Surface forces and wetting features in drops and capillaries

Using the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory, which accounts for quantum mechanics and electrostatics at the macroscopic level, the thermodynamic expressions for (thermodynamic) equilibrium contact angles of drops on solid substrates and menisci in solid wall capillaries are, operationally and unambiguously, expressed in terms of the corresponding Derjaguin pressure. The latter’s S-shape is responsible for microdrops and other phenomena appearing on flat solid substrates

Simultaneous spreading and evaporation: recent developments

The recent progress in theoretical and experimental studies of simultaneous spreading and evaporation of liquid droplets on solid substrates is discussed for pure liquids including nanodroplets, nanosuspensions of inorganic particles (nanofluids) and surfactant solutions. Evaporation of both complete wetting and partial wetting liquids into a nonsaturated vapour atmosphere are considered. However, the main attention is paid to the case of partial wetting when the hysteresis of static contact angle takes place. In the case of complete wetting the spreading/evaporation process proceeds in two stages. A theory was suggested for this case and a good agreement with available experimental data was achieved. In the case of partial wetting the spreading/evaporation of a sessile droplet of pure liquid goes through four subsequent stages: (i) the initial stage, spreading, is relatively short (1-2 min) and therefore evaporation can be neglected during this stage; during the initial stage the contact angle reaches the value of advancing contact angle and the radius of the droplet base reaches its maximum value, (ii) the first stage of evaporation is characterised by the constant value of the radius of the droplet base; the value of the contact angle during the first stage decreases from static advancing to static receding contact angle; (iii) during the second stage of evaporation the contact angle remains constant and equal to its receding value, while the radius of the droplet base decreases; and (iv) at the third stage of evaporation both the contact angle and the radius of the droplet base decrease until the drop completely disappears. It has been shown theoretically and confirmed experimentally that during the first and second stages of evaporation the volume of droplet to power 2/3 decreases linearly with time. The universal dependence of the contact angle during the first stage and of the radius of the droplet base during the second stage on the reduced time has been derived theoretically and confirmed experimentally. The theory developed for pure liquids is applicable also to nanofluids, where a good agreement with the available experimental data has been found. However, in the case of evaporation of surfactant solutions the process deviates from the theoretical predictions for pure liquids at concentration below critical wetting concentration and is in agreement with the theoretical predictions at concentrations above it

Simultaneous spreading and evaporation: recent developments

The recent progress in theoretical and experimental studies of simultaneous spreading and evaporation of liquid droplets on solid substrates is discussed for pure liquids including nanodroplets, nanosuspensions of inorganic particles (nanofluids) and surfactant solutions. Evaporation of both complete wetting and partial wetting liquids into a nonsaturated vapour atmosphere are considered. However, the main attention is paid to the case of partial wetting when the hysteresis of static contact angle takes place. In the case of complete wetting the spreading/evaporation process proceeds in two stages. A theory was suggested for this case and a good agreement with available experimental data was achieved. In the case of partial wetting the spreading/evaporation of a sessile droplet of pure liquid goes through four subsequent stages: (i) the initial stage, spreading, is relatively short (1-2 min) and therefore evaporation can be neglected during this stage; during the initial stage the contact angle reaches the value of advancing contact angle and the radius of the droplet base reaches its maximum value, (ii) the first stage of evaporation is characterised by the constant value of the radius of the droplet base; the value of the contact angle during the first stage decreases from static advancing to static receding contact angle; (iii) during the second stage of evaporation the contact angle remains constant and equal to its receding value, while the radius of the droplet base decreases; and (iv) at the third stage of evaporation both the contact angle and the radius of the droplet base decrease until the drop completely disappears. It has been shown theoretically and confirmed experimentally that during the first and second stages of evaporation the volume of droplet to power 2/3 decreases linearly with time. The universal dependence of the contact angle during the first stage and of the radius of the droplet base during the second stage on the reduced time has been derived theoretically and confirmed experimentally. The theory developed for pure liquids is applicable also to nanofluids, where a good agreement with the available experimental data has been found. However, in the case of evaporation of surfactant solutions the process deviates from the theoretical predictions for pure liquids at concentration below critical wetting concentration and is in agreement with the theoretical predictions at concentrations above it

Evaporation of droplets of surfactant solutions

The simultaneous spreading and evaporation of droplets of aqueous trisiloxane (super-spreader) solutions onto a hydrophobic substrate has been studied both experimentally, using a video-microscopy technique, and theoretically. The experiments have been carried out over a wide range of surfactant concentration, temperature and relative humidity. Similar to pure liquids, four different stages have been observed: the initial one corresponds to spreading till the contact angle, , reaches the value of the static advancing contact angle, θad. Duration of this stage is rather short and the evaporation during this stage can be neglected. The evaporation is essential during next three stages. The next stage after the spreading, which is referred to below as the first stage, takes place at constant perimeter and ends when reaches the static receding contact angle, θr. During the next, second stage, the perimeter decreases at constant contact angle =θr for surfactant concentration above critical wetting concentration (CWC). The static receding contact angle decreases during the second stage for concentrations below CWC because the concentration increases due to the evaporation. During the final stage both the perimeter and the contact angle decrease till the drop disappears. Below we consider only the longest stages one and two. The developed theory predicts universal curves for the contact angle dependency on time during the first stage, and for the droplet perimeter on time during the second stage. A very good agreement between theory and experimental data has been found for the first stage of evaporation, and for the second stage for concentrations above CWC, however, some deviations were found for concentrations below CWC

Humectacion: conceptos y cuestiones basicas [Wetting: concepts and basic features]

Resumen Cuando se coloca una gota sobre una superficie sólida plana y lisa puede que no moje o que se esparza mojando parcial o totalmente al sólido, cuya caracterización se da mediante el correspondiente valor de un ángulo de conjunción, en equilibrio termodinámico (y consecuentemente mecánico). Introduciendo fuerzas superficiales se calcula este ángulo en función de la presión (isoterma) de Derjaguin, minimizando la adecuada energía libre de exceso de Gibbs. La clásica ecuación de Young para el ángulo de conjunción es recordada y su validez conceptual delimitada. Se discute también la posible histéresis que da lugar a un ángulo de conjunción de avance y otro de retroceso aun en el caso de una superficie sólida homogénea, debido a la peculiar forma de la presión (isoterma) de Derjaguin. Abstract For a small sessile liquid drop on a plane, smooth solid substrate three possibilities exist: complete wetting, partial wetting and non wetting. They are characterized by their corresponding (thermodynamic and hence mechanical) equilibrium contact angle. The latter is given here as a function of Derjaguin’s (disjoining or conjoining) pressure (isotherm) using the surface forces, and minimizing the appropriate excess Gibbs free energy. Young’s classic contact angle equation is recalled and its conceptual validity is assessed. Contact angle (advancing and receding) hysteresis is also discussed pointing out its appearance even for homogeneous solid surfaces

Surface forces and wetting features in drops and capillaries

Using the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory, which accounts for quantum mechanics and electrostatics at the macroscopic level, the thermodynamic expressions for (thermodynamic) equilibrium contact angles of drops on solid substrates and menisci in solid wall capillaries are, operationally and unambiguously, expressed in terms of the corresponding Derjaguin pressure. The latter’s S-shape is responsible for microdrops and other phenomena appearing on flat solid substrates

Surface forces and wetting phenomena

Conditions for thermodynamic equilibrium of liquid drops on solid substrates are presented. It is shown that if surface force (disjoining/conjoining Derjaguin pressure) action in a vicinity of the three-phase contact line is taken into account the condition of thermodynamic equilibrium is duly satisfied. Then the thermodynamic expressions for equilibrium contact angles of drops on solid substrates and menisci in thin capillaries are expressed in terms of the corresponding Derjaguin isotherm. It is shown that equilibrium contact angles of drops vary significantly depending on the vapour pressure in the ambient atmosphere, while there is a single, unique equilibrium contact angle in thin capillaries. It is also shown that the static advancing contact angle of a drop depends on its volume, in agreement with experimental data. In the case of menisci in capillaries, the expression for the receding contact angle is deduced, with results that are also in agreement with known experimental data

Computer simulations of quasi-steady evaporation of sessile droplets

Computer simulations of quasi-steady evaporation of sessile droplet

Evaporation of pinned sessile microdroplets of water: Computer simulations

The aim of present work is to describe results of computer simulations, which show the influence of kinetic effects on evaporation of pinned sessile submicron droplets of water. The suggested model takes into account both diffusive and kinetic regimes of evaporation. The obtained results show a smooth transition between kinetic and diffusive regimes of evaporation as the droplet size decreases from millimetre to micrometer size

Droplets evaporation: problems and solutions

The evaporation of single droplets and sprays into gaseous atmosphere and the evaporation of sessile liquid droplets on solid substrates are here considered. We argue that if thermodynamics is augmented with Derjaguin’s (disjoining/conjoining) pressure to handle phenomena in a vicinity of the three-phase contact line, problems like the singularity of the evaporation flux and of the viscous stress at the three-phase contact line of a sessile droplet are ruled out