maximum relative error 4.9%) our algorithm for layer thicknesses ranging from 30µm to 200µm. Furthermore, we tested our method in the high-temperature environment of polymer processing by measuring the clearance between screw and barrel in the plasticisation unit of an injection moulding machine. The results of such measurements can indicate (i) the wear status of the tribo-mechanical screw-barrel system and (ii) unsuitable process conditions.

Negative Pressures and Spallation in Water Drops Subjected to Nanosecond Shock Waves.

Most experimental studies of cavitation in liquid water at negative pressures reported cavitation at tensions significantly smaller than those expected for homogeneous nucleation, suggesting that achievable tensions are limited by heterogeneous cavitation. We generated tension pulses with nanosecond rise times in water by reflecting cylindrical shock waves, produced by X-ray laser pulses, at the internal surface of drops of water. Depending on the X-ray pulse energy, a range of cavitation phenomena occurred, including the rupture and detachment, or spallation, of thin liquid layers at the surface of the drop. When spallation occurred, we evaluated that negative pressures below -100 MPa were reached in the drops. We model the negative pressures from shock reflection experiments using a nucleation-and-growth model that explains how rapid decompression could outrun heterogeneous cavitation in water, and enable the study of stretched water close to homogeneous cavitation pressures.