Finite Contact Angle Research Articles

Wetting and interfacial behavior of aqueous Deep eutectic solvents on Graphite, silica and calcite surfaces: Molecular dynamics study

Understanding the wetting behavior of liquids/fluids on solid surfaces at the nanoscale can help develop more effective and efficient technologies in various applications, including microfluidics, biomedical, enhanced oil recovery, and many others. Deep eutectic solvents (DESs) have recently gained attention due to their physical and chemical properties and tunability, making them useful in many applications. This article reports the investigation of the wetting behavior of aqueous deep eutectic solvent (DES) on graphite, silicon dioxide (SiO2), and calcite (CaCO3) using molecular dynamics (MD) simulation. The study's main objective is to understand the interfacial behavior of aqueous DES on these surfaces. In the current study, two different DESs, choline-chloride (ChCl) as hydrogen bond acceptor (HBA) and two different hydrogen bond donors (HBD), urea (U) and ethylene glycol (EG) with molar ratio 1:2are used. Aqueous DES has been studied for three different concentrations of10, 20, and 30 wt% of DES molecules to explore the effect of concentration.AqueousDES system on the graphite makes a finite contact angle for all concentrations of DES.In contrast, for SiO2 and CaCO3, droplets spread and attain a contact angle below ∼ 10° due to high hydrophilicity. We confirmed that our simulation had attained equilibrium by computing the interaction energy with the substrate and system molecules. The wetting behavior of aqueous DES thin film on the surfaceis characterizedby a density profile along the z-axis (normal to the surface). Furthermore, hydrogen bond (HB) analysisis performedto elucidate the structural relationship of these aqueous-DES systems. Overall, this study provides insights into the wetting behavior of nanodroplets/film of aqueous DES on different surfaces and illustrates the structural characteristics of these systems through various properties such as contact angle, density profiles, and hydrogen bonds.

Atomic wetting of oil droplets into hexagons and stripes

HypothesisPrevious studies showed that liquid droplets can be spherical caps or polygonal shapes on patterned surfaces. We hypothesize that these phenomena can be narrowed down to nanoscale and affecting liquid molecular wetting. We anticipate observing preferential wetting on the underlying substrate with atomic crystalline lattice. ExperimentsThis study explores the wetting behavior of oil nanodroplets deposited on highly ordered pyrolytic graphite (HOPG) with atomic force microscope (AFM). Oil droplets were scanned with hydrophobic and hydrophilic AFM tips in water in peak force mode AFM, carefully controlling the scanning force. Simple modeling was carried out using Surface Evolver to simulate how oil droplets wet on HOPG with molecular lattice. FindingsThe results showed two distinct wetting behaviors: the hexagonal nanodroplets and stripe nanometric films. In both cases, the droplets appear to wet preferentially along preferred orientations which reflect the atomic periodicity of the underlying substrate. This preferential wetting occurs because the atomic roughness is significant at the length scale of the nanodroplets, while the eventual outcome of the wetting (i.e. whether the droplet settles to a finite contact angle, or wets completely into a nanometric film) is dependent on the spreading parameter of the droplet-substrate system, independent of scanning direction.

Autothermotaxis of volatile drops

When a drop of a volatile liquid is deposited on a uniformly heated wettable, thermally conducting substrate, one expects to see it spread into a thin film and evaporate. Contrary to this intuition, due to thermal Marangoni contraction, the deposited drop contracts into a spherical-cap-shaped puddle, with a finite apparent contact angle. Strikingly, this contracted droplet, above a threshold temperature, well below the boiling point of the liquid, starts to spontaneously move on the substrate in an apparently erratic way. We describe and quantify this self-propulsion of the volatile drop. It arises due to spontaneous symmetry breaking of thermal Marangoni convection, which is induced by the nonuniform evaporation of the droplet. Using infrared imaging, we reveal the characteristic interfacial flow patterns associated with Marangoni convection in the evaporating drop. A scaling relation describes the correlation between the moving velocity of the drop and the apparent contact angle, both of which increase with the substrate temperature. Published by the American Physical Society 2024

The Force Required to Detach a Rotating Particle from a Liquid-Fluid Interface.

The force required to detach a particle from a liquid–fluid interface is a direct measure of the capillary adhesion between the particle and the interface. Analytical expressions for the detachment force are available but are limited to nonrotating particles. In this work, we derive analytical expressions for the force required to detach a rotating spherical particle from a liquid–fluid interface. Our theory predicts that the rotation reduces the detachment force when there is a finite contact angle hysteresis between the particle and the liquid. For example, the force required to detach a particle with an advancing contact angle of 120° and a receding contact angle of 80° (e.g., polydimethylsiloxane particle at a water–air interface) is expected to be 25% lower when the particle rotates while it is detached.

Effect of finite contact angle on the structure and shear behavior of two-dimensional wet foam

We present simulations of a 2D liquid foam with finite contact angles, over a wide range of liquid fraction. It is shown that, with the increase of liquid fraction, the variation of average coordination number may be characterized by three different regimes (the “flat” regime, the approximate linear regime and the gentle decreasing regime). The detailed distribution of the coordination number is displayed; the fraction of bubbles with the same number of neighbours, as a function of the average coordination number, follows the Gaussian distribution. In addition, we present the variation of the length of the gas-liquid-gas interfaces (films) with the liquid fraction, as well as the distribution of the Plateau border sides. Furthermore, in response to shear, the stress level of the wet foam rises with the increase of contact angle. We find a linear relationship between the yield stress and the cosine of the contact angle.

203 Developing and Fine-Tuning Novel 3D-Printed Biodegradable Scaffolds to Promote Auricular Cartilaginous Regeneration for Surgical Implantation

Abstract Introduction Microtia, a congenital cartilaginous defect, poses major challenges in cosmetic surgery. Biodegradable polymers promote chondrogenesis, with promises of seeding cells into synthetic-polymer-implants for surgical fixation. However, existing polymers used in auricular reconstruction present limitations including inflammation, fibrosis, and extrusion. This study aimed to modulate the mechanical properties of the novel polylactic-acid/polyhydroxyalkanoate (PLA/PHA) blend by 3D-printing and hence, evaluate its suitability to the auricular microenvironment in developing next-generation reconstructs. Method Digitally defined PLA/PHA scaffolds were free-form 3D-printed at various infill densities and thicknesses. Through tensile testing, tensile moduli, yield point, maximum strength, tensile toughness, and stiffness were calculated, alongside Finite Element Analysis (FEA) and contact angle tests. Finally, preliminary cell seeding was conducted. Results Increasing infill densities of PLA/PHA scaffolds from 30%-60% significantly increased tensile moduli, yield point and maximum strength (P < 0.01). Tensile stiffness increased significantly with scaffold thicknesses between 1mm-2mm (P < 0.05). Cell studies showed promising proliferative activity. Conclusions The mechanical properties and structural stiffness of 3D-printed PLA/PHA scaffolds can be significantly tailored by altering infill density and thickness, respectively. The digitally defined interconnected pores within printed PLA/PHA scaffolds reduce stiffness mismatches between surgical-synthetic polymers and auricular cartilage, potentially promoting cell migration and nutrition transportation in future reconstructs.

Variation of average coordination number with liquid fraction for two-dimensional foams with finite contact angle

ABSTRACT We present Surface Evolver computer simulations for two-dimensional (2d) foams in which the contact angle at which liquid films and Plateau borders meet is non-zero. We determine the average coordination number Z of bubbles over a wide range of finite contact angles θ and liquid fractions ϕ. We find a linear scaling of Z for values of ϕ between about 0.07 and 0.15, corroborating previous data obtained for the case of zero contact angle. The results highlight the importance of attractive bubble interactions for 2d foams.

Contact angles for perfectly wetting pure liquids evaporating into air: Between de Gennes-type and other classical models

Even when a liquid is perfectly wetting on a smooth substrate (zero Young's angle), it can manifest finite contact angles if evaporating. These angles are of a dynamic, evaporation-induced nature, a compromise between the tendencies to spread and to evaporate. They form as a result of the resolution of a macroscopic evaporation-flux singularity at a microscopic scale in a tiny vicinity of the contact line referred to as the microregion. A critical evaluation generalizes and unifies the various models thereof, for nonpolar liquids undergoing diffusion-limited evaporation into air, and compares them with experiment.

Contact angle measurements on cyclopentane hydrates

The wetting characteristics of clathrate hydrates influence their behavior in flow assurance situations, but these properties are not well-characterized. In this work, a new experimental method is proposed for measuring the contact angle of various liquids on cyclopentane hydrate, a structure II clathrate hydrate that forms at atmospheric pressure. This method includes a protocol to obtain a smooth flat hydrate surface, followed by standard image-based contact angle measurements. The protocol involves a succession of flipping and resting steps of a solid block, initially ice, immersed in liquid cyclopentane and subject to temperature variations around water freezing point, in order to allow progressive transformation to cyclopentane hydrate. Contact angle measurements on the flat hydrate surface were conducted for two sets of systems. First, water drops immersed in cyclopentane or various oils were observed to fully wet the hydrate surface. This result agrees with the surface tension values estimated by Aman et al. (2013) for the cyclopentane/hydrate/water system. Second, the contact angle of halogenated organic (dichloromethane, dichloroethane, and chloroform) drops immersed in brine was measured on cyclopentane hydrate and ice. Both the hydrate and ice surfaces were found to be water-wetting. Finite contact angles in the range of 125⩽θ⩽149° were obtained on the hydrate substrate. The ice substrate however, was observed to be fully wetted by the aqueous phase. Using the measured contact angles and interfacial tension of the liquid pairs, the work of adhesion for the hydrate surface and each of the halogenated organics was determined, and is comparable to values found in the work of Asserson et al. (2009) on freon hydrates.

Dynamics of moving contact line on a transversely patterned inclined surface

We study the dynamics of moving contact lines and film deposition on a chemically patterned plate withdrawn from a liquid bath obliquely. The plate is patterned with transverse stripes and characterized by alternating wettability. We assume that the inclination of the plate is small enough, so lubrication theory can be employed. The finite element method is used to solve the one-dimensional unsteady lubrication equation, and it is combined with the precursor film model and disjoining pressure to realize the moving contact line with finite contact angles. When the width of the strips is relatively large, four typical modes of contact line dynamics are observed as the withdrawal speed of the plate increases. In particular, if the withdrawal speed is smaller than the critical value of the wetting transition on the more wettable strip, the contact line would periodically move between the equilibrium positions of the more wettable and less wettable regions, which is known as a “stick-slip” motion. In accordance with the Cox–Voinov law, a quantitative analysis of the “stick-slip” motion is conducted, which predicts the critical condition of the “stick” process and the relaxation time of the “slip” process. When the stripe width is sufficiently small, the evolution of the contact line and liquid film is similar to that on an equivalent homogeneous substrate, whose contact angle can be predicted via Cassie theory.

Dynamical wetting transition on a chemically striped incline

We study the dynamics of moving contact lines and film deposition on a chemically heterogeneous plate withdrawn from a liquid bath. The plate is patterned with vertical stripes characterized by alternating wettabilities. It is assumed that the interfacial slope with respect to the plate is small such that lubrication theory can be employed. The finite element method is used to solve the two-dimensional unsteady lubrication equation, which is coupled with the precursor film model and disjoining pressure to realize moving contact lines with finite contact angles. At low withdrawal velocity, a three-dimensional stable meniscus is formed. If the velocity exceeds a threshold, liquid films and droplets are produced on the more wettable stripes. When the width of the more wettable stripes is small, the contact line always loses its stability at the center of these stripes. For a sufficiently large width of the more wettable stripes, a trapezoidal film can be observed, in analogy to the wetting transition on a homogeneous plate of finite width; however, the onset of meniscus instability is found to occur at a location whose distance to the stripe boundary is independent of the stripe width, corresponding to a three-dimensional mechanism of wetting transition. Furthermore, the dynamic evolution of the liquid film is also analyzed.

Combined effect of relative humidity and substrate temperature on evaporation of methanol droplet

Understanding and manipulating the mechanisms involved in the evaporation of organic solvent droplets in a humid environment are of great importance for numerous industrial and biological applications. The combined effect of substrate temperature and relative humidity of the environment on evaporation of pure methanol droplets has been experimentally investigated. A drop shape analyzer was used to record the evolution of contact angle, base diameter, and volume of the droplet. At low relative humidity, the droplet experiences three stages of evaporation: constant, slightly increasing, and sharply decreasing constant angle. Substrate temperature has no effect on the qualitative behavior of the droplet at low relative humidity. At high relative humidity, droplet evolution is influenced by substrate temperature. At relatively high substrate temperature, the droplet undergoes two stages of evaporation where the contact angle increases to a maximum then decreases until the end of evaporation. At low substrate temperature, the droplet experiences a single stage where it reaches an equilibrium state with finite volume, diameter, and contact angle. In humid environment, water vapor gets adsorbed–absorbed and/or condensed on the droplet. Decreasing substrate temperature causes the temperature at the liquid–gas interface to fall below the dew point which enhances the water condensation. The evaporation rate diminishes as the relative humidity rises and its effect is more pronounced for lower substrate temperature. Relative humidity of the surroundings seems to have a negligible effect on the initial evaporation rate which is mainly attributed to the evaporation of methanol. A regime map is proposed based on the different evolution of droplets under different environmental conditions as well as substrate temperature.

Ideal wet two-dimensional foams and emulsions with finite contact angle.

We present simulations that show that the equilibrium structure of an ideal two-dimensional foam with a finite contact angle develops an inhomogeneity for high liquid fraction φ. In liquid-liquid emulsions this inhomogeneity is known as flocculation. In the case of an ordered foam this requires a perturbation, but in a disordered foam inhomogeneity grows steadily and spontaneously with φ, as demonstrated in our simulations performed with the Surface Evolver.

Insights From Ion Adsorption and Contact-Angle Alteration at Mineral Surfaces for Low-Salinity Waterflooding

Summary Most solid surfaces acquire a finite surface charge after exposure to aqueous environments caused by desorption and/or adsorption of ionic species. The resulting electrostatic forces play a crucial role in many fields of science and technology, including colloidal stability, self-assembly, wetting, and biophysics. Enhanced oil recovery (EOR) is an example of a large-scale industrial process that hinges in many respects on these phenomena. In this paper, we present a series of experiments illustrating fundamental aspects of low-salinity waterflooding in well-defined model systems. We show how pH and ion content of the water phase as well as the presence of model polar components (fatty acids) in the oil phase affect the wettability (i.e., contact-angle distribution) of oil/water/rock systems. Specifically, we discuss high-resolution atomic-force microscopy (AFM) experiments demonstrating the preferential adsorption of multivalent cations to mineral surfaces such as mica and gibbsite. Cation adsorption leads to increased and, in some cases, reversed surface charge at the solid/liquid interface. In particular, the adsorption of divalent cations gives rise to charge reversal and thereby induces a transition from complete water-wetting in the absence to finite contact angles in the presence of such ions. Although already dramatic for pure alkanes as base oil, adding fatty acids to the oil phase enhances the effect of divalent ions on the oil/water/rock wettability even more. In this case, contact-angle variations of more than 70 ° can be observed as a function of the salt concentration. This enhancement is caused by the deposition of a thin film of fatty acid on the solid surface. AFM as well as surface plasmon-resonance spectroscopy measurement in a microfluidic continuous flow cell directly demonstrate that adsorbed Ca2+ ions promote secondary adsorption of acidic components from the oil phase. The combination of the effects discussed provides a rational scenario explaining many aspects of the success of low-salinity waterflooding.

The effect of the degree of ionicity of ceramic materials on their wettability by melted sodium chloride

The sessile drop method has been used to study the wettability of hexagonal boron nitride, sapphire, quartz, and polycrystalline silicon carbide by melted sodium chloride in a reducing He–H2 atmosphere. Melted NaCl completely spreads over sapphire and quartz surfaces and form finite contact angles equal to 51° ± 10° and 77° ± 5° on silicon carbide and hexagonal boron nitride, respectively. The calculated works of salt adhesion to the ceramic substrates increase with the magnitude of the ionic component of ceramic material surface energy.

Effect of Marangoni Flows on the Shape of Thin Sessile Droplets Evaporating into Air.

Freely receding evaporating sessile droplets of perfectly wetting liquids, for which the observed finite contact angles are attributed to evaporation, are studied with a Mach-Zehnder interferometer. The experimentally obtained droplet shapes are found to depart, under some conditions, from the classical macroscopic static profile of a sessile droplet. The observed deviations (or the absence thereof) are explained in terms of a Marangoni flow due to evaporation-induced thermal gradients along the liquid-air interface. When such a Marangoni effect is strong, the experimental profiles exhibit a maximum of the slope at a certain distance from the contact line. In this case, the axisymmetric flow is directed from the contact line to the apex (along the liquid-air interface), hence delivering more liquid to the center of the droplet and making it appear inflated. These findings are quantitatively confirmed by predictions of a lubrication model accounting for the impact of the Marangoni effect on the droplet shape.

A new experiment for investigating evaporation and condensation of cryogenic propellants

Passive and active technologies have been used to control propellant boil-off, but the current state of understanding of cryogenic evaporation and condensation in microgravity is insufficient for designing large cryogenic depots critical to the long-term space exploration missions. One of the key factors limiting the ability to design such systems is the uncertainty in the accommodation coefficients (evaporation and condensation), which are inputs for kinetic modeling of phase change.A novel, combined experimental and computational approach is being used to determine the accommodation coefficients for liquid hydrogen and liquid methane. The experimental effort utilizes the Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland to image evaporation and condensation of hydrogenated propellants inside of metallic containers. The computational effort includes numerical solution of a model for phase change in the contact line and thin film regions as well as an CFD effort for determining the appropriate thermal boundary conditions for the numerical solution of the evaporating and condensing liquid. Using all three methods, there is the possibility of extracting the accommodation coefficients from the experimental observations. The experiments are the first known observation of a liquid hydrogen menisci condensing and evaporating inside aluminum and stainless steel cylinders. The experimental technique, complimentary computational thermal model and meniscus shape determination are reported. The computational thermal model has been shown to accurately track the transient thermal response of the test cells. The meniscus shape determination suggests the presence of a finite contact angle, albeit very small, between liquid hydrogen and aluminum oxide.

Line-tension-induced scenario of heterogeneous nucleation on a spherical substrate and in a spherical cavity.

Line-tension-induced scenario of heterogeneous nucleation is studied for a lens-shaped nucleus with a finite contact angle nucleated on a spherical substrate and on the bottom of the wall of a spherical cavity. The effect of line tension on the free energy of a critical nucleus can be separated from the usual volume term. By comparing the free energy of a lens-shaped critical nucleus of a finite contact angle with that of a spherical nucleus, we find that a spherical nucleus may have a lower free energy than a lens-shaped nucleus when the line tension is positive and large, which is similar to the drying transition predicted by Widom [B. Widom, J. Phys. Chem. 99, 2803 (1995)]. Then, the homogeneous nucleation rather than the heterogeneous nucleation will be favorable. Similarly, the free energy of a lens-shaped nucleus becomes negative when the line tension is negative and large. Then, the barrier-less nucleation with no thermal activation called athermal nucleation will be realized.

Interaction between Air Bubbles and Superhydrophobic Surfaces in Aqueous Solutions.

Superhydrophobic surfaces are usually characterized by a high apparent contact angle of water drops in air. Here we analyze the inverse situation: Rather than focusing on water repellency in air, we measure the attractive interaction of air bubbles and superhydrophobic surfaces in water. Forces were measured between microbubbles with radii R of 40-90 μm attached to an atomic force microscope cantilever and submerged superhydrophobic surfaces. In addition, forces between macroscopic bubbles (R = 1.2 mm) at the end of capillaries and superhydrophobic surfaces were measured. As superhydrophobic surfaces we applied soot-templated surfaces, nanofilament surfaces, micropillar arrays with flat top faces, and decorated micropillars. Depending on the specific structure of the superhydrophobic surfaces and the presence and amount of entrapped air, different interactions were observed. Soot-templated surfaces in the Cassie state showed superaerophilic behavior: Once the electrostatic double-layer force and a hydrodynamic repulsion were overcome, bubbles jumped onto the surface and fully merged with the entrapped air. On nanofilaments and micropillar arrays we observed in addition the formation of sessile bubbles with finite contact angles below 90° or the attachment of bubbles, which retained their spherical shape.

Ion adsorption-induced wetting transition in oil-water-mineral systems.

The relative wettability of oil and water on solid surfaces is generally governed by a complex competition of molecular interaction forces acting in such three-phase systems. Herein, we experimentally demonstrate how the adsorption of in nature abundant divalent Ca2+ cations to solid-liquid interfaces induces a macroscopic wetting transition from finite contact angles (≈10°) with to near-zero contact angles without divalent cations. We developed a quantitative model based on DLVO theory to demonstrate that this transition, which is observed on model clay surfaces, mica, but not on silica surfaces nor for monovalent K+ and Na+ cations is driven by charge reversal of the solid-liquid interface. Small amounts of a polar hydrocarbon, stearic acid, added to the ambient decane synergistically enhance the effect and lead to water contact angles up to 70° in the presence of Ca2+. Our results imply that it is the removal of divalent cations that makes reservoir rocks more hydrophilic, suggesting a generalizable strategy to control wettability and an explanation for the success of so-called low salinity water flooding, a recent enhanced oil recovery technology.