Substrate Wettability Research Articles

The reversal of carbonate wettability via alumina nanofluids: Implications for hydrogen geological storage

Underground hydrogen storage (UHS) has been recognized as a key enabler of the industrial-scale implementation of a hydrogen-based economy. However, the efficiency and storage capacity of hydrogen (H2) in carbonate aquifers can be influenced by the presence of organic acids. Nevertheless, the existing literature contains few investigations of H2/calcite/brine wettability and the influence of organic acids on H2 storability in carbonate reservoirs. Therefore, the present study examines the influence of stearic acid on the dynamic H2/brine wettability of calcite substrates (as a proxy of carbonate formation) under various geological conditions (0.1–20 MPa at 323 K), equilibrated in 10 wt% NaCl brine. In addition, the application of various alumina nanofluid concentrations (0.05, 0.1, 0.25, and 0.75 wt%) is evaluated under the same experimental conditions for enhancing the organic-aged calcite wettability. The results demonstrate a significant impact of stearic acid on the dynamic (advancing and receding) contact angles of the calcite substrates, thereby resulting in a shift from intermediate water-wet to H2-wet conditions, representing an unfavorable state for H2 geological storage. Conversely, the application of alumina nanofluid on the organic-aged calcite substrate enhances the H2/brine/calcite wettability towards an intermediate water-wet state, which is more favorable for H2 residual trapping in carbonate formations. The optimal concentration of alumina nanofluid for the wettability modification of the organically aged calcite samples is found to be 0.25 wt%. These findings highlight the influence of organic acid contamination and the potential of alumina nanofluid application in enhancing H2 geo-storage conditions in carbonate reservoirs.

Quasi-static responses of marine mussel plaques detached from deformable wet substrates under directional tensions

Quantifying the response of marine mussel plaque attachment to wet surfaces remains a significant challenge to a mechanistic understanding of plaque adhesion. Here, we develop a novel, customized microscope system, combined with two-dimensional in situ digital image correlation (DIC), to quantify the in-plane deformation of a deformable substrate that interacts with a mussel plaque under directional tension. By examining the strain field within the substrate, we acquired an understanding of the mechanism by which in-plane traction forces are transmitted from the mussel plaque to the underlying substrate. Finite-element (FE) models were developed to assist in the interpretation of the experimental measurement. Our study revealed a synergistic effect of pulling angle and substrate stiffness on plaque detachment, with mussel plaques anchoring to a ‘stiff’ substrate at small pulling angles, i.e. natural anchoring angles, having mechanical advantages with higher load-bearing capacity and less plaque deformation. We identify two distinct failure modes, i.e. shear-traction-governed failure (STGF) and normal-traction-governed failure (NTGF). It was found that increasing the stiffness of the substrate or reducing the pulling angle results in a change of the failure mode from NTGF to STGF. Our findings offer new insights into the mechanistic understanding of mussel plaque–substrate interaction, providing a plaque-inspired strategy to develop high-performance and artificial wet adhesion.

Understanding the Role of Loss Modulus of Viscoelastic Substrates in the Evaporation Dynamics of Sessile Drops.

Viscoelastic properties of soft substrates play a crucial role in the evaporation dynamics of sessile drops. Recent studies have revealed that the modification of the viscoelastic properties of substrates changes the dynamics of the three-phase contact line, consequently affecting the evaporation behavior of sessile drops. Notably, these modifications occur without any noticeable changes to the substrate's wetting characteristics or surface topography. However, the individual role of storage (G') and loss (G″) moduli of substrates on drop evaporation dynamics remains unexplored. In this study, we investigate the evaporation dynamics of water drops on two groups of poly(dimethylsiloxane)-based viscoelastic substrates possessing either identical G' with varying G″ or identical G″ with varying G'. Our study reveals that on a substrate with constant shear modulus (G'), a reduction of an order of magnitude in loss modulus shifts the evaporation process from the constant contact radius mode to the constant contact angle mode. We hypothesize that this observed shift in behavior stems from the varying viscoelastic dissipation influenced by the plateau modulus and characteristic relaxation time of polymer gels. Our hypothesis is further supported from the observation that the evaporation process persists on the substrate with constant loss modulus (G″). Our study advances the current understanding of drop evaporation on soft substrates that may find potential applications involving soft composites, biological entities, tissue engineering, and wearable electronics.

Micro-PIV study on the influence of viscosity on the dynamics of droplet impact onto a thin film

This work presents a systematic experimental study of droplet impact onto a wet substrate. Four different silicone oils are used, covering a range of Reynolds number between 116<Re<1106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$116< \ ext{Re} <1106$$\\end{document} at two different initial wall film heights. The objective is to characterize the temporal and radial evolution of the velocity field within the crown crater by means of micro-PIV. Our findings show that the velocity field has the structure of an axisymmetric stagnation point flow with decaying strength a(t). The latter exhibits an exponential decay and can be explained in terms of the exponential decay of the pressure force exerted by the impacting droplet onto the wall film. In this context, the commonly accepted functional dependence a(t)∝t-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$a(t) \\propto t^{-1}$$\\end{document} represents only the first-order Taylor approximation of the exponential decay and has therefore only a limited temporal validity. The analysis also corroborates the existence of an inertial regime concerning the velocity field for Re>270\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{Re}} > 270$$\\end{document}. This is not observed at lower Re numbers due to the increased pressure losses caused by the extensional (normal) strain during the radial spreading of the lamella. To validate these findings a holistic approach is chosen, which combines numerical results, analytical solutions and experimental data from literature. In particular, by using the continuity equation, it is shown that the experimental decay of the wall film height can be reconstructed from the velocity measurements. Consilience of results from different approaches provides a robust validation of the micro-PIV data obtained in this work.Graphical abstract

Influence of seawater immersion on acrylic adhesive properties and bond strength on wet composites

This paper describes a study on an acrylic based adhesive developed for marine repair applications. The adhesive alone was aged for over 12 months and tensile samples were tested periodically to characterize the influence of seawater aging at 40 °C. The adhesive alone plasticizes in seawater, losing around 40 % of both modulus and strength after 12 months, but these are largely recovered after drying. In parallel, adhesively bonded glass and carbon fibre composite assemblies were tested after similar aging times. Both retain over 80 % of unaged apparent shear strength after 12 months in natural seawater at 40 °C. Adhesive bonding of wet composite substrates, which had been immersed in seawater for up to 12 months before bonding, was also evaluated to determine residual bond strength. The break strengths of assemblies of wet glass fibre composites were not affected by substrate immersion for up to 12 months before bonding, while strengths of carbon fibre composite assemblies dropped to around 50 % after prolonged substrate immersion. Reasons for this difference are discussed. The results suggest that this adhesive shows good durability and should be considered for marine repair applications.

Good vibrations: remote‐tactile foraging success of wading birds is positively affected by the water content of substrates they forage in

Some taxa of wading birds can locate buried prey by detecting vibratory cues in their foraging substrates while probe‐foraging, using a sensory modality called ‘remote‐touch'. As more saturated substrates transmit vibrations better, we predict that these birds can detect prey in wetter substrates more easily. We used sensory assays to test whether substrate water content affects the remote‐touch foraging success rate of Hadeda ibises, Bostrychia hagedash. The birds were more successful at locating prey using vibratory cues than when relying on random direct contact with the beak alone. Their remote‐touch foraging success rate was positively affected by increasing water contents of the soil, but water content had no effect on their direct contact foraging success (indicating this is not an artefact of ease of probing). This may partially explain the link between the range expansion of this species in southern Africa and increased soil irrigation, as it is easier for the birds to detect prey in wetter substrates. Thus, it is likely that the distribution of other remote‐touch foraging birds is affected by substrate water content, and as many of these species are endangered and rely on sensitive wetland habitats, it is vital to understand their sensory requirements for foraging.

Method for Calculating the Engineering Design of a Mobile Bioreactor for Organic Waste Processing

The article discusses the engineering calculation methodology for developing the design of a mobile bioreactor for processing organic waste. The calculations for processing organic waste were based on SNiP, instructions and methods of leading scientists. When calculating the mobile bioreactor, we took into account: the limit of decomposition of fats, proteins, carbohydrates and the formation of biogas with the density of the mixture. A diagram of the phase process of ongoing and sequential decomposition with the maximum possibility of fermentation for an organic mixture with activated sludge over time, under aerobic and anaerobic conditions during waste disposal, is presented. The advantages and disadvantages of dry and wet waste processing methods are analyzed. From the calculations obtained, we can draw the following conclusion: in a bioreactor with a volume of 7.5 liters, in thermophilic mode, and with a volume of 15 liters, in mesophilic mode, approximately 50% of the organic mixture is fermented, and with activated sludge with a wet substrate, about 95%.

Study on the control of electrowetting and permeation of ionic liquids on porous substrates via molecular dynamics simulations

The electrowetting and permeation behaviors of ionic liquids, which enable the control of their transport at low flow rates without external mechanical forces, significantly promote the development of microfluidic technology. A micro-valve system developed with the combination of porous substrates and micro-channels allows the control of ionic liquid on–off states and the adjustment of its flow rates solely using electrical energy. However, the nanoscale mechanisms of electrowetting of ionic liquid clusters on the surface of porous materials have not been fully revealed. In this study, molecular dynamics simulations are employed to investigate the impact of substrate wettability, pore characteristics, electric field features, and ionic liquid types on electrowetting. The results indicate that the substrates should be maintained hydrophobic initially to decrease spontaneous permeation and avoid over-wetting. Increasing the electric field strength promotes permeation, but an excessively intense electric field intensifies the ion field evaporation. Furthermore, the electrowetting and permeation mechanisms of various substrate pore sizes and pore spacings can be explained by the motion characteristics of cations and anions. Kinematic analysis of the ionic liquids climbing the capillary sidewalls under an alternative electric field at a controllable velocity is provided. These conclusions can provide theoretical guidance and reference for the electrowetting and permeation of ionic liquids.

Surface Modification of Polymers by Plasma Treatment for Appropriate Adhesion of Coatings.

In this study, recent advances in tailoring the surface properties of polymers for the optimization of the adhesion of various coatings by non-equilibrium gaseous plasma are reviewed, and important findings are stressed. Different authors have used various experimental setups and reported results that scatter significantly and are sometimes contradictory. The correlations between the processing parameters and the adhesion are drawn, and discrepancies are explained. Many authors have explained improved adhesion with the adjustment of the surface free energy or wettability of the polymer substrate and the surface tension of liquids used for the deposition of thin films. The adhesion force between the polymer substrate and the coating does not always follow the evolution of the surface wettability, which is explained by several effects, including the aging effects due to the hydrophobic recovery and the formation of an interlayer rich in loosely bonded low molecular weight fragments.

Inspired square micro-texture structures designed by nanosecond laser sculpture for enhancing adhesion strength of plasma spraying Si-HfO2 bond coat on SiC substrates

In order to enhance the adhesion strength between the Si-HfO2 bond coat and substrate, square micro-textures inspired by straw-checkered sand fixation are fabricated using nanosecond laser sculpture. The adhesion strength of the laser surface texture (LST) sample is up to 27.31 ± 3.34 MPa, which is more than 200 % higher than the conventional grit-blasted one (8.12 ± 0.70 MPa). The promoted interface bonding is attributed to the following three aspects: First, the mechanical anchoring established by micro-texture positively improves the adhesion strength. Second, the LST diminishes the contact angle to 16.5 ± 1.7°, improving the wetting of the SiC substrate. Third, the high-pressure plasma is implemented to produce enormous compressive stress (205.6 ± 14.6 MPa) on the SiC substrate surface, greatly aiding substrate/coating adhesion. The above three mechanisms work synergistically to enable the LST treatment to exhibit better adhesion than the conventional grit-blasted treatment on SiC substrate.

Revisiting palaeolithic combustion features of Theopetra Cave: A diachronic use of dung and peat as fuel

The chemical diagenesis and palaeoenvironmental reconstruction of the Palaeolithic sequence of Theopetra Cave have been studied extensively, but little information is available regarding the details of its combustion structures. The cave is characterized by extensive beds of multi-sequence combustion layers dated between 140 and 50 ka BP, and thick, often stratigraphically complex bodies of ash and charred remains dated between 16 and 13 ka BP. All combustion features contain large amounts of charred and partially charred fibrous organic matter of non-wood plant material and very little charcoal. The structure, fabric, composition, and chemistry of these remains suggest that a mixture of peat and dung was used as a fuel, occasionally enriched with amounts of wood fuel. The integrity of the sequence of combustion structures precludes the possibility that dung was produced by animals inside the cave as their traffic on its wet substrate would have destroyed the burnt layers and homogenized the sediment. The mixture of peat and dung was most likely collected from peatlands associated with swamps of the former so called Karditsa Lake that probably existed in the area until the beginning of the Holocene. Peat and dung were used as a fuel when wood fuel was not available, during the relatively cold intervals of the glacial periods, but also during the last interglacial when the area close to the cave was wooded. Of great interest is that this same fuel was used during both the Middle and the Upper Palaeolithic, presumably by different human species. Although Theopetra appears to be the first site that peat and dung was used as a fuel during the Middle Palaeolithic, it is suggested that other sites may have used this fuel as well. This has important consequences in understanding the evolution of human pyrotechnology particularly during glacial periods.

Impact dynamics of water droplets on oil-covered dielectrowetting substrate: Effects of oil film thickness and surface wettability

Droplet impacting on solid or liquid film is a ubiquitous phenomenon in relevant processes such as spray cooling and inkjet printing on primer layers. In this study, the dynamics of water droplets impacting a solid substrate covered by a silicon oil film is investigated under the influences of the surface wettability of the dielectrowetting substrate (69.3°-117.4°) and the thickness of the silicon oil film (7.79–21.8 μm) at We=6.81. A phase diagram is established, which reflects the relationships between the impact dynamic behavior, equilibrium contact angle, and the oil film thickness. The maximum spreading factor βmax and maximum out-of-plane height Hmax/D0 for rebound of sub-droplet behaviors was smaller than 13 % and greater than 3 times than those droplet deposition behaviors, respectively. Finally, the retraction dynamics of the droplet after reaching its maximum spreading diameter is analyzed and discussed based on retraction rate γw=vr/Dmax=σw(cosθrmin-cosθeq)/fkDmax through the force balance at the three-phase contact line. Results of the study indicate that the surface wettability of the dielectrowetting substrate and thickness of the silicon oil film significantly affect the impact dynamics of water droplets on a solid substrate covered by the silicone oil film, especially in the retraction stage. This study sheds new light on controlling the dynamic impact of the droplets on silicon oil films, which is of importance to the control or optimization of processes such as inkjet printing on a primer layer.

Nature‐Inspired Directional Microneedle Structures for Reversible Gripping on Skin and Fibrous Materials

Plants such as Galium aparine show directional gripping to various surfaces because of their microscale hook structures. A bio‐inspired artificial version of microhooks holds potential as a bidirectional, reversible, and reusable dry adhesive for soft, wet, or fibrous substrates such as skin or textiles. However, current methods for fabricating 3D structures are often costly, time‐consuming, or require precise alignment steps. Herein, a facile, cost‐effective, scalable, and unconventional microfabrication technique is proposed using tilted photolithography on a rotary stage and shear molding to fabricate a soft polydimethylsiloxane polymer mold containing the inverse replica of directional microneedle structures. To demonstrate the proof‐of‐concept, polyurethane is then added to the mold to obtain directional microneedles, referred to as microhooks. Friction force measurements using a nanotribometer show anisotropic performance (strong attachment in one direction and easy detachment in the opposite direction over multiple cycles) similar to the natural microhooks. Tribological characterization of the microhooks shows a gripping force 2.25 times greater than the sliding force. The fabricated microhook structures also provide an adhesion force of ≈0.62 N cm−2 to artificial skin. This biomimetic approach promises precise adhesion control and material versatility for attaching wearable devices and patches to the skin in biomedical applications.

Selective Vapor Condensation for the Synthesis and Assembly of Spherical Colloids with a Precise Rough Patch.

Patchy particles occupy an increasingly important space in soft matter research due to their ability to assemble into intricate phases and states. Being able to fine-tune the interactions among these particles is essential to understanding the principles governing the self-assembly processes. However, current fabrication techniques often yield patches that deviate chemically and physically from the native particles, impeding the identification of the driving forces behind self-assembly. To overcome this challenge, we propose a new approach to synthesizing spherical colloids with a well-defined rough patch on their surface. By treating polystyrene microspheres with vapors of a good solvent, here an acetone-water mixture, we achieve selective polymer corrugation on the particle surface resulting in a chemically similar yet rough surface patch. The key step is the selective condensation of the acetone-water vapors on the apex of the polystyrene microparticles immobilized on a substrate, which leads to rough patch formation. We leverage the ability to tune the vapor-liquid equilibrium of the volatile acetone-water mixture to precisely control the polymer corrugation on the particle surface. We demonstrate the dependence of patch formation on particle and substrate wettability, with the condensation occurring on the particle apex only when it is more wettable than the substrate, which is consistent with Volmer's classical nucleation theory. By combining experiments and molecular dynamics simulations, we identify the role of the rough patch in the depletion interaction-driven self-assembly of the microspheres, which is crucial for designing programmable supracolloidal structures.

Interfacial Hydrogen Bond-Reinforced Adhesion and Cohesion Enabling an Ultrastretchable and Wet Adhesive Hydrogel Strain Sensor.

Historically, research on silicotungstic-acid-based hydrogels has primarily focused on their adhesive properties, often at the expense of mechanical strength (cohesion). In this study, we present a novel approach to fabricate a polysaccharide hydrogel that harmoniously balances both adhesion and cohesion via interfacial hydrogen bonds. This hydrogel, composed of carboxymethyl cellulose (CMC), polyacrylamide (PAM), silicotungstic acid (SiW), and lithium chloride (LiCl), showcases a unique combination of properties: strain-responsive ionic conductivity, superior transparency, remarkable stretchability, and robust adhesion. Contrary to conventional PAM hydrogels, our PAM-SiW networked hydrogel addresses the common challenge of achieving good adhesion without compromising on cohesion. Specifically, our hydrogel demonstrates a maximum toughness of 20.3 MJ/m3 and a strain of 4079%, an accomplishment rarely observed in other adhesive hydrogel. Furthermore, the hydrogel's adhesion is both reversible and versatile, adhering effectively to a variety of wet and dry substrates. This makes it a promising candidate for advanced healthcare applications, particularly as a mechanically reinforced underwater adhesive with unparalleled stability. We also provide insights into the role of LiCl in the hydrogel matrix, emphasizing its influence on electrostatic interactions without affecting the hydrogen bonds. This study serves as a testament to the potential of harmonizing adhesive and cohesive properties in hydrogels, paving the way for future innovations in the field.

Surface Nanodroplet‐Confined Engineering of Gold (I) ‐Thiolate Nanostructures

AbstractGold(I)‐thiolate complexes have served as primary building blocks for diverse Au nanostructure synthesis strategies. A delicate approach to characterize and control Au(I)‐thiolate motif formation and assembly on the surface is needed as it can potentially solve challenges associated with utilizing gold nanomaterials in many applications. Here, the controllable generation of flower‐shaped surface gold nanostructures (FSGNs) is demonstrated by manipulating the formation‐assembly process of Au(I)‐dodecanethiolate motifs within nanoscale surface droplets. The morphology and structure of the resulting Au nanostructures are governed by internal convection flows and interfacial energy, modulated by the nanodroplet composition and substrate wettability. The obtained FSGNs are proven to act as versatile scaffolds for the selective generation of Au spiky nanostars. These FSGNs can also be utilized to functionalize nanodroplet‐based reactors, boosting the fluorescent intensity of Nile red (NR) fluorophores and decomposing NR via catalytic reaction. Remarkably, with FSGN functionalized droplets smaller than a radius of 500 nm, the decomposition rate of NR can reach ≈0.01 s−1. These results highlight a miniaturized, controllable, and automated method for the in situ production of 3D gold nanostructures on substrates, offering prospects for fast surface nanostructure fabrication and efficient environmental pollutant treatment.

Wetting of laser-textured tungsten substrate by molten tin

In the new generation of nuclear reactors, plasma facing components tend to be designed as Sn/W composite components. Considering the combination of tin and tungsten, the wettability evaluation of Sn/W system and its influencing factors are urgent work. Wetting experiments of Sn/W system were carried out in high vacuum by using modified sessile drop method. The results show that after vacuum annealing at 1100℃, good wettability (∼ 48° intrinsic contact angle) can be obtained quickly at 900℃ below the critical temperature of 1050℃. After laser surface texturing, the greater the roughness of the tungsten substrate surface, the better the wettability can be obtained. In essence, this correspondence is closely related to the geometric parameters of the grooves formed by laser etching, which corresponds to the predicted results in Wenzel model. The phenomenon of pinning and depinning of triple lines in the later stage of spreading is related to the spreading of liquid in the groove, and the spreading rate of the latter is faster than that of macro triple line, which leads to this phenomenon.

Enhanced interfacial bonding strength in K-300 adhesive joint between aluminum alloy and mild steel substrates through effective resin precoating treatment

In this work, two different surface treatments, such as grinding and acid-pickling, were applied to both mild steel and aluminum alloy substrates for performance comparison. A novel method known as resin precoating (RPC) has been used effectively to seal the micro-cavities surface and enhance substrate wetting. The RPC solution consists of 10 % resin, and 90 % acetone (without hardener) is applied onto the grinding and acid-pickling metal substrate. The acetone solution facilitates resin penetration into the micro-cavities formed through grinding and acid pickling. It enables effective coating and wetting of micro-debris, ultimately eliminating micro-voids or gaps between the metal substrate and the adhesive joint. Then, the standard K-300 adhesive (with hardener) is applied to the metal substrate surface. The adhesive bonding strength was determined using a single-lap shear test on varied surface conditions. The topography of surface-treated samples was analyzed using X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Contact Angle Goniometer. The grinding and acid-picking surface treatment increased the surface roughness of the substrate, which reduced the contact angle and improved the mechanical interlocking effect. The single lap shear test results show that the bonding strength of the grinded samples was superior to that of the acid-pickled samples. The RPC treatment further enhanced the bonding strength of all the grinded samples by 16.83–19.77 % and for acid-pickled samples by 20.79–44.97 %, which is higher than the values observed without RPC treatment. The combination of grinding and RPC surface-treated samples exhibited a superior bonding strength of 14.05 MPa. This value was 12.38 % greater than the bond strengths of acid-pickled samples, and the failure mode shifted from adhesive to cohesive failure mode. Thus, RPC treatment enhanced the wettability of metal substrates and maximized the utilization of contact areas on roughened substrate surfaces, consequently improving adhesive bond strength. These grinding and acid-pickling surface treatments are effective and well-suited for aerospace, automobile, and aviation applications.

Thermocapillary migration of a droplet on a substrate with wettability difference: A comparison between slip and precursor film models in three dimensions

Moving contact line dynamics calculations include two models: precursor film models and slip models. The lubrication approximation method is used to establish a three-dimensional mathematical model to analyze the droplet thermocapillary migration behavior on a non-uniformly heated solid substrate with a wettability track. The contact line dynamics in the slip model and the disjoining pressure effect in the precursor model are proposed to regulate the substrate wettability. Both models are numerically implemented to investigate droplet spreading for three cases: free spreading on an isothermal substrate, thermocapillary migration on a uniform wettability substrate, and thermocapillary migration on a wettability-confined track. For the case of free spreading on an isothermal substrate, the three-dimensional results of the slip and precursor contact line models are essentially consistent with two-dimensional slip model results. For the case of thermocapillary migration on a uniform wettability substrate, the results of the two models essentially agree with the experimental results. Decreasing the thermal gradient reduces the discrepancies between the two models that result from the coordinate transformation method used in the slip model, which reduces the contact angles measured in the y-direction and enlarges the advancing contact angle in the migration direction. For the case of thermocapillary migration on a wettability-confined track, the slip model gradually shows a “dynamic-pinning” behavior with increasing equilibrium contact angle in the hydrophobic region. By contrast, the precursor film model maintains a stationary pinning behavior but separates a residual liquid outside the track. The precursor film model is preferred over the slip model in lubrication approximations for three-dimensional fluids when calculating complex moving contact dynamics caused by wettability differences. However, the precursor film model must be further optimized to prevent numerical instability.

Coalescence of sessile aqueous droplets laden with surfactant

With most of the focus to date having been on the coalescence of freely suspended droplets, much less is known about the coalescence of sessile droplets, especially in the case of droplets laden with surfactant. Here, we employ large-scale molecular dynamics simulations to investigate this phenomenon on substrates with different wettability. In particular, we unravel the mass transport mechanism of surfactant during coalescence, thus explaining the key mechanisms present in the process. Close similarities are found between the coalescence of sessile droplets with equilibrium contact angles above 90° and that of freely suspended droplets, being practically the same when the contact angle of the sessile droplets is above 140°. Here, the initial contact point is an area that creates an initial contact film of surfactant that proceeds to break into engulfed aggregates. A major change in the physics appears below the 90° contact angle, when the initial contact point becomes small and line-like, strongly affecting many aspects of the process and allowing water to take part in the coalescence from the beginning. We find growth exponents consistent with a 2/3 power law on strongly wettable substrates but no evidence of linear growth. Overall bridge growth speed increases with wettability for all surfactant concentrations, but the speeding up effect becomes weaker as surfactant concentration grows, along with a general slowdown of the coalescence compared to pure water. Concurrently, the duration of the initial thermally limited regime increases strongly by almost an order of magnitude for strongly wettable substrates.