Evaporation of a liquid sessile droplet subjected to forced convection

Experiments on measuring the rate of evaporation of liquid sessile droplets into air show that the rate of evaporation increases in the presence of forced convection flows. However, data on the effect of convection on evaporation are often contradictory and should be clarified. The paper presents a numerical analysis of evaporation from the surface of a water droplet subjected to forced convection in the gas phase. The drop is located on a smooth horizontal isothermal substrate; the mode with constant contact angle is considered. The shape of the drop has axial symmetry, the same for the velocities and pressure. Forced convection compatible with the symmetry conditions are represented by flows directed downward along the axis of the system and diverging along the sides near the drop and the substrate. The mathematical model is constructed for evaporation controlled by diffusion in the gas phase and takes into account surface tension, gravity, and viscosity in both media, buoyancy and Marangoni convection. The results indicate the existence of the mutual influence of liquid and gaseous media. Thus, a drop vibrates under the influence of movements in the atmosphere, which generates a density wave in the gas: the drop «sounds». The magnitude of the velocity in a liquid is 50 times less than the characteristic velocity in air. It is found that the evaporation rate does not change in the presence of forced convection flows, which contradicts most of the experimental works. The reason for the discrepancies is supposed to be the appearance of nonequilibrium conditions at the boundary of the condensed phase: under these conditions, the evaporation regime ceases to be diffusional.

Correction to “Contact Angle of Sessile Drops in Lennard-Jones Systems”

Molecular dynamics simulations are used for studying the contact angle of nanoscale sessile drops on a planar solid wall in a system interacting via the truncated and shifted Lennard-Jones potential. The entire range between total wetting and dewetting is investigated by varying the solid–fluid dispersive interaction energy. The temperature is varied between the triple point and the critical temperature. A correlation is obtained for the contact angle in dependence of the temperature and the dispersive interaction energy. Size effects are studied by varying the number of fluid particles at otherwise constant conditions, using up to 150 000 particles. For particle numbers below 10 000, a decrease of the contact angle is found. This is attributed to a dependence of the solid–liquid surface tension on the droplet size. A convergence to a constant contact angle is observed for larger system sizes. The influence of the wall model is studied by varying the density of the wall. The effective solid–fluid dispersive interaction energy at a contact angle of θ = 90° is found to be independent of temperature and to decrease linearly with the solid density. A correlation is developed that describes the contact angle as a function of the dispersive interaction, the temperature, and the solid density. The density profile of the sessile drop and the surrounding vapor phase is described by a correlation combining a sigmoidal function and an oscillation term.

Adhesion of a Thin Soft Matter Layer: The Role of Surface Tension

We consider an adhesive contact between a thin soft layer on a rigid substrate and a rigid cylindrical indenter (“line contact”) taking the surface tension of the layer into account. First, it is shown that the boundary condition for the surface outside the contact area is given by the constant contact angle—as in the case of fluids in contact with solid surfaces. In the approximation of thin layer and under usual assumptions of small indentation and small inclination angles of the surface, the problem is solved analytically. In the case of a non-adhesive contact, surface tension makes the contact stiffer (at the given indentation depth, the contact half-width becomes smaller and the indentation force larger). In the case of adhesive contact, the influence of surface tension seems to be more complicated: For a flat-ended punch, it increases with increasing the surface tension, while for a wedge, it decreases. Thus, the influence of the surface tension on the adhesion force seems to be dependent on the particular geometry of the contacting bodies.

Investigation of Wettability, Drying and Water Condensation on Polyimide (Kapton) Films Treated by Atmospheric Pressure Air Dielectric Barrier Discharge

In this study, we report on the investigation of influence of air atmospheric pressure dielectric barrier discharge on polyimide (Kapton) films. It is shown that plasma treatment causes a significant increase of Kapton wettability that is connected with alterations of its chemical composition (oxidation) induced by dielectric barrier discharge. Observed variations in the wettability of Kapton were also found to be accompanied by changes in the dynamics of water droplets drying on plasma-treated Kapton, namely by the reduction of the constant contact angle phase of the droplet drying. This effect may be ascribed to the higher surface heterogeneity of plasma-treated Kapton that causes pinning of the edges of drying droplet on the Kapton surface. Finally, the differences in wettability induced by the plasma treatment led to a different way, how the water condensates on the Kapton surface: while the condensing water forms large amount of small droplets on untreated Kapton, much bigger water structures were found on the Kapton exposed to atmospheric plasma.

MORPHOLOGICAL FEATURES AND WETTABILITY OF POLYTETRAFLUOROETHYLENE SURFACE TEXTURED BY LASER ABLATION

In this paper, the morphological features of textures with non-uniform wettability created using femtosecond laser ablation of polytetrafluoroethylene substrates have been studied. Covering the surface of polytetrafluoroethylene with microcraters in accordance with a proper design a texture in the form of periodically located microcollets could be created. The period of the location of the columns is the same over the entire surface and is selected in the range from 15 to 100 microns. In the case when the period lies within 30–100 µm, the diameter of the bars is ~ 20 µm. If in the range of 15–20 µm, then this diameter decreases accordingly to ~ 10 µm. Depending on the pulse energy, the height of the pillars could be smoothly changed from 0 to 60 μm. However, to create a superhydrophobic concentrator, textures with the greatest depth were used so that the height of the columns does not limit the stability of the superhydrophobic state by the sagging mechanism. It was established that on the surface of each pillar during the process of laser ablation, a relief with a two-modal roughness in the form of short drop-shaped projections of the material covered with spherical globules is additionally formed. Thus, in one stage of laser micromachining, it is possible to create a surface with a three-modal roughness – microcolumns, drop-shaped projections and spherical globules. The process of droplet evaporation is represented by two main modes of constant contact angle and constant contact diameter, when the latter ceases to decrease and remains constant until the complete evaporation of the drop. As a result, a precipitate of the substance dissolved in a drop is formed on the substrate. It has been established that in the interval 0 <τ <0.9, evaporation occurs in the constant contact angle mode.

Effects of Wettability on Capillary Flow of Non-Newtonian Fluid in Microchannels

In this study, capillary filling of diluted bitumen was evaluated using glass etched microchannel. Glass microchannel was treated using Trichloro(1H,1H,2H,2H-perfluorooctyl) silane that makes the microchannel lyophobic (not favorable for neither hydrophilic nor hydrophobic liquids). Water contact angle, as a degree of hydrophilicity, was changed from 15° for untreated microchannel to 115° for treated microchannel. Measured Capillary filling speed of bitumen in hexane (10% to 60%) was experimentally monitored using white light microscope and compared with Washburn theoretical model. For all samples, a linear relation between square of propagation distance and time was found. However, a deviation between experimental and theoretical values of penetration as a function of time was recorded. Experimental results indicated slower velocity compared to theoretical prediction due to simplifications of the Washburn model. Advancing dynamic contact angle of capillary-driven flow was measured and compared with static contact angle using MATLAB®. It was found that dynamic contact angle was increasing during the penetration in microchannel and application of a constant contact angle leads to higher deviation between experimental and theoretical results.

Well productivity enhancement by applying nanofluids for wettability alteration

Water blocking is a frequent cause for gas productivity decline in unconventional and conventional fields. It is a result of the capillary end effect near the wellbore vicinity. It creates significant formation damage and decreases gas well productivity. The alteration of the rock wettability by nanofluids is an effective way to reduce water blockage and enhance gas production. Presently, several types of surfactants and nanofluids are used in the industry for contact angle alteration. In this study, we developed an analytical model and analysed the sensitivity to several parameters. After the treatment, the porous medium in the well vicinity (or along the core) will have a stepwise constant contact angle profile. We derive analytical models for compressible steady-state two-phase linear and axi-symmetric flows, accounting for the piecewise-constant contact angle and contact-angle-dependent capillary pressure and relative permeability. The modelling reveals a complex interplay between the competing effects of compressibility, viscous and capillary forces, which influence the optimal contact angle for treatment. The optimal contact angle for treatment will depend on the initial wettability of the formation, the water cut and the capillary-viscous ratio.