Smooth Solid Surface Research Articles

Success and failure of the spreading law for large drops of dense granular suspensions

The spreading of large viscous drops of density-matched suspensions of non-Brownian spheres on a smooth solid surface is investigated experimentally at the global drop scale. The focus is on dense suspensions with a solid volume fraction equal to or greater than $40\,\%$ , and for drops larger than the capillary length, i.e. for which the spreading is governed by the balance of gravitational and viscous forces. Our findings indicate that all liquids exhibit a power-law behaviour typical of gravity-driven dynamics, albeit with an effective suspension viscosity that is smaller than the bulk value. When the height of the drop is of the order of the particle size, the power law breaks down as the particles freeze while the contact line continues to advance.

Estimating contact angle of pure and mixed liquids on smooth solid surfaces using dispersive-to-attractive surface energy ratio from PCP-SAFT model

This study introduces a novel methodology using the PCP-SAFT model and Young equation to estimate contact angles of pure and mixed liquids on solid surfaces. By considering the dispersion and non-dispersion contributions to the Helmholtz energy, our approach estimates the ratio of dispersion to total surface energy, that is crucial to predict contact angles. Comparing our results with literature data demonstrates high accuracy; the average absolute relative deviation (AARD) of 6.65% is achieved for estimating the ratio of dispersion surface tension to total surface tension of 13 liquids and 9.27% for estimating contact angle for 70 systems at room condition. Moreover, our model effectively captures the temperature and composition variations for binary mixtures on various surfaces, showing its applicability in understanding the wetting behavior. The PCP-SAFT model offers a promising tool for estimating the ratio of dispersion surface tension to total surface tension, required to estimate contact angle as an important measure of surface wetting.

Waldron linkage-inspired deployable cylindrical mechanisms with smooth surfaces

Deployable mechanisms with solid surfaces, which possess the advantage of high accuracy, have great potential in engineering applications. This paper proposes a design method to construct a deployable cylindrical mechanism with smooth solid surfaces based on Waldron hybrid linkage. Joints above the desired surface are replaced by remote central mechanisms. The equivalence of joints ensures the surface smooth and keeps the motion characteristics unchanged. A largescale deployable mechanism is obtained by networking the proposed units along axial and radial directions, while the characteristics of 1-DOF, smooth surface, and compact folding remain. A correlation model is established to describe the relationship among design parameters. A design method is proposed to obtain the optimal mechanism parameters for constructing various types of cylindrical surfaces with maximum deployment ratio. Three cases of a circular cylindrical surface, a parabolic cylindrical surface and an elliptical cylindrical surface are developed. The effectiveness of the proposed method is verified by the deployment function and precision tests of the prototype.

Study on trapping behaviour of SiO2-containing droplets on the solid surface

The dynamic behaviour of droplets impacting solid surfaces exists in nature, production, and life. In this paper, for the first time, the molecular dynamics method is used to simulate capturing the behaviour of nanodroplets containing SiO2 on solid surfaces. The dynamic behaviour of nanodroplets impacting solid surfaces is analysed when the initial falling velocity v0 of the droplet is 0.3–1.5 nm·ps−1 and for different particle size ds, mass fraction φs of SiO2, and solid surfaces with various wettability. The contact angles of droplets containing SiO2 nanoparticles on the solid surface were measured, and the simulated values closely matched experimental results. The results show that the droplets on the smooth solid surface are completely captured after impact when v0 = 0.3–1.1 nm·ps−1. When v0 = 1.2–1.5 nm·ps−1, the droplet is fragmented, and the fragmentation ratio ns increases with the mass fraction φs. The droplet fragments when v0 = 1.4–1.5 nm·ps−1 on the rough solid surface when the ratio of the side length of the square column to the droplet diameter is 0.07, while the pure water droplet directly bounces and does not fragment. There is a linear relationship between the minimum height of the centre of mass and the mass fraction in the process of droplet impact. The maximum droplet’s spreading factor value in the spreading stage βmax increases with mass fraction φs and v0. When the spreading factor of droplets increases to βc ≈ 3.02, the droplet fragments. The droplet spreading presents the Wenzel state on the rough solid surface. Moreover, the pinning effect affects the droplet falling into the square column gap surface, hindering droplet spreading. Therefore, the rough surface has better catching ability for the droplet than the smooth surface, while the speed range that can be caught is larger. These results provide a theoretical basis for related engineering applications.

Mechanism of the Polysiloxane Concentration-Dependent Hydrophobicity in Epoxy Resin Based on a Thermodynamic Ring Model With Saturation Effect

To deeply understand the controversial results about nonpolar organics concentration-dependent hydrophobicity in polar polymers, we establish a thermodynamic ring model combined with the saturation effect of surface tension for a smooth multiphase surface. It is found that the static contact angle of the water droplet on the smooth solid surface is significantly affected by the area proportion of nonpolar dispersed phases near the three-phase contact line. The proportion of the nonpolar dispersed phase on the surface is affected by the concentration of the nonpolar dispersed phase and the particle size of the dispersed phase. Due to the high concentration and large particle size of the dispersed phase, the proportion of the nonpolar dispersed phase on the surface has obvious dispersibility, which may lead to the increase or decrease of the proportion of the nonpolar dispersed phase on the surface. When the proportion of the surface nonpolar dispersed phase is low, the static contact angle increases with the increase of the proportion of the surface nonpolar dispersed phase. However, the further increase of the proportion of nonpolar dispersed phase enhances the saturation effect of surface tension, contributing to a saturated static contact angle. Our findings afford a thorough explanation for the reported discordant results about the effect of nonpolar organics concentration on hydrophobicity in polar polymers, which is critical to rational design for polar polymers modified by nonpolar organics.

Omniphobic liquid-like surfaces.

Liquid-repellent surfaces, especially smooth solid surfaces with covalently grafted flexible polymer brushes or alkyl monolayers, are the focus of an expanding research area. Surface-tethered flexible species are highly mobile at room temperature, giving solid surfaces a unique liquid-like quality and unprecedented dynamical repellency towards various liquids regardless of their surface tension. Omniphobic liquid-like surfaces (LLSs) are a promising alternative to air-mediated superhydrophobic or superoleophobic surfaces and lubricant-mediated slippery surfaces, avoiding fabrication complexity and air/lubricant loss issues. More importantly, the liquid-like molecular layer controls many important interface properties, such as slip, friction and adhesion, which may enable novel functions and applications that are inaccessible with conventional solid coatings. In this Review, we introduce LLSs and their inherent dynamic omniphobic mechanisms. Particular emphasis is given to the fundamental principles of surface design and the consequences of the liquid-like nature for task-specific applications. We also provide an overview of the key challenges and opportunities for omniphobic LLSs.

Manipulating Trapped Nanobubbles Moving and Coalescing with Surface Nanobubbles.

Trapped nanobubbles are observed nucleating at nanopits on a pitted substrate, while surface nanobubbles are usually formed on the smooth solid surface in water. In this work, trapped nanobubbles and surface nanobubbles were captured by a tapping-mode atomic force microscope (AFM) on a nanopitted substrate based on the temperature difference method. A single trapped nanobubble was manipulated to change into a surface nanobubble, then to change into the trapped nanobubble again. At the same time, surface nanobubbles can be moved to merge into a trapped nanobubble. Our results show that the scan load and the size of the scan area were the main factors that significantly affect the mobility of surface/trapped nanobubbles. The coalescence and mutual transformation of the two kinds of nanobubbles indicate that trapped nanobubbles and surface nanobubbles have the same chemical nature, which also provides vital experimental proof of the existence of nanobubbles in the course of contact line depinning. Our results are of great significance for understanding nanobubble stability and providing guidelines in some industrial applications.

Structural superlubricity with a contaminant-rich interface

Structural superlubricity, which results from the cancelation of lateral force between contacted smooth solid surfaces with weak interaction, achieves ultralow friction and wear, inspiring disruptive innovations in applications on microscale. It is generally believed that this phenomenon requires the contact interface being ultra-clean, ensured by either a clean environment or mechanical cleaning of the contact. However, in this paper, the experimental observation of structural superlubricity with a contaminant-rich interface is unambiguously observed. After intentionally introducing airborne contaminants into microscale structural superlubric incommensurate graphite contacts, we find that the important features of structural superlubricity, the nearly-zero friction coefficient, ultralow friction, and symmetry in friction, are all well preserved. Moreover, contaminants are found to reduce the friction of a commensurate contact but increase the friction of an incommensurate contact, i.e., playing opposite roles. A theoretical model is proposed to fully grasp the contaminant effect. Our results bring challenges to the current understanding about the mechanism of structural superlubricity and provide solid support for its application under practical conditions.

Dynamic and kinematic characteristics of unsteady motion of a water-in-oil emulsion droplet in collision with a solid heated wall under conditions of convective heat transfer

The paper presents the experimental results of spreading, fingering and corona-like splashing of water-in-oil emulsion droplets based on n-decane, isoparaffin oil and distilled water in the range of Weber numbers We = 100–900, wall temperatures Tw = 20–80 °C and in collision with a smooth solid surface. The spreading features and separate convective rolling structures in emulsion droplets are examined at the late stages of wetting. The effect of Tw on the viscous dissipation energy and the maximum spreading coefficient is established. The validity of the assumptions explaining the contradictory behavior of the maximum spreading diameter of emulsion droplets with increasing temperature is studied. The assumption of unstable behavior of the emulsion droplet rim during spreading is proposed. The maximum emulsion corona diameter and height decrease linearly with the Tw growth due to the longer corona lifetime. The maximum corona diameter increases following the power law with increasing the Brinkman number due to an increase in the liquid viscosity and a decrease in Tw. The results help to predict the corona's behavior by determining the relationship between the viscous dissipation of the spreading liquid and the energy transferred to the liquid from the wall due to molecular (thermal) conduction.

Wetting failure in the early stage of water drop impact on a smooth solid surface

A water drop impacting a dry solid surface can eject a thin liquid sheet, which is forced to expand on the surface to wet the solid surface. Wetting failure, which produces defects in applications based on the impact of drops, including coating, cooling, cleaning, and printing, may occur with a sufficiently large liquid-sheet velocity. However, the exact onset of wetting failure when a drop impacts the surface has yet to be determined. Therefore, we examine the dependence of rim instability immediately after liquid-sheet ejection on the static contact angle of the solid surface at the instant of water drop impact. This study is the first attempt to solve this problem and is made possible only by using an ultra-high-speed camera. We revealed that wetting failure can occur by investigating the rim instability of the liquid sheet.

Surface morphology regulates the sorption-induced deformation of mesoporous media

Sorption in mesoporous media can induce either solid expansion or contraction, with the governing mechanism at molecular scale still unclear. The coupling between sorption and deformation is studied by an atomistic slit pore model with the help of molecular simulations. It is found that the sorption-induced deformation can shift between expansion and contraction by simply tuning the surface morphology. The underlying mechanism is revealed by a theoretical analysis conducted on a simplified two-dimensional model. It shows that the change in surface morphology alters the directions of atomistic forces between solid and fluid atoms, resulting in the change of sign of the strain. The theoretical analysis predicts that contraction happens on relatively smooth solid surfaces while expansion on corrugated surfaces, which is validated by molecular simulations. Moreover, roles of different solid-fluid interaction parameters on the coupling behavior are characterized. The interaction strength parameter influences the magnitude of the strain but has no effect on the sign, while the interaction distance parameter takes effect on both the magnitude and the sign. The current research explains both sorption-induced expansion and contraction and paves ways to material design for targeted sorption-induced deformation behavior.

Interfacial Segregation, Structure, and Diffusion ofn-Alkane Mixture Films Adsorbed on Smooth and Rough Gold Surfaces

The compositional, structural, and diffusional properties of equal-weight mixtures of liquid n-hexane and n-hexadecane films supported on atomically smooth and rough gold surfaces are explored with the use of temperature-controlled, canonical, molecular dynamics simulations at 315 K. Preferential adsorption of n-hexadecane molecules is found in the region next to the solid surface. The selective molecular segregation effect is much stronger at the smooth Au(111) surface, where it is accompanied by pronounced layering exhibited in the density profile of the adsorbed film normal to the surface, which shows the formation of two well-defined interfacial liquid layers. For both the smooth and rough solid surfaces, in the region of the first layer located about 5 Å from the solid surface, the alkane molecules order with their axis oriented parallel to the adsorbing surface, thus maximizing the attractive physisorption energy. The orientational ordering is accompanied by the formation of ordered domains in the first liquid-film layer, with the molecules in each domain lying parallel to each other. The interfacial domain ordering is frustrated in the alkane mixture film adsorbed on the rough surface. Reduced diffusive motion is found for molecules located near the solid surfaces. However, in the region of the first adsorbed layer of the liquid film adsorbed on the smooth surface, the rate of diffusion, particularly of the hexane molecules, is larger than at the rough surface. It is found that the smaller, hexane, molecules diffuse through channels at the boundaries between the ordered domains formed by the hexadecane molecules. For the rough surface case, where the domain ordering is frustrated, such interdomain boundary channels do not form and consequently the domain boundary-assisted diffusion of the smaller molecules is not operative there, resulting in a smaller diffusion rate.

Influence of hydrophobicity and roughness on the wetting and flow resistance of water droplets on solid surface: A many-body dissipative particle dynamics study

Hydrophobicity and roughness are crucial properties for determining rock wetting and flow resistance. The wetting and flow behavior of water droplets on smooth and rough solid surfaces are investigated by performing a many-body dissipative particle dynamics simulation. The results show that, in terms of the roughness, a solid surface can be divided into three regions. In Region I, as inherent hydrophobicity increases, roughness can enhance the apparent hydrophilicity of a hydrophilic surface. In Region II, the increased hydrophobicity improves the apparent hydrophobicity of a more hydrophobic surface. In Region III, when the hydrophobicity is beyond the superhydrophobic boundary, the enhancement effect of the rough surface on the apparent hydrophobicity is weakened or even disappears. With an increase in the roughness, the boundaries among these three regions and their ranges show nonmonotonic changes, making the contact mode (Wenzel state/Cassie and Baxter state), contact angle hysteresis and drag reduction effect change nonmonotonically.

Data-driven splashing threshold model for drop impact on dry smooth surfaces

We propose a data-driven threshold model to redefine the boundary between deposition and splashing for drop impact on dry smooth surfaces. The starting point is the collection and digitization of multiple experimental sources with varying impact conditions. The model is based on the theory of Riboux and Gordillo [Riboux and Gordillo, “Experiments of drops impacting a smooth solid surface: A model of the critical impact speed for drop splashing,” Phys. Rev. Lett. 113, 024507 (2014)] and is obtained by an uncertainty quantification analysis coupled with machine learning. The uncertainty quantification analysis elucidates the relevance of the impact condition uncertainties when estimating the splashing parameter. The proposed threshold model is trained using a support vector machine algorithm variant that includes uncertainty as a hyperparameter. This threshold model is generalized by complexity reduction and is eightfold cross-validated on the reference data. The results reveal a dependency of the splashing threshold on the impact velocity, the liquid viscosity, the surface tension, and the gas density. Detailed quantification of the prediction performance and a comparison with state-of-the-art models show that the proposed threshold model is the most accurate model to describe the boundaries between deposition and splashing for a wide range of impact conditions. The simplicity and accuracy of this model make it an alternative to existing approaches.

Dropwise Condensation Mechanism on Smooth Hydrophilic Surfaces

The objective of this work is to identify the mechanism of dropwise condensation on a smooth solid surface. We investigate the stable dropwise condensation that occurs at a droplet growth rate of 1 µm/s in diameter on a gold-coated glass surface. Additionally, we present our observations on unstable dropwise condensation, i.e., degradation of dropwise condensation of steam on a gold surface. Surface Plasmon Resonance Imaging (SPRi) is used to evaluate the existence and structure of thin films and occurrence of initial nuclei during condensation. SPRi approach used in this study has lateral resolutions of 4–10 µm, thickness resolutions of 0.1–1 nm, and temporal resolutions of 200–10,000 frames per second (FPS). Visualization of the onset of stable dropwise condensation suggests droplets form at heterogeneous nucleation sites and that no film greater than a monolayer exists on the surface before the formation of droplets. Observation of the unstable dropwise condensation of steam shows the existence of water films that are several nanometers thick between droplets. This work shows that neither the nucleation theory nor film rupture theory can individually explain the physics of dropwise condensation. Therefore, there is a need for a more comprehensive theory that can explain the mechanism of dropwise condensation.

Structure of a nanodrop of a binary mixture on a solid surface

The structure of a nanodrop composed of the binary mixture and sitting on the smooth solid surface is examined on the basis of a nonlocal canonical ensemble density functional theory. The molecules of two components have different diameters, masses and energy parameters of Lennard–Jones interaction potential. The properties of density distributions of the fluid components and local composition of the mixture inside and on the boundary of the drop are analysed in details for various net compositions of the mixture at various temperatures, parameters of intermolecular interaction and gravitational acceleration. The results might be of interest in developing drug delivery methods as one component of the mixture can be considered as a solvent and another one (solute) as a drug.

Experimental Study on Droplet Splash and Receding Breakup on a Smooth Surface at Atmospheric Pressure.

Droplet impact on a smooth solid surface at atmospheric pressure was experimentally studied and physically interpreted. A particular emphasis of the study is on the effects of liquid viscosity on the transition between droplet deposition (or droplet spreading without breakup) and droplet disintegration (including droplet splash and receding breakup). Specifically, the critical Weber number separating droplet deposition from droplet disintegration decreases and then increases with increasing Ohnesorge number (Oh). The splash in the low-Oh region and the receding breakup in the high-Oh region were analyzed qualitatively based on the unbalanced forces acting on the rim of the spreading or receding liquid film. A semiempirical correlation of droplet deposition/disintegration thresholds is proposed and well fits the experimental results from previous and present studies over a wide range of liquid viscosity.

Newtonian and Non-Newtonian Effects on the Collision Dynamics of a Liquid Drop with a Static Drop Located on Smooth Solid Surface

Newtonian and Non-Newtonian Effects on the Collision Dynamics of a Liquid Drop with a Static Drop Located on Smooth Solid Surface

Analytical Consideration for the Maximum Spreading Factor of Liquid Droplet Impact on a Smooth Solid Surface

Based on the energy conservation approach, this study develops a universal model to predict the maximum spreading factor of liquid droplet impact on a smooth solid surface. Validated with the present simulations and experiments in the literature, this model effectively overcomes the limitation of previous models in the viscous regime and greatly reduces the computing errors from over 30% to below 6%. It is demonstrated that the underestimated maximum spreading factor by previous models results from the overestimation of viscous dissipation. By replacing the conventional model of spreading time, tm = 8D0/3U0, with a more precise one, tm = 1.47τiWe-0.44, the formulation to compute the viscous dissipation of entire spreading is improved. Finally, we examine the applicability of present model in the capillary regime and good performance is also shown.

Objective-Driven Solid-Surface-Roughness Characterization for Enhanced Nuclear-Magnetic-Resonance Petrophysics

Summary Typically, smooth solid surfaces of reservoir rocks are assumed in formation evaluation, such as nuclear-magnetic-resonance (NMR) petrophysics and reservoir-wettability characterization through contact-angle measurements. Measuring the degree of surface roughness (R), or smoothness, and evaluating its effects on formation evaluation are topics of much research. In this paper, we primarily focus on details in characterizing solid-surface roughness and its applications in NMR pore-size analysis. R can be measured by contact techniques and noncontact techniques, such as stylus profilometer, atomic-force microscopy, and different kinds of optical measurements. Each technique has different sensitivities, measurement artifacts, resolutions, and field of view (FOV). Intuitively, although a finer resolution measurement provides the closest account of all surface details, the correspondingly small FOV might compromise the representativeness of the measurement, which is particularly challenging for charactering heterogeneous samples such as carbonates. To balance the FOV and measurement representativeness, and to minimize artifacts, laser scanner confocal microscopy (LSCM) is selected in this study. Results for the more than 27 rock samples tested indicate that rocks of similar rock types have similar R-values. Grainy limestones have relatively higher R-values compared with dolostones, consistent with the dolostone’s crystallization surface features. Muddy limestones have smoother surfaces, resulting in the lowest R-values among the rocks studied. For sandstones, R varies with clay types and content. For rocks containing two distinct minerals, two R-values are observed from the R profiles, which for these rock types justifies the use of two NMR surface relaxivity (ρ2) parameters for determining the pore-size distribution (PSD) from the NMR T2 distribution. The novelty here is the integration of LSCM and NMR to obtain an NMR PSD relevant for permeability, capillary pressure, and other petrophysical parameters. Typically, ρ2 is calibrated using the total surface area from Brunauer-Emmett-Teller (BET; Brunauer et al. 1938) gas adsorption, but this underestimates the NMR pore size because of surface-roughness effects. In our novel approach, we use R measured from LSCM to correct ρ2 for surface-roughness effects, and thereby obtain the NMR pore size more relevant for permeability and other petrophysical parameters. We then compare the roughness-corrected NMR PSD against pore size from microcomputed tomography (micro-CT) scanning (which is roughness independent). The good agreement between roughness-corrected NMR and micro-CT pore sizes in the micropore region validates our new technique, and highlights the importance of surface-roughness characterization in NMR petrophysics.