Wetting In Systems Research Articles

Aluminum droplets on Ni substrates: Evaluating high-temperature oxidation at the liquid/gas interface

In reactive wetting aluminum (Al)/nickel (Ni) systems, oxides at liquid/gas (L/G) interfaces received less attention compared to intermetallic compounds forming at liquid Al/solid Ni interfaces yet potentially significantly influencing wetting properties. Between 900 and 1250 °C, we observed that the shape of sessile Al droplets with dense oxides at the L/G interface and intense interactions at liquid Al/solid Ni interfaces deviates from a spherical cap and can be used as a way to characterize the oxide evolution. This approach reveals a critical temperature of 1100 °C under a PO2 level of 3.9 × 10−9 atm and a vacuum level of ∼2 × 10−2 mbar. Above it, Al2O3 oxides at the L/G interface are eliminated as Al2O gas, while at lower temperatures, they develop as a mixture of Al2O3 oxides and Al-Ni intermetallic compounds with Ni being dissolved from the substrate. At high temperatures (above 1200 °C), despite oxide-free L/G interfaces, non-spherical cap shapes were also obtained due to the formation of sharp edges at the contact line triggered by the growth of intermetallic compounds. The coupling between the droplet shapes and the interfacial interactions is further confirmed by in-situ observation of oxides, analysis of quenched samples and thermodynamic calculations. The proposed method can be potentially applied to multiple reactive wetting systems (e.g. Sn/Cu, Al/Fe, Mg/Ni systems), which are crucial for high-temperature processes such as soldering, coating, and processing of metal matrix composites.

Step flow mechanism in dissolutive wetting Cu/Ni systems

The Cu-Ni system is a typical dissolutive system due to its mutual dissolution across a wide range of temperatures and compositions. We characterized the effects of Ni dissolution on the wetting behavior of liquid Cu by combining high-temperature wetting experiments, in-situ observation of spreading and solidification, microstructure analysis of the quenched droplets, and computational fluid dynamic (CFD) simulations. In the very early moment, at 1100 °C, when the Cu droplet is brought in contact with the Ni substrate, it oscillates due to capillarity and is dampened by inertial effects, while the significant Ni dissolution at 1150 °C largely reduced the initial oscillations. Later, a peculiar spreading behavior is observed and we propose to describe it through a 4-step mechanism: pinning of the contact line by a newly formed solid solution layer at the interface acting as a physical barrier, driving of liquid towards the solidified edge due to a Ni-concentration induced Marangoni flow, forming of a precursor film ahead of the solidified edge caused by the strong Cu-Ni interactions and Marangoni flow, and finally depinning due to overflow as a result of liquid accumulation at the solidified edge. The formation of a solid solution layer is confirmed by in-situ observation and quenching. The Ni-concentration induced Marangoni flow is characterized experimentally and further investigated by CFD simulations. The proposed step flow mechanism can be potentially relevant to other dissolutive wetting systems (e.g. Bi/Sn, Ag/Cu and Cu/Fe systems), which are crucial for high-temperature processing techniques.

Spreading of rotating liquid annular ring with hysteresis effects

The spreading of a liquid, subjected to centrifugal forces or an air jet, leads to a lowering of the liquid height at the center. If the system is partially wetting, experiments show a break up of the liquid and the appearance of a dry patch at the center. In this article, the spreading of a liquid droplet or a liquid film, featuring a dry patch at the center, is investigated. Via a generalized Tanner’s law, we allow for a contact-angle hysteresis in partially wetting systems. By means of the lubrication approximation, an analytical quasi-steady solution can be derived in the limit of small capillary numbers. We find a power-law regime for the spreading, in which at least one of the contact lines follows a power law in time, almost independent of both static contact angles. The influence of the static contact angles remains restricted to the beginning of the spreading and to transition periods. Four different types of spreading can be identified, namely spreading (i) with a very thin central liquid layer, (ii) as annular ring with central dry patch, (iii) into a static equilibrium, and (iv) with closing of the central dry patch. In a flow map, these types of spreading are mapped as function of both static contact angles. Moreover, the dependency from gravitational and centrifugal forces is investigated and included in the flow map. For a perfectly wetting system, a disjoining-pressure correction in combination with Tanner’s law prohibits negative liquid heights at the center. Hereby, the magnitudes of the Hamaker constants have a negligible influence and arbitrary-small values can be used, since the dynamics of the contact line remains controlled by Tanner’s law.

Mechanism study on the influence of wettability on CO2 displacement of shale oil in nanopores

A four-phase model of an oil–gas-water-wall system was constructed using the molecular dynamics simulation method. On this basis, the effect of shale reservoir wettability on the mechanism of CO2 flooding of oil will be investigated. The study found that in hydrophilic and mixed wetting systems, the front of the CO2 molecules had a semilunar shape. In lipophilic systems, however, it advanced as a whole. In addition, water molecules in the mixed wet system will form a water bridge, preventing the forward movement of CO2 molecules. CO2 molecules can easily displace oil molecules in pores of different wettability, but cannot remove the water molecules adsorbed on hydrophilic and mixed wetting systems. Oil molecules interact weakly with the hydroxyl group on the wall, but strongly with the methyl group on the wall. CO2 molecules interact strongly with both the methyl and hydroxyl groups on the wall. The difficulty of CO2 in displacing shale oil in different wetting systems is ranked in the following order: mixed wetting system (41.90 %), lipophilic system (91.51 %), hydrophilic system (98.77 %). Additionally, the storage efficiency of the three nanopores for CO2 was 50.53 %, 77.53 %, and 74.24 %, respectively. The research results can provide theoretical support for the development of CO2 in shale reservoirs.

Imaging flow focusing and isolation of aqueous fluids in synthetic quartzite: implications for permeability and retained fluid fraction in deep-seated rocks

The microstructure of realistic fluid–rock systems evolves to minimize the overall interfacial energy, enabling local variations in fluid geometry beyond ideal models. Consequently, the permeability–porosity relationship and fluid distribution in these systems may deviate from theoretical expectations. Here, we aimed to better understand the permeability development and fluid retention in deep-seated rocks at low fluid fractions by using a combined approach of high-resolution synchrotron radiation X-ray computed microtomography imaging of synthesized rocks and numerical permeability computation. We first synthesized quartzite using a piston-cylinder apparatus at different fluid fractions and wetting properties (wetting and non-wetting systems with dihedral angles of 52° and 61°–71°, respectively) under conditions of efficient grain growth. Although all fluids should be connected along grain edges and tubules in the homogeneous isotropic wetting fluid–rock system enabling segregation by gravitational compaction in natural settings, the fluid connectivity rapidly decreased to ~ 0 when the total fluid fraction decreased to 0.030–0.037, as the non-ideality of quartzite, including the interfacial energy anisotropy (i.e., grain faceting), became critical. In non-wetting systems, where the minimum energy fluid fraction based solely on the dihedral angle is ~ 0.015–0.035, the isolated (disconnected) fractions was 0.048–0.062. A streamline computation in the non-wetting system revealed that with decreasing total porosity, flow focusing into fewer channels maintained permeability, allowing the effective segregation of the connected fluids. These results provide insight into how non-wetting fluids segregate from rocks and exemplify the fraction of retained fluids in non-wetting systems. Thus, the findings suggest a potential way for wetting system fluids to be transported into the deep Earth's interior, and the amount of fluids dragged down to the Earth’s interior could be higher than what was previously estimated.

New computational method for calculating of water droplet–clay contact angles: Based on molecular dynamics

The measurement of the water contact angle (θ) on the surface of clay minerals poses a significant challenge due to their microscopic particle sizes ranging from micrometers to nanometers and their extensive specific surface areas. We introduce an advanced computational framework based on molecular dynamics simulations. This framework leverages the coordination number (CN) differences of the water molecules and effectively eliminates the gaseous water molecules in the droplet equilibrium system. Additionally, our framework uses an alpha-shaped probe ball to accurately determine the three-dimensional (3D) coordinates of the droplet interface and utilizes a feed-forward neural network (FFNN) to optimize the objective function of the 3D-interface; this ensures highly accurate characterization of the interface contour and an accurate computation of θ. Our approach exhibits outstanding performance in typical clay mineral surface-water wetting systems; it significantly overcomes the limitations of the transformed cylindrical coordinate system algorithm and accurately captures the wettability variations under complex environmental conditions. This new θ computational framework provides crucial assistance for the in-depth exploration of the wettability characteristics of clay minerals in complex environments.

Impact and wetting of polysilazane droplets on a metal surface

Spray coating with liquid polymers is a crucial step in producing polymer-derived ceramic (PDC) coatings before curing the precursor. However, the interactions between polymer droplets and target surfaces during the spray coating process have not been adequately characterized in existing literature. This experimental study aims to comprehensively examine the previously unknown droplet impact dynamics for a specific ceramic precursor, Polysilazane, on Inconel surfaces. The diameter of Polysilazane droplets was kept constant, while their impact velocity was varied to achieve different impact conditions. The same parameter range was also explored with water, water-glycerol, and ethanol droplets for comparison purposes. High-speed videos were captured to observe the instantaneous impact process, while low-speed videos were recorded at specific instances to document the gradual wetting process at later stages. The investigation revealed unique spreading dynamics of Polysilazane droplets, including smooth contact angle variation during the transition from impact to lateral spreading, absence of delayed wetting, no formation of wavy structures on the droplet interface during spreading, and no retraction process within the studied conditions (Weber number up to 1200). Conversely, Polysilazane displayed similarities in the normalized spreading curves with other liquids. Additionally, the maximum spread factor of Polysilazane could be well predicted using existing universal scaling models designed for different liquid types. Lastly, Polysilazane exhibited a distinctive post-impact wetting process that persisted for an extended duration. The wetting dynamics of Polysilazane could be effectively described by an empirical exponential law originally developed for partial wetting systems.

Reactive Spreading Dynamics of Molten Polymer Liquids

Despite the key to the adhesion between polymers and a plethora of materials, our understanding of reactive wetting is still rudimentary. In the present work, we report the reactive wetting dynamics of molten maleic anhydride-grafted polypropylene (MA-g-PP) liquids modeled by molecular-kinetic theory (MKT). The dependencies of the contact line friction (ζ) and viscosity (η) were compared for reactive and nonreactive wetting polymer systems. Compared with linear dependencies of ζ and η for the nonreactive wetting systems, the reactive wetting system presented a nonlinear relationship. In the capillary regime, the chemistry-affected regime is followed by a regime that is nearly not affected by chemical reactions, which is identified by the linear MKT. The formation of a new interface and the resulting entanglements of polymer chains in the chemistry-affected regime contribute to the dependence of ζ on η in the capillary regime.

Influence of porous high entropy alloy coating on wetting behavior and interfacial microstructure of Al-Si alloy on steel substrate

A porous FeCoNiCr high entropy alloy (HEA) coating was prepared on steel substrate via vacuum sintering. Wetting behavior of liquid Al-12Si alloy over coated and uncoated steel substrate was comparatively analyzed. The results show that the liquid Al-12Si alloy droplets were quickly spread and infiltrated into the porous structures under the enhanced capillary force by the micro-channels in the porous coating. A complete wetting ~ 0° contact angle (CA) was thus achieved; however, the CA was up to 41.2° in the case of uncoated steel substrate. In the case of uncoated steel substrate, the interfacial microstructure consisted of θ-Al13Fe4, η-Al5Fe2 and τ1-Al2Fe3Si3, while it changed to Cr-enriched FCC, AlFe-enriched BCC and AlNi-enriched B2 +Al-enriched BCC eutectic-like structure embedding in the HEA skeleton. The suppression of the formation of intermetallic compounds (IMCs) at the interface was mainly attributed to the sluggish diffusion and HEA effects of the porous coating. The altered interfacial metallurgical reactions and the enhanced capillary force were contributed to the improvement of wettability. This research provides a new method for improving the wettability and suppressing the interfacial IMCs in reactive metallic wetting systems.

Results of a simulation to propose an automated irrigation & monitoring system in crop production using fast charging & solar charge controller

In daily operations related to farming or gardening, watering is the most important cultural practice. Especially in rice cultivation huge amount of water is used. In conventional method continuous standing water is kept above paddy field during whole growing period of rice crop, though it is not necessary to keep continuous standing water in the field. The alternate Wetting and Drying (AWD) irrigation systems are a modern technology which saves water as well as increases crop yield. This AWD is being practiced manually in Bangladesh. This proposed model aims to make the AWD irrigation system into automated AWD irrigation system. However huge amount of electric power and fuel are being consumed by irrigation system in the country. The other aims of the proposed model are use of renewable energy instead of electric and fuel energy to save and secure future resources. To fulfil the country's feed, enhance the economy and develop availability of water automation in AWD is an essential requirement. Smart irrigation system occupies an exact amount of water at the correct location. It saves time and decreases hard work. It also makes a country self-sufficient in the food section. The traditional watering system is long time consumed where automation in AWD mitigate the time duration. It can also sense the weather behavior. During the rainy season it checks the possibility of rain and acts like that with help of the multi-level sensors. So, it can improve production efficiency as well. For this reason, an automatic irrigation control system has been designed with sensors, solar panels, fast charger, and battery. The sensors detect the dynamic water above and below ground surface. All this set up is controlled by Arduino microcontroller. Online Wi-fi ESP8266 module is used to control automated online monitoring to get feedback from sensors. A fast charger is used as back up part in AWD method. This whole circuit is more efficient and works automatically and manually.

Wetting Behavior of the Ag-5CuO Brazing Alloy on ZTA Composite Ceramic with/without CuO Coating in Air

The wetting of Ag-5 wt.% CuO (Ag-5CuO) alloy on initial/CuO-coated zirconia toughened alumina (ZTA) composite ceramic in air was studied in detail. The results showed that the contact angle of the Ag-5CuO/ZTA system rapidly decreased from 81° at 970 °C to 45° at 990 °C during the heating process, however, moderate reductions in contact angle were observed in the subsequent heating and temperature holding stages. In comparison with the contact angle of pure Al2O3, an increment of about 4° of the stable contact angle of Ag-5CuO alloy on the heterogeneous ZTA was observed. The reaction between Al2O3 and CuO can reduce the damage of the CuO-rich liquid to ZrO2 in the ZTA substrate. Both oxygen and CuO were helpful in reducing the contact angle of Ag on ZTA and enhancing the bonding of the Ag/ZTA interface. The continuous CuO coating on ZTA and the monotectic liquid containing more CuO in the region near the triple line induced reductions of more than 40° and about 10° in the contact angle, respectively, between the initial and the CuO coating-improved wetting systems.

Wetting of Laser Textured Cu Surface by Ethylene Glycol and Sn

The effect of microcosmic morphologies of textured Cu surface by nanosecond laser on the inert wetting and reactive wetting, i.e., ethylene glycol/copper and tin/copper wetting systems, was studied by using modified sessile drop methods. To create different surface roughness, the microcosmic morphologies with different spacing of grooves were constructed by nanosecond laser. The results showed that the inert wetting (ethylene glycol/copper) was consistent with Wenzel model, while the reactive wetting results deviated from the model. In Sn/Cu reactive wetting system, the interfacial evolution in the early stage and the pinning of triple line by the precipitated h-Cu6Sn5 caused the rougher surface and the worse final wettability. When the scale of artificial roughness exceeded the roughness that was caused by interfacial reaction after reaching the quasi-equilibrium state at interface, the final wettability could be improved.

Brazing of Tungsten Heavy Alloy and Fe-Ni-Co Based Superalloy by a Novel Cu-Ti Based Amorphous Filler

Cu-Ti based amorphous fillers were designed based on cluster-plus-glue-atom model to join tungsten heavy alloy (WHA) and Fe-Ni-Co based superalloy (GH907). The wettability of brazing fillers on WHA substrates was analyzed, and the interfacial microstructures and mechanical properties of brazed joints were also investigated. High constituent homogeneity and excellent flowability of amorphous fillers facilitated the superior wettability and brazeability effectively. In addition, WHA substrate/β-Ti/(Cu, Ni)(Ti, Zr)+(Cu, Ni)(Ti, Zr)2/(Fe, Ni)Ti+Fe2Ti/GH907 substrate were identified as typical microstructures of brazed joints, and the maximum joint shear strength of 340 MPa was obtained at 1050°C for 10 min. The underlying joining mechanisms were addressed in view of diffusion-reaction behavior and dissolution-dilution effect of multiple alloying elements. This study shed light on a new sight of WHA/amorphous filler/GH907 wetting and brazing systems.

Scheduling rice irrigation using soil moisture thresholds for furrow irrigation and intermittent flooding

AbstractAlternative methods of rice (Oryza sativa L.) irrigation are gaining increasing popularity across the U.S. Mid‐South, but an allowable soil moisture deficit stress has not been defined. This study was conducted in 2018 and 2019 to determine the soil moisture threshold for furrow‐irrigated rice (FIR) and alternate wetting and drying (AWD) rice production systems to minimize irrigation water use while maximizing grain and milling yield. Soil moisture thresholds of −15, −30, and −45 kPa, as determined by Watermark sensors at a 10‐cm depth in the top third of the field, were compared with conventional flood rice (CFR) on a Calloway silt loam and a Sharkey clay in eastern Arkansas. Soil moisture threshold had no effect on grain or milling yield across irrigation thresholds; however, the bottom third of FIR, where a flood was held, consistently produced higher grain yield (10,652–10,965 kg ha−1) than the top third (9,442–9,722 kg ha−1). Significant differences in irrigation water use efficiency (IWUE) were achieved under AWD and FIR. The IWUE did not differ across AWD irrigation thresholds but was 58.1–103.8% greater than that of CFR. The −15 and −30 kPa FIR thresholds did not improve IWUE, but using a −45 kPa irrigation threshold resulted in a 42.7% greater IWUE than CFR and 66.0% greater than the −15 kPa threshold. Results suggest that using a −45 kPa irrigation threshold at the 10‐cm depth can potentially maintain grain and milling yield while increasing IWUE under AWD and FIR.

Wetting and spreading behaviors of Al-Si alloy on surface textured stainless steel by ultrafast laser

An ultrafast laser was used to ablate the surface of stainless steel. Periodical surface micro-textures including micro-grooves, micro-pits and micro/nano-ripples were successfully fabricated. As a typical reactive wetting system, Al-Si alloy was used to study the wetting and spreading behaviors on the laser patterned surfaces of stainless steel. The results showed that initial, rapid spreading and gradient balance stages were found in the spreading process. By comparing to the primitive surface, Al-Si alloy exhibited worse wettability on the surface with micro-grooves and micro-pits because of the geometrical characteristics and nano-scale oxides residuals after laser processing; however, Al-Si alloy showed better wettability on the surface with micro/nano-ripples owing to the formation of micro/nano hierarchically patterns and the resultant improved capillary actions. The interfacial reaction layer formed during wetting and spreading processes were also investigated. The surface micro-textures were found to enhance the interfacial metallurgical reactions, thus increasing the thickness of the reaction layer. This work provides a new method to improve the wettability, spreadability and metallurgical reactions of Al-Si/stainless steel reactive wetting system and may extend its use in other reactive wetting systems. This method may be able to improve brazing, soldering, coating and other processes involving solid/liquid interfacial interactions.

Wetting Transition in a Molten Metal and Solid Substrate System in High Magnetic Fields

Wetting transitions between molten metals and different solid substrates were investigated using the sessile drop method to evaluate the possibilities of regulating wettability by high magnetic fields (HMFs) during wetting. For most wetting counterparts, the molten-metal droplet outline showed an apparent change because of the influence of HMFs. Contact angles that were measured with HMFs decreased compared with contact angles that were measured without HMFs. The temperature and magnetic-flux density had an evident but more complicated effect on the wetting transition. For reactive wetting systems, the effect of HMFs on changes of element distributions at the metal/substrate interface may lead to a variety of wetting transitions. For non-reactive wetting systems, solidified metal droplets can move from the solid substrates after wetting. No detailed and comprehensive explanations on HMF wetting-transition mechanisms exist, and further work is required. This study contributes to perfect wetting mechanisms and theories and provides a scientific approach to control wettability.

High-temperature reactive wetting systems: Role of lattice constant

The high-temperature spreading dynamics of Al-Cu(l)/Ni(s) and Al-Ni(l)/Ni(s) wetting systems was studied using molecular dynamics simulations. Accompanied with spreading, a significant dissolution reaction was noted to take place for the two studied systems. The dissolution reaction involves both diffusion of solid atoms into the droplet and incorporation of liquid atoms into the substrate. No precursor film is generated during spreading, since its formation is suppressed by the dissolution reaction. The dissolution reaction induces an enhanced spreading rate, and spreading becomes faster when droplets contain less Cu or Ni atoms. The dissolution of solid atoms into the droplet is faster than that of liquid atoms into the substrate, so that the dissolution of the former is the dominant driving force for the spreading enhancement. The spreading of Al-Cu droplets correlates with that of Al-Ni droplets so long as the Cu concentration in Al-Cu droplets is equal to the Ni concentration in Al-Ni droplets, since both Cu and Ni atoms have the identical lattice constants.

A Comparative Study of the Diffuse-Interface Model and Sharp-Interface Model in the Soldering Related Wetting Spreading Systems

A typical dissolution wetting system, Bi-Sn eutectic filler metal over a Bi substrate in a high-purity argon atmosphere was investigated first using real-time in situ hot stage microscopy for the extensive use of the sharp-interface model and the diffuse-interface model in the modeling of brazing/soldering related wetting systems. Subsequently, the similarities and differences between the aforementioned models in describing the issues of the wetting and spreading interfaces were discussed in terms of soldering definition and theoretical formula derivation. It is noted that (i) the mutual dissolution diffusion between the liquid Bi-Sn solder and Bi substrate were obvious. As a result, the composition and volume of the liquid solder is constantly changing during the wetting and spreading process; (ii) the sharp-interface model is a special case of the diffuse-interface model of the Cahn-Hilliard nonlinear diffuse-equation under the convective dominant condition; (iii) although there are differences between the sharp-interface model and the diffuse-interface model, both of them could be used in brazing/soldering related processes; and, (iv) the agreement between the experimental and simulation results of the sharp-interface model is not as good as that of the diffuse-interface model, which can be attributed to the effects of the elements’ diffusion and the phase transformation.

Microbubble flows in superwettable fluidic channels.

The control of bubble adhesion underwater is important for various applications, yet the dynamics under flow conditions are still to be unraveled. Herein, we observed the wetting dynamics of an underwater microbubble stream in superwettable channels. The flow of microbubbles was generated by integrating a microfluidic device with an electrochemical system. The microbubble motions were visualized via tracing the flow using a high-speed camera. We show that a vortex is generated in the air layer of the superaerophilic surface under laminar conditions and that the microbubbles are transported on the superaerophilic surface under turbulent conditions driven by the dynamic motion of the air film. Furthermore, microbubbles oscillated backward and forward on the superaerophobic surface under turbulent conditions. This investigation contributes to our understanding of the principles of drag reduction through wettability control and bubble flow.

The wetting phenomenon and precursor film characteristics of Sn-37Pb/Cu under ultrasonic fields

The wetting phenomenon of Sn-37Pb alloy solder spreading on Cu substrate under different ultrasonic fields was investigated. Sn-37Pb/Cu wetting systems showed significant precursor film characteristics with the assist of ultrasonic energy. A short-time ultrasonic field evidently enhanced the wettability of Sn-37Pb/Cu compare to a common wetting field which had no introduced ultrasonic. At the temperature of 213 °C, spreading area of a Sn-37Pb solder drop in a 20 kHz, 1000 W ultrasonic field reached maximum at 2 s, and showed significant precursor film characteristics. Atomistic dynamic models showed the relevance between wettability and precursor film formation. The observed dynamic results agreed with the experimental results.