Film Thickness Fluctuations Research Articles

Experimental study and modeling of water film thickness in aero-engine under air–water mist flow

The water film not only plays an important role in the mass, momentum, and energy transfer between the air–water-surface but also determines the on-wing washing effect of the aero-engine. In view of this, air–water mist flow visualization experiments have been conducted at different gas velocities and water-to-air ratios in a compressor cascade, and the microscopic water film images have been analyzed to extract the transient water film thickness data by the Matlab code. It was found that the transient water film thickness fluctuation has no obvious association with the gas velocity, and the water film fluctuation is more affected by the water-to-air ratio. As the water-to-air ratio increases, the fluctuation magnitude of the water film thickness increases. The average water film thickness has been studied in relation to gas velocity and water flow rate, i.e., the average thickness of water film decreases with increasing gas velocity and increase with the increasing water flow rate. On the basis of the water film flow equation and taking the droplet collection efficiency into account, i.e., from the perspective of the physical mechanism of water film formed, a new model for predicting the water film thickness of a compressor blade surface under the air–water mist flow condition was proposed and validated. This model predicted, with a root mean square error and the mean absolute percentage error of 11.6% and 9.15%, respectively, under the present experimental flow conditions.

Influence of liquid column impact wall on liquid film thickness fluctuation outside the horizontal tube

Abstract A 3D falling film flow model outside the horizontal tube is established to study the influence of the liquid column impact the wall on the liquid film thickness distribution. It is found that the liquid film thickness fluctuates significantly at 0°–60° in single liquid column, and the minimum thickness appears. In the jet flow, there is a critical value of 70° due to the liquid film fluctuation. The liquid film thickness change before 70° was negatively correlated with Reynolds number and positively correlated after 70°. And the fluctuation amplitude negatively correlates with the Reynolds number.

Acoustic resonant imaging technique for visualizing the acoustic properties and thickness of polymer film on substrate.

This paper proposes an acoustic resonant imaging technique for visualizing the acoustic properties and thickness of a polymer film on a substrate. When ultrasound passes through a thin layer, transmission and reflection coefficients of sound pressure attain their extreme values at the resonant frequency. By obtaining the area information of the extreme value and resonant frequency and matching them with a theoretical model, the acoustic properties and thicknesses of a polymer film on a substrate can be visualized. Herein, this technique was applied to a photoresist film coated on a Si wafer, and in addition to visualizing fluctuations in film thickness, the differences in the film hardness that may have occurred during the curing process were successfully detected as the differences in the acoustic impedance of the film. The acoustic resonant imaging technique was successfully used to determine the frequency dependence of both the transmission and reflection coefficients.

Experiments in free falling and downward cocurrent annular flows – Characterization of liquid films and interfacial waves

Falling liquid films and downward cocurrent flows in rounded shape pipes have been experimentally studied during the last decades, estimating the evolution of its major characteristics. The most important variables during the formation and growth of surface waves in falling downward flows have been measured using conductance probes.The main objective of the current research paper is to study the dependency of the characteristics of the thin liquid layer for downward cocurrent annular flows. The GEPELON experimental facility consists of a vertical pipe with 3.8 m of useful test length. Two pipe diameters have been analysed in this experimental study, 42 and 30 mm, in which the range covered by the liquid Reynolds number varies between 570 and 8500 and 800–7900 respectively, while the gas Reynolds numbers vary from 0 to 7.9·104 and from 0 to 1.1·105 respectively for the mentioned pipe diameters. Up to five conductance probes have been placed along the pipes test sections to capture the liquid film thickness fluctuations along time at different distances of the pipe entrance for both developing and fully developed regions.After the study and analysis of the experimental data, the central point of this paper has been the development of new correlations for the liquid film thicknesses and the two major properties of the interfacial waves. Their adjustment procedure has been carried out in terms of dimensionless numbers, aiming to provide more general relationships. In particular, the magnitudes that characterise the interface behavior have been measured, particularly film thicknesses, average disturbance wave amplitudes, and disturbance wave frequencies for each boundary condition.An additional part of the document contains an extensive comparison between the results obtained in this study and the data and expressions of other authors. It has been confirmed the significant dispersion existing among different researchers, especially when analysing variables related to the interfacial waves. This highlights the lack of knowledge in some aspects even today. The different correlations proposed have been calculated based on the best fit of the data from all the series of experiments carried out in this study. Comparisons of the behaviour of these correlations with data from other researchers have also been included.

Statistical characterization of liquid film fluctuations during gas-liquid two-phase counter-current flow in a 1/30 scaled-down test facility of a pressurized water reactor (PWR) hot leg

The present work investigates the statistical characteristics of the fluctuations of the liquid film thickness obtained by resistive-conductance probe signals of a complex geometry representing a 1/30 down-scaled of a PWR hot leg. Here, four locations at the horizontal part of the test section were assessed, while the flow development was investigated through a close observation by using a high-speed video camera. Next, the normalized voltages, indicating the liquid film thickness fluctuations, are shown in the time-series. From the fluctuations, the amplitudes representing the film thickness were recorded. Hence, such statistical diagnostic tools were applied on the basis of time-domain, frequency-domain and also time–frequency domain analyses. From the probability density function, the distributions of the fluctuations were assessed, whereas the power spectral density function describes the dominant frequencies. In addition, the Kolmogorov entropy denotes the chaotic level of the signal, meanwhile the wavelet energy corresponds to several details on the wavy interfaces on the basis of its frequency scales. Next, those parameters were statistically employed to elaborate the interfacial behaviors due to the signal characteristics. The results indicate that the low dominant frequency corresponds to the occurrence of either liquid blockage or slugs during the acquisition, meanwhile the largest scale fluctuations reveal the wave movements.

Axisymmetry breaking, chaos, and symmetry recovery in bubble film thickness profiles due to evaporation-induced Marangoni flows

Understanding the dynamics of evaporating thin liquid films is of practical and fundamental interest. Practically, this understanding is crucial for tuning bubble stability, while fundamentally thin films are an excellent platform to study the characteristics of evaporation-driven two-dimensional (2D) flows. Here, we experimentally study, across a wide range of volatile species concentrations (c0), the spatial and temporal dynamics of film thickness profiles [h(r, θ, t)] over bubbles in binary liquid mixtures subjected to evaporation-induced Marangoni flows. Initially, we probe the spatial structure and show that the spatial symmetry of the film thickness profiles is non-monotonic functions of volatile species concentration with profiles being axisymmetric for both very low (∼1%) and very high (∼90%) concentrations. The temporal evolution of the film thickness fluctuations reveals a similar non-monotonic dependence between the species concentration and the spatial prevalence of fluctuation stochasticity. At a tested intermediate species concentration of 50%, we observe a complete breakdown in spatial symmetry and obtain film thickness fluctuations that are chaotic everywhere in space with spatially invariant fluctuation statistics and rapidly decaying spatial correlation. The observed non-monotonic behavior is a result of the system sensitivity to ambient perturbations scaling as Δγc0(1 − c0)/μ, where Δγ is the difference in equilibrium surface tension between the two species in the mixture and μ is the dynamic viscosity. These insights along with the reported experimental setup serve as an excellent platform to further investigate evaporation-driven 2D chaotic flows.

Study of oil flow rates effects on lubricant oil behaviour during minimum quantity lubrication milling process

Minimum quantity lubrication (MQL) technology has been utilized extensively on these days as an alternative method to conventional cooling technique. A study to understand the know-how of tiny lubricant oil mists to effectively penetrate the cutting zone is important. However, such study is still scarce since the setup of the experimental probe during the machining process is on-going is difficult. The objective of this study is to analyse the lubricant oil film behaviour accumulated on AL6061 workpiece during MQL milling process by using a non-intrusive technique called as laser-induced fluorescence (LIF) method. The experiment was conducted under different oil flow rates to measure the thickness fluctuation of the lubricant oil film. Moreover, the surface roughness of workpiece after the milling process was also measured. As a result, the lubricant oil thickness was found to fluctuate in between 0.2 mm and 0.4 mm for two lower oil flow rates due to the tendency of evaporation. At higher oil flow rate, the lubricant oil fluctuated at higher thickness, i.e. 0.4 mm and 0.5 mm. On the other hand, the obtained mean surface roughness values did not reveal any remarkable difference from each other. However, lower mean surface roughness values were detected at 90 mm distance from machining starting point due to the accumulation of lubricant oil towards the end of workpiece. Therefore, the results of the oil behavior were successfully confirmed and thus MQL milling performance under different oil flow rates condition was clarified.

Vertical Miscibility of Bulk Heterojunction Films Contributes to High Photovoltaic Performance

AbstractThe power conversion efficiencies (PCEs) of organic photovoltaic cells employing donor (polymer):acceptor (small molecule) bulk heterojunction (BHJ) are being rapidly improved. The phase segregation along vertical (film‐depth) direction of the active layer, referring to vertical immiscibility between donor and acceptor, leads to vertically varied and nonlinearly distributed composition, optical and electronic properties. However, the correlation between vertical miscibility and photovoltaic performance is still confusing. Here, it is semi‐empirically found that such vertical variations induced by vertical immiscibility deteriorate PCE. Subsequently, using PM6:Y6‐based binary and ternary BHJ as examples, a combined statistical, experimental, and numerical investigation on the dependence of photovoltaic performance on vertical miscibility is reported. The vertical phase evolution of BHJ significantly depends on solvents and processing methods. As compared with other donor:acceptor systems, polymer:Y6 deposited from appropriate solvents could have the best vertical miscibility which is nearly independent on film‐depth, leading to a higher PCE. PM6:Y6:fullerene ternary blends also have a good and uniform vertical miscibility, forming spatially well‐mixed ternary BHJ. Consequently, under this design guideline, the optimized film‐depth‐dependent miscibility contributes to optimized vertical distribution of optical and electronic properties, leading to an optimized PCE 17.1% with a low sensitivity to fluctuation of film thickness.

Measurement of Liquid Film Thickness in Vertical Multiphase Slug and Churn Flows Using Distributed Ultrasonic Method

A distributed ultrasonic method for measuring the liquid film thickness of vertical multiphase slug flow and churn flow widely existed in industrial production is proposed in this paper. First, according to the proportional relationship between transit time of ultrasonic echo and liquid film thickness, we use the twin-crystal ultrasonic probe to measure the local thickness of liquid film. In view of the non-uniform circumferential distribution of the liquid film, three ultrasonic probes are installed on the same pipe section to measure the circumferential distribution of liquid film thickness. Then, a high-speed data acquisition system suitable for multi-channel and a data transmission system based on gigabit network are designed. The experiments of gas-liquid two-phase flow and oil-gas-water three-phase are carried out in a vertical pipe with a diameter of 20 mm. The fluctuation of local liquid film thickness has been measured by the distributed ultrasonic sensor, and the average thickness of liquid film of slug flow and churn flow under different flow conditions is obtained. The result indicates that the average liquid film thickness of slug flow and churn flow gradually thickens with the increase of liquid superficial velocity, and the distributed ultrasonic sensor can accurately measure the change of film thickness.

Rearrangement of liquid metal surface waves by a uniform transverse magnetic field

In the fusion engineering, the liquid metal flows are considered to be an alternative solution against the solid surface in designing the plasma facing components. However, it remains unclear about the magneohydrodynamics influences on the liquid metal flows due to the lacking of experimental observations that the limited measurement method for the opaque liquid metal film should be criticized. In the present study, a laser profilometer based on the laser triangulation technique is adapted to measure the thickness of liquid GaInSn film, while a visualization method is introduced to monitor the topography of the free surface, which is formed on a horizontal substrate in the presence of a uniform transverse magnetic field. We will firstly demonstrate the applicability of a laser profilometer by measuring the thickness of GaInSn in a container with a quasi-static or dynamic free surface, to validate that the result is in good agreement with theoretical data. Using the visualization method and the laser profilometer, the evolutions of free surface waves and the time-dependent film thickness across a fixed line are measured by varying the flow rate and the magnetic field continuously. It is found that, under the influences of transverse magnetic field, the free surface waves are strongly suppressed along the spanwise direction, leaving the surface waves to propagate in the streamwise direction. More quantitatively, the measured thickness of the free surface reveals that a moderate magnetic field tends to decrease the averaged value and the root mean square value of the film thickness fluctuations while a strong magnetic field will conversely increase them. Besides, the dominant fluctuation frequencies of surface wave are observed to decrease with the increase of magnetic intensity, indicating that greater suppression is induced on the motion of small waves by the transverse magnetic field.

Numerical analysis of transverse reciprocating velocity effects in EHL point contacts

Abstract Experimental results by Fryza et al. [1] on the effect of transverse harmonic vibrations over the film thickness in EHL point contacts are directly compared here with numerical simulations assuming isothermal conditions and either Newtonian or Non-Newtonian fluid rheology. The simulations agree well with the experiments at the highest reciprocating speed frequency applied. It is then apparent from the comparison that with the oils and contact conditions analyzed, phenomena such as shear and thermal thinning, starvation and rig inertia have a minimal impact if not entirely absent. The simulated time-dependent film thickness fluctuations also show that, whereas the mean central film thickness decreases as reported by Fryza et al., both the global minimum and average film values increase as a result of adding a transverse reciprocating speed. Possible explanations for the obtained results are provided, highlighting the need for a broader analysis to better understand this type of problems.

Experimental study of starvation in grease-lubricated finite line contacts

PurposeThe purpose of this paper is to study starvation in grease-lubricated finite line contacts and to understand film-forming mechanisms of grease-lubricated finite line contacts.Design/methodology/approachA multiple-contact optical elastohydrodynamic (EHL) test rig is constructed to investigate the influences of lubricant properties on film thickness and lubrication conditions at different working conditions. The film thickness is calculated according to the relative light intensity principle. The degree of starvation is evaluated by the air–oil meniscus distance and the corresponding film thickness.FindingsThe experimental results show that for greases with high-viscosity base oil, the high-frequency fluctuation of film thickness is observed in low-speed operating conditions. Reducing the viscosity of the base oil and improving running speed can weaken the fluctuation of film thickness. The degree of starvation increases with increasing base oil viscosity, rolling speed and the crown drop. In addition, reducing the replenishment time by reducing the gap between the rollers also can increase the degree of starvation.Originality/valueStarvation is often to occur in finite line contacts, such as roller bearings and gears; there are still limited finite line contact EHL test rigs, much less multiple-contact optical test rigs. Therefore, the present work is undertaken to construct the multiple-contact test rig and to evaluate the mechanism of starvation in finite line contacts.

In situ measurement method for film thickness using transparency resin sheet with low refractive index under wet condition on chemical mechanical polishing

We suggest that a transparency resin sheet with low refractive index can be applied to the measurement of a silicon dioxide (SiO2) film on a silicon wafer under wet condition for a film thickness measurement system on chemical mechanical polishing (CMP). By adjusting the refractive indices of the resin sheet and water, stable measurements of the SiO2 film can be expected, irrespective of slurry film thickness fluctuation because it has robustness against the slurry film. This result indicates that the transparency resin sheet with low refractive index is a useful for monitoring system of CMP.

Optical Elements: Effect of structural defects of aperiodic multilayer mirrors on the properties of reflected (sub)femtosecond pulses

The effect of structural defects (for example, of interlayer roughness, layer thickness fluctuations and departures of Mo film density from the tabular one) on the amplitude and phase of the complex reflection coefficient as well as on the amplitude and duration of reflected pulses is numerically studied by the example of a model aperiodic Mo/Si multilayer mirror intended for the compression of a chirped pulse with a spectrum lying in a 50 – 80 eV photon energy range. The departures of Mo film density from the tabular values and film thickness fluctuations are shown to exert the strongest effect on the amplitude and duration of the reflected pulses. The interlayer roughness has a comparable effect on the amplitude of the reflection coefficient, but its effect on the duration of reflected pulses is weaker. Even small film thickness fluctuations may give rise to additional reflected pulses of high intensity, which are delayed in time relative to the principal pulse. The Mo-film density in a Mo/Si mirror is shown to vary from 0.77 to 0.97 (in units of the tabular value for massive molybdenum) as the film thickness varies from 1.5 to 5.5 nm. We discuss the key problems that have to be solved in the development of the fabrication technology of multilayer mirrors with desired characteristics.

Interfacial Transitional Layer in SiO2 Film Thermally Grown on SiC(000-1)

SiC is one of the materials which attracts much attention for its application to power devices because of its excellent electrical, thermal, and mechanical properties. It is an advantage that SiC is a wide gap semiconductor, and then it is expected to be used in high power, high temperature, and high frequency devices. In addition, it is also an advantage that insulating SiO2 films can be easily formed directly on SiC substrates by thermal oxidation. However, SiO2/SiC interface properties, which determines the electrical characteristics of them, has not been well controlled. One of the problems is higher density of interface traps which might be related to C atoms at the interface. To control the interface properties, oxidation mechanism needs to be understood in detail. We have reported that two dimensional fluctuation of thermal oxide film thickness on SiC brings about dispersion of leakage current for MOS capacitors. However, there was no relation between the film thickness fluctuation and lifetime uniformity characterized by TDDB (Time-Dependent-Dielectric-Breakdown) measurement. It is considered that oxide film quality of SiC has an effect on the lifetime uniformity. In this research, we focused on thermal oxide film quality in the vicinity of the interface and investigated non-uniformity of the film with etching characteristics in diluted HF solution. The substrate in our experiments was 4H-SiC substrate with the miscut angle of 4°. After modified RCA cleaning, thermal oxidation was carried out at 1200°C in O2 ambient. The grown oxide thickness was measured by spectroscopic ellipsometry. The morphology of the oxide surface was characterized by Atomic Force Microscopy (AFM). Then, step-etching, at 3 ~ 5 nm intervals, using 0.5 % diluted HF solution was done for the oxide film. The morphology of the surface after each step- etching was characterized by AFM. Figure is surface roughness growth with step-etching of the oxide films on SiC(000-1) C-face. The oxide thickness is 30 nm. The surface roughness obtained in the case of Si substrate is also plotted for comparison. In the case of Si, the surface roughness increases with etching in the initial stage, which means that the film is not two-dimensionally uniform giving rise to fluctuation of the etching rate. As being close to the SiO2/Si interface, the surface roughness RMS maintains at a constant value, which means that the film is uniform at this region. The drastic decrease in the roughness at the interface is because it is the roughness of the substrate surface obtained after complete removal of the SiO2 film. For SiC case, on the other hand, the surface roughness still increases even at near-interface region. This means that the film quality at near interface is not two-dimensionally uniform. However, the tendency of the roughness growth at near-surface and middle regions are comparable to the Si case, indicating the middle region is uniform for SiC substrate, too. It should be also emphasized that the specific characteristics of SiO2 films at near-interface region was also seen for other oxide thicknesses 10 and 50 nm. These results apparently show that SiO2film, after its formation at the interface, keeps non-uniformity for a while, then gradually turns to be two-dimensionally uniform during subsequent oxidation time. The non-uniform region at near interface, which we call interfacial-transitional layer, was found to be ~10 nm-thick in our present experiments. The non-uniformity might come from existence of C-related impurity and it non-uniform distribution. Homogenization and uniformization might proceed by out-diffusion of the impurities. Figure 1

Non uniform thermal oxidation of SiC and its impact on electrical characteristics

The surface and interface roughnesses of SiO2 films thermally grown on 4H-SiC were investigated by atomic force microscopy. The roughness increases with oxide thickness in the initial oxidation stage, and it saturates at a certain thickness. An increase in roughness reflects nonuniform thermal oxidation. In addition, the two-dimensional fluctuation of oxide film thickness becomes marked for the thin film. Furthermore, current density vs voltage characteristics were investigated using MOS capacitors with various oxide thicknesses. The dispersion of the leakage current was found to be related to the oxide surface and interface roughnesses. On the other hand, time-zero dielectric breakdown voltage showed no relation to the roughness.

Non-linear dynamics of inlet film thickness during unsteady rolling process

The inlet film thickness directly affects film and stress distribution of rolling interfaces. Unsteady factors, such as unsteady back tension, may disturb the inlet film thickness. However, the current models of unsteady inlet film thickness lack unsteady disturbance factors and do not take surface topography into consideration. In this paper, based on the hydrodynamic analysis of inlet zone an unsteady rolling film model which concerns the direction of surface topography is built up. Considering the small fluctuation of inlet angle, absolute reduction, reduction ratio, inlet strip thickness and roll radius as the input variables and the fluctuation of inlet film thickness as the output variable, the non-linear relationship between the input and output is discussed. The discussion results show that there is 180° phase difference between the inlet film thickness and the input variables, such as the fluctuant absolute reduction, the fluctuant reduction ratio and non-uniform inlet strip thickness, but there is no phase difference between unsteady roll radius and the output. The inlet angle, the steady roll radius and the direction of surface topography have significant influence on the fluctuant amplitude of unsteady inlet film thickness. This study proposes an analysis method for unsteady inlet film thickness which takes surface topography and new disturbance factors into consideration.

A physical model of hole mobility for germanium-on-insulator pMOSFETs**Project supported by the National Natural Science Foundation of China (Nos. 61274112, 61176100, 61404055).

A physical model of hole mobility for germanium-on-insulator pMOSFETs is built by analyzing all kinds of scattering mechanisms, and a good agreement of the simulated results with the experimental data is achieved, confirming the validity of this model. The scattering mechanisms involved in this model include acoustic phonon scattering, ionized impurity scattering, surface roughness scattering, coulomb scattering and the scattering caused by Ge film thickness fluctuation. The simulated results show that the coulomb scattering from the interface charges is responsible for the hole mobility degradation in the low-field regime and the surface roughness scattering limits the hole mobility in the high-field regime. In addition, the effects of some factors, e.g. temperature, doping concentration of the channel and the thickness of Ge film, on degradation of the mobility are also discussed using the model, thus obtaining a reasonable range of the relevant parameters.

Crossover from quantum to classical electron transport in ultrathin metal films

Using the Boltzmann approach we have developed one dimensional (1D) model of metal films conductivity for the quantum electron transport. Thickness fluctuation of film has an effect on the electron spectra via carriers scattering in quantum size regime. In the framework of the developed model size dependent conductivity of metal films has been calculated. Here, we employed above mentioned model to explain experimental data on single crystalline CoSi2 films and fine-grained gold films. Wide film thickness ranges applicable by the described model cover also the crossover from quantum to quasiclassical electron transport.

An experimental study of spatiotemporally resolved heat transfer in thin liquid-film flows falling over an inclined heated foil

This paper describes the development of an experimental technique that combines simultaneous planar laser-induced fluorescence (PLIF) and infrared (IR) thermography imaging, and its application to the measurement of unsteady and conjugate heat-transfer in harmonically forced, thin liquid-film flows falling under the action of gravity over an inclined electrically heated-foil substrate. Quantitative, spatiotemporally resolved and simultaneously conducted measurements are reported of the film thickness, film free-surface temperature, solid–liquid substrate interface temperature, and local/instantaneous heat flux exchanged with the heated substrate. Based on this information, local and instantaneous heat-transfer coefficients (HTCs) are recovered. Results concerning the local and instantaneous HTC and how this is correlated with the local and instantaneous film thickness suggest considerable heat-transfer enhancement relative to steady-flow predictions in the thinner film regions. This behaviour is attributed to a number of unsteady/mixing transport processes within the wavy films that are not captured by laminar, steady-flow analysis. The Nusselt number Nu increases with the Reynolds number Re; at low Re values the mean Nu number corresponds to 2.5, in agreement with the steady-flow theory, while at higher Re, both the Nu number and the HTC exhibit significantly enhanced values. Evidence that the HTC becomes decoupled from the film thickness for the upper range of observed film thicknesses is also presented. Finally, smaller film thickness fluctuation intensities were associated with higher HTC fluctuation intensities, while the amplitude of the wall temperature fluctuations was almost proportional to the amplitude of the HTC fluctuations.