Solid Surface Research Articles

Ultrafast bounce of particle-laden droplets.

The rebound of liquid droplets on solid surfaces exhibits behavior reminiscent of elastic spheres, albeit with distinct contact dynamics. While the rapid detachment of droplets from surfaces holds significant relevance for various applications, previous endeavors relying on engineered surfaces can only reduce the contact time to several milliseconds, primarily due to capillary effects dominating droplet bounce. Here, we present ultrafast rebound by designing heterogeneous core-shell droplets encapsulating a particle (DEP), which achieves an unprecedentedly short contact time of 0.3 ms and 0.05 ms with polydimethylsiloxane and glass particles, respectively. This remarkable contact-time reduction is universally applicable to diverse systems, including both water and oil droplets, elastic and rigid particles, super-repellent and superlyophilic surfaces, and is effective across a wide range of impact velocities. Beyond exhibiting liquid-like dynamics, DEP manifests solid-like behavior owing to asynchronized motions between the particle and the droplet, which effectively breaks down the dominance of capillarity. With systematic experimental and analytical studies, we delineate contact times in three bouncing regimes and identify critical conditions governing regime transitions. DEP amalgamates the bouncing dynamics of both solids and liquids, offering a robust and versatile strategy for tailoring contact time to suit diverse applications involving solid-liquid composite systems.

Signature of Proton-Hole Transfer in Hydrogen-Bonded Solids at 10 K.

Although proton transport in water ice is well understood, proton-hole transfer (PHT) involving proton abstraction by anions remains less explored. This study investigates PHT in H2S and NH3 solids at low temperatures, aiming to determine whether these solids exhibit negative charge transport similar to that in ice. In H2S and NH3 solids at 10 K, surface HS- and NH2- anions in hydrogen-bonded systems trigger negative current flow, providing a clear signature of PHT. This negative current is controlled by electron flow and 193 nm ultraviolet irradiation, which generates HS- and NH2- anions on the solid surfaces. In bilayer H2S/H2O and NH3/H2O solids, a significant negative current is observed only in the NH3/H2O solid, which is attributed to the exothermic proton abstraction by NH2- from H2O at the bilayer interface, a process not available for H2S on ice. This study is the first to demonstrate PHT-induced electrochemical behavior in hydrogen-bonded solids other than ice.

A chronological catalog of methods and solutions in the Space–Time Computational Flow Analysis: I. Finite element analysis

AbstractThe Space–Time Computational Flow Analysis (STCFA) started in 1990 with the inception of the Deforming-Spatial-Domain/Stabilized Space–Time (DSD/SST) method. The DSD/SST was introduced as a moving-mesh method for flows with moving boundaries and interfaces, which is a wide class of problems that includes fluid–particle interactions, fluid–structure interactions (FSI), and free-surface and multi-fluid flows. The first 3D computations were reported in 1992. The original DSD/SST method is now called “ST-SUPS,” reflecting its stabilization components. As the STCFA evolved, advanced mesh moving methods, FSI coupling methods, and problem-class-specific methods were introduced to increase its scope and the ST Variational Multiscale was introduced to upgrade its stabilization components to the VMS. Complementary general-purpose methods developed in the evolution of the STCFA include the ST Isogeometric Analysis (ST-IGA) and the ST Slip Interface (ST-SI) and ST Topology Change (ST-TC) methods. The ST-IGA delivers superior accuracy through IGA basis functions not only in space but also in time. The ST-SI enables high-fidelity moving-mesh computations even over meshes made of patches with nonmatching meshes at the interfaces between those patches. The ST-TC enables high-fidelity moving-mesh computations even in the presence of topology changes in the fluid mechanics domain, such as an actual contact between moving solid surfaces. The STCFA brought first-of-its-kind solutions in many classes of problems, ranging from fluid–particle interactions in particle-laden flows to FSI in parachute aerodynamics, flapping-wing aerodynamics of an actual locust to ventricle-valve-aorta flow analysis to car and tire aerodynamics with near-actual geometries, road contact, and tire deformation. With the success we see in so many classes of problems, we can conclude that the STCFA has reached a level of remarkable sophistication, scope, and practical value. We present a chronological catalog of the methods and solutions in the STCFA. In Part I of this two-part article, we focus on the methods and solutions in finite element analysis.

The effect of single walled carbon nanotubes on conductivity and thermal properties of glass fiber reinforced composites

Abstract This paper reports on a comprehensive study of the effect of single-walled carbon nanotube (SWCNT) on the alternating current (AC) conductivity, thermal and morphological properties of the unsaturated polyester based glass fiber reinforced polymer composite (GFRPC). AC conductivity measurements were carried out using the impedance spectrum and thermal measurements were carried out using differential scanning calorimetry (DSC) at a temperature range of 24-900 °C and heating rates of 20 °C/min. Impedance results showed that the conductivity behavior in the nanotube-loaded composite laminate obeys a Jonscher-type mechanism. At low frequencies, the conductivity value remains almost constant for the doped material and takes the value of 10-5 S/cm. It is observed that the AC conductivity starts to increase after the critical frequency value of approximately 103 Hz and increases up to 10-2 S/cm due to hopping and tunneling mechanisms caused by space charge polarization accumulated in the local regions at high frequencies. The pure material with an insulating nature also exhibited a typical insulating behavior. Thermal testing showed that nanotube reinforcement increases thermal conductivity in three different directions. DSC thermocurves analysis also revealed that the addition of carbon nanotubes increased the glass transition temperature of the material from 180°C to 190°C. The scaling and fractal analysis methods were also applied to obtain hetero morphological structure of materials. The fractal analysis results indicated that carbon nanotube doping to the standard sample increases the coating rates, scalability and heterogeneity of the solid phase surface of the sample. The coating rates of composite surfaces were calculated as 45% and 36%, respectively. Morphology analysis revealed that the probability of finding surface particles for the nanotube-doped sample decreased compared to the undoped sample, but the fractal dimension value increased. While this value was 1.83 in the pure sample, it increased to 1.92 in the nanotube material.

Azo-Enhanced Raman Scattering Probing Proton Transfer between Water and Nanoscale Zero-valent Iron.

The interaction between a solid and water at their interface, especially proton transfer, impacts molecular-scale catalysis, macroscopic environmental science, and geoscience. Although being highly desired, directly probing proton transfer between a solid and water is a great challenge, given the subnanometer to nanometer scale of the interface. The fundamental challenge lies in the lack of a measurement tool to sensitively observe local proton concentration without introducing an exogenous electrode or nanoparticle with a minimum size of tens of nanometers. Here, we demonstrate an azo-enhanced Raman scattering strategy to design a 2 nm long small-molecule pH probe with a chelating group anchoring to the solid surface. Empowered by the intramolecular Raman enhancing sensitivity, the probe directly observes proton transfer between water and nanoscale zero-valent iron (nZVI), a famous environmental material for pollution control. This molecular-scale interfacial probing methodology offers a powerful tool to pave the way for advanced environmental and geochemical discernment and management.

Functionalization of Emulsion Interfaces: Surface Chemistry made Liquid.

Disperse systems, and emulsions in particular, are currently massively used in fields as varied as food industry, cosmetics, health care and environmentally-friendly materials. To meet increasingly precise needs or targeted applications, these systems need to be endowed with new functionalities at their interfaces, in addition to their composition and structural properties. However, due to the fragility of drops and the low reactivity of their surface, conventional solid surface chemistry cannot be used for such a purpose. Several specific emulsion interface functionalization techniques have thus been developed for targeted systems and applications, but a general framework has yet to be drawn. In this review, we attempt to present these methods in a unified way through the prism of what we may call "liquid surface chemistry". We propose to categorize existing methods into drop-coating strategies, including layer-by-layer techniques and polymer coating, and emulsifier-carrier approaches involving particles and/or amphiphilic molecules. They are discussed in a transversal way, highlighting the underlying physico-chemical principles and providing a comparative analysis of their advantages, current limitations and potential for improvement. We also propose future directions and opportunities, involving for instance DNA-based programmability or artificial intelligence, which could make liquid surface chemistry more versatile and controlled.

A Photoelectric Synergistic Flexible Solid Slippery Surface for All-Day Anti-Icing/Frosting.

The accumulation of ice on surface has caused great harm to lots of fields such as transportation or aerospace. Nowadays, various equipment or tools used in low-temperature environments, which face the risk of interface icing, usually have irregular shapes. Traditional rigid anti-icing materials are difficult to meet practical application requirements. Thus, it is crucial to develop flexible anti-icing materials that can be applied to various shape surfaces (curved surfaces, flat surfaces). In this paper, a photoelectric synergistic flexible solid slippery surface (FSSS) is prepared by using flexible basalt fiberglass cloth, flexible copper foil, flexible polyurethane/carbon nanotubes mixture, and flexible solid lubricant (the mixture of coconut wax and coconut oil). Even under harsh conditions of the temperature as low as -80 °C, the FSSS exhibits excellent all-day anti/de-icing performance whether on flat or curved surface. Moreover, the FSSS shows long-term stability both on flat and curved surface: situated in air for 60 days, submerged in water for 60 days, kept in acid environment (pH 1) and base environment (pH 13) for 30 days. Besides, the FSSS can also achieve self-healing function under -80 °C. This flexible surface provides a novel approach for de-icing/frosting of multi-shaped objects in the future.

Nuclear burning in an accretion flow around a stellar-mass black hole embedded within an AGN disk

Abstract A stellar-mass black hole, embedded within the accretion disk of an active galactic nuclei (AGN), has the potential to accrete gas at a rate that can reach approximately ∼109 times the Eddington limit. This study explores the potential for nuclear burning in the rapidly accreting flow towards this black hole and studies how nucleosynthesis affects metal production. Using numerical methods, we have obtained the disk structure while considering nuclear burning and assessed the stability of the disk. In contrast to gas accretion onto the surface of a neutron star or white dwarf, the disk remains stable against the thermal and secular instabilities because advection cooling offsets the nuclear heating effects. The absence of a solid surface for a black hole prevents excessive mass accumulation in the inner disk region. Notably, nuclear fusion predominantly takes place in the inner disk region, resulting in substantial burning of $\rm ^{12}C$ and $\rm ^{3}He$, particularly for black holes around M = 10 M⊙ with accretion rates exceeding approximately ∼107 times the Eddington rate. The ejection of carbon-depleted gas through outflows can lead to an increase in the mass ratio of oxygen or nitrogen to carbon, which may be reflected in observed line ratios such as $\rm N\, V/C\, IV$ and $\rm O\, IV/C\, IV$. Consequently, these elevated spectral line ratios could be interpreted as indications of super-solar metallicity in the broad line region.

Antimicrobial Surfaces: Stainless Steel Functionalized with the Essential Oil Component Vanillin

Pathogenic microorganisms can adhere to solid surfaces, leading to the formation of biofilms, thus building a physical barrier hindering the penetration and diffusion of antimicrobial compounds. In this context, the use of natural antimicrobial compounds, such as essential oil components, as substitutes for common synthetic antimicrobials in the fight to prevent antimicrobial resistance is explored. As stainless steel is one of the most widely used surfaces in different industries, we have developed an innovative antimicrobial treatment for stainless steel surfaces based on a multi-step functionalization process, in which the stainless steel surface is coated with a silica layer to which a vanillin derivative is covalently attached. The surface was analyzed by microscopy studies, indicating the correct immobilization on the surfaces. Antimicrobial studies (viability and bacterial adhesion assays) were performed against the bacteria Staphylococcus epidermidis, which is one of the most frequent causes of nosocomial infections. The results of the microbiological studies showed that vanillin-functionalized stainless steel surfaces reduce the bacteria viability by 100% and the biofilm formation on the stainless steel surface by 75% compared with non-functionalized surfaces, highlighting the contact-killing and adhesion resistance properties of the developed surface. Additional cycles using the functionalized surfaces showed good maintenance of the antimicrobial coating efficacy. Moreover, the surfaces coated with an intermediate silica layer demonstrated much greater antimicrobial activity than surfaces in which the active molecule was directly functionalized on the stainless steel surface.

Drop impact onto wettability-patterned solid surfaces: A phase field approach

Drop impact onto wettability-patterned surfaces is of great significance in industries. Self-propulsion, self-splitting, and directional rebounding can be realized when drops impact on such surfaces. This paper established a diffuse interface/phase field model to delve into drop impact onto wettability-patterned surfaces, with two typical surfaces considered, one having a step change in the contact angle and the other having a smooth change in it. The diffuse interface model used the phase field to track the liquid–gas interface, was discretized on a half-staggered grid, and was run in a parallel manner. The model was validated first against an impact onto a uniform surface and then against an impact onto a hydrophilic surface coated with a superhydrophobic strip. A mesh independence study was conducted for the phase field modeling. Grid independence was achieved while the phase field mobility was kept fixed in meshes of varied resolutions. The major findings are as follows. The spreading of a spherical drop on gradient wettability surfaces resembles that of an ellipsoidal drop on a uniform surface, and axis-switching was observed. On the other hand, directional rebounding on multi-region wettability surfaces is enhanced with increased wettability contrast.

Assessment of Cavitation Erosion Using Combined Numerical and Experimental Approach

This paper aims to numerically assess the cavitation damage of hydrodynamic machines and hydraulic components and its development in time, based on cavitation erosion tests with samples of used materials. The theoretical part of this paper is devoted to the numerical simulation of unsteady multiphase flow by means of computational fluid dynamics (CFD) and to the prediction of the erosive effects of the collapses of cavitation bubbles in the vicinity of solid surfaces. Compressible unsteady Reynolds-averaged Navier–Stokes equations (URANS) are solved together with the Zwart cavitation model. To describe the destructive collapses of vapor bubbles, the modeling of cavitation bubble dynamics along selected streamlines or trajectories is applied. The hybrid Euler–Lagrange approach with one-way coupling and the full Rayleigh–Plesset equation (R–P) are therefore utilized. This paper also describes the experimental apparatus with a rotating disc used to reach genuine hydrodynamic cavitation and conditions similar to those of hydrodynamic machines. The simulations are compared with the obtained experimental data, with good agreement. The proposed methodology enables the application of the results of erosion tests to the real geometry of hydraulic machines and to reliably predict the locations and magnitude of cavitation erosion, so as to select appropriate materials or material treatments for endangered parts.

Conformational Switching of a Nano-Size Urea-Bridged Zn(II)Porphyrin Dimer by External Stimuli.

For the first time, explicit stabilization of all the three conformers, viz. (cis,cis), (cis,trans) and (trans,trans), of a 'nano-sized' highly-flexible urea-bridged Zn(II)porphyrin dimer have been achieved via careful manipulations of external stimuli such as solvent dielectrics, temperature, anionic interactions, axial ligation and surface-induced stabilization. The conformers differ widely in their structures, chemical and photophysical properties and thus have vast potential applicability. X-ray structural characterizations have been reported for the (cis,cis) and (cis,trans)-conformers. While (cis,cis) conformer stabilized exclusively in dichloromethane, more polar solvents resulted in the stabilization of (cis,trans) and (trans,trans)-conformers. Low temperature promotes the stabilization of (cis,trans)-conformer while rise in temperature facilitates flipping to the (cis,cis) one. Significantly, exclusive stabilization of the (trans,trans)-isomer has been illustrated using acetate anion which facilitates H-bonding with the two amide linkages of the urea spacer. Remarkably, HOPG surface facilitates stabilization of the energetically challenging (trans,trans)-conformer via CH⋅⋅⋅π and π⋅⋅⋅π interactions with the solid surface to the porphyrinic cores. DFT calculations demonstrate that the relative stability of the conformers can be modulated upon slight external perturbations as also observed in the experiment. Several factors contributing towards the conformational landscape for the highly flexible urea-bridged porphyrin dimers have been mapped.

Amino Acid Adsorption Onto Magnetic Nanoparticles Reveals Correlations With Physicochemical Parameters

AbstractWe analyze the adsorption of the proteinogenic amino acids (AAs) glutamine, glutamic acid, lysine, tyrosine, proline, and valine onto bare iron oxide nanoparticles (approx. 10 nm). Aiming to identify the governing principles of low molecular weight coronae, which remain underinvestigated, our study covers broad concentration ranges up to the solubility limit of the AAs. Isothermal experiments reveal that the highly soluble AAs valine, proline, and lysine form extensive multilayers on the nanoparticle surface, and infrared measurements indicate intermolecular interactions, particularly with valine and lysine, for higher AA contents. Conversely, the low solubility of tyrosine and glutamic acid restricts their adsorption capacity, despite their higher partitioning on the solid surface. Parameters derived from fitting a classic saturation model seem to align with well‐documented physicochemical properties such as the hydrophobicity and the complexity indices – a promising first step towards formulating design principles. Scaling these parameters by the AA solubility reveals a clear correlation with the adsorption behavior. In adsorption experiments with AA model mixtures, sequential incubation increases the adsorption capacity for valine and proline, whereas simultaneous incubation with these AAs reduces tyrosine's capacity. Future studies should seek to elucidate adsorption patterns to advance our understanding of corona growth and evolution mechanisms.

Competing Bifurcations Determine Symmetry Breaking During Droplet Snaps on Smooth Patterned Surfaces.

The shape and stability of a droplet in contact with a solid surface is affected by the chemical composition and topography of the solid, and crucially, by the droplet's size. During a variation in size, most often observed during evaporation, droplets on smooth patterned surfaces can undergo sudden shape and position changes. Such changes, called snaps, are prompted by the surface pattern and arise from fold and pitchfork bifurcations which respectively cause symmetric and asymmetric motions. Yet, which type of snap is likely to be observed is an open fundamental question that has relevance in the rational design of surfaces for managing droplets. Here we show that the likelihood of observing symmetric or asymmetric snaps depends on the distance between fold and pitchfork bifurcation points and on how this distance varies for droplets that grow or shrink in size on surfaces patterned with a smooth topography. Our results can help develop strategies to control droplets by exploiting smooth surface patterns but also have broader relevance in situations where different types of bifurcations compete in determining the stability of a system, for instance in snap-through instabilities observed in elastic media.

Drying of Soft Colloidal Films.

Thin films made of deformable micro- and nano-units, such as biological membranes, polymer interfaces, and particle-laden liquid surfaces, exhibit a complex behavior during drying, with consequences for various applications like wound healing, coating technologies, and additive manufacturing. Studying the drying dynamics and structural changes of soft colloidal films thus holds the potential to yield valuable insights to achieve improvements for applications. In this study, interfacial monolayers of core-shell (CS) microgels with varying degrees of softness are employed as model systems and to investigate their drying behavior on differently modified solid substrates (hydrophobic vs hydrophilic). By leveraging video microscopy, particle tracking, and thin film interference, this study shed light on the interplay between microgel adhesion to solid surfaces and the immersion capillary forces that arise in the thin liquid film. It is discovered that a dried replica of the interfacial microstructure can be more accurately achieved on a hydrophobic substrate relative to a hydrophilic one, particularly when employing softer colloids as opposed to harder counterparts. These observations are qualitatively supported by experiments with a thin film pressure balance which allows mimicking and controlling the drying process and by computer simulations with coarse-grained models.

Numerical Thermal Analysis of Greased Rolling Bearing Considering Surface Topography and Plastic Deformation

ABSTRACTIn numerical studies of grease lubrication, thermal effect is often neglected and surface plastic deformation is almost not considered. This paper has developed a deterministic thermal plasto‐elastohydrodynamic lubrication (PEHL) model for grease‐lubricated rolling bearing. The influence of tangent modulus, rheological index and texture orientation on lubrication characteristics and temperature rise is analysed. The results show that increasing the rheological index of grease and decreasing the wavelength factor are obviously positive for improving lubrication behaviour. Since surface plastic deformation in the EHL state is at nanoscale, film thickness, friction coefficient and temperature rise of solid surfaces decline slightly with the decrease of tangent modulus.

Differential gliding motility responses of Chryseobacterium sp. strain PMSZPI isolated from uranium ore deposit on hard and soft substrates

The Bacteroidota bacterium, Chryseobacterium sp. strain PMSZPI isolated from sub-surface soil of uranium ore deposit was shown to move on solid surfaces via gliding motility resulting in the formation of thin spreading colonies. In this study, we attempted to understand the influence of the surfaces, soft or hard/rigid, on the motility behaviour of PMSZPI cells. The computational tool T9GPred in combination with LC-MS/MS analysis established the presence of orthologs of vital gliding motility proteins in PMSZPI. We analyzed the single cell or population motility phenotypes of PMSZPI under spreading and non-spreading conditions. A low percentage of agar or soft agar (0.35%) with low nutrient levels induced more active gliding motility in individual cells leading to increased colony spreading. Microscopic analyses indicated the self-assembly of the gliding cells into irregular edged or spherical microcolonies based on the agar concentration. Cells moved at a speed of 0.2 to 0.6 µm s-1 on low-percentage gliding permissive agar (0.35%) surface in contrast to significant inhibition of motility on rigid or hard agar (1.5%) surface. RNA sequencing and real-time quantitative PCR (qPCR) analysis revealed increased expression of gliding motility genes under low agar conditions consistent with increased spreading behaviour. These findings provide the first glimpse into the gliding motility behaviour of a Bacteroidota bacterium from metal enriched environment that apparently could have implications on bacterial adaptation to changing surface environments.

Prominent Mid-infrared Excess of the Dwarf Planet (136472) Makemake Discovered by JWST/MIRI Indicates Ongoing Activity

We report on the discovery of a very prominent mid-infrared (18–25 μm) excess associated with the trans-Neptunian dwarf planet (136472) Makemake. The excess, detected by the Mid-Infrared Instrument of the James Webb Space Telescope, along with previous measurements from the Spitzer and Herschel space telescopes, indicates the occurrence of temperatures of ∼150 K, much higher than what solid surfaces at Makemake’s heliocentric distance could reach by solar irradiation. We identify two potential explanations: a continuously visible, currently active region powered by subsurface upwelling and possibly cryovolcanic activity covering ≤1% of Makemake’s surface or an as-yet-undetected ring containing very small carbonaceous dust grains, which have not been seen before in trans-Neptunian or Centaur rings. Both scenarios point to unprecedented phenomena among trans-Neptunian objects and could greatly impact our understanding of these distant worlds.

Wetting dynamics and heat transfer of droplet evaporation around boiling point: Facile manipulation by surface roughness

The evaporation of water–ethanol droplets on solid surfaces has vast potential for applications in thermal management, microfluidics, and biomedical sciences, while the approach of manipulating wetting dynamics and heat transfer of droplet evaporation around the boiling point remains open for investigations. The present study proposes a facile approach: fabricating solid surfaces with diverse roughness by sandpaper milling with different grit densities, which can alter the wetting and heat transfer characteristics of droplet evaporation. We discover that when the temperature of the heated surface exceeds the droplet boiling point, the existence of bubbles affects the solid–liquid wetting state; surface roughness alters the ability of bubble departure, thus determining the solid–liquid contact and the contact line retraction time. Consequently, in this case, the solid–liquid contact area and the heat absorbed from heated surfaces for droplets changes with surface roughness, altering the droplet evaporation time and the average heat flux at the solid–liquid interface. Bridged by droplet wetting dynamics, the heat transfer of droplet evaporation above the boiling point can be manipulated by adjusting surface roughness, offering a facile approach to tuning the process of droplet evaporation in the related industrial applications.

Particle impact in high-pressure homogenizer valves – a step towards understanding wear and cell breakup in food and beverage processing

In many liquid food processing applications using high-pressure homogenizers (HPHs), particles impact with the solid surfaces of the homogenization device. This may lead to costly wear. For some applications, impact is also postulated to control the desired cell disruption. This contribution uses computational fluid dynamics to study impact of particles on solid surfaces in HPHs, as a step towards design optimization. Effects of particle diameter, density, homogenizing pressure, and impact distance are studied. Results show impacts both on the forcer and on the impingement ring. Few particles hit the forcer, at low velocities and with low angles (‘bracing impacts’). More particles hit the impact ring. These impacts are with higher velocities and typically occur head-on. The effect of both homogenizing pressure and impact ring distance scales according to a previously suggested stagnation pressure model. Results are discussed in the light of wear and cell disruption observations.