Wide Range Of Contact Angles Research Articles

Impact of wettability on capillary phase trapping using pore-network modeling

The impact of rock wettability on phase mobility and capillary trapping is significant in the valuation of geologic carbon sequestration scenarios. Wetting conditions of CO2/brine/rock systems can vary from site to site and over time due to changes in temperature, pressure, salinity, mineral composition and surface roughness. Therefore, effective predictability of CO2 mobility and trapping in underground formations requires a thorough exploration of the relationship between wetting conditions and phase saturation over a wide range of contact angles. In this paper, we use pore-network modeling to investigate the effect of wettability on phase mobility and capillary trapping. We simulate different wetting and nonwetting conditions during secondary flooding cycles where the initial phase saturations are varied significantly. Characteristic initial-residual (IR) saturation curves and the locus of residual phase connectivity and saturation are determined for each wettability condition.Simulations show that when the receding phase is the wetting phase, there is reduced trapping of that phase owing to piston-like advance by the nonwetting phase and sustained layer flow of the wetting phase. Such flow regimes cause nonlinearity in the characteristic initial-residual (IR) saturation curve, which is not captured by traditional trapping models. A proposed extended Land-based model matches the IR trends for all wettabilities. Simulations also show that the loci of residual phase saturation and phase connectivity are a strong function of wettability. When the receding phase is strongly nonwetting, the locus remains nearly constant in phase connectivity, while at increasingly wetting conditions, the locus forms a closed-loop in the saturation-connectivity space, where the endpoint of the locus is constrained by pore topology as residual saturations approach low values.

Heat and Mass Transfer Processes and Evaporation of a Liquid Droplet on a Structured Surface

The characteristics of water droplet heating and evaporation on structured hydrophobic and hydrophilic surfaces in the range of static contact angles from 73° to 155° were studied experimentally using high-speed video recording. Two fundamentally different technologies for applying coatings on a metal surface were used in comparison with the results on a polished surface. Microscopic studies were conducted to identify the features of the formed coatings. The wetting properties were characterized by means of the static contact angle and the contact angle hysteresis: on polished surface No. 1 (contact angle—73°, hysteresis—11°), on structured surface No. 2 (contact angle—125°, hysteresis—9°), and on structured surface No 3 (contact angle—155°, hysteresis—7°). The experimental dependences of the droplet evaporation rate on the different surfaces under normal conditions (ambient air temperature—293 K, atmospheric pressure, humidity—35%) were obtained. The evaporation regimes of droplets on the surfaces under study were identified. Water droplets evaporated in the pinning mode on surfaces No. 1 and No. 2. When a water droplet evaporated on surface No 3, the droplet was in the constant contact angle regime for ≈90% of its lifetime. Based on the experimental data obtained, a two-dimensional model of conjugate heat and mass transfer was developed, which describes the heating and evaporation of a liquid droplet on structured hydrophobic and hydrophilic surfaces at a wide range of contact angles. Satisfactory agreement was obtained between the numerical simulation results and experimental data. Using the model, the fields of temperature, concentration and other key characteristics were established at different points in time. Recommendations for its application in the development of gas–vapor–droplet applications were formulated.

Systematic Study of Wettability Alteration of Glass Surfaces by Dichlorooctamethyltetrasiloxane Silanization-A Guide for Contact Angle Modification.

To investigate the effects of wettability on multiphase flow in porous media, glass bead packs or micromodels are commonly used. Their wettability can be altered by the surface treatment method-silanization. Although silanization is widely used for glass wettability modification, comparable systematic approaches over a large range of geometries, treatment conditions, and measurement systems are scarce. In this work, dichlorooctamethyltetrasiloxane (Surfasil) treatment was systematically investigated, resulting in a guide for achieving a wide range of contact angles. Initially, the influence of the Surfasil solvent, treatment time, and Surfasil-to-solvent ratio was investigated on glass plates using the sessile drop method. By varying these variables, it was possible to achieve a wide range of comparable, repeatable, and stable contact angles, from approximately 20-95° for air-water systems. Due to the linear increase of contact angle with larger Surfasil exposure, either due to the time or concentration, contact angle tuning is possible until the critical point. Beyond the critical point of exposure, a system-specific plateau value is reached, independent of the approach. After establishing a clear relationship between the parameters and contact angles, the same treatment parameters were applied to single beads, micromodels, and beadpacks with heptane as the chosen solvent. Optical image analysis was used for the microchips, and micro CT data analysis was used for the bead packs. The treatment appeared to be transferable to all geometries, resulting in similar wetting conditions within the limitations of the measurements. It is concluded that a glass plate can be used as an analogue for obtaining the contact angle alteration trends for more complex porous media with similar compositions. Data analysis methods and surface roughness could have an effect on the obtained contact angle spread.

Effects of gravity on natural oscillations of sessile drops

Natural oscillations of sessile drops with a free or pinned contact line in different gravity environments are studied based on a linear inviscid irrotational theory. The inviscid Navier–Stokes equations and boundary conditions are reduced to a functional eigenvalue problem by the normal-mode decomposition. We develop a boundary element method model to numerically solve the eigenvalue problem for predicting the natural frequencies. Emphasis is placed on the frequency shifts of modes due to gravity for a wide range of contact angles $\alpha$ and Bond numbers $Bo$ . Three types of $\alpha$ – $Bo$ diagrams reflecting how gravity shifts the frequency are identified. Specifically, the frequency of zonal modes shifts downwards (upwards) when $\alpha$ is smaller (larger) than a critical value, while the frequencies of most sectoral modes are shifted downwards regardless of $\alpha$ . As a result, gravity can transform the lowest mode from a zonal mode to a sectoral mode. The spectral degeneracy of hemispherical drops inherited from the Rayleigh–Lamb spectrum is also broken by gravity. However, we discover that gravity has no effect on the mode associated with the horizontal motion of the centre of mass, whose frequency is always zero regardless of $\alpha$ and $Bo$ . This implies that the ‘walking’ drop instability reported in previous literature does not exist.

Structure induced wide range wettability: Controlled surface of micro-nano/nano structured copper films for enhanced interface

The wettability of materials used in the production of devices employed in various technological domains have attracted significant attentions. Therefore, it is important to design the surfaces of these materials such that they can provide the required surface free energy and simplify the interfacial structure. Herein, various Cu films with a highly controllable surface wettability and a wide range of contact angles ranging from 6° to 152° were fabricated, and the corresponding mechanism was discussed. A wide range of wettability was realized by controlling the surface structure of the Cu film. The nanogap structure of the vertical nanowire-array film led to a high surface free energy. Similarly, the oblique nanowire-array film increased the surface free energy; however, the surface free energy was dependent on the size of the nanowires rather than on the nanogaps owing to the crystallinity of the film. Additionally, cluster-nanowire-array films were designed to realize a wettability transition from hydrophilicity to hydrophobicity with a constant surface free energy. The Cu foam possessed a superhydrophilic surface owing to its high porosity, whereas the cluster-nanoparticle structure possessed a superhydrophobic surface. In addition, we noted that the structure-induced wettability played an important role in tuning the semiconductor and metal interfacial stress and simplifying the interfacial structure. Furthermore, the outstanding electrical conductivity of the Cu films indicates its promising potential as an electrode. The structure-induced wettability proposed in this study can be applied for a wide range of materials, particularly for films used for advanced applications.

Natural oscillations of a sessile drop on flat surfaces with mobile contact lines

Oscillation of sessile drops is important to many applications. In the present study, the natural oscillation of a sessile drop on flat surfaces with free contact lines (FCL) is investigated through numerical and theoretical analysis. The FCL condition represents a limit of contact line mobility, i.e. the contact angle remains constant when the contact line moves. In the numerical simulation, the interfaces are captured by the volume-of-fluid method and the contact angle at the boundary is specified using the height-function method. The oscillation frequencies for sessile drops with FCL are mainly controlled by the contact angle and the Bond number and a parametric study is carried out to characterize their effects on the frequencies for the first and high-order modes. Particular attention is paid to the frequency of the first mode, since it is usually the dominant mode. An inviscid theoretical model for the first mode is developed. The model yields an explicit expression for the first-mode frequency as a function of the contact angle and the Bond number, with all parameters involved fully determined by the equilibrium drop theory and the simulation. The predicted frequencies for a wide range of contact angles agree very well with the simulation results for small Bond numbers. The frequencies for both the first and high-order modes decrease with the contact angle and increase with the Bond number. For the high-order modes, the frequencies for different modes generally scale with the Rayleigh frequencies. The scaling relation performs better for small Bond numbers and large contact angles. A simple model is proposed to predict the frequencies of high-order modes for large contact angles and a good agreement with the simulation results is observed.

Pore-Scale Modeling of the Effect of Wettability on Two-Phase Flow Properties for Newtonian and Non-Newtonian Fluids.

The Darcy-scale properties of reservoir rocks, such as capillary pressure and relative permeability, are controlled by multiphase flow properties at the pore scale. In the present paper, we implement a volume of fluid (VOF) method coupled with a physically based dynamic contact angle to perform pore-scale simulation of two-phase flow within a porous medium. The numerical model is based on the resolution of the Navier–Stokes equations as well as a phase fraction equation incorporating a dynamic contact angle model with wetting hysteresis effect. After the model is validated for a single phase, a two-phase flow simulation is performed on both a Newtonian and a non-Newtonian fluid; the latter consists of a polymer solution displaying a shear-thinning power law viscosity. To investigate the effects of contact angle hysteresis and the non-Newtonian nature of the fluid, simulations of both drainage and imbibition are carried out in order to analyze water and oil saturation—particularly critical parameters such as initial water saturation () and residual oil saturation () are assessed in terms of wettability. Additionally, the model sensitivities to the consistency factor (), the flow behavior index (n), and the advancing and receding contact angles are tested. Interestingly, the model correctly retrieves the variation in and wettability and predicts behavior over a wide range of contact angles that are difficult to probe experimentally.

New Insights Into Complex Interactions Between Heterogeneity and Wettability Influencing Two‐Phase Flow in Porous Media

AbstractCoupled effects of wettability and pore geometry on pore‐scale dynamics of immiscible two‐phase fluid displacement were investigated using a multiphase lattice Boltzmann model in homogeneous and heterogeneous porous media under a wide range of contact angle θ (25° ≤θ≤ 175°). The observed invasion patterns indicate that the complex interactions between wettability and pore morphology play crucial roles in controlling the displacement patterns. The detailed pore‐scale analysis enabled us to delineate how wettability influences the recovery efficiency of the defending fluid in homogeneous and heterogeneous porous media. As the wetting condition changes from strong drainage to imbibition, we observed a transition from burst to corner flow leading to a compact displacement and hence a higher recovery efficiency in the heterogeneous sample. The present study sheds new insights on how the interaction between wettability and heterogeneity influences fluid displacement in porous media.

Microfluidic slug transport on traveling-wave surface topographies by mechanowetting

Nature often uses capillary forces to manipulate fluids, which has inspired scientists to develop new applications, such as in microfluidics and laboratory-on-a-chip systems. Here we present a method to transport fluids in microfluidic channels, by exploiting the capillary interaction of fluid interfaces with traveling surface waves, called mechanowetting. We found that the three-phase lines of fluid slugs dynamically attach to the crests of the waves, resulting in fluid velocities that are equal to the wave speed. By comparing this microfluidic slug flow to conventional peristaltic fluid propulsion, we demonstrate that fluid velocities can be reached that are one order of magnitude larger. We quantified the efficiency numerically and theoretically in terms of the generated pressure gradient using a closed channel and measured the evolution of the pressure distribution as the wave progresses. The method was shown to work for a very wide range of contact angles. We anticipate that our results will lead to new microfluidic applications based on switchable topography technology.

Effect of wettability on the water entry problem of aluminum spheres

We investigated the hydrodynamics of water entry by aluminum spheres varying in wettability, assisted by nano/microscaled surface morphology. A wide range of contact angle (C.A.: 0° ≤ θ0 < 170°) on the test spheres was prepared via inexpensive single-step anodization method. The water entry events: Splash and cavity formation, were visualized by a high-speed camera to understand the fast transient features affected by the surface wettability. In terms of hydrodynamics, cavity formation and air entrainment during the water entry were analyzed via dynamic wetting of liquid film flow along the sphere surface. It was confirmed that wettability was a determinant factor of both splash shape and cavity formation, especially in hydrophobic condition (90° ≤ θ0). Furthermore, the analysis of dynamic contact angle of liquid film front in this study was able to classify the water entry events more detail than previous literature. In addition, we also recorded audio signals to evaluate acoustic pressures produced by the impact between spheres and water pool. The audio signals, which are expected to be proportional to acoustic pressures, differed when hydrophilic and hydrophobic spheres entered water.

Integration of Self-Sealing Suction Cups on the FLEXotendon Glove-II Robotic Exoskeleton System

This article presents a hand exoskeleton using self-sealing suction cup modules to assist and simplify various grasping tasks. Robotic hands, grippers, and hand rehabilitation exoskeletons require complex motion planning and control algorithms to manipulate various objects, which increases system complexity. The proposed hand exoskeleton integrated with self-sealing suction cup modules provides simplified grasping with the assistance of suction. The suction cup has a self-sealing mechanism with a passive opening valve and it reduces vacuum consumption and pump noise. The gimbal mechanism allows the suction cup to have a wide range of contact angles, which increases adaptability of grasping of the exoskeleton. The fabrication process of the device is introduced with the suction cup design and material selection. The vacuum canister and solenoid valve that comprise the proposed pneumatic circuit provide a continuous vacuum supply source without continuous operation of a vacuum pump and autonomous suction/release motion, respectively. The performance of the hand exoskeleton was demonstrated with various grasping tasks and it provided stable grasping and pick-and-place task without complex finger manipulation. The proposed hand exoskeleton has the potential to simplify the grasping process and allow patients with hand dysfunction to expand their versatility of grasping tasks.

Heat transfer and convection of evaporating sessile droplets in transition from superhydrophilic to superhydrophobic structured wall: Optimization of functional properties

Abstract The free convection and heat transfer of evaporating water droplets on a textured wall have been studied. The analysis of the influence of micro- and macrotextures on the heat transfer coefficient has been performed. Structured surfaces were created by two different methods: mechanical surface treatment and laser exposure. The behavior of heat transfer on a structured wall was compared with that on a smooth surface. The wettability of the wall varied over a wide range of contact angles (the transition from a superhydrophilic to a superhydrophobic surface was considered). Simple relations have been obtained to estimate the separate influence of several key factors (the drop height, the surface area of the wetted spot of the drop, the convection velocity in the liquid and the temperature difference in the drop). The obtained data are important for designing heat exchangers with structured walls and for optimizing the functional properties.

Lattice Boltzmann method for contact-line motion of binary fluids with high density ratio.

Within the phase-field framework, we present an accurate and robust lattice Boltzmann (LB) method for simulating contact-line motion of immiscible binary fluids on the solid substrate. The most striking advantage of this method lies in that it enables us to handle two-phase flows with mass conservation and a high density contrast of 1000, which is often unavailable in the existing multiphase LB models. To simulate binary fluid flows, the present method utilizes two LB evolution equations, which are respectively used to solve the conservative Allen-Cahn equation for interface capturing, and the incompressible Navier-Stokes equations for hydrodynamic properties. Besides, to account for the substrate wettability, two popular contact angle models including the cubic surface-energy model and the geometrical one are incorporated into the present method, and their performances are numerically evaluated over a wide range of contact angles. The contact-angle hysteresis effect, which is inherent to a rough or chemically inhomogeneous substrate, is also introduced in the present LB approach through the strategy proposed by Ding and Spelt [J. Fluid Mech. 599, 341 (2008)10.1017/S0022112008000190]. The present method is first validated by simulating droplet spreading and capillary intrusion on the ideal or smooth pipes. It is found that the cubic surface-energy and geometrical wetting schemes both offer considerable accuracy for predicting a static contact angle within its middle region, while the former is more stable at extremely small contact angles. Besides, it is shown that the geometrical wetting scheme enables us to obtain better accuracy for predicting dynamic contact points in capillary pipe. Then we use the present LB method to simulate the droplet shearing processes on a nonideal substrate with contact angle hysteresis. The geometrical wetting model is found to be capable of reproducing four typical motion modes of contact line, while the surface-energy wetting scheme fails to predict the hysteresis behaviors in some cases. At last, a complex contact-line dynamic problem of three-dimensional microscale droplet impact on a wettable solid is simulated, and it is found that the numerical results for droplet shapes agree well with the experimental data.

Lattice Boltzmann modeling of wall-bounded ternary fluid flows

• A novel wetting boundary scheme in the lattice Boltzmann method is proposed for ternary fluids. • The proposed scheme can preserve the reduction consistency property of the diphasic case. • The wetting scheme is demonstrated to be accurate for a wide range of contact angles. • Fascinating interfacial phenomena are found in simulations of compound drop impact dynamics. Starting from the phase-field perspective, we first formulate a novel wetting boundary condition to describe the interactions among ternary fluids and a solid and then we propose a boundary scheme for its implementation in the framework of the lattice Boltzmann (LB) method. This scheme for three-phase fluids can preserve the reduction consistency property of the diphasic case such that it can give physically relevant results. Combining this wetting boundary scheme and the LB ternary fluid model based on multicomponent phase-field theory, we simulated several ternary fluid flow problems involving a solid substrate, including the spreading of binary drops on a substrate, the spreading of a compound drop on a substrate, the capillary intrusion of ternary fluids, and the shear of a compound liquid drop on a substrate. The numerical results are found to be good agreement with the analytical solutions and some available results. Finally, as an application, we use the LB model coupled with the present wetting boundary scheme to numerically investigate the impact of a compound drop on a solid circular cylinder. It is found that the dynamics of a compound drop can be remarkably influenced by the wettability of the solid surface and the dimensionless Weber number .

Controlling the Wettability of Steel Surfaces Processed with Femtosecond Laser Pulses.

The wettability of a material surface is an essential property that can define the range of applications it can be used for. In the particular case of steel, industrial applications are countless but sometimes limited because of the lack of control over its surface properties. Although different strategies have been proposed to tune the wetting behavior of metal surfaces, most of them require the use of processes such as coatings with different materials or plasma/chemical etching. In this work, we present two different laser-based direct-write strategies that allow tuning the wetting properties of 1.7131 steel over a wide range of contact angles using a high repetition rate femtosecond laser. The strategy consists in the writing of parallel and crossed lines with variable spacing. A detailed morphological analysis confirmed the formation of microstructures superimposed with nanofeatures, forming a hierarchical surface topography that influences the wetting properties of the material surface. Contact angle measurements with water confirm that this behavior is mostly dependent on the line-to-line spacing and the polarization-dependent orientation of the structures. Moreover, we demonstrate that the structures can be easily replicated in a polymer using a laser-fabricated steel master, which enables low-cost mass production. These findings provide a practical route for developing user-defined wetting control for new applications of steel and other materials functionalized by rapid laser structuring.

Controlled Surface Silanization for Actin-Myosin Based Nanodevices and Biocompatibility of New Polymer Resists.

Molecular motor-based nanodevices require organized cytoskeletal filament guiding along motility-promoting tracks, confined by motility-inhibiting walls. One way to enhance motility quality on the tracks, particularly in terms of filament velocity but also the fraction of motile filaments, is to optimize the surface hydrophobicity. We have investigated the potential to achieve this for the actin-myosin II motor system on trimethylchlorosilane (TMCS)-derivatized SiO2 surfaces to be used as channel floors in nanodevices. We have also investigated the ability to supress motility on two new polymer resists, TU7 (for nanoimprint lithography) and CSAR 62 (for electron beam and deep UV lithography), to be used as channel walls. We developed a chemical-vapor deposition tool for silanizing SiO2 surfaces in a controlled environment to achieve different surface hydrophobicities (measured by water contact angle). In contrast to previous work, we were able to fabricate a wide range of contact angles by varying the silanization time and chamber pressure using only one type of silane. This resulted in a significant improvement of the silanization procedure, producing a predictable contact angle on the surface and thereby predictable quality of the heavy meromyosin (HMM)-driven actin motility with regard to velocity. We observed a high degree of correlation between the filament sliding velocity and contact angle in the range 10-86°, expanding the previously studied range. We found that the sliding velocity on TU7 surfaces was superior to that on CSAR 62 surfaces despite similar contact angles. In addition, we were able to suppress the motility on both TU7 and CSAR 62 by plasma oxygen treatment before silanization. These results are discussed in relation to previously proposed surface adsorption mechanisms of HMM and their relationship to the water contact angle. Additionally, the results are considered for the development of actin-myosin based nanodevices with superior performance with respect to actin-myosin functionality.

A study of wall boundary conditions in pseudopotential lattice Boltzmann models

The effect of fluid–solid interactions on the hydrodynamics of non-ideal fluids and wettability of surfaces is investigated. We integrate the interaction forces, simulated by pseudopotentials, into two on-site boundary conditions: standard bounce-back (SBB) and Zou and He (ZH) [12] to determine the distribution functions of the boundary nodes. Three different interaction forces are tested: pseudopotential-based interaction (ψ), modified pseudopotential-based interaction (mψ), and a ZH-based interaction, which is proposed by this study based on the ZH method. Therefore, the schemes are ψ-SBB, mψ-SBB, mψ-ZH, and ZH-ZH. The first criterion is the achievement of macroscopic Poiseuille flow. The second criterion is the achievement of a wide range of contact angles. The main method of simulation is multipseudopotential interaction [30]. It is found that the scheme of ψ-SBB creates a relatively large fluctuation of density across the channel. Whilst, the schemes of mψ-SBB, mψ-ZH, and ZH-ZH generate much less density variation across the channel. Among them, ZH-ZH treatment is superior based on density fluctuation and the error associated with the resolution, relaxation time, and compressibility. We found that all four boundary conditions can form a wide of range of contact angles. The ψ-SBB scheme creates largest density fluctuation inside a drop on wettable surfaces. The schemes of mψ-SBB and mψ-ZH create almost the same density fluctuation which is larger than ZH-ZH. Moreover, mψ interaction generates spurious velocities as high as six times a free drop with SBB and eight times with ZH while spurious velocities in ψ-SBB and ZH-ZH are very close to the free drop. Therefore, ZH-ZH performs best, also, in wettability tests.

A liquid bridge model for spherical particles applicable to asymmetric configurations

The modeling of wet granular materials is an important research topic in chemical engineering. This study focuses on the numerical model for the capillary liquid bridge force in sphere-sphere and sphere-plane configurations, which is an indispensable ingredient of wet particle simulations. The first part of this study describes a direct approach via optimization of the interfacial energy of the liquid bridge system. The results, referred to as the optimal solutions, can be used as reference values for the exact solutions of static liquid bridges. In the second part, a force model is presented based on a toroidal approximation of the liquid bridge profile. Particularly, its advantage resides in the generality, which is applicable to a wide range of liquid volumes, contact angles and radius ratios. Predictions by the proposed model are validated against experimental data and optimal solution, from which good agreements are obtained. Numerical tests indicate that the relative error by the proposed model is generally below 10% within a wide range of contact angles and liquid volumes, showing its satisfactory accuracy for calculating the capillary force. It is expected that the present model will be useful for practical numerical analysis of wet granular materials.

The role of surface wettability on natural convection frosting: Frost growth data and a new correlation for hydrophilic and hydrophobic surfaces

In refrigeration systems, frost accumulation on the evaporator demands periodic defrosting. Chemical coatings (either hydrophilic or hydrophobic) are sometimes used to mitigate frost build-up and/or improve drainage during the defrost cycle. Although several studies have been performed on frost growth behavior, relatively few studies have specifically examined the effect of surface wettability on the frost growth rate. This paper focuses on the influence of environmental operating conditions on the growth of a frost layer on surfaces which span a wide range of contact angles under natural convection conditions. A new semi-empirical correlation which includes the surface contact angle θ and the modified Jakob number Λ as parameters is reported with an average predictive error of 11.7% (N = 930) for all non-zero experimental data. The correlation was developed using multivariable regression analysis and data which span different surface temperatures (−13 °C to −5°C), relative humidities (40–80%), and static contact angles ranging from 45° (hydrophilic) to 160° (hydrophobic).

Hydrophobic CuO Nanosheets Functionalized with Organic Adsorbates.

A new class of hydrophobic CuO nanosheets is introduced by functionalization of the cupric oxide surface with p-xylene, toluene, hexane, methylcyclohexane, and chlorobenzene. The resulting nanosheets exhibit a wide range of contact angles from 146° (p-xylene) to 27° (chlorobenzene) due to significant changes in surface composition induced by functionalization, as revealed by XPS and ATR-FTIR spectroscopies and computational modeling. Aromatic adsorbates are stable even up to 250-350 °C since they covalently bind to the surface as alkoxides, upon reaction with the surface as shown by DFT calculations and FTIR and 1H NMR spectroscopy. The resulting hydrophobicity correlates with H2 temperature-programmed reduction (H2-TPR) stability, which therefore provides a practical gauge of hydrophobicity.