Pre-wetted Surface Research Articles

Exploring the dynamic mechanism of water wetting induced corrosion on differently pre-wetted surfaces in oil–water flows

Water wetting induced corrosion is the core issue for uncovering the corrosion mechanism in multiphase flow environments, relevant to many industrial applications. Here, we experimentally investigated the dynamic failure of an oil film attached on the pre-wetted model surfaces by the electrochemical current detection using an “Alternate Wetting Cell” and the direct visualization of near-wall fluid states. The oil pre-wetted surface performed a superior corrosion mitigation efficiency, exhibiting a protective oil film with a duration time at least 5 times longer than the water pre-wetted surface. It confirms that the oil film rupture is a combined process of the local penetration and pinning of micro-droplets and the phase redistribution of the near-wall fluids. Corrosion finally initiates and propagates on the surface once the droplets pin there or damage the oil film. The result suggests new control strategies for materials corrosion in complex systems by surface modification and fluid management.

Viscoplastic lines: printing a single filament of yield stress material on a surface

This study presents the spreading of a single filament of a yield stress (viscoplastic) fluid extruded onto a pre-wetted solid surface. The filaments spread laterally under surface tension forces until they reach a final equilibrium shape when the yield stress dominates. We use a simple experimental set-up to print the filaments on a moving surface and measure their final width using optical coherence tomography. Additionally, we present a scaling law for the final width and determine the corresponding prefactor using asymptotic analysis. We then analyse the level of agreement between the theory and experiments, and discuss the possible origins of discrepancies. The process studied here has applications in extrusion-based thermoplastic and bio-three-dimensional printing.

Tailoring micro/nanostructured porous polytetrafluoroethylene surfaces for dual-reversible transition of wettability and transmittance

Materials with dynamic transition are current mostly limited to two-state transition of single property. Herein, a textured polytetrafluoroethylene (PTFE) film with hierarchical porous micro/nanostructures is fabricated by one-step femtosecond laser line-by-line scanning technology. The as-prepared sample is superhydrophobic and opaque, and its wettability and optical transmittance can be simultaneously controlled. As the sample prewetted with alcohol, it displayed superhydrophilicity with high transparency. In contrast, since the prewetted surface was dried, it recovered its original superhydrophobicity and opacity. The switching repeatability, applicability of solutions, stability of the samples are also demonstrated. By using its controllable switching performances, potential applications such light pass/block controlling, anti-fogging, cooling were realized on the as-prepared surfaces. This work reports a smart surface with dual-functional transition ability, which could open fresh ideas for a broad range of multifunctional applications.

Capillary ripples in thin viscous films

Capillary ripples in thin viscous films are important features of coating and lubrication flows. Here, we present experiments based on digital holographic microscopy, measuring with nanoscale resolution the morphology of capillary ripples ahead of a viscous drop spreading on a prewetted surface. Our experiments reveal that upon increasing the spreading velocity, the amplitude of the ripples first increases and subsequently decreases. Above a critical spreading velocity, the ripples even disappear completely and this transition is accompanied by a divergence of the ripple wavelength. These observations are explained quantitatively using linear wave analysis, beyond the usual lubrication approximation, illustrating that new phenomena arise when the capillary number becomes of the order of unity.

Cleaning of gypsum-rich black crusts on granite using a dual wavelength Q-Switched Nd:YAG laser

Gypsum black crust is one of the most severe deterioration forms affecting the built Cultural Heritage (CH) worldwide. Its removal from granitic substrates is necessary since gypsum layer favours the deterioration of the rock. This study investigates the laser assisted removal of gypsum black crust, from a Hercynian granite from the North-West (NW) Iberian Peninsula. The suggested laser cleaning methodology involves the combination of two wavelengths (1064 nm and 355 nm) from a QS Nd:YAG laser system and is based on their simultaneous interaction with the involved materials at various fluence values and FIR/FUV ratios (1/1, 1/3 and 3/1). This ablative scheme was initially introduced in order to overcome undesirable discoloration issues recorded upon single wavelength laser cleaning of black crusts but its role to effectively remove different undesired over-layers was also acknowledged.The results of this study were comparatively evaluated with surfaces irradiated using one of the two specific wavelengths, while the presence of water as an enhancing cleaning agent was also considered. A multi-analytical evaluation methodology based on stereomicroscopy, Raman spectroscopy, Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), was carried out to determine the optimum cleaning strategy in terms of crust removal effectiveness and minimisation of damages to the substrate.The optimum irradiation condition determined in this study is the simultaneous application of the two wavelengths in FIR/FUV = 1/1 ratio (with FIR = FUV = 0.3 J·cm−2) on a pre-wetted surface. Under this irradiation condition a high removal level of carbonaceous pollution particles and gypsum was achieved, without any damage to the granitic minerals in the form of melting and spallation and/or exfoliation of planes of biotite and K-feldspar and cleavage fracture of quartz.

Contact Angle Determination on Hydrophilic and Superhydrophilic Surfaces by Using r-θ-Type Capillary Bridges.

To avoid the restrictions of the captive bubble and the Wilhelmy plate techniques, a method was introduced for contact angle measurements under equilibrium conditions. It enables to determine even ultralow contact angles with high precision without prewetting the investigated surface because in this case, the capillary bridge of the test liquid is formed from a pendant drop and used as a probe. The contact angle is determined from the measured capillary force and liquid bridge geometry by using Delaunay's analytical solution. The method was experimentally proved to be valid. As a demonstration, contact angles less than 1° were measured with the uncertainty down to 0.1° on lightly corroded glass surfaces. Moreover, a new observation was obtained in complete wetting situations: the receding contact line starts to advance again during the increase of the bridge length. The contact angle is much lower in this readvancing phase compared to the advancing and receding values because the contact line finds prewetted surface in front of itself. Further advantage of the method is that the existing contact angle goniometers can be developed further into the presented measurement setup.

Study of water wetting and water layer thickness in oil-water flow in horizontal pipes with different wettability

Two-phase oil-water pipe flow is common in oil production and transportation. Appropriate estimation of phase wetting (oil wet or water wet) of internal pipe walls can significantly reduce corrosion control costs, and increase confidence in measures taken to ensure the integrity of pipelines. Water wetting can be avoided by fully dispersing the water phase into the oil phase. It has been suggested that pipe wettability may affect oil-water flow patterns; particularly, water-in-oil dispersed flow transition boundaries. However, there are no systematic studies in the literature on this matter for carbon steel pipes, which are the preferable choice for economic reasons in the oil and gas industry. Moreover, traditional and widely used models to predict the onset of dispersed flow do not consider the effect of pipe wettability. This work studies phase wetting and water layer thickness in large scale oil-dominated oil-water horizontal flow in carbon steel and PVC pipes of similar internal diameter (0.1 m) and roughness, but different wettability. The effect of wetting hysteresis (oil or water pre-wetted pipe surface) on phase wetting is also investigated. It is demonstrated that pipe wettability plays a very important role on the transition boundaries for phase wetting (oil wet to water wet) and the transition to fully dispersed flow. Water droplet deposition and spreading are identified as the main mechanisms for incipient segregation in a hydrophilic pipe. In a hydrophobic pipe, poor surface wettability hinders the sticking and spreading of water droplets. Water wetting in a hydrophobic pipe requires a sufficient low flow velocity at which local droplet accumulation and coalescence becomes the dominant segregation mechanism. Predictions from available hydrodynamic models are compared with the experimental results and recommendations are provided.

Heat transfer characteristics of double, triple and hexagonally-arranged droplet train impingement arrays

In this study, hydrodynamics and heat transfer of multiple droplet trains impinging a pre-wetted solid surface have been investigated experimentally. A piezo-electric droplet generator has been designed and constructed, which is capable of producing double, triple and hexagonally-arranged droplet trains. A translucent sapphire substrate coated with a thin layer of indium tin oxide (ITO) was used as flat heating element. The effects of droplet Weber number, impact spacing and impingement pattern on liquid film hydrodynamics and heat transfer have been evaluated using high speed optical imaging and IR thermal imaging techniques. High speed images show that a hump was formed between two impact craters for double droplet train impingement. Surface jet flows were observed among impact craters for triple and hexagonally-arranged droplet train impingement arrays. Heat transfer results reveal that horizontal impact spacing and impingement pattern play significant roles in cooling performance. For double droplet train impingement, it was found that higher impact spacing leads to better cooling performance both locally (i.e. within the impingement zone) and globally (i.e. outside the droplet impingement zone). For triple droplet train impingement, there is an optimum impact spacing for heat transfer. For hexagonally-arranged droplet train impingement arrays, lower impact spacing leads to better cooling performance locally. However, higher impact spacing leads to better cooling performance globally. Comparisons have been made between droplet train impingement and circular jet impingement for various impingement patterns. Heat transfer measurements show that droplet train impingement leads to better cooling performance for various impingement patterns. In summary, results reveal that the combined effects of the droplet Weber number and impingement pattern are significant factors in the study of droplet-induced surface heat transfer phenomena.

Pre-wetted treatment for optimized multicrystalline Si wafer texturing and its solar cell efficiency improvement

We report a simple way to optimize texturing of multi-crystalline Si (mc-Si) wafer by pre-wetted surface in the wet etching. In the range of 300–1200 nm, a better texturing surface was achieved with average reflectivity 40 mA) and efficiency (0.1% absolute) has been achieved compared with control cell without deterioration of other performance parameters (such as open circuit voltage and fill factor). Internal quantum efficiency measurement also proves the lower front surface recombination velocity of pre-wetted based solar cell. We believe that the improved wetting surface property of multi-crystalline Si prior to etching is important for optimizing texturing process, and pre-wetted treatment is easy integrated for industrial mass production.

Simple, Reversible, and Fast Modulation in Superwettability, Gradient, and Adsorption by Counterion Exchange on Self-Assembled Monolayer.

A simple fabrication method for preparation of surfaces able to switch from superhydrophobic to superhydrophilic state in a reversible and fast way is described. A self-assembled monolayer (SAM) consisting of quaternary ammonium group with aliphatic tail bearing terminal thiol functionality was created on gold nano/microstructured and gold planar surfaces, respectively. A rough nano/microstructured surface was prepared by galvanic reaction on a silicon wafer. The reversible counterion exchange on the rough surface resulted in a switchable contact angle between <5° and 151°. The prewetted rough surface with Cl(-) as a counterion possesses a superoleophobic underwater character. The kinetics of counterion exchanges suggests a long hydration process and strong electron ion pairing between quaternary ammonium group and perfluorooctanoate counterion. Moreover, a wettability gradient from superhydrophobic to superhydrophilic can be formed on the modified rough gold surface in a robust and simple way by passive incubation of the substrate in a counterion solution and controlled by ionic strength. Furthermore, adsorption of gold nanoparticles to modified plain gold surface can be controlled to a high extent by counterions present on the SAM layer.

Liquid spreading on a pre-wetted biomimetic surface inspired by a dog’s tongue

Liquid film flow widely exists in industries due to its high thermal efficiency and low flow flux. The spreading of the liquid film is strongly influenced by the properties of the working surface. A biomimetic surface with multi-scale structures inspired by the skin of a dog’s tongue is proposed in this paper for the enhancement of heat and mass transfer. The spreading and flow behaviors of a gravity-driven liquid falling down the pre-wetted biomimetic surface are compared with that on the smooth surfaces, via the combination of numerical simulations using the volume of fluid (VOF) method, and experimental measurements using high-speed imaging. On the pre-wetted smooth substrate, liquid merges with two droplets before the free surface of the liquid slowly develops into a parabolic shape. In contrast, on the biomimetic surface, liquid rapidly and uniformly spreads into a thin film which could effectively enhance mass transfer in both spanwise and streamwise directions. The characteristics and distribution of the microstructures on the proposed biomimetic surface are potentially to be used to guide the design of the surface in high efficiency heat exchangers and reactors.

Effects of High Frequency Droplet Train Impingement on Crown Propagation Dynamics and Heat Transfer

The objective of this study is to investigate the hydrodynamics and heat transfer phenomena due to high frequency droplet train impingement on a pre-wetted solid surface for electronic cooling applications. The effects of crown propagation dynamics and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of mono-dispersed HFE-7100 droplets was generated using a piezo-electric droplet generator at a frequency ( f ) of 6000 Hz with a droplet Weber number (We) of 280. Droplet-induced crater and crown were imaged using a high speed camera system. Numerically, the ANSYS Fluent CFD tool was used to simulate the droplet train impingement process. A reasonable agreement was reached between experimental and numerical data in terms of crown propagation dynamics. Numerical simulations reveal that at the instant of initial spot formation, the magnitude of droplet velocity is almost identical to the crown's radial velocity. The instantaneous temperature field obtained by numerical simulations shows that heat transfer was most effective within the crown propagation region due to the radial momentum generated by the droplets, which leads to a large velocity gradient within the liquid film. A significant increase in surface temperature was observed beyond a radial position of 500 μm. In summary, high frequency droplet impingement leads to a very small temperature gradient in the radial direction within the droplet-induced impact crater. This study will benefit in understanding the relationship between the droplet parameters and surface heat transfer for different cooling applications involving impinging droplets.

Numerical and experimental investigations of crown propagation dynamics induced by droplet train impingement

In this study, hydrodynamics of HFE-7100 droplet train impinging on a pre-wetted solid surface was investigated experimentally and numerically. Experimentally, single stream of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet velocity. Crown propagation events were imaged using a high-speed camera system given the high-frequency of droplet impingement. Relationships between droplet-induced crown propagation and crater formation were investigated experimentally. The high-frequency droplet impingement process was simulated numerically using CFD tool. Crown propagation dynamics were evaluated and analyzed experimentally and numerically, with reasonable agreement between the two methods. A revised theoretical crown propagation model based on numerical results is proposed in this paper, which takes into account the impinged liquid velocity distribution and film thickness at the moment of initial spot formation. The revised theoretical crown propagation model gives predictions with improved accuracy, which are in better agreement with the numerical results.

Numerical simulations of droplet train and free surface jet impingement

The cooling behavior of the impingement of a droplet train, and free surface jets over a heated and pre-wetted surface is explored employing an Algebraic Volume-of-Fluid methodology. The code is based on a modified version of the two-phase numerical solver interFoam (OpenFOAM) (Trujillo and Lewis, 2012). Two versions of the free surface jet are studied. The first consists of a fully-developed profile exiting the nozzle, and the second is characterized by a uniform velocity distribution. Results show that both jet configurations have higher cooling performance than the droplet train locally and globally, with the fully-developed case being the most effective of the two jet arrangements. Locally, the performance is measured by radial profiles of the boundary-layer-displacement thickness and heat transfer coefficient. Globally, the cooling effectiveness is directly proportional to the surface area that resides within the high-convection region, i.e. before the boundary layer separation point. On a temporal basis, the liquid film within the impingement region of the droplet train exhibits pronounced variations in velocity magnitude and film thickness. This is directly attributed to the nature of continuous droplet impacts affecting the impingement region, and gives rise to an unsteady cooling and heating of the fluid near the wall. In contrast for the jets, the film and the corresponding free surface are nearly steady with only minor perturbations.

Thermal boundary layer analysis corresponding to droplet train impingement

Simulations of droplet train impingement on a pre-wetted solid surface heated from below are used to study the thermal boundary layer behavior over a parameter space which includes variations in Reynolds, Peclet, and Weber numbers, as well as variations in inter-droplet spacing and initial liquid film thickness. Computationally, a modified version of the Volume-of-Fluid method is developed and employed in this study. The solver is validated against closed form solutions and additional experimental data from the literature. In combination with the simulations, an analytical representation is also developed and compared to the computations yielding favorable agreement. Results show that the boundary layer thickness is mostly affected by changes in inter-droplet spacing, Reynolds, and Peclet number, and influenced minimally by variations in Weber number and initial film thickness. In fact, it is explicitly demonstrated in the analysis that the impact velocity has the greatest effect in local heat transfer. An analytical expression for the Nusselt number radial profile is also developed. It shows that the Nusselt number scales as ∼ Re1/2, and its radial dependence is \documentclass[12pt]{minimal}\begin{document}$\sim \sqrt{r}$\end{document}∼r, which is the same as the circular jet impingement case. The notable difference in the present Nusselt number relationship is the role of inter-droplet spacing, which plays a significant role in the current configuration.

Numerical Simulations and Experimental Characterization of Heat Transfer From a Periodic Impingement of Droplets

In this combined experimental and simulation investigation, a stream of HFE-7100 droplets striking a prewetted surface under constant heat flux was studied. An implicit free surface capturing technique based on the Volume-of-Fluid (VOF) approach was employed to simulate this process numerically. Experimentally, an infrared thermography technique was used to measure the temperature distribution of the surface consisting of a 100 nm ITO layer on a ZnSe substrate. The heat flux was varied to investigate the heat transfer behavior of periodic droplet impingement at the solid–liquid interface. In both experiments and simulations, the morphology of the impact zone was characterized by a quasi-stationary liquid impact crater. Comparison of the radial temperature profiles on the impinging surface between the experiments and numerical simulations yielded reasonable agreement. Due to the strong radial flow emanating from successive droplet impacts, the temperature distribution inside the crater region was found to be significantly reduced from its saturated value. In effect, the heat transfer mode in this region was governed by single phase convective and conductive heat transfer, and was mostly affected by the HFE-7100 mass flow rates or the number of droplets. At higher heat fluxes, the minimum temperature, and its gradient with respect to the radial coordinate, increased considerably. Numerical comparison between average and instantaneous temperature profiles within the droplet impact region showed the effect of thermal mixing produced by the liquid crowns formed during successive droplet impact events.

Stability of under-compressive waves with second and fourth order diffusions

This is a continuation of our work on the nonlinear stability of traveling shock fronts arising in multidimensional conservation laws with fourth order regularization only. Our motivating example is the thin film equation for which planar waves correspond with fluid coating a pre-wetted surface. Under only the fourth order regularization, we established the nonlinear stability of compressive waves for dimensions $d\geq 2$, and of under-compressive waves for dimensions $d\geq 3$ under general spectral conditions. The case of stability for under-compressive waves in the thin film equations for the critical dimensions $d=1,2$ remained open. In this paper we study the nonlinear stability of under-compressive waves by assuming both the second and fourth order regularization. We present a step toward the open problem by establishing the nonlinear stability of under-compressive waves in dimensions $d\geq 2$ under general spectral conditions. We emphasize the above mentioned stability question remains still open.

Field, laboratory and estimated soil-water content limits

For the purpose of irrigation scheduling, estimates of soil-water content limits are determined using field or laboratory measurements or empirically-based regression equations. In this study the field method involved measuring simultaneously the soil-water content (using a frequency domain reflectometer with the PR1 profile probe that relies on changes in the dielectric constant of soil), and soil-water potential (using Watermark granular matrix sensors and tensiometers) at three depths (100, 300 and 600 mm) from a 1 m2 bare plot. A retentivity relationship was developed from these measurements and the drained upper limit was estimated to be 0.355 m3·m-3 when the drainage from the pre-wetted surface was negligibly small. The lower limit, corresponding to -1 500 kPa, was estimated to be 0.316 m3·m-3. In the laboratory, soil-water content and soil matric potential were measured on undisturbed soil samples taken from the edge of the bare plot. The undisturbed soil samples were saturated and exposed to different matric potentials between -1 and -1 500 kPa. A retentivity relationship was then developed from these measurements. The laboratory method realized a drained upper limit value of 0.390 m3·m-3 at -33 kPa and a lower limit value of 0.312 m3·m-3 at -1 500 kPa. A regression equation, which uses the soil bulk density and the clay (

Hysteresis and non-uniqueness in the speed of the onset of instability in curtain coating

The maximum speed of stable coating is determined experimentally by the onset of air entrainment in curtain coating onto a pre-wetted surface over a broad range of dimensionless parameters (Reynolds number: ). We show not only that the substrate speed at the onset of instability is flow-rate dependent for a pre-wetted surface, but that the speed at which instability occurs is not always unique for a given flow rate.

Nonlinear Stability for Multidimensional Fourth-Order Shock Fronts

We consider the question of stability for planar wave solutions that arise in multidimensional conservation laws with only fourth-order regularization. Such equations arise, for example, in the study of thin films, for which planar waves correspond to fluid coating a pre-wetted surface. An interesting feature of these equations is that both compressive, and undercompressive, planar waves arise as solutions (compressive or undercompressive with respect to asymptotic behavior relative to the un-regularized hyperbolic system), and numerical investigation by Bertozzi, Munch, and Shearer indicates that undercompressive waves can be nonlinearly stable. Proceeding with pointwise estimates on the Green's function for the linear fourth-order convection–regularization equation that arises upon linearization of the conservation law about the planar wave solution, we establish that under general spectral conditions, such as appear to hold for shock fronts arising in our motivating thin films equations, compressive waves are stable for all dimensions d≧2 and undercompressive waves are stable for dimensions d≧3. (In the special case d=1, compressive waves are stable under a very general spectral condition.) We also consider an alternative spectral criterion (valid, for example, in the case of constant-coefficient regularization), for which we can establish nonlinear stability for compressive waves in dimensions d≧3 and undercompressive waves in dimensions d≧5. The case of stability for undercompressive waves in the thin films equations for the critical dimensions d=1 and d=2 remains an interesting open problem.