Solid Surface Research Articles

Sustainable Antibacterial Surface of Transparent PMMA Membranes with α-ZrP Nanosheets Adsorbing Tetraalkylammonium Ions.

We fabricated composite membranes containing inorganic nanosheets (NSs) and polymers and demonstrated their outstanding antibacterial performance against several opportunistic pathogens. Layered α-zirconium phosphate [Zr(HPO4)2, α-ZrP] as a pristine compound of NS was exfoliated by ion-exchanging protons in the interlayer space of α-ZrP with bulky tetraalkylammonium ions (TRA+: R = butyl, hexyl, and octyl). During the exfoliation process, TRA+ was electrostatically adsorbed onto α-ZrP NS with a negative surface charge (ZrP-TRA-NS). The produced PMMA membrane including α-ZrP NS (PM-ZrP-TRA-NS) was optically transparent and prohibited bacterial growth, and the effect was stronger for Gram-positive Staphylococcus aureus than Gram-negative Escherichia coli. The antibacterial activity of PM-ZrP-TRA-NS was based on physical damage induced by both 2D ceramic NSs and sharp alkyl chains of TRA+. Despite the inherent flexibility of alkyl chains, when adsorbed onto the NSs, they can act in a manner that effectively pierces the bacterial cell wall. The piercing force of TRA+ was greater for the longer alkyl chains (TBA+ < THA+ < TOA+). Focusing on the difference in the cell wall structure between these bacteria, the growth of Gram-positive S. aureus with loose peptidoglycan layers as an outer membrane could be easily inhibited by contact with the composite film. In contrast, Gram-negative bacteria E. coli, surrounded by a relatively dense outer cell wall composed of peptidoglycan and lipopolysaccharide layers, could not be damaged easily. In this study, the antibacterial mechanism of PM-ZrP-TRA-NS membranes was elucidated, and their usefulness as antimicrobial coatings for existing solid surfaces was demonstrated.

Gradient Nonwettability Controls Water-Film Flowing Features.

The flow of the water film on solid surface depended on the film Reynolds number and wind speed. Moreover, environmental factors had an impact on the flow process. This study explored how surface wettability impacts the stability and detachment of water film under varying conditions. Superhydrophobic surface played a critical role in speeding up the detachment of water film, especially under conditions of strong wind and specific flow dynamics, such as a wind speed of 19 m/s and a Reynolds number of 83. The efficiency in water removal was due to a combination of forces: capillary action, which pulls the water into smaller areas, reduced surface tension, and decreased adhesion between the liquid and surface. These factors worked together to shrink the area of contact, allowing the film to break and detach more easily. The findings suggested that enhancing nonwettability can improve water removal efficiency, with potential applications in fields like aerospace. Furthermore, additional trials across varying wind speeds and Reynolds numbers consistently supported the superior performance of superhydrophobic surface in driving rapid water-film detachment. To advance this effect, a gradient nonwetting surface was introduced, specifically engineered within the superhydrophobic regime, to amplify the velocity of film separation. This design innovation leveraged variations in surface energy across the gradient, which fostered directional water-film movement and accelerated detachment. A comprehensive analysis of the underlying physical mechanisms further elucidated its potential in enhancing water-shedding applications across a range of environmental conditions.

High-Energy Spectra of Black Hole and Neutron Star Low-Mass X-Ray Binaries

In the case of low-mass X-ray binaries, the companion star is often too faint for detection; therefore, there is no chance for dynamical studies to independently determine the mass of the compact object. In the absence of a mass estimate, one cannot make a distinction as to whether the binary hosts a black hole or neutron star. Therefore, the question arises whether this distinction can be made based on the X-ray data alone, even when there are no bursts or pulsations. These would automatically imply a neutron star, but they are not always present. Black hole systems are known to emit radiation with an unbroken power–law shape up to several hundred keV energies in their high/soft states. If the non-thermal Comptonization processes that are responsible for this are somehow related to the lack of a solid surface, and to the fact that more gravitational potential energy can be released for a black hole, then there would be a definite method to reliably distinguish between the two sources. This work intends to review the available observations and studies to compare how these two populations behave during their different spectral states. A conclusion can be made that high/soft-state spectra are really different for black holes and neutron stars, for the low/hard state; however, the same conclusion cannot be safely made.

Boiling performance enhancement and self-recovery of nucleate boiling regime on micro- and nanostructured porous surfaces

In this paper, we report the pool boiling experimental results for significant enhancements in the boiling heat transfer coefficient (BHTC) and CHF on various microporous surfaces. The microporous surfaces were fabricated by metal (Copper) powder sintering process, and nanostructured porous surfaces were additionally fabricated through the thermal oxidation of the metal porous surfaces. Among the examined substrates, a boiling surface with a thin metal porous layer and millimeter-sized porous pillar structures exhibited optimal boiling performance; the BHTC and CHF of the microporous surface were measured to be approximately 500 % and 270 % higher than those of a smooth reference surface, respectively. We conjecture that the results come from the combined effect of active nucleation site density enhancement, capillary wicking promotion, and separation of liquid-vapor flow paths on the microporous surface with porous pillars. On the other hands, the porous pillar surface with needle-like nanostructures showed the interesting phenomena that can be regarded to the recovery of the boiling regime from transition boiling to nucleate boiling. At the CHF, the temperature increase on the surface was slower than that on the other surfaces. Additionally, at high heat fluxes below the CHF, the overheated surface self-recovered to the nucleate boiling state, indicating the rewetting of local dry spots. We consider that the capillary wicking capability strongly improved by the nanostructures on the surface was presumably responsible for inhibiting the irreversible expansion of local dry spots. From the experimental observations throughout this study, we propose the following key requirements to design an ideal boiling surface: increasing the nucleation site density, ensuring a liquid supply path by capillary wicking, separating the liquid and vapor flow paths, and improving the liquid wettability of the solid surface by forming nanostructures.

Modeling melt relocation with solidification and remelting using a coupled level-set and enthalpy-porosity method

A numerical model to simulate molten metal relocation with phase change is proposed, coupling the level-set method to track the metal-gas interface and an enthalpy-porosity model to handle phase changes between solid and liquid metal. This coupling simultaneously solves the evolution of the metal-gas interface and liquid-solid metal. The numerical model is validated by a melting experiment involving a Sn-Bi eutectic alloy on a copper substrate, wherein the alloy’s transient morphology and spreading diameter are measured. The numerical simulation effectively replicates the observed melting and spreading behaviors of the metal on the solid surface. Further validations, including a melt infiltration simulation and experiment, are consistent with findings from previous research. These simulations affirm the model’s capability and efficiency in accurately representing the dynamics of melt relocation across various geometries, even within complex porous structures.

Multi-scale evaluation of surfactant effects on asphalt desorption behavior from oil sand surfaces

This study created a hydrophobic oil sand model to investigate the impact of surfactants on the desorption behavior of asphalt from solid surface. Cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and anionic surfactant sodium dodecyl benzene sulfonate (SDBS) were used to soak the model to observe the effect of surfactants on asphalt desorption. The effect of surfactant pretreatment on the wettability of the substrate was evaluated by measuring the contact angle of ultrapure water on the substrate and observing the morphological changes of the solid surface by atomic force microscopy. The results show that the contact angle of the model treated with SDBS decreases to 10° after 3 days, which significantly improves the wettability of the substrate surface and promotes the desorption of asphalt. However, CTAB has no obvious effect on asphalt desorption. The standard deviation of the gray value of the sand surface image is significantly reduced to 38.693 after the SDBS treatment, which can be seen by binarizing the image. This suggests that the asphalt and solution are evenly distributed and that there is hardly any discernible boundary. Nevertheless, neither of the two surfactants can improve the water-based extraction efficiency of oil sands after soaking treatment. The asphalt yield is less than 5% after 30 days of pretreatment with CTAB solution, while only 45% asphalt yield is obtained after 30 days of pretreatment with SDBS solution. Despite these variations, the quality of asphalt foam remains relatively the same. This may be related to the failure of surfactants to improve the surface wettability of hydrophobic sand. Therefore, the surface wettability of the sand is considered to be a key factor affecting the asphalt desorption and final yield. This study can provide a scientific basis for oil sand recovery to a certain extent.

Application of metal organic frameworks ZIF-8-based solid catalyst with hierarchical porous structure and Brønsted-Lewis dual acid sites ionic liquids for sustainable biodiesel production from acidic soybean oil

In order to alleviate the mass transfer limitation of triglyceride macromolecules on the solid catalyst surface during the catalytic oil transformation process, the utilization of hierarchical porous solid catalyst is a feasible method. For reaching this goal, the hierarchical porous support of H-ZIF-8 was initially prepared by incorporating the three-dimensional ordered macro and mesoporous into the ZIF-8 using polystyrene spheres (PS) as a hard template. Subsequently, the phosphotungstate anion PW12O403- (PW) decorated sulfonated 4,4'-dipyridinium ionic liquid with Keggin structure, namely [DPySO3H]1.5PW, was encapsulated in the H-ZIF-8 support, with the formation of the novel [DPySO3H]1.5PW@H-ZIF-8 catalyst. The so-synthesized solid catalysts featured with ordered hierarchical porous structure and dual Lewis and Brønsted acidic properties, with large BET surface area of 267.38 m2/g. This catalyst had high catalytic performances for the concurrent transformation of triglycerides and free fatty acids to biodiesel in a heterogeneous manner. Under the optimal reaction parameters (methanol/oil molar ratio: 30:1; catalyst dosage: 4 wt%; reaction time: 8 h; reaction temperature: 130 °C), the oil conversion level of 91.2% and entire conversion of free fatty acids could be attained by applying this [DPySO3H]1.5PW@H-ZIF-8 catalyst. The high catalytic activity of this catalyst was due primarily to the increased mass transfer efficiency and synergism of Lewis and Brønsted acid sites. Moreover, the good FFA and moisture-resistance capacity was also shown for this catalyst even in the case of moisture content of 5 % and FFA content of 40 % in the oil feedstock. This solid catalyst could be reused by simple filtration and displayed good reusability, still attaining over 80% oil conversion at the fourth reuse cycles thanks to the robust interactions between the acidic active centers and the hierarchical porous support. The kinetic analysis indicated that the activation energy Ea and Arrhenius constant A for this solid acid-catalyzed transesterification process were 56.0 kJ/mol and 7.8×104 min-1, respectively. This research would provide a new approach for the development of hierarchical porous solid catalysts, enabling the one-step transformation of low-grade acidic oils into biodiesel in a more sustainable and environmentally friendly way.

Measurement of surface tension and contact angle of solar salt at high temperature with axisymmetric drop-shape analysis

As interfacial properties of common materials enabled many practical applications, those for molten salts would do the same. While their properties are important for the safety and design of thermal energy storage and modular nuclear reactors, they are hard to obtain because of the measurements done in confined spaces inside a furnace at high temperatures. Moreover, a small amount is preferred due to the high cost of salts. To overcome these limitations, an accurate method to measure surface tension, γ, and contact angles, θ, of molten salts were sought. As a steppingstone toward using more complex salts, solar salt was used because of safety and availability. Because molten salts typically show low θ, they provide large errors by incorrectly predicting volume and/or geometry. Therefore, various solid materials on which solar salts show high θ were sought and boron nitride was identified to provide θ > 60°. Later, solar salt sessile droplets were analyzed with the axisymmetric drop-shape analysis (ADSA) method to measure γ over a temperature range, which showed a good agreement with literature. Thus, this paper sets an experimental protocol that can be possibly used for other complex and hazardous salts that are scarcer and hard to handle. This paper extends the current knowledge by identifying solid surfaces on which the salts show low wettability for accurate measurements. Compared to other accurate methods, such as maximum bubble pressure method, this method used a small amount of salt (< 100 μL) while keeping a high accuracy.

Modification as a Method of Regulation of Energy Characteristics and Functionalization of Solid Surfaces

The surfaces of solids (gold, silver, polymers) were modified with adsorption layers of various compounds. Optimal modification conditions were determined using methods of contact angle measuring and quartz crystal microbalance. The degree of filling of surfaces with the adsorption layer was calculated and the data obtained were compared with the results of direct measurements of adsorption. The surface energy of the modifying layers was determined and the potential applications of modified solids as functional materials are demonstrated.

Imaging Dihydrogen Bond-Driven Assembly of Borazine on Au(111).

Dihydrogen bondingis a peculiar type of attractive interaction occurring between a partially positively charged hydrogen atom and a partially negatively charged hydrogen atom. Borazine represents a prototypical molecule exhibiting dihydrogen bonding in both gas phase, as well as in its crystalline form. For borazine assemblies on solid surfaces, a direct observation and characterization of dihydrogen bonding has remained elusive, possibly due to an intricate interplay of substrate-molecule and intermolecular interactions. Here we present evidence of dihydrogen bonding occurring in borazine assemblies on a Au(111) surface. By means of low-temperature scanning tunneling microscopy, we unveiled distinct configurations, exhibiting single and double dihydrogen bonding. Density functional theory calculations elucidate the interplay between substrate adsorption and intermolecular interactions to stabilize the formation of borazine dimers on Au(111), being the building blocks for the formation of larger assemblies.

Direct activation of Toll-like receptor 2 signaling stimulated by contact with the interfacial structures of chitin nanofibers

The innate immune system, which eliminates pathogens and abnormal cells, is involved in the pathogenesis of various diseases and infections, where Toll-like receptors (TLRs) play a critical regulatory role. In this study, we investigated the potential of chitin nanofiber (CtNF) to induce an immune response, which is expected to act as an agonist of TLR2. Crab-derived CtNF, surface-deacetylated CtNF, and surface-carboxylated cellulose NF were employed as TLR2-mediated immune stimulator, signal regulator, and cell adhesion promoter, respectively, to fabricate cell culture scaffolds for HEK293 cells with TLR2 and human monocyte THP-1 cells with or without TLR2. Surface deacetylation of CtNF drastically diminished the immunological response of HEK293 cells, suggesting that the N-acetyl groups on the solid CtNF surface were pivotal for TLR2-mediated stimulation. A comparison of wild-type and TLR2-KO THP-1 cells on cell culture substrates with N-acetyl groups ranging from 0 to 1.39 mmol g−1 revealed that immune signaling for nuclear factor-κB and interferon regulatory factor pathways was strongly dependent on the surface N-acetyl group content. The immunostimulatory level at the interface of solid CtNF and immune cells could be regulated by simply mixing CtNF and surface-deacetylated CtNF, which is a significant advantage for its potential use as a novel immunostimulant.

Nebulous landscape and the aesthetics of indeterminacy

A landscape is commonly conceived as an arrangement of items on solid or liquid surfaces. Despite its entanglement with the landscape, the weather condition tends to be overlooked. The liminal case of a thick fog that suppresses the view of the environment challenges this standard understanding. The paper examines the experience of being lost in fog with respect to perceptual, spatiotemporal and emotional aspects. Fog suspends the everyday scopic regime and the inconspicuous ‘immateriality’ of air as medium of perception, distorting the patterns of perception and temporarily installing an own spatiotemporality. Moreover, fog is an atmospheric phenomenon par excellence, a quasi-thing and a quasi-agent that is usually felt as distressing and maleficent yet may trigger a sense of wonder as well. While it is considered a deficit in epistemic issues, nebulosity has a strong aesthetic expressivity, whose sources range from semantic ambiguity to defamiliarization and the transfiguration of the ordinary.

Impact of Bulk Nanobubble Water on a TiO2 Solid Surface: A Case Study for Medical Implants.

In the field of medical implants, enhancing the wettability of artificial surfaces is crucial for improving biocompatibility. This study investigates the potential of ozone nanobubble water, an aqueous solution known for its strong oxidizing and sterilizing properties, to modify the surface of titanium dental implants. By immersing the implants in ozone nanobubble water for ∼10 min, we observed a significant transformation of their surface characteristics. Implant surfaces that had become hydrophobic over time, likely due to organic contaminants, became superhydrophilic, exhibiting a contact angle near zero. Fresh implant material is initially hydrophilic but becomes hydrophobic within a few days of drying. In sharp contrast, the hydrophilicity induced by ozone nanobubble water treatment persisted for more than one month. This durability suggests not only the removal of organic matter through cleaning but also a substantial alteration in the surface properties of the implants. The generation of ozone nanobubble water involved releasing ozone microbubbles into an aqueous solution containing trace amounts of iron and manganese, resulting in spherical particles with an average diameter of ∼10 nm. These particles could be bulk nanobubbles, a stabilized gas body surrounded by a solid shell composed of iron hydroxide. Termed "nanoshells" in our previous study, these particles demonstrated exceptional dispersibility without the need for stabilizing agents such as surfactants or capping agents, attributed to their inherent high wettability. The sustained hydrophilicity of the implant surfaces might be attributed to the adherence of these hydrophilic nanoshells to the implant's surface. This study highlights the potential of ozone nanobubble water for long-term surface modification of dental implants, offering a promising avenue for enhancing implant biocompatibility.

Force analysis of bubble-particle detachment colliding with a solid surface

This is the second in a series of papers concerned with bubble-particle detachment colliding with a solid surface. Part I [22] explored, experimentally, the detachment of a particle from the surface of a bubble colliding with a solid surface in mimic of the pulp-froth interface. Detachment was seen to be dependent on interactions between the bubble-particle aggregate and the solid surface, which is characterized by the inclination angle. A neck was formed during the detachment process between a particle and a bubble. Zoom in images of bubble-particle aggregate are used to extract parameters on contact angle and three-phase contact, which can be used to calculate forces acting on the particle.Part II presents calculations of the forces in the particle detaching processes, which is essential to the modelling of coarse particle flotation processes. It is found that advancing contact angle serves as a prerequisite for the particle detachment in both particle detachment modes, i.e. rebound detachment and slip detachment. The advancing contact angle makes the three-phase contact line contract and move over the location of the maximum capillary force. After which the particle is likely on the track of detachment.This study provides detailed experimental observations and calculations of the forces acting on particles during separation, analyzes the forces between particles attached to bubbles when aggregates collide with a flat wall, and provides the first quantitative analysis of the impact of contact angle hysteresis on the separation mechanism, providing new insights for applications in the field of transportation and beneficiation.

Design and Validation of Specific Oligonucleotide Probes on Planar Magnetic Biosensors.

Planar DNA biosensors employ surface-tethered oligonucleotide probes to capture target molecules for diagnostic applications. To improve the sensitivity and specificity of biosensing, hybridization affinities should be enhanced, and cross-hybridization with off-targets must be minimized. To this end, assays can be designed using the thermodynamic properties of hybridization between probes and on-targets or off-targets based on Gibbs free energies and melting temperatures. However, the nature of heterogeneous hybridization between the probes on the surface and the targets in a solution imposes challenges in predicting precise hybridization affinities and the degree of cross-hybridization due to indeterminable thermodynamic penalties induced by the solid surface and its status. Herein, we suggest practical and convenient guidelines for designing oligonucleotide probes based on data obtained from planar magnetic biosensors and thermodynamic properties calculated by using easily accessible solution-phase prediction. The suggested requirements comprised Gibbs free energy ≥ -7.5 kcal mol-1 and melting temperature ≤10 °C below the hybridization temperature, and we validated for the absence of cross-hybridization. Additionally, the effects of secondary structures such as hairpins and homodimers were investigated for better oligonucleotide probe designs. We believe that these practical guidelines will assist researchers in developing planar magnetic biosensors with high sensitivity and specificity for the detection of new targets.

The microlayer and force balance of bubbles growing on solid in nucleate boiling

This study uses interface-resolved computational fluid dynamics simulations to investigate the dynamics of bubble growth on a horizontal solid surface, with a focus on the characteristics of the microlayer and the forces involved in the process. The simulations exclude phase change effects to concentrate on hydrodynamics and employ an external mass source for controlled bubble inflation. This mass source follows a time-dependent bubble radius growth law, R(t)∝t1/2, which is typical for heat-transfer-controlled growth of a vapour bubble during nucleate boiling of water at atmospheric pressure. The numerical framework is validated against recent experimental measurements of bubble shape and microlayer profile. The results of this work indicate that the rate of bubble growth significantly influences microlayer formation. Faster growth rates produce a near-hemispherical bubble shape with an extended radial microlayer on the solid, while slower rates yield a taller, more spherical bubble with a shorter microlayer. All microlayer profiles exhibit an outwardly-curved shape, with a maximum microlayer thickness increasing with the growth rate. The study also examines the force balance on the bubble, revealing that the net vertical force of the bubble does not equal zero even when the bubble remains attached to the solid surface. Our analysis of the bubble motion demonstrates that previous force balance models used for determining bubble detachment lack robustness. The microlayer profile obtained in this work is important for boiling heat transfer studies as the microlayer contributes significantly to local heat transfer. The force balance analysis shows the need for a new approach to determine bubble detachment behaviour, which is vital for predicting flow boiling rates.

Ultralow Resistivity of the Graphite/Au van der Waals Heterostructure in the Structural Superlubric State.

Structural superlubricity, a state of zero wear and ultralow friction between two sliding solid contact surfaces, offers transformative potential for NEMS/MEMS switches, relays, logic devices, and sensors. Ultralow resistivity in two-dimensional (2D)/three-dimensional (3D) van der Waals heterostructures is critical for most applications. Using high-vacuum thermal evaporation of Au, we fabricate defect-free single-crystal graphite/Au van der Waals heterostructures in the SSL state, achieving a record-low resistivity of 2.542 × 10-12 Ω m2, which is 6 times lower than that of prior reports. The resistance remains stable under varying loads and speeds, attributed to the all-atom contact and defect-free single-crystal graphite. Thermal evaporation of Au preserves the single-crystal graphite 2D lattice, enabling lower resistivity. The 2D/3D SSL heterostructure is simpler, more stable, and broadly more applicable than 2D/2D systems. It establishes the electrical foundation for applying superlubric technology in micro-nano devices and provides a platform for studying fundamental behaviors at 2D/3D sliding interfaces, including electron friction, phonon dissipation, and electron transfer.

Non-Invasive Follicular Thyroid Neoplasm With Papillary-Like Nuclear Features – An Illustrative Case

A 54-year-old woman underwent total thyroidectomy for a one-year history of anterior neck mass. The specimen was a 29.88–gram thyroid gland that on sectioning showed a single 2.0 x 1.5 x 1.5 cm encapsulated nodule with tan-brown solid cut surfaces noted on the right thyroid lobe. No gross lesions were noted on sectioning of the isthmus and left thyroid lobe. Microscopic examination showed a thinly encapsulated nodule composed of tightly-packed follicles. (Figure 1) Examination of the entire capsule did not show capsular or vascular invasion. The follicles were lined by follicular cells that had crowded and enlarged nuclei with pale chromatin and some nuclear membrane irregularities such as grooving. (Figure 2) There were no papillae, psammoma bodies, necrosis, mitotic figures and solid or trabecular architecture seen. Based on these features, the diagnosis rendered was non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP). NIFTP is a term adopted in 2016 to replace the nomenclature of a thyroid tumor previously termed “non-invasive encapsulated follicular variant of papillary thyroid carcinoma (FVPTC)”.1 In the most recent edition of the World Health Organization (WHO) Classification of Tumors, NITFP is considered a low-risk follicular cell-derived neoplasm whose definition also reflects the diagnostic criteria of this tumor.2

Three-stage Lobatto analysis on hydrodynamic Eyring-powell flow of nanofluid: Impact of viscous dissipation, thermal radiation, and wall slip dynamics

Thermal radiation and viscous dissipation play crucial roles in the phenomenon of boundary layer flow, especially in engineering and industrial applications involving high temperatures, such as in combustion engines, gas turbines, and furnaces, thermal radiation becomes a significant mode of heat transfer whereas viscous dissipation signifies the conversion of kinetic energy into thermal energy due to the frictional forces in the fluid. The key findings of the current investigations is to explore 3D magneto-hydrodynamics radioactive Eyring-Powell nanofluid flowing towards a stretchable porous surface using a three-stage Lobatto numerical. The influence of velocity and thermal slip under convective boundary constraints are also incorporated in the present investigations. The Eyring-Powell model, known for its relevance in non-Newtonian fluid mechanics, is used to simulate a nanofluid's flow dynamics and heat transfer characteristics. The mathematical Navies-Stokes equations are transformed into a system of ordinary differential equations (ODEs) by adopting a similarity variable, which are then solved numerically with the aid of the MATLAB bvp4c package. Results highlight the significance of physical parameters involved in the model like magnetic field strength , slip parameter, Eckert number, Lewis number, thermophoresis parameter, Brownian motion parameter and their impact on velocity temperature, and concentration profiles are displayed in the form of graphically. The data reveals an 11.2% increase in the heat transfer rate and a 6.65% increment in mass transfer when the radiation parameter is raised from 0.1 to 0.4. Results also elucidate that fluid temperature at the boundary rises as the Biot number increases because the rate of heat transfer between a solid surface and the surrounding fluid becomes more efficient. To check the validity and reliability of the present study, our calculated results are compared with the previous one, which shows stable agreement.

Vapor-mediated impact of droplet on superheated powder bed

Droplet impacting on a sufficiently heated powder bed resembles those on a superheated solid surface, as the surface deformation is mediated by the spontaneously generated vapor flow from the bottom surface of the droplet. This emerged impacting behavior is denoted as vapor-mediated impact to differentiate from the wetting impact, which involves the wetting and absorption of the particles due to capillarity. We systematically vary the impacting velocity and the temperature of the powder bed to characterize the impacting dynamics for these two behaviors. For the vapor-mediated impact, the contact time and the maximum spreading diameter are found to have the same scaling laws derived for impact on the superheated surface. We construct a phase diagram of the impacting behaviors based on experimental observation, and propose a simplified model to predict the transition between these two behaviors. The predicted values match well with the experimental results, suggesting the proposed model captures the physical mechanism of the vapor-mediated impact.