Liquid Spreading Research Articles

Flow and solute transport in dissolutive wetting

Liquid spreading on a soluble substrate is known as dissolutive wetting, a common phenomenon in the medical and industrial fields. However, the dynamic process of dissolutive wetting is not fully understood due to the complex transport of mass and momentum. In this article, we present a hydrodynamic model that accounts for both Marangoni and buoyancy effects to analyze interface shape, flow, and solute transport in dissolutive wetting. Based on our theoretical analysis and experiments, we propose two dimensionless numbers, Ya and Pe′, to plot a phase diagram of dissolutive wetting. We investigate the evolution of the flow field, concentration field, and solid–liquid interface shape for different types of dissolutive wetting. Furthermore, we discovered a new type of dissolutive wetting resulting from the competition between Marangoni convection and buoyancy, using both experimental and theoretical approaches. Our study provides significant insight into dissolutive wetting, which may has applications in joining processes of materials, material synthesis, structural fabrication, and so on.

Spreading Characteristics of Volatile Liquid Film on the Liquid-solid Substrates

The spreading characteristics of volatile liquid film on the liquid-solid substrate have been experimentally investigated. Unlike the liquid film spreading on the liquid substrate, there exists a dip at the top of the solid substrate, as a result, the spreading behaviors are inhibited. It is found that dip characteristics of R’/ R slightly decrease at the beginning then it keeps constant R’/ R=0.83 until the end. Moreover, the comparison of spreading behaviors on two different substrates is investigated. When the volatile film spreads on the liquid-solid substrate, the maximum radii of the outer and inner rings are smaller, but the process lasts longer. Although the evolutions of radii are different, the variation trend of nondimensional ring width is consistent. We further study the spreading rate; results show there exist three stages due to the relative importance of different forces. Our results provide important missing pieces to the rich mechanisms of the multiple-phase flow driven by the Marangoni effect.

Designing Versatile Superhydrophilic Structures via an Alginate-Based Hydrophilic Plasticene.

The rational design of superhydrophilic materials with a controllable structure is a critical component in various applications, including solar steam generation, liquid spontaneous transport, etc. The arbitrary manipulation of the 2D, 3D, and hierarchical structures of superhydrophilic substrates is highly desirable for smart liquid manipulation in both research and application fields. To design versatile superhydrophilic interfaces with various structures, here we introduce a hydrophilic plasticene that possesses high flexibility, deformability, water absorption, and crosslinking capabilities. Through a pattern-pressing process with a specific template, 2D prior fast spreading of liquids at speeds up to 600 mm/s was achieved on the superhydrophilic surface with designed channels. Additionally, 3D superhydrophilic structures can be facilely designed by combining the hydrophilic plasticene with a 3D-printed template. The assembly of 3D superhydrophilic microstructure arrays were explored, providing a promising route to facilitate the continuous and spontaneous liquid transport. The further modification of superhydrophilic 3D structures with pyrrole can promote the applications of solar steam generation. The optimal evaporation rate of an as-prepared superhydrophilic evaporator reached ~1.60 kg·m-2·h-1 with a conversion efficiency of approximately 92.96%. Overall, we envision that the hydrophilic plasticene should satisfy a wide range of requirements for superhydrophilic structures and update our understanding of superhydrophilic materials in both fabrication and application.

When health and environment are entwined: An analysis of green, conventional and government bonds during the COVID-19 variants of concern (VOC)

The awareness of environmental and social issue has become one of the primary considerations among investors recently. This has sparked interest for social impact studies among many researchers. Among all the green-based financial products, green bond has become an instrumental mechanism of the financial landscape in overcoming the climate-related effects. Thus, we investigated the liquidity spread, measured by bid and ask spread among bonds in developed countries. Then, we use COVID-19 VOC to look at the behavior of liquidity during the specific VOC. The findings show that the bid-ask spread of the green bonds has been mostly equivalent across the multiple COVID-19 variants’ periods. Meanwhile, the conventional bonds have been mostly contractionary across the multiple COVID-19 variants periods; and the government bonds are mostly expansionary across the multiple COVID-19 variants’ periods.

Selective liquid directional steering enabled by dual-scale reentrant ratchets

Achieving well-controlled directional steering of liquids is of great significance for both fundamental study and practical applications, such as microfluidics, biomedicine, and heat management. Recent advances allow liquids with different surface tensions to select their spreading directions on a same surface composed of macro ratchets with dual reentrant curvatures. Nevertheless, such intriguing directional steering function relies on 3D printed sophisticated structures and additional polishing process to eliminate the inevitable microgrooves-like surface deficiency generated from printing process, which increases the manufacturing complexity and severally hinders practical applications. Herein, we developed a simplified dual-scale structure that enables directional liquid steering via a straightforward 3D printing process without the need of any physical and chemical post-treatment. The dual-scale structure consists of macroscale tilt ratchet equipped with a reentrant tip and microscale grooves that decorated on the whole surface along a specific orientation. Distinct from conventional design requiring the elimination of microgrooves-like surface deficiency, we demonstrated that the microgrooves of dual-scale structure play a key role in delaying or promoting the local flow of liquids, tuning of which could even enable liquids select different spreading pathways. This study provides a new insight for developing surfaces with tunable multi-scale structures, and also advances our fundamental understanding of the interaction between liquid spreading dynamics and surface topography.

Fingering Instability of Binary Droplets on Oil Pool

The interfacial instability of a complex fluid in a multiphase flow system is ubiquitous in both nature and industry. We experimentally investigated the spreading and interfacial instability dynamics of a binary droplet (a water and 2-propanol (IPA) mixture) on an immiscible (sunflower oil) pool. For droplets of 40 wt% IPA solution on sunflower oil, fingering instability occurred at the spreading liquid front. To reveal the interfacial characteristics of the spreading and fingering processes, we analyzed the interplay among the speed, diameter, and number of fingers on the spreading front. Based on our observations, the finger length, wavelength between the fingers, head length, and neck length were quantified. Our experimental results clearly demonstrate that fingering instability can be driven by the capillary effect for a liquid–liquid system as well as the Plateau–Rayleigh instability. We hope that our results will inspire further experimental and numerical investigations to provide deeper insights into the interfacial dynamics of multicomponent droplets in a liquid pool.

DESIGN OF PLATE HEAT EXCHANGER FOR HEAT TREATMENT INDUSTRY

This study discusses the design of plate type heat exchangers (PHE). Conventional heat exchangers have many drawbacks, including difficult maintenance, high cost, and increased area usage. In contrast, plate heat exchangers have a number of benefits over traditional heat exchangers. Because the liquid spreads across the plate, PHE has a bigger surface area. The study discusses and explains the design of the plate type heat exchanger, plate material, chevron angle, and gasket material since the plate allows for faster heat transmission. Key Words: Plate heat exchanger, Design, materials.

Wetting of Cu-SiC Composite Material Modified by Nanosecond Laser Radiation and Liquid Spreading over It

In the framework of this work, the surface properties of Cu-SiC composite material were studied when spreading micro- and nanoliter liquids. The Cu-SiC samples with a SiC content of 5 to 20 wt.% were fabricated by spark plasma sintering at temperatures from 700 to 850 °C. The Cu-SiC surfaces were processed by two different methods: using abrasive materials and nanosecond laser radiation. Surface analysis was performed by scanning electron microscopy, profilometry, energy dispersive spectroscopy and Vickers methods. The surface properties (wetting and dynamic characteristics of spreading) were studied using a shadow optical technique when interacting the Cu-SiC surfaces with water (up to 10 μL). It was proved that the recorded deterioration of the wettability properties of Cu-SiC surfaces processed by abrasive materials with an increase in their sintering temperature and the reason for the spontaneous hydrophobization of the Cu-SiC composite materials modified by nanosecond laser radiation, are due to the adsorption of airborne hydrocarbon contaminants, similar to the known wetting inversion of metal surfaces. It was established that the wetting properties of materials prior to modification by laser radiation do not affect the intensity, duration of stages, and steady-state values of contact angles upon wetting inversion of Cu-SiC composite materials. It was also found that the processing of Cu-SiC surfaces by laser radiation makes it possible to change the dynamic characteristics of the liquid spreading (at a flow rate of 5 μL/min, the liquid front speed is more than three times, and the dynamic contact angles are in the range of 30°).

4D Printing of Butterfly Scale-Inspired Structures for Wide-Angle Directional Liquid Transport.

Unidirectional liquid transport has been extensively explored for water/fog harvesting, electrochemical sensing, and desalination. However, current research mainly focuses on linear liquid transport (transport angle α= 0°), which exhibits hindered lateral liquid spreading and low unidirectional transport efficiency. Inspired by the wide-angle (0°< α< 180°) liquid transport on butterfly wings, this work successfully achieves linear (α= 0°), wide-angle, and even ultra-wide-angle (α= 180°) liquid transport by four-dimensional (4D)printing of butterfly scale-inspired re-entrant structures. These asymmetric re-entrant structures can achieve unidirectional liquid transport, and their layout can control the Laplace pressure in the forward (structure-tilting) and lateral directions to adjust the transport angle. Specifically, high transport efficiency and programmable forward/lateral transport paths are simultaneously achieved by the ultra-wide-angle transport, where liquid fills the lateral path before being transported forward. Moreover, the ultra-wide-angle transport is also validated in 3D space, which provides an innovative platform for advanced biochemical microreaction, large-area evaporation, and self-propelled oil-water separation.

Measuring the Adhesion Force and the Spreading Radius between Droplets and a Solid Surface during Short-Time Spreading

When a droplet contacts a solid surface, the liquid spreads over the solid surface to minimize the total surface energy. This phenomenon is widespread in industrial production and nature, so research on droplet spreading is of great significance. Here, the adhesion force and the spreading radius during droplet spreading can be quantified using a highly sensitive photoelectric method. It is possible to study droplet spreading from two dimensions at the microscale. The adhesion force is measured by an optical lever, and the spreading radius is measured by an ultrafast electrical method. The measurement method allows the force resolution and the space-time resolution to reach the nanonewton lever and the nanosecond lever, respectively. We obtain the maximum spreading radius and the maximum adhesion force during short-time spreading through our technique. Moreover, we numerically simulate the droplet spreading process through the lattice Boltzmann solver and confirm the observed results. This study provides a new experimental technique for studying droplet spreading dynamics from multiple perspectives, which can deepen our understanding of droplet spreading and provide guidance for the development of new techniques.

Combining experiments and modelling to predict the competition between liquid spreading and impregnation in porous media for Metal Binder Jetting applications

A predictive model accounting for binder spreading and impregnation in powders during Metal Binder Jetting was set. Presintered cylindrical samples were 3D printed considering main printing orientations and two types of wetting tests were carried out on samples : imbibition or wicking tests using a tensiometric method and spreading-impregnation tests using a sessile drop method with a goniometer. Tensiometric tests showed results on impregnation rate in powders and goniometric tests gave the impregnated volume of the binder drop into the powder. Both those experiments allowed to identify physical parameters depending on the binder and powder bed interaction. All parameters injected in the model have been identified in the present study. The proposed model accurately described experiments and showed the dependence of impregnated volume on wetting parameters but also on printing orientation.

From the teapot effect to tap-triggered self-wetting: a 3D self-driving sieve for whole blood filtration

Achieving passive microparticle filtration with micropore membranes is challenging due to the capillary pinning effect of the membranes. Inspired by the teapot effect that occurs when liquid (tea) is poured from a teapot spout, we proposed a tap-triggered self-wetting strategy and utilized the method with a 3D sieve to filter rare cells. First, a 3D-printed polymer tap-trigger microstructure was implemented. As a result, the 3 µm micropore membrane gating threshold (the pressure needed to open the micropores) was lowered from above 3000 to 80 Pa by the tap-trigger microstructure that facilated the liquid leakage and spreading to self-wet more membrane area in a positive feedback loop. Then, we implemented a 3D cone-shaped cell sieve with tap-trigger microstructures. Driven by gravity, the sieve performed at a high throughput above 20 mL/min (DPBS), while the micropore size and porosity were 3 µm and 14.1%, respectively. We further filtered leukocytes from whole blood samples with the proposed new 3D sieve, and the method was compared with the traditional method of leukocyte isolation by chemically removing red blood cells. The device exhibited comparable leukocyte purity but a higher platelet removal rate and lower leukocyte simulation level, facilitating downstream single-cell analysis. The key results indicated that the tap-triggered self-wetting strategy could significantly improve the performance of passive microparticle filtration.

Capillary-driven horseshoe vortex forming around a micro-pillar

Hypothesis: Horseshoe vortices are known to emerge around large-scale obstacles, such as bridge pillars, due to an inertia-driven adverse pressure gradient forming on the upstream-side of the obstacle. We contend that a similar flow structure can arise in thin-film Stokes flow around micro-obstacles, such as used in textured surfaces to improve wettability. This could be exploited to enhance mixing in microfluidic devices, typically limited to creeping-flow regimes.Experiments: Numerical simulations based on the Navier–Stokes equations are carried out to elucidate the flow structure associated with the wetting dynamics of a liquid film spreading around a 50 μm diameter micro-pillar. The employed multiphase solver, which is based on the volume of fluid method, accurately reproduces the wetting dynamics observed in current and previous (Mu et al., Langmuir, 2019) experiments.Findings: The flow structure within the liquid meniscus forming at the foot of the micro-pillar evinces a horseshoe vortex wrapping around the obstacle, notwithstanding that the Reynolds number in our system is extremely low. Here, the adverse pressure gradient driving flow reversal near the bounding wall is caused by capillarity instead of inertia. The horseshoe vortex is entangled with other vortical structures, leading to an intricate flow system with high-potential mixing capabilities.

Spreading of water microdroplets simulating human sweating in polyester yarns and fabrics

Thermo-physiological comfort of human body is highly influenced by the moisture transmission behaviour of the fabric. For active sportswear, intimate apparels and inner layer of extreme cold weather clothing, moisture transport is very crucial. The liquid moisture transport of fabrics is analysed using standardised test methods like vertical wicking, absorbency test and moisture management through moisture management tester. These test methods use large reservoir of liquid in contrast to human sweating behaviour. The human skin generates sweats in the form of microdroplets through sweating glands. Hence, the standard test procedures employed to assess the fabric’s liquid moisture transport fail to capture preciously the sweat transport from human skin to the fabrics. Hence, in this work, the spreading behaviour of water microdroplets simulating human sweat through yarns which are the building block of fabrics and knitted fabrics that are widely used as sportswear or inner layer of extreme cold weather clothing are investigated. The purpose of this study is to understand how the microfluid move along the capillary channels in yarns (in the forms of network of yarns crossing over each other and fabric) by keeping yarn count and twist same for all yarns. A single micro drop representing accumulated sweat droplets and multiple drops showing continuous sweating through sweat glands were allowed to wick through the yarns made up of coarser and finer fibres to understand the role of fibre diameter on wicking kinetics. Results from the conventional wicking test were compared with microfluidic wicking. It is found that the initial wicking speed in coarser-fibres yarn is faster in comparison to finer-fibres yarn, but the final wicking height is longer in finer fibre yarn. Overall liquid spreading length found to be similar for both the yarns. Yarn to yarn liquid transfer at crossover junctions is faster with finer-fibre yarn compared to coarser-fibre yarn. Same trends are found in fabrics constructed with these yarns. The effect of yarn tortuosity on liquid spreading behaviour has also been studied. As the yarn path becomes torturous (straight yarn to fabric form) wicking length also decreases.

When does an impacting drop stop bouncing?

Non-wetting substrates allow impacting liquid drops to spread, recoil and take-off, provided they are not too heavy (Bianceet al.,J. Fluid Mech., vol. 554, 2006, pp. 47–66) or too viscous (Jhaet al.,Soft Matt., vol. 16, no. 31, 2020, pp. 7270–7273). In this article, using direct numerical simulations with the volume of fluid method, we investigate how viscous stresses and gravity oppose capillarity to inhibit drop rebound. Close to the bouncing to non-bouncing transition, we evidence that the initial spreading stage can be decoupled from the later retraction and take-off, allowing us to understand the rebound as a process converting the surface energy of the spread liquid into kinetic energy. Drawing an analogy with coalescence-induced jumping, we propose a criterion for the transition from the bouncing to the non-bouncing regime, namely by the condition${Oh}_{c} + {Bo}_{c} \sim 1$, where${Oh}_{c}$and${Bo}_{c}$are the Ohnesorge number and Bond number at the transition, respectively. This criterion is in excellent agreement with the numerical results. We also elucidate the mechanisms of bouncing inhibition in the heavy and viscous drop limiting regimes by calculating the energy budgets and relating them to the drop's shape and internal flow.

국제자본조달유동성과 우리나라의 경기변동에 대한 연구

This study investigates the significance of the U.S. funding liquidity measure, TED spread, obtained by subtracting 3-month T-bill rate from 3-month Libor, in explaining the business cycle of South Korea and takes a look at recent changes pertaining to the transition to SOFR(Secured Overnignt Financing Rate) from Libor. This study contributes to the existing literature by empirically demonstrating the statistical significance of the global funding liquidity on the business cycle of South Korea, thereby enhancing the understanding of the business cycle in South Korea, and by examining SOFR which is gaining increasing attention. In applying the probit estimation model, a binary dependent variable is generated based on the business cycle stages and descriptive variables are constructed using term spread, short rate, and funding liquidity measures. The estimation results show that TED spread is significant in explaining the business cycle of South Korea, while the domestic funding liquidity measure proves to be insignificant, and considering U.S. term spread and TED spread along with domestic variables remarkably enhances the explanatory power. Under the circumstance that TED spread is no longer available due to the transition to SOFR, it is necessary to reassess the measurement of funding liquidity, considering recent studies which demonstrate that SOFR is less informative than Libor concerning credit and liquidity risks. Especially in case of the term SOFR, which covers vulnerabilities of overnignt rate and promotes practical use of the new benchmark rate, further research is required to figure out its informational contents on the funding conditions.

Effect of Micropillar Array Morphology on Liquid Propagation Coefficient Enhancement.

Roughness on hydrophilic surfaces allows for fast propagation of liquids. In this paper, the hypothesis is tested which theorizes that pillar array structures with nonuniform pillar height levels can enhance wicking rates. In this work, within a unit cell, nonuniform micropillars were arranged with one pillar at constant height, while other shorter pillars were varied in height to study these nonuniform effects. Subsequently, a new microfabrication technique was developed to fabricate a nonuniform pillar array surface. Capillary rising-rate experiments were conducted with water, decane, and ethylene glycol as working liquids to determine the behavior of propagation coefficients that were dependent on pillar morphology. It is found that a nonuniform pillar height structure leads to a separation of layers in the liquid spreading process and the propagation coefficient increases with declining micropillar height for all liquids tested. This indicated a significant enhancement of wicking rates compared to uniform pillar arrays. A theoretical model was subsequently developed to explain and predict the enhancement effect by considering capillary force and viscous resistance of nonuniform pillar structures. The insights and implications from this model thus advance our understanding of the physics of the wicking process and can inform the design of pillar structures with an enhanced wicking propagation coefficient.

Evidence-based hand hygiene: Liquid or gel handrub, does it matter?

BackgroundRecent studies put under scrutiny the prevailing hand hygiene guidelines, which incorporate quantitative parameters regarding handrub volume and hand size. Understanding the criticality of complete (i.e., efficient) hand hygiene in healthcare, objectivization of hand hygiene related parameters are paramount, including the formulation of the ABHR. Complete coverage can be achieved with optimal Alcohol-Based Hand Rub (ABHR) provided. The literature is limited regarding ABHR formulation variances to antimicrobial efficiency and healthcare workers’ preference, while public data on clinically relevant typical application differences is not available. This study was designed and performed to compare gel and liquid format ABHRs (the two most popular types in Europe) by measuring several parameters, including application time, spillage and coverage.MethodologySenior medical students were invited, and randomly assigned to receive pre-determined ABHR volumes (1.5 or 3 ml). All the 340 participants were given equal amounts of gel and liquid on two separate hand hygiene occasions, which occurred two weeks apart. During the hand hygiene events, by employing a digital, fully automated system paired with fluorescent-traced ABHRs, disinfectant hand coverage was objectively investigated. Furthermore, hand coverage in relation to the participants’ hand sizes was also calculated. Additional data collection was performed regarding volume differences and their effect on application time, participants’ volume awareness (consciousness) and disinfectant spillage during the hand hygiene events.ResultsThe 1.5 ml ABHR volume (commonly applied in healthcare settings) is insufficient in either formulation, as the non-covered areas exceeded significant (5%+) of the total hand surface area. 3 ml, on the contrary, resulted in almost complete coverage (uncovered areas remained below 1.5%). Participants typically underestimated the volume which they needed to apply. While the liquid ABHR spreads better in the lower, 1.5 ml volume compared to the gel, the latter was easier handled at larger volume. Drying times were 30/32 s (gel and liquid formats, respectively) when 1.5 ml handrub was applied, and 40/42 s when 3 ml was used. As the evaporation rates of the ABHR used in the study are similar to those available on the market, one can presume that the results presented in the study apply for most WHO conform ABHRs.ConclusionThe results show that applying 1.5 ml volume was insufficient, as large part of the hand surface remained uncovered (7.0 ± 0.7% and 5.8 ± 1.0% of the hand surface in the case of gel and liquid, respectively) When 3 ml handrub was applied drying times were 40 and 42 s (gel and liquid, respectively), which is a very long time in daily clinical practice. It looks like we cannot find a volume that fits for everyone. Personalized, hand size based ABHR volumes may be the solution to find an optimal balance between maximize coverage and minimise spillage and drying time. 3 ml can be a good volume for those who have medium size hands. Large handed people should use more handrub to reach appropriate coverage, while small-handed ones may apply less to avoid massive spillage and not to take unrealistically long to dry.

Interfacial microstructure and strengthening mechanism of dissimilar laser al/steel joint via a porous high entropy alloy coating

A porous high entropy alloy (HEA) coating was prepared on a steel surface by vacuum sintering. The coating was then used as a transition layer during dissimilar laser joining of Al to steel. Compared with the uncoated laser joints, the liquid alloy spread and infiltrated into the porous structure, the contact angle of the weld reduced from 65.8° to 56.7°, and the brazed width increased from 5.1 mm to 5.9 mm, which improved the wettability and spreadability of the molten filler wire on the substrate. In the case of the uncoated steel, the fusion zone/steel interfacial microstructure consisted of laminated Al7·2Fe1·8Si and Fe(Al,Si)3, while it changed to a composite-like structure containing a soft HEA skeleton and hard IMCs which included Al7·2Fe1·8Si, Al3Ni, and (Al,Si)2Cr. In addition, due to the sluggish diffusion effect of HEAs, a layer of gradient nanocrystalline composed of Al7·2Fe1·8Si was generated, which significantly strengthened the dissimilar laser joints with improvements in both the fracture load (∼26.5%) and the displacement (∼101.8%). The fracture mode changed from brittle to ductile failure when the porous HEA coating was applied, with fracture propagating through the HEA skeleton. This work provides a novel solution for the strengthening of hard-to-join dissimilar combinations.

Spreading dynamics of the viscous droplet impacting on a spherical particle

The dynamic behavior of droplets in the impingement process with particles has attracted extensive interest due to its widespread industrial applications. In this study, collision experimentation was carried out to investigate the spatial and temporal variation of droplets on a target particle surface by utilizing high-speed photography. Energy conversion and force analysis were conducted through theoretical analysis. Moreover, we captured the microscopic and evolutionary features of droplets in detail by image processing. The dimensionless liquid film thickness in the maximum spreading state is primarily determined by its spreading area. The Weber number can be used to calculate the maximum spreading area of droplets for liquids with a specific viscosity and droplet-to-particle size ratio. The time-evolved liquid film morphology and spreading area of a highly viscous liquid show a different trend compared to that of the low-viscous liquid. When the liquid film is not broken, the maximum dimensionless spreading area is linearly related to the Weber number. At low Weber numbers and high Reynolds numbers, droplets exhibit more pronounced oscillation characteristics. The oscillation period of the collision is related to the droplet-to-particle size. The liquid film thickness decreases as the Weber and Reynolds numbers rise. As for the low-viscous liquid, a low Weber number leads to a periodic change in the dynamic contact angle. A decrease in the Reynolds number for the highly viscous droplets generates a greater dynamic contact angle. The recoiling of the liquid film results in a more significant reduction in the dynamic contact angle.