Liquid Spreading Research Articles

Multiscale Investigation of Liquid Imbibition on an Electrodeposited Rough Surface.

Hydrophilic and superhydrophilic surfaces are often used in various applications, such as biomedical surface modification, heat pipe design, lab-on-chip devices, microelectronic structure design, etc. Deposition of molecules on surfaces and chemical modification of the surface are two of the frequently adopted techniques to fabricate hydrophilic surfaces. However, achieving controlled and precise surface roughness is an expensive process, and the multiscale nature of liquid imbibition on rough surfaces complicates the system's physics. To address this challenge, we propose an alternative approach for creating controlled rough surfaces by electrodeposition of copper ions on copper electrodes, explore the influence of various length scales encountered in roughened surfaces, ranging from micrometers to angstroms, and examine the associated physics. The microscale roughnesses are controlled by adjusting the deposition time. A molecular level investigation is employed to probe the intriguing physics of liquid imbibition on the created rough surface. The molecular analysis of droplet contact line dynamics, including the contact angle and footprint radius, shows qualitative alignment with the real systems. Additionally, this study provides new insights into the flow velocities during imbibition within surface asperities at the molecular level. Directional velocities, such as axially downward and horizontally outward flows, which are otherwise challenging to measure experimentally, are also evaluated on a molecular scale. These findings provide a fundamental understanding of the intricate phenomena of liquid spreading and imbibition on metallic surfaces with randomly distributed rough textures, deepened by molecular-scale insights. The agreement, on a qualitative scale, between the experiments and simulations is successfully established, providing a fundamental understanding of the complex phenomena of water droplet imbibition dynamics on an electrodeposited surface.

Electricity-driven selectivity in the photocatalytic oxidation of methane to carbon monoxide with liquid gallium-semiconductor composite

Metal co-catalysts in oxide semiconductors are essential to enhance photocatalytic reaction, but the selectivity is often low due to the conversion of the target molecules as more reactive in comparison with the reagents. Here, we explore the influence of electricity on the activity and selectivity of liquid gallium as a co-catalyst loaded in TiO2 and ZSM-5 supports the photocatalytic oxidation of methane. We first show that the polarization of the photocatalytic chips allows a more efficient wetting of liquid gallium into the semiconductor matrices, which increases the availability of the photogenerated charge carriers for reactions and improves the overall activity of the composites. More importantly, the oxide skin surrounding the spread liquid is found to be charged, resulting in a significant reduction of the CO oxidation. An increase of the CO selectivity from 10 % to 80 % upon polarization to 50 V, with a stable photocatalytic activity reaching 0.6 ± 0.01 mmol.g-1·h-1, makes the use of electricity quite appealing to tailor liquid metals in catalysis.

Eulerian multi-fluid simulations of local liquid distribution in a bed packed with cylindrical particles

Packed bed reactors (PBRs) are extensively used in chemical process industries to perform solid-catalyzed gas–liquid reactions. Their performance is significantly influenced by liquid distribution which is a strong function of particle shape and size. In the present work, we simulated the steady-state local liquid distribution, pressure drop (ΔP/L), and overall liquid holdup (〈εL〉) for the beds packed with spherical and cylindrical particles using a three-dimensional (3D) Eulerian multifluid model and validated the predictions using corresponding high-speed imaging measurements performed in pseudo-2D packed beds. We showed that the Eulerian model with existing closures significantly underpredicts the liquid distribution for the bed randomly packed with cylindrical particles. To address this, we proposed a modification in existing mechanical dispersion force (F→D) model as a function of average particle orientation angle (θ). The modification significantly improved agreement between the predictions and the corresponding measurements. Using the modified model, we showed that the increased magnitude of F→D in the lateral direction leads to higher lateral liquid spreading for cylindrical particles as compared to that for the spherical particles. Importantly, the modified model also predicted the local liquid distribution and ΔP/L for beds packed manually with cylindrical particles oriented at different θ in a satisfactory agreement with the corresponding measurements. On increasing θ, the F→D in lateral direction (i.e., perpendicular to flow) as well as liquid–solid interaction force (F→LS) in axial direction (i.e., parallel to flow) found to increase significantly. As a result, the lateral liquid spreading as well as ΔP/L decrease substantially on increasing θ.

Boosting Droplet Transport for Fog Harvest.

Wedge-shaped superhydrophilic tracks have been considered as one of the most effective ways to transport droplets for diverse cutting-edge applications, e.g., energy harvesting and lab-on-a-chip devices. Although significant progress, such as serial wedge-shaped tracks with curved edges, has evolved to advance the liquid transport, the ultrafast and long-distance transporting of drop-shaped liquid remains challenging. Here, inspired by the cactus spine that enables fast droplet transport and the serial spindle knot of spider silk, which is capable of collecting condensate from a wide range of distances, we created serial wedge-shaped superhydrophilic patterns and optimized their side edges with a convex brachistochrone curve to boost the acceleration. The junctions of the serial patterns were meanwhile reformed into concave brachistochrone curves to lower the energy barrier for sustained transport. For transporting the liquid in drop shapes to the long distance at high velocity, the wedge-shaped tracks were slenderized to the greatest extent to suppress the liquid spreading and thus prevent the degradation of the Laplace driving force. Moreover, the junction that determines the energy barrier of droplet striding was carefully designed based on the principle of minimizing momentum loss. The exquisite architecture design pushed the droplet transport to a maximum instantaneous velocity of 207.7 mm·s-1 and an outermost transport distance of 120.5 mm, exceeding most wettability or geometric gradient based reports. The transported volume of the droplets can be readily regulated by simply scaling the created architectures. The enhanced droplet transport facilitates the motion and departure of the cohered droplets, enabling a 1.9-fold rise of the water collection rate and 12-fold increase of the heat transfer coefficient during the fog harvest test. This scalable, controllable, and easily fabricatable surface design provides an essential pathway in realizing high-performance manipulation of droplets and possibly pioneers substantial innovative applications in multidisciplinary fields. Those include but are not limited to energy harvesting, lab-on-a-chip devices, and MEMS systems.

Investigation of droplet splashing behavior during oblique jet impact onto a wall

For the issue of jet impingement on the wall in industrial cooling processes, an experimental setup based on high-speed photography for oblique jet impingement onto the wall was constructed. The experimental focus was on the study of liquid droplet splashing behavior after oblique jet impingement on the wall, discussing the liquid droplet splashing behavior under three jet impingement modes: Rayleigh regime, first wind-induced regime, and secondary wind-induced regime. By employing methods such as trajectory imaging and particle image velocity for liquid droplet parameter image measurement, obtaining the particle size, velocity, and distribution of splashed droplets after oblique jet impact on walls under different working conditions. The impact of jet impingement velocity and angle on droplet splashing parameters was analyzed. The results showed that when the impingement point is before the breakup length, with increasing flow velocity, the surface wave of the liquid column, and the spreading liquid film became more pronounced, but the loss of liquid-phase components due to splashing was relatively small. When the impingement point is after the breakup length, the secondary breakup resulting in a “crown”-shaped liquid film after droplet impingement leads to a significant loss of liquid-phase components through splashing. As the inlet velocity of the jet increases, there is a decreasing trend in droplet size and an increasing trend in droplet velocity. With an increase in jet angle, there is a decreasing trend in droplet size and velocity. Based on the concentration, size, and velocity distribution characteristics of splashing droplets, the area after oblique jet impingement on the wall can be divided into the impingement zone, low-concentration low-velocity zone, high-concentration high-velocity zone, and lateral splashing zone. This has significant implications for understanding the splashing mechanism after oblique jet impingement on the wall and optimizing operating conditions.

Continuous Homogeneous Thin Liquid Film on a Single Cross-Shaped Profiled Fiber with High Off-Circularity: Toward Quick-Drying Fabrics.

Quick-drying fabrics, renowned for their rapid sweat evaporation, have witnessed various applications in strenuous exercise. Profiled fiber textiles exhibit enhanced quick-drying performance, which is attributed to the excellent wicking effect within fibrous bundles, facilitating the rapid transport of sweat. However, the evaporation process is not solely influenced by macroscopic liquid transport but also by microscopic liquid spreading on the fibers where periodic liquid knots induced by spontaneous fluidic instability significantly reduce the evaporation area. Here, a cross-shaped profiled fiber with high off-circularity, featured as multiple concavities along the fibrous longitude-axis, which enables the formation of a homogeneous thin liquid film on a single fiber without any periodic liquid knots, is developed. The high off-circularity cross-sections help overcoming Plateau-Rayleigh instability by tuning the Laplace pressure difference, further facilitated by capillary flow along the concave surface. The homogeneous thin liquid film on a single fiber is responsible for maximizing the evaporation area, resulting in excellent overall evaporation capacity. Consequently, fabrics made from such fibers exhibit rapid evaporation behavior, with evaporation rates ≈50% higher than those of cylindrical fabrics. It is envisioned that profiled fibers may provide inspiration for the manipulating homogeneous liquid films for applications in fluid coatings and functional textiles.

Femtosecond laser writing of durable open microfluidic channels via a mode-switchable strategy

Open microfluidic systems offer significant advantages, including the elimination of external pumps and facilitating fluid access at any point along the channel. However, their deployment in harsh environments is commonly compromised due to the delicate nature of hydrophilic chemical coatings and the vulnerability of open microchannels to clogging and contamination. Here, a bioinspired, demand-responsive mode-switchable strategy is proposed to enhance the mechanical durability of open microfluidic systems. Specifically, under harsh conditions or when long-term storage is necessary, this strategy allows the open microfluidic device to transition to a protective mode simply through releasing the strain, thereby preserving the integrity of the structure and hydrophilic coatings. The stretched open microfluidic mode enables spontaneous liquid spreading along a hydrophilic microchannel scribed by femtosecond laser. This mode-switchable strategy provides the open microfluidic device with robustness to maintain spontaneous liquid flow, even under severe testing conditions such as 2000 cycles of cotton swab rubbing, sand impact, sandpaper abrasion, tape peeling, twisting, and finger rubbing. A proof-of-concept application involving blood type analysis on this mode-switchable open microfluidic device showcases its superior mechanical durability under severe environmental conditions. The proposed strategy paves the way for the broader use of open microfluidic devices in various practical applications.

Enhancing spontaneous and continuous liquid directional transport on peristome-mimetic surface with hierarchical microgrooves

Directional liquid transport function discovered on the peristome of Nepenthes alata has attracted considerable attention for its diverse potential applications. Despite the extensive efforts made for the peristome-mimetic surface fabrication and the anisotropic liquid spreading regulation, it remains a daunting challenge to reveal the synergistic effect of hierarchical structures on the liquid spreading and pinning dynamics. Here, we demonstrate the first-tier microgroove morphology, as well as the presence of second-tier microgrooves, play an important role in homogenous film formation and the directional liquid transport control. Through experimental investigation and theoretical analysis, the enhanced spreading and pinning effect is validated. Moreover, the preferential directional liquid spreading will collapse on the peristome-mimetic surface without a rational parameter design, and the threshold value for the transition of liquid propagation dynamics is determined. Spontaneous directional liquid transport from the cold region to the hot region and smart liquid transport regulation was also realized on the peristome-mimetic surface. This work will provide guidance to the design of effective open microfluidic systems, and open a new way for thermal management and lab-on-chip applications.

Non-simultaneous impact of droplet pairs on solid surfaces

This study delves into the dynamics of non-simultaneous droplet impacts on solid substrates, focusing on interactions between identical impacting droplets. Comparisons between non-simultaneous and simultaneous impacts are presented to understand the phenomena comprehensively. An in-house-built microcontroller-based droplet generator releases two equal-sized droplets on demand, allowing for simultaneous or non-simultaneous impacts. The interaction between impacting droplets generates an uprising sheet, whose characteristics vary with time lag between impacts, impact Weber number, and inter-droplet spacing. The evolution of central sheet characteristics, lamellae spreading dynamics, splashing mechanism, and secondary atomization is evaluated. Findings reveal that central sheet morphology varies with the time lag between impacts, transitioning from a two-dimensional (2D) “semilunar” sheet (vertical or inclined) with a linear base to a three-dimensional (3D) sheet with a curved base, increasing the probability of secondary atomization. The temporal evolution of the central sheet position, height, and inclination angle is governed by the momentum of spreading lamellae. A novel scaling law for maximum sheet extension and a theoretical expression for surface liquid spread are proposed, consistent with the measurements. The characteristics of secondary droplets generated during non-simultaneous impacts are similar to those from simultaneous impacts, with the size of the secondary droplets being one order of magnitude larger than those expected from isolated single-droplet impacts.

A label-free sensing of creatinine using radio frequency-driven lab-on-chip (LoC) system

This paper presents a promising avenue of Radio Frequency (RF) biosensors for sensitive and real-time monitoring of creatinine detection. Knowing creatinine levels in the human body is related to its possible association with renal, muscular, and thyroid dysfunction. The detection was performed using an Inter-Digitated Capacitor (IDC) made of copper (Cu) metal over an FR4 substrate. To demonstrate our methodology, we have chosen Phosphate Buffer (PB) as our solvent for making the creatinine solutions of different concentrations. Moreover, Assayed Chemistry Control (ACC), a reference control consisting of human serum-based solutions has been mixed with the different concentrations of creatinine in a ratio of 1:9 to spike the creatinine value in the ACC solution. The sensor has been designed using a High-Frequency Structure Simulator (HFSS) tool with an operating frequency of 2.53 GHz. Then the design is fabricated over the FR4 printed circuit board (PCB) and tested using a Vector Network Analyzer (VNA). However, the sensitive area of the IDC is introduced to grade 4 Whatman filter paper for the Sample Under Test (SUT) handling unit. The main advantage of using Whatman filter paper is that the uniform spreading of liquid reduces experimental error, and less volume is required for testing the sample. The principal idea implemented in the biosensor design is to track the shift in the operating frequency in the presence of different concentrations of creatinine mix in ACC solution with Phosphate Buffer (PB) solution as a reference.

Bioinspired Capillary Transistors.

Inspired by the unidirectional liquid spreading on Nepenthes peristome, Araucaria leaf, butterfly wings, etc., various microfluidic devices have been developed for water collection, irrigation, physical/chemical reaction, and oil-water separation. Despite extensive progress, most natural and artificial structures fail to enhance the Laplace pressure difference or capillary force, thus suffering from a low unidirectional capillary height (<30mm). In this work, asymmetric re-entrant structures with long overhangs and connected forward/lateral microchannels are fabricated by 3D printing, resulting in a significantly increased unidirectional capillary height of 102.3mm for water, which approximately corresponds to the theoretical limit. The overhangs can partially overlap the forward microchannels of the front structures without direct contact, thus enhancing the Laplace pressure difference and capillary force simultaneously. Based on asymmetric and symmetric re-entrant structures, capillary transistors are proposed and realized to programmably adjust the capillary direction, height, and width, which are envisioned to function as switches/valves and amplifiers/attenuators for highly efficient liquid patterning, desalination, and biochemical microreaction in 3D space.

Bank Specific Factors that Affect Credit Risk

Indonesian’s banking has taken steps to improve financial performance by managing credit risk and supporting economic recovery in 2023, efforts carried out by banks include providing credit and acting as intermediaries with very vulnerable credit risk challenges.This research has purpose to find out which bank spesific internal factor can significantly affect the credit risk in a bank. The independent variables in this study are efficiency, leverage, liquidity, ROA, ROE, solvency, banking spread, banking size, and ownership concentration. While the dependent variable is credit risk using NPL and Z-Score. The study used panel data regression models to test the data collected from 98 banks registered with OJK over three years (2020-2022). The results stated that leverage, ROA, ROE, solvency, bank size, have a positive influence on ZScore, while efficiency, liquidity, and bank spread have no influence on Zscore. The results also show that efficiency and ROA have a positive influence on NPL. Liquidity, ROE, solvency, bank spread, bank size, and ownership concentration variables have an influence on NPLs and leverage has no influence on NPL. Additionally, investors should be able to use this research's insights about company performance to get a general understanding of the credit risk associated with linked companies, which they can use to inform their investment decisions.

Experimental characterization of dynamics of bed‐scale liquid spreading in a trickle bed

AbstractWe report measurements performed to understand the effects of gas (QG) and liquid (QL) flow rates, surface tension (σGL), liquid viscosity (μL), and particle diameter (dp) on dynamics of local liquid spreading, pressure drop, and overall liquid holdup in a pseudo‐2D trickle bed. We show that an increase in the gas‐phase inertia leads to a decrease in the lateral liquid spreading, whereas an increase in the liquid‐phase inertia leads to an increase in the lateral liquid spreading. We also show that an increase in dp causes a reduction in the lateral liquid spreading. Using dimensionless numbers (AB and We), we propose a regime map showing contributions of different forces to the local liquid spreading. We show that the interplay between the inertia and capillary forces governs the liquid distribution near the inlet, whereas the relative contribution of gravitational force increases toward the outlet. Finally, we propose a relation between AB and We for “bed‐scale” liquid spreading.

Rapid Spreading of Yield-Stress Liquids.

When a liquid drop makes initial contact with any surface, an unbalanced surface tension force drives the contact line, causing spreading. For Newtonian liquids, either liquid inertia or viscosity dictates these early regimes of spreading, albeit with different power-law behaviors of the evolution of the dynamic spreading radius. In this work, we investigate the early regimes of spreading for yield-stress liquids. We conducted spreading experiments with hydrogels and blood with varying degrees of yield stress. We observe that for yield-stress liquids, the early regime of spreading is primarily dictated by their high shear rate viscosity. For yield-stress liquids with low values of high shear rate viscosity, the spreading dynamics mimics that of Newtonian liquids like water, i.e., an inertia-capillary regime exhibited by a power-law evolution of spreading radius with exponent 1/2. With increasing high shear rate viscosity, we observe that a deceptively similar, although slower, power-law spreading regime is obeyed. The observed regime is in fact a viscous-capillary where viscous dissipation dominates over inertia. The present findings can provide valuable insights into how to efficiently control moving contact lines of biomaterial inks, which often exhibit yield-stress behavior and operate at high print speeds, to achieve desired print resolution.

Topographic variation and fluid flow characteristics in rough contact interface

Understanding flow characteristics of fluid near rough contact is important for the design of fluid-based lubrication and basic of tribology physics. In this study, the spreading and seepage processes of anhydrous ethanol in the interface between glass and rough PDMS are observed by a homemade optical in-situ tester. Digital image processing technology and numerical simulation software are adapted to identify and extract the topological properties of interface and thin fluid flow characteristics. Particular attention is paid to the dynamic evolution of the contact interface morphology under different stresses, the distribution of microchannels in the interface, the spreading characteristics of the fluid in contact interface, as well as the mechanical driving mechanism. Original surface morphology and the contact stress have a significant impact on the interface topography and the distribution of interfacial microchannels, which shows that the feature lengths of the microchannels, the spreading area and the spreading rate of the fluid are inversely proportional to the load. And the flow path of the fluid in the interface is mainly divided into three stages: along the wall of the island, generating liquid bridges, and moving from the tip side to the root side in the wedge-shaped channel. The main mechanical mechanism of liquid flow in the interface is the equilibrium between the capillary force that drives the liquid spreading and viscous resistance of solid wall to liquid. In addition, the phenomenon of “trapped air” occurs during the flow process due to the irregular characteristics of the microchannel. This study lays a certain theoretical foundation for the research of microscopic flow behavior of the liquid in the rough contact interface, the friction and lubrication of the mechanical system, and the sealing mechanism.

Revealing the Relationship between Critical Inlet Velocity and a Double-Layer Oxide Film Combined with Low-Pressure Casting Technology

In the context of low-pressure casting, an excessive inlet velocity may result in the introduction of an oxide film and air into a liquid metal, leading to the formation of a two-layer film structure within the casting. Such defects can significantly degrade the mechanical properties of the castings. In order to optimize the advantages of low-pressure casting, an empirically designed equation for the inlet velocity was formulated and the concept of critical inlet velocity was further refined. A comprehensive numerical simulation was conducted to meticulously analyze the liquid metal spreading phase within the cavity. Subsequently, low-pressure casting experiments were carried out with actual castings of an A357 alloy, using two different entrance velocities—one critical and the other exceeding the critical entrance velocity. Tensile test specimens were extracted from the castings for the comparative evaluation of mechanical properties. It was observed that the average tensile strength of specimens cast at the critical inlet velocity exhibited a notable 16% enhancement. In contrast, specimens cast at velocities exceeding the critical inlet velocity manifested the presence of double oxide film defects. This evidence suggests that casting at a velocity faster than the critical inlet velocity leads to the formation of double oxide film defects, which in turn reduces the mechanical properties of the castings.

Stretch-Controlled Branch Shape Microstructures for Switchable Unidirectional Self-Driven Spreading of Oil Droplets.

Unidirectional transport of liquids has attracted the attention of researchers in recent years for its wide application foreground. However, it is still a challenge to control the spreading of liquid, especially for oils with relatively high viscosity. In this paper, a flexible surface textured with branch-shaped microstructures is proposed. These asymmetric microstructures exhibit excellent unidirectional spreading behaviors for various oils. By suitably stretching the flexible surface to different stretch ratios, the spreading length of the oil droplets can be controlled. Moreover, the ongoing forward spreading of oil droplets can be suspended dynamically when the surface is stretched to 40%. Corresponding mechanism analysis demonstrates that surface stretching can narrow and close the microvalves between adjacent branches, which restrain the flow of the precursor film and the primary droplet. The switchable unidirectional spreading behavior enables the surface with such microstructures to be used for oil transportation, oil-water separation, and controllable lubrication.

Vertical-supercooling-controlled interfacial instability for a spreading liquid film

Vertical-supercooling-controlled interfacial instability for a spreading liquid film

Molecular Dynamics Simulation of Dual Nanodroplet Impacts on a Cylindrical Surface.

Droplet impact behavior is ubiquitous in various fields. However, the dynamics and spreading mechanisms of micro- and nanoscale droplet impact on curved surfaces, particularly in the case of multiple droplets, have yet to be fully elucidated. In this study, molecular dynamics (MD) methods are employed to investigate the dynamic evolution of double nanodroplet impact on a nano cylindrical wall. The effects of droplet spacing, initial impact velocity, and wall wettability on droplet impact characteristics are analyzed. The results demonstrate that there are five impact modes of nanoscale double-droplet impacts with nanocylinders: spreading-partial wrapping-splitting-complete detachment (SPSC), spreading-complete wrapping-complete attachment (SCC), spreading-partial wrapping-complete attachment (SPC), spreading-partial wrapping-partial attachment (SPP), and spreading-partial wrapping-fragmentation-partial attachment (SPFP). The droplet spacing has an insignificant effect on the impact modes but affects the droplets' spreading shape in both the axial and radial directions. The initial velocity and wall wettability have significant impacts on the droplet impact modes and liquid film spreading characteristics. As the initial velocity increases, the liquid film's radial and axial spreading distances gradually increase. Under hydrophobic conditions, the spreading of the droplet is dominant in the radial direction, while under hydrophilic conditions, the spreading is dominant in the axial direction. Properly reducing the droplet spacing, increasing the impact velocity, and enhancing the wall hydrophobicity can promote detaching the droplet from the cylindrical wall.

One-step laser preparation of unidirectional liquid spontaneous transport structures

The Tilted Wedge Microcavity Array (TWMA) structure of the Nepenthes alata peristome allows for unidirectional fluid transport without any surface energy gradient. However, current methods of creating bionic Nepenthes alata peristome structures have significant improvement room in terms of structural shape, machining efficiency, and cost-effectiveness. To address these issues, in this paper, bionic Nepenthes alata peristome TWMA structures on 7075 aluminium alloy was built via a nanosecond laser with highly efficiency, low cost and flexible processing pattern. A unilateral tilted microcavity structure was fabricated by designing the laser processing path, and at the same time, the samples were tilted to change the angle at which the laser beam hit the working surface. This ensures that the microcavity is machined without destroying the microcavity structure underneath. The variation law of aluminum alloy ablation threshold under the influence of laser incidence angle was studied. The dimensional parameters of the TWMA structure were analysed in relation to laser incidence angle θ, laser processing power P, number of laser scans n, and laser scanning speed V. Additionally, liquid spreading tests were carried out on different surfaces of bionic structures, and the influence of surface wettability on the spreading results was investigated. The results indicate that the ablation threshold decreases initially and then increases with an increase in the laser incidence angle. This reflects the trend of the aluminium alloy absorption rate, which initially increases and then decreases with an increase in the incidence angle. For the prepared TWMA structure, the optimum speed of achieving the unidirectional spreading effect was found to be 2.41 mm s−1 with a surface contact angle (CA) of 35°. This study presents a low-cost and rapid preparation method for creating liquid unidirectional transport structures without any energy input.