Trapped Liquid Drop at the End of Capillary

The dynamic behavior of impinging water droplets is studied in the context of varying surface wettability and wickability on smooth and nanostructured superhydrophilic surfaces. This study distinguishes the separate effects of wetting (contact angle), wickability, and inertia on the spreading and vaporization of water droplets deposited on nanoporous surfaces by considering experimental results in tandem with axisymmetric, volume of fluid (VOF) simulations of droplet spreading. High speed videos were obtained for water droplets spreading on nanoporous surfaces which exhibit very low (< 15°) contact angle and high wickability. In this study, the effect of wickability was assessed by comparing the experimental results, which include the low contact angle and high wickability effects, to predictions of the VOF model, which include only the ultralow contact angle. While a droplet touched to the nanostructured surface demonstrates spreading driven by wicking, droplets which hit the surface with a non-zero impact velocity demonstrate spreading characteristics similar to the smooth surface, which are driven by inertia and ultra-low contact angle. The presence of the nanoporous layer impacts the equilibrium position of the contact line and the final spread radius changes with impact velocity on the nanostructured surface. These results provide fundamental input for modeling of spray cooling systems with nanostructured surfaces.

In this work, we have studied the wettability of zinc oxide (ZnO) nanorods grown on fluorine-doped tin oxide (FTO) by highlighting the effect of polar and non-polar ZnO facets on contact angle (CA) results. The variation in the wettability behaviors of the synthesized surfaces is mainly related to physical and chemical surface texturing which influenced the liquid drop penetration. Indeed, three main penetration states can be deduced: total, partial, and null-penetration. Where, low CA (100.9°) with high contact angle hysteresis (CAH) (13°) is observed for total penetration of the liquid drop. While, high CA (139.6°) with low CAH (7°) is observed for null-penetration. Moreover, we have found that the chemical texturing of ZnO, especially the hydrophilicity of ZnO tips, responsible for liquid drop sticking, prevents the liquid slipping over the surface. In order to promote the liquid rolling on the ZnO surface, we reported the physical modification of the ZnO structures. Therefore, the rolling of the liquid drop on the inclined surface of ZnO is achieved by using a new structure based on double scale roughness. This surface exhibits superhydrophobic behavior with a CA of 153° and CAH of 3°.

We conduct experimental investigations of macroscopic capillary forces between two flat rigid substrates characterized by their advancing and receding contact angles with water. Our results exhibit excellent agreement with theoretical predictions obtained by the numerical solution of the capillary equation. On the basis of this comparison, we use the measurements of the capillary force to investigate the phenomenon of contact angle hysteresis. We present examples of force measurements for surfaces that display low, moderate, and high contact angle hysteresis and compare results for a larger variety of substrates. Finally, we show that for the case of water, the role of viscosity is insignificant within the range of force and velocity measured in the present work.

In this study, a thermodynamic analysis is conducted to investigate the chemical effect, in terms of intrinsic contact angle (CA), on the superhydrophobic behavior. It is theoretically revealed that the essential effect of intrinsic CA is to promote the composite transition. In particular, a large intrinsic CA more than 90° is necessary for such transition. Furthermore, for a pillar system with an intrinsic CA smaller than 90°, composite states are not impossible but is thermodynamically unstable. Once composite states are achieved, the advancing or maximum CA always approaches 180° whether an intrinsic CA is larger or smaller than 90°. In addition, the role of intrinsic CA in the water-repellent or self-cleaning behavior such as contact angle hysteresis (CAH) and equilibrium CA has been discussed in detail.

ABSTRACTAn old procedure used to carry out a graphical derivation of curves, which is based on the optical properties of plane mirrors, has been adapted for the measurement of the contact angle (θ) formed between a liquid drop and a flat solid substrate in wettability experiments carried out by the so-called “sessile drop” method. The method was tested for mercury on soda-lime glass at room temperature in air as well as for Cusil (Ag-28wt.%Cu) and Incusil-ABA (Ag-27wt.%Cu-12wt.%In-2wt.%Ti) brazing alloys on pressureless-sintered silicon carbide (PLS-SiC) at 850 °C, under a vacuum of 10-4/10-5 Torr. The proposed method is fast, simple and accurate enough from high (∼140°) to relatively low (∼10°) contact angles. Although the proposed method has been tested for metal-ceramic systems, it is of general application, so that it would be useful for any liquid-solid system. The method is applicable for any temperature, pressure and atmospheric experimental conditions employed, as well as for any chemical composition of liquid and solid. It is also useful for both low and high contact angles, as well as for reactive and non-reactive systems, as long as a photograph of a liquid drop resting on a flat solid surface is available for the studied system.

The amazing water repellency of many biological surfaces, exemplified by lotus leaves, has recently received a great deal of interest. These surfaces, called superhydrophobic surfaces, exhibit water contact angles larger than 150 degrees and a low contact angle hysteresis because of both their low surface energy and heterogeneously rough structures. In this paper, we suggest a biomimetic method, "biosilicification", for generating heterogeneously rough structures and fabricating superhydrophobic surfaces. The superhydrophobic surface was prepared by a combination of the formation of heterogeneously rough, nanosphere-like silica structures through biosilicification and the formation of self-assembled monolayers of fluorosilane on the surface. The resulting surface exhibited the water contact angle of 160.1 degrees and the very low water contact angle hysteresis of only 2.3 degrees, which are definite characteristics of superhydrophobic surfaces. The superhydrophobic property of our system probably resulted from the air trapped in the rough surface. The wetting behavior on the surface was in the heterogeneous regime, which was totally supported by Cassie-Baxter equation.

Dynamic wetting characteristics during droplet formation in a microfluidic T-junction

Liquid bridges with moving contact lines are found in a variety of settings such as capillary feeders and high-speed printing. Although it is often assumed that the length scale for these flows is small enough that inertial effects can be neglected, this is not the case in certain applications. To address this issue, we solve the Navier-Stokes equations with the finite element method for the stretching of a liquid drop between two surfaces for non-zero Reynolds numbers. We consider an axisymmetric liquid bridge between a moving flat plate and either a stationary flat plate or a cavity. The contact lines are allowed to slip, and we evaluate the effect of the Reynolds number and contact angles on the transfer of liquid to the moving plate. In the case of two flat plates, we find that inertia forces the interface to map onto a similarity solution in a manner that shifts the breakup point toward the more wettable surface. Inertia and wettability are thus competing effects, with inertia driving fluid toward the surface with the higher contact angle and wettability driving fluid toward the surface with the lower contact angle. When a cavity is present, contact line pinning on the cavity wall biases breakup toward the cavity as the Reynolds number is increased, leading to improved cavity emptying. As the flat plate is made more wettable, a second pinch-off point can form near that plate, leading to a satellite drop and a reduction in liquid transfer to the plate. Therefore, higher liquid transfer is not always obtained with a more wettable substrate when inertia is present, in contrast to Stokes flow. We also compare our results to those obtained using a model based on the long-wave approximation and find good qualitative agreement, with the long-wave model overpredicting the amount of liquid transferred after breakup.

On the effect of static and dynamic contact angles on humid air condensation heat transfer

Even though there are quite large studies on wettability of aqueous surfactants and a few studies on effects of nanoparticles on wettability of colloids, to the best of authors' knowledge, there is no study reported on the combined effect of surfactant and nanoparticles in altering the wettability. The present study, for the first time, reports an extensive experimental and theoretical study on the combined effect of surfactants and nanoparticles on the wettability of complex fluids such as nanocolloids on different substrates, ranging from hydrophilic with a predominantly polar surface energy component (silicon wafer and glass) to near hydrophobic range with a predominantly dispersive component of surface energy (aluminum and copper substrates). Systematically planned experiments are carried out to segregate the contributing effects of surfactants, particles, and combined particle and surfactants in modulating the wettability. The mechanisms and the governing parameters behind the interactions of nanocolloids alone and of surfactant capped nanocolloids with different surfaces are found to be grossly different. The article, for the first time, also analyzes the interplay of the nature of surfaces, surfactant and particle concentrations on contact angle, and contact angle hysteresis (CAH) of particle and surfactant impregnated colloidal suspensions. In the case of nanoparticle suspensions, the contact angle is observed to decrease for the hydrophobic system and increase for the hydrophilic systems considered. On the contrary, the combined particle and surfactant colloidal system shows a quasi-unique wetting behavior of decreasing contact angle with particle concentration on all substrates. Also interestingly, the combined particle surfactant system at all particle concentrations shows a wetting angle much lower than that of the only-surfactant case at the same surfactant concentration. Such counterintuitive observations have been explained based on the near-surface interactivity of the particle, fluid, and surfactant molecules based on effective slip length considerations. The CAH analyses of colloidal suspensions at varying surfactant and particle concentrations reveal in-depth physical insight into contact line pinning, and a unique novel relationship is established between the contact angle and differential energy for distorting the instantaneous contact angle for a pinned sessile droplet. A detailed theoretical analysis of the governing parameters influencing the wettability has been presented invoking the principles of DLVO (Derjaguin-Landau-Verwey-Overbeek), surface energy and interaction parameters influencing at the molecular scale, and the theoretical framework is found to support the experimental observations.

Determination of line tension for systems near wetting

The hydrostatic shape, transient deformation, and asymptotic shape of a small liquid drop with uniform surface tension adhering to a planar wall subject to an overpassing simple shear flow are studied under conditions of Stokes flow. The effects of gravity are considered to be negligible, and the contact line is assumed to have a stationary circular or elliptical shape. In the absence of shear flow, the drop assumes a hydrostatic shape with constant mean curvature. Families of hydrostatic shapes, parameterized by the drop volume and aspect ratio of the contact line, are computed using an iterative finite-difference method. The results illustrate the effect of the shape of the contact line on the distribution of the contact angle around the base, and are discussed with reference to contact-angle hysteresis and stability of stationary shapes. The transient deformation of a drop whose viscosity is equal to that of the ambient fluid, subject to a suddenly applied simple shear flow, is computed for a range of capillary numbers using a boundary-integral method that incorporates global parameterization of the interface and interfacial regriding at large deformations. Critical capillary numbers above which the drop exhibits continued deformation, or the contact angle increases beyond or decreases below the limits tolerated by contact angle hysteresis are established. It is shown that the geometry of the contact line plays an important role in the transient and asymptotic behaviour at long times, quantified in terms of the critical capillary numbers for continued elongation. Drops with elliptical contact lines are likely to dislodge or break off before drops with circular contact lines. The numerical results validate the assumptions of lubrication theory for flat drops, even in cases where the height of the drop is equal to one fifth the radius of the contact line.

Wettability alteration in shale formations can be an important factor in improving the performance of hydraulic fracturing treatments. The use of surfactants in the frac fluid, at proper concentrations, has shown to change wettability in Unconventional Liquid Reservoirs (ULR) favoring the process of imbibition. This study evaluates and compares the efficiency of anionic and nonionic surfactants in recovering hydrocarbons in carbonate and siliceous preserved side-wall core. The techniques developed also open the door for investigation of low concentration surfactants for enhanced oil recovery (EOR) in ULR. Contact angle (CA) experiments were performed, using the captive bubble method, to measure the magnitude of wettability alteration on intermediate to oil-wet ULR core at reservoir temperature (165 °F). Different types of anionic and nonionic surfactants at field concentrations were used. The results showed that all surfactants lower the CA at the concentration tested. However, anionic surfactants showed better results as observed by lower contact angles. IFT measurements were also performed, using the pendant drop and spinning drop methods, at reservoir temperature using reservoir crude oil and anionic and nonionic surfactants at the same concentrations. The IFT reduction was similar for each type of surfactant compared to regular frac fluid without any surfactant, but anionic surfactant showed slightly better capability of reducing IFT than nonionic surfactants. Computed tomography (CT) scan methods were used to gauge the performance of these surfactants in improving oil recovery. The magnitude of penetration or imbibition into artificially-fractured ULR cores was studied for both anionic and nonionic surfactants. Frac fluids containing surfactants were mixed with a dopant salt to trace the movement of these fluids and measure the penetration numerically. Both, anionic and nonionic, surfactants have higher penetration magnitudes compared to slick water without surfactant. However, anionic surfactants displaced a higher observable amount of liquid hydrocarbon from the shale cores. This observation agrees qualitatively with the results observed in the CA experiments where anionic surfactants showed the lowest contact angles. From the results obtained, it can be concluded that anionic surfactants alter wettability in these ULR core, giving lower CA, better spontaneous imbibition and higher oil recovery than nonionic surfactants. These observed wettability changes induced by surfactants mixed in the frac fluids can improve matrix penetration with spontaneous imbibition which opens further discussions for EOR potential in shale formations.

Salinity effect on micro-scale contact angles using a 2D micromodel for geological carbon dioxide sequestration

Numerical investigations of water droplet dynamics in micro-channels considering contact angle hysteresis