Wide Range Of Contact Angles Research Articles

Direct Measurement of Static and Dynamic Contact Angles Using a Random Micromodel Considering Geological CO2 Sequestration

The pore-level two-phase fluids flow mechanism needs to be understood for geological CO2 sequestration as a solution to mitigate anthropogenic emission of carbon dioxide. Capillary pressure at the interface of water–CO2 influences CO2 injectability, capacity, and safety of the storage system. Wettability usually measured by contact angle is always a major uncertainty source among important parameters affecting capillary pressure. The contact angle is mostly determined on a flat surface as a representative of the rock surface. However, a simple and precise method for determining in situ contact angle at pore-scale is needed to simulate fluids flow in porous media. Recent progresses in X-ray tomography technique has provided a robust way to measure in situ contact angle of rocks. However, slow imaging and complicated image processing make it impossible to measure dynamic contact angle. In the present paper, a series of static and dynamic contact angles as well as contact angles on flat surface were measured inside a micromodel with random pattern of channels under high pressure condition. Our results showed a wide range of pore-scale contact angles, implying complexity of the pore-scale contact angle even in a highly smooth and chemically homogenous glass micromodel. Receding contact angle (RCA) showed more reproducibility compared to advancing contact angle (ACA) and static contact angle (SCA) for repeating tests and during both drainage and imbibition. With decreasing pore size, RCA was increased. The hysteresis of the dynamic contact angle (ACA–RCA) was higher at pressure of one megapascal in comparison with that at eight megapascals. The CO2 bubble had higher mobility at higher depths due to lower hysteresis which is unfavorable. CO2 bubbles resting on the flat surface of the micromodel channel showed a wide range of contact angles. They were much higher than reported contact angle values observed with sessile drop or captive bubble tests on a flat plate of glass in previous reports. This implies that more precaution is required when estimating capillary pressure and leakage risk.

In situ characterization of mixed-wettability in a\xa0reservoir rock at subsurface conditions

We used X-ray micro-tomography to image the in situ wettability, the distribution of contact angles, at the pore scale in calcite cores from a producing hydrocarbon reservoir at subsurface conditions. The contact angle was measured at hundreds of thousands of points for three samples after twenty pore volumes of brine flooding.We found a wide range of contact angles with values both above and below 90°. The hypothesized cause of wettability alteration by an adsorbed organic layer on surfaces contacted by crude oil after primary drainage was observed with Scanning Electron Microscopy (SEM) and identified using Energy Dispersive X-ray (EDX) analysis. However, not all oil-filled pores were altered towards oil-wet conditions, which suggests that water in surface roughness, or in adjacent micro-porosity, can protect the surface from a strong wettability alteration. The lowest oil recovery was observed for the most oil-wet sample, where the oil remained connected in thin sheet-like layers in the narrower regions of the pore space. The highest recovery was seen for the sample with an average contact angle close to 90°, with an intermediate recovery in a more water-wet system, where the oil was trapped in ganglia in the larger regions of the pore space.

Marangoni convection instability in a sessile droplet with low volatility on heated substrate

In order to investigate Marangoni convection in a sessile droplet on heated substrate, a series of three-dimensional numerical simulations are carried out in a wide range of Marangoni number. A low volatile silicone oil is adopted and the effect of evaporation is neglected. Bénard-Marangoni convection and thermocapillary convection may coexist in droplet because of its curve surface. When Marangoni number is low the flow is steady axisymmetric thermocapillary convection whereas the flow becomes to be asymmetric and loses its stability and Hopf bifurcation occurs when Marangoni number high. The oscillatory transition convection, steady Bénard-Marangoni convection and irregular oscillatory Bénard-Marangoni convection appear in sequence with increasing Marangoni number. The characteristics of each kind of convection modes are described and the critical Marangoni numbers for the incipience of them are determined in a wide range of contact angles. One of the most interesting phenomenon is that the shape of the Bénard-Marangoni convection cell in sessile droplet is circular, which is different from that of classical hexagonal cell in flat layer. With increasing contact angle, the critical Marangoni number for the onset of convection instabilities first decreases and then increases. Critical cell number of the steady Bénard-Marangoni convection keeps on only one for contact angle less than 15° whereas it increases sharply after angle lager than 15°. The numerical results are qualitatively verified by experiment.

A Semi‐Automated Technique for Repeatable and Reproducible Contact Angle Measurements in Granular Materials using the Sessile Drop Method

Core Ideas Large standard deviations in contact angle measurements are frequently encountered. Image exposure and baseline position considerably affect contact angle measurements. Techniques to fit contact angles differ significantly from each other at high contact angles. A new technique has been developed to enhance contact angle measurements. Improvements in standard deviations by 30 to 40% have been found in hydrophobic materials. Contact angles (CAs) are used to measure the extent to which a material is wettable. Granular materials such as natural soils and crushed minerals, which are commonly assumed wettable, can exhibit non‐wetting characteristics. The sessile drop method (SDM) is a direct method widely used to generate and measure CAs, however, the procedure involved in their determination is often overlooked leading to very large standard deviations in their measurements. In this study, a close examination of the steps involved in extracting the CAs on granular materials shows that two factors, the image exposure and the position of the baseline, can affect CAs measurements significantly. Seven methods of fitting CAs were compared. It was found that the discrepancy between the methods became more and more significant as CAs increase in magnitude. A semi‐automated technique has therefore been proposed through this study to improve the standard deviations of CAs measurements. The new technique uses five steps and involves an adjustment of the image exposure and manual movement of the baseline. The proposed method was tested on flat surfaces as well as granular materials (chemically treated sand and a naturally occurring hydrophobic mineral). The results have shown that the method can be applied for both flat and granular materials with a wide range of CAs. In particular, the standard deviations of flat surfaces (e.g., hydrophobized microscope slides) with CA in the range of 90 to 135° recorded improvements of 37%. For granular materials (e.g., fluorspar) with CA in the range of 105 to 120°, improvements of 33% in standard deviations have been observed.

The change in contact angle at unsaturated CO2‐water conditions: Implication on geological carbon dioxide sequestration

AbstractThe performance of a geologic carbon storage site strongly depends on the capillary pressure of sealing rock and formations. While wettability of minerals is a key factor in capillary pressure, published contact angles are inconsistent. This study explores the discrepancy of published contact angles in order to reduce the uncertainty of measured laboratory contact angles, and understand the variation of contact angles at unsaturated CO2‐water conditions. A ratio of droplet dimension and triple line (or contact line) are used to explain the observed wide range of contact angles and the variation of contact angle at unsaturated conditions. Results show that the shape factor has a good agreement with contact angle change during CO2 dissolution in water. Silica substrate has clear two pinned and slip stages of triple line during CO2 droplet dissolution, which cause contact angle on silica substrate to increase from 34.5° to 42.1°. However, mica substrate has the repeated pinned and slip stages due to the heterogeneity of mica surface, which cause contact angle to increase dramatically from 25.4° to 68.1°. Thus, both the impact of the unsaturated CO2‐water conditions on the wide range of contact angle and the heterogeneity of mineral surface should be considered when one estimates capillary pressure based on contact angle in geological CO2 sequestration.

Wettability control on multiphase flow in patterned microfluidics

Multiphase flow in porous media is important in many natural and industrial processes, including geologic CO2 sequestration, enhanced oil recovery, and water infiltration into soil. Although it is well known that the wetting properties of porous media can vary drastically depending on the type of media and pore fluids, the effect of wettability on multiphase flow continues to challenge our microscopic and macroscopic descriptions. Here, we study the impact of wettability on viscously unfavorable fluid-fluid displacement in disordered media by means of high-resolution imaging in microfluidic flow cells patterned with vertical posts. By systematically varying the wettability of the flow cell over a wide range of contact angles, we find that increasing the substrate's affinity to the invading fluid results in more efficient displacement of the defending fluid up to a critical wetting transition, beyond which the trend is reversed. We identify the pore-scale mechanisms-cooperative pore filling (increasing displacement efficiency) and corner flow (decreasing displacement efficiency)-responsible for this macroscale behavior, and show that they rely on the inherent 3D nature of interfacial flows, even in quasi-2D media. Our results demonstrate the powerful control of wettability on multiphase flow in porous media, and show that the markedly different invasion protocols that emerge-from pore filling to postbridging-are determined by physical mechanisms that are missing from current pore-scale and continuum-scale descriptions.

Marangoni convection in an evaporating droplet: Analytical and numerical descriptions

The stationary single vortex Marangoni convection in an axially symmetrical sessile drop of capillary size is considered. The detailed description of the fluid flows is presented for a wide range of contact angles, which takes into account the boundary conditions and the mass balance equation, without explicitly solving the Navier–Stokes equations. The analytical approach developed is compared with the results of numerical simulations and demonstrated to describe reasonably well the single-vortex Marangoni flows. This indicates the substantial role of the boundary conditions in the problem.

Wettability of crushed air-dried minerals

The importance of soil particle wettability has been recognised in mining engineering, soil science and geomorphology but to a lesser extent in geotechnical engineering, perhaps due to the assumption that soils, or in general most common minerals, are always wettable. From the soil science and geomorphology literature, changes in soil wettability are known to influence soil processes such as infiltration, erosion and post-wildfire debris flows. Organic matter has been identified as one of the most important factors controlling the wettability of natural soils. However, its occurrence is very shallow and often shallower than the depth of interest for geotechnical processes. Therefore, it becomes essential to determine the wettability of minerals, in particular because it can be linked to processes that involve crushing or abrasion of soil particles. This paper examines the wettability of 21 common, freshly crushed minerals (sulfides, oxides and silicates) by measuring wettability via contact angles with the sessile drop method and the Wilhelmy plate method, including particle attributes such as aspect ratio and sphericity. The results revealed a wide range of contact angles for the uncoated minerals, with the sulfides showing strong water-repellent behaviour and the silicates remaining very wettable. Higher contact angles are associated with mineral particles that converge to the shape of a sphere and for those that have a larger aspect ratio.

Wettability modification of human tooth surface by water and UV and electron-beam radiation.

The wettability of the human tooth enamel and dentin was analyzed by measuring the contact angles of a drop of distilled water deposited on the surface. The samples were cut along the transverse and longitudinal directions, and their surfaces were subjected to metallographic mirror-finish polishing. Some samples were also acid etched until their microstructure became exposed. Wettability measurements of the samples were done in dry and wet conditions and after ultraviolet (UV) and electron beam (EB) irradiations. The results indicate that water by itself was able to increase the hydrophobicity of these materials. The UV irradiation momentarily reduced the contact angle values, but they recovered after a short time. EB irradiation raised the contact angle and maintained it for a long time. Both enamel and dentin surfaces showed a wide range of contact angles, from approximately 10° (hydrophilic) to 90° (hydrophobic), although the contact angle showed more variability on enamel than on dentin surfaces. Whether the sample's surface had been polished or etched did not influence the contact angle value in wet conditions.

Silica-based aerogels as adsorbents for phenol-derivative compounds

The simultaneous presence of hydrophobic and hydrophilic chemical moieties in phenolic compounds imply some challenges in the choice of the more selective absorbent for their removal from industrial wastewaters. In this work, the adsorption capacities of chemically tailored aerogels/xerogels towards several phenolic compounds – phenol, p-cresol and 4-clorophenol – were evaluated to study the various kinds of interactions in the adsorption process as a function of the functional groups in the gel structure. The surfactant sodium dodecylbenzenesulfonate (SDBS) was also tested as an adsorbate due to its amphiphilic character. Highly porous silica-based aerogels/xerogels with different degrees of hydrophobicity/hydrophilicity were synthesized, by using a combination of methyltrimethoxysilane (MTMS) and tetramethyl orthosilicate (TMOS) precursors in different proportions, in order to provide a variable number of methyl and hydroxyl groups in the aerogels structure. β-cyclodextrin functionalization of the aerogels was also tested due to the recognized suitability of this macrocycle cavity for phenol retention. In this way, a systematic and innovative study was carried out based on a wide range of contact angles of the adsorbent matrices and different hydrophobic/hydrophilic nature of adsorbates, in order to have an increased understanding of the dominant adsorption interactions of phenolic compounds with these silica-based adsorbents. The sol–gel synthesis followed a one-step NH4OH-catalyzed procedure and the drying of the gels was accomplished by supercritical fluid drying/extraction with CO2, to obtain aerogels, and evaporative drying, to produce xerogels. When using SDBS as adsorbate in the xerogels with intermediate wetting behaviour, good adsorption capacity is achieved and the adsorption kinetics is well described by a pseudo-second order model. The aerogels with intermediate and high contact angles show higher adsorption capacities, with favourable isotherms, for the phenolic compounds with higher hydrophobicity, which shows the important role of hydrophobic interactions in this process. The best results were obtained for the aerogel functionalized with β-cyclodextrin when adsorbing the 4-chlorophenol, proving the positive effect of the presence of the β-cyclodextrin hydrophobic cavity in the adsorption of this highly hydrophobic pollutant.

A fractal dropwise condensation heat transfer model including the effects of contact angle and drop size distribution

In this paper, a new dropwise condensation heat transfer model is developed on the basis of Rose model for a drop heat transfer and random fractal functions for the drop size distributions of direct condensation drops and coalescence drops. Compared to the well-established Rose’s model, the new model considers the contact angle and its hysteresis, the nucleation site density, the fractal dimension for drop sizes and maximum and minimum drop radii. Expressions of the distributions functions for both the direct condensing drops and the coalescence drops are derived and calculated by introducing fractal theory, respectively. The total heat flux of dropwise condensation on the entire condensing surface is calculated as the sum of the contributions from the condensations of above mentioned two kinds of drops. The simulation results indicate that the fractal dimension for drop sizes distribution decreases with increase in contact angle, resulting in smaller departure diameters and even shorter life cycle, which plays an active role in higher heat transfer coefficients. But the decline of fractal dimension leads to the decrease in the number density of droplets, which has negative effects on heat transfer process. Attributing to the combined influences mentioned above, the maximum heat flux value is obtained at θ=135°. The model proposed is applicable to predict the heat flux of the condensing drops with a wide range of contact angles, and the prediction results can agree well with different experimental dropwise condensation heat transfer data obtained from experiments and majority of literature results by choosing the proper contact angle parameters. The study can also reasonably explain the cause of the diversities of the dropwise condensation heat transfer data from different references.

Effect of contact angle on bubble formation at submerged orifices

This paper presents a theoretical model and experimental results of the generation of bubbles due to the injection of a constant flow rate of gas through an orifice submerged in liquid. The bubble formation process can be identified into three distinct stages termed (1) nucleation stage, (2) growth stage, and (3) detachment stage by analyzing the evolution of interface equilibrium and force balance conditions. Influence of contact angle on bubble formation at each stage is quantitatively elucidated. Experimental investigations in the preparation process of aluminum foams by gas injection method were conducted, and the generated bubble size was measured. The theoretical prediction of the present model suits well with the experimental results when a contact angle of 30° is introduced. The present model covers a wide range of contact angle (θ < 90°) at different gas flow rates and orifice radii in both aqueous and metallic systems.

Moving Water Droplets Over Nanoscaled (Super) hydrophobic Wettability Contrasts: Experimental Test of a Simple Model Describing Driving Forces

Applying advanced nanolithography techniques, various arrays of nanopillars on top of Si‐wafers are fabricated with all geometric parameters on the nanoscale. Additional chemical functionalization together with control over areal pillar density, height, and diameter allows the preparation of superhydrophobic surfaces exhibiting a wide range of contact angles (CA). Further improvement of this approach enables the production of step‐like wettability contrasts involving various CB–CB (Cassie‐Baxter) and CB–S (Smooth substrate)‐transitions. Such samples in combination with a high‐speed camera allow studying under optimized conditions quantitatively additional driving forces acting on a water droplet due to CA gradients. Experimentally it turns out that the maximum driving force on the droplet is well predicted by a simple model assuming circularly‐shaped base lines during the passage of a step‐like gradient of wettability. The provided study permits a comparison between maximum retention forces when tilting the substrate up to a critical angle and the presently determines maximum driving forces acting on a droplet due to a step‐like CA gradient. Both situations can be nicely described by a joint linear relation between normalized forces and CA hysteresis values with a slope close to theoretical values.

Methods and preliminary measurement results of liquid Li wettability

A test of lithium wettability was performed in high vacuum (< 3 × 10(-4) Pa). High magnification images of Li droplets on stainless steel substrates were produced and processed using the MATLAB(®) program to obtain clear image edge points. In contrast to the more standard "θ/2" or polynomial fitting methods, ellipse fitting of the complete Li droplet shape resulted in reliable contact angle measurements over a wide range of contact angles. Using the ellipse fitting method, it was observed that the contact angle of a liquid Li droplet on a stainless steel substrate gradually decreased with increasing substrate temperature. The critical wetting temperature of liquid Li on stainless steel was observed to be about 290 °C.

A smoothed particle hydrodynamics model for droplet and film flow on smooth and rough fracture surfaces

Flow on fracture surfaces has been identified by many authors as an important flow process in unsaturated fractured rock formations. Given the complexity of flow dynamics on such small scales, robust numerical methods have to be employed in order to capture the highly dynamic interfaces and flow intermittency. In this work we use a three-dimensional multiphase Smoothed Particle Hydrodynamics (SPH) model to simulate surface tension dominated flow on smooth fracture surfaces. We model droplet and film flow over a wide range of contact angles and Reynolds numbers encountered in such flows on rock surfaces. We validate our model via comparison with existing empirical and semi-analytical solutions for droplet flow. We use the SPH model to investigate the occurrence of adsorbed trailing films left behind droplets under various flow conditions and its importance for the flow dynamics when films and droplets coexist. It is shown that flow velocities are higher on prewetted surfaces covered by a thin film which is qualitatively attributed to the enhanced dynamic wetting and dewetting at the trailing and advancing contact lines. Finally, we demonstrate that the SPH model can be used to study flow on rough surfaces.

A Robust Polynomial Fitting Approach for Contact Angle Measurements

Polynomial fitting to drop profile offers an alternative to well-established drop shape techniques for contact angle measurements from sessile drops without a need for liquid physical properties. Here, we evaluate the accuracy of contact angles resulting from fitting polynomials of various orders to drop profiles in a Cartesian coordinate system, over a wide range of contact angles. We develop a differentiator mask to automatically find a range of required number of pixels from a drop profile over which a stable contact angle is obtained. The polynomial order that results in the longest stable regime and returns the lowest standard error and the highest correlation coefficient is selected to determine drop contact angles. We find that, unlike previous reports, a single polynomial order cannot be used to accurately estimate a wide range of contact angles and that a larger order polynomial is needed for drops with larger contact angles. Our method returns contact angles with an accuracy of <0.4° for solid-liquid systems with θ < ~60°. This compares well with the axisymmetric drop shape analysis-profile (ADSA-P) methodology results. Above about 60°, we observe significant deviations from ADSA-P results, most likely because a polynomial cannot trace the profile of drops with close-to-vertical and vertical segments. To overcome this limitation, we implement a new polynomial fitting scheme by transforming drop profiles into polar coordinate system. This eliminates the well-known problem with high curvature drops and enables estimating contact angles in a wide range with a fourth-order polynomial. We show that this approach returns dynamic contact angles with less than 0.7° error as compared to ADSA-P, for the solid-liquid systems tested. This new approach is a powerful alternative to drop shape techniques for estimating contact angles of drops regardless of drop symmetry and without a need for liquid properties.

Fabrication of inkjet-printed SU-8 photoresist microlenses using hydrophilic confinement

Inkjet-printed microlens arrays (MLAs) were fabricated using hydrophilic confinement by UV/ozone treatment. The MLAs were made from negative photoresist SU-8 (n = 1.63 at 530 nm). A film of 10 µm thick SU-8 shadow mask was used to define UV/ozone-treated hydrophilic zones on an SU-8 photoresist base layer. An inkjet print head was used to jet SU-8 photoresist drops onto these zones. After UV-curing, MLAs with diameters of 150, 200, 400, 800 and 1000 µm were successfully fabricated. Contact angles of MLAs increased from 22° (on MLAs fabricated using an SU-8 photoresist base layer without any surface treatment) to 45.5°, 47.7°, 52.4°, 51.3° and 54.2°, for 150, 200, 400, 800 and 1000 µm lens diameters, respectively. Using hydrophilic confinement, MLAs with a wide range of contact angles can be fabricated with diameters from 150 µm to 1 mm. This method provides a simple, cost-effective fabrication process without need for etch-transfer.

Direct numerical simulation of an oscillating droplet in partial contact with a substrate

Small-amplitude oscillations of a viscous drop that is in a partial contact with a flat substrate are investigated computationally using a finite-difference/front-tracking method. Emphasis is placed on the first mode resonance frequency response of the droplet for a wide range of contact angles. It is found that numerical results converge to the theoretical predictions of Strani and Sabetta (Strani M, Sabetta F. J Fluid Mech 1984;141:223–47) for high contact angles, whereas considerable discrepancy is observed as contact angle decreases. However, the dependence of the frequency on the drop radius, drop density and surface tension coefficient remains the same as predicted by the Strani and Sabetta theory. It is also found that the effects of density and viscosity ratio become insignificant for the density and viscosity ratios larger than 10. The oscillations are found to be damped exponentially in time due to viscous dissipation similar to the case of an isolated droplet and the damping rate decreases with increasing contact angle.

Modelling unidirectional liquid spreading on slanted microposts

A lattice Boltzmann algorithm is used to simulate the slow spreading of drops on a surface patterned with slanted micro-posts. Gibb's pinning of the interface on the sides or top of the posts leads to unidirectional spreading over a wide range of contact angles and inclination angles of the posts. Regimes for spreading in no, one or two directions are identified, and shown to agree well with a two-dimensional theory proposed in Chu, Xiao and Wang. A more detailed numerical analysis of the contact line shapes allows us to understand deviations from the two dimensional model, and to identify the shapes of the pinned interfaces.

Surface wettability and contact angle analysis by dissipative particle dynamics

A dissipative particle dynamics (DPD) simulation was presented to analyze surface wettability and contact angles of a droplet on a solid platform. The many-body DPD, capable of modeling vapor-liquid coexistence, was used to resolve the vapor-liquid interface of a droplet. We found a constant density inside a droplet with a transition along the droplet boundary where the density decreased rapidly. The contact angle of a droplet was extracted from the isosurfaces of the density generated by the marching cube and a spline interpolation of 2D cutting planes of the isosurfaces. A wide range of contact angles from 55 o to 165 o predicted by the normalized parameter (|ASL|/BSL) were reported. Droplet with the parameters |ASL| > 5.84BSL 0.297 was found to be hydrophilic. If |ASL| was much smaller than 5.84BSL 0.297 , the droplet was found to be superhydrophobic.