Evaporation of sessile droplets

Recent developments in the studies of evaporation of liquid droplets placed on a solid substrate are reviewed for the droplet size typically larger than 1 μm, so that kinetics effects of evaporation are neglected. The attention is paid to the limits of applicability of classical diffusion model of evaporation, effect of substrate, evaporation of complex fluids and applicability for its description of the theory developed for pure liquids, and hydrothermal waves accompanying evaporation

Kinetics of spreading over porous substrates

The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) is discussed in the case of both complete wetting (silicone oils of different viscosities over nitrocellulose membranes and blood over a filter paper) and partial wetting (aqueous SDS (Sodium dodecyl sulfate) solutions of different concentrations and blood over partially wetted substrates). Filter paper and nitrocellulose membranes of different porosity and different average pore size were used as a model of thin porous layers, sponges, glass and metal filters were used as a model of thick porous substrates. Spreading of both Newtonian and non-Newtonian liquid are considered below. In the case of complete wetting, two spreading regimes were found (i) the fast spreading regime, when imbibition is not important and (ii) the second slow regime when imbibition dominates. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations was derived in the case of complete wetting for both Newtonian and non-Newtonian liquids to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. The deduced system of differential equations does not include any fitting parameter. Experiments were carried out by the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions) and blood over dry filter paper. The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactorily for the experimental data. According to the theory prediction, (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation in the case of complete wetting. These conclusions again are in good agreement with experimental observations in the case of complete wetting for both Newtonian and non-Newtonian liquids. Addition of surfactant to aqueous solutions, as expected, improve spreading over porous substrates and, in some cases, results in switching from partial to complete wetting. It was shown that for the spreading of surfactant solutions on thick porous substrates there is a minimum contact angle after which the droplet rapidly absorbs into the substrate. Unfortunately, a theory of spreading/imbibition over thick porous substrates is still to be developed. However, it was shown that the dimensionless time dependences of both contact angle and spreading radius of the droplet on thick porous material fall on to a universal curve in the case of complete wetting

Surfactant enhanced spreading: Catanionic mixture

The spreading behaviour of aqueous solutions of mixture of two surfactants sodium 1-decane sulfonate and dodecyltrimethylammoniumbromide is investigated on two hydrophobic substrates. The solutions demonstrate rapid complete wetting on polyethylene film and only partial wetting on silanized glass. It is shown that the spreading behaviour depends crucially on the age of the mixture and is determined by the crystal growth affecting the surface tension of solution. An increase of surface tension with time results in an interesting phenomenon — a transition from complete to partial wetting, that is, a droplet of freshly prepared mixture first spreads completely but after some time the solution assembles into the droplet agai

Sessile droplets on deformable substrates

Wetting of deformable substrates has gained significant interest over the past decade due to a multiplicity of industrial and biological applications. Technological advances in the area of interfacial science have given rise to the ability to capture interfacial behavior between a liquid droplet and an elastic substrate. Researchers have developed several theories to explain the interaction between the two phases and describe the process of wetting of deformable/soft substrates. A summary of the most recent advances on static wetting of deformable substrates is given in this review. It is demonstrated that action of surface forces (disjoining/conjoining pressure) near the apparent three-phase contact line should be considered. Any consideration of equilibrium droplets on deformable (as well as on non-deformable) substrates should be based on consideration of the excess free energy of the system. The equilibrium shapes of both droplet and deformable substrate should correspond to the minimum of the excess free energy of the system. It has never been considered in the literature that the obtained equilibrium profiles must satisfy sufficient Jacobi’s condition. If Jacobi’s condition is not satisfied, it is impossible to claim that the obtained solution really corresponds to equilibrium. In recently published studies, equilibrium of droplets on deformable substrates: (1) provided a solution that corresponds to the minimum of the excess free energy; and (2) the obtained solution satisfies the Jacobi’s condition. Based on consideration of disjoining/conjoining pressure acting in the vicinity of the apparent three-phase contact line, the hysteresis of contact angle of sessile droplets on deformable substrates is considered. It is shown that both advancing and receding contact angles decrease as the elasticity of the substrate is increased and the effect of disjoining/conjoining pressure is discussed. Fluid inside the droplet partially wets the deformable substrate. It is shown that just these forces coupled with the surface elasticity determine the deformation of the deformable substrates

Surfactant enhanced spreading: Catanionic mixture

The spreading behaviour of aqueous solutions of mixture of two surfactants sodium 1-decane sulfonate and dodecyltrimethylammoniumbromide is investigated on two hydrophobic substrates. The solutions demonstrate rapid complete wetting on polyethylene film and only partial wetting on silanized glass. It is shown that the spreading behaviour depends crucially on the age of the mixture and is determined by the crystal growth affecting the surface tension of solution. An increase of surface tension with time results in an interesting phenomenon — a transition from complete to partial wetting, that is, a droplet of freshly prepared mixture first spreads completely but after some time the solution assembles into the droplet agai

Influence of haematocrit level on the kinetics of blood spreading on thin porous medium during dried blood spot sampling [poster]

Influence of haematocrit level on the kinetics of blood spreading on thin porous medium during dried blood spot sampling [poster

Determining electroosmotic velocity in a free liquid film

The flow field within a free liquid film under an applied external electric field was measured using confocal micro-PIV system. Free liquid films of thickness ∼ 200 μm were formed in a rectangular frame with electrodes in direct contact with the fluid and stabilised by cationic surfactant. The flow field induced by an external electric field of ∼1600 V/m was visualised using 2 μm tracer particles on several depth wise planes. The observed particle velocities were used to determine the fluid velocities within the film by accounting for the electrophoresis of the tracer particles

Influence of haematocrit level on the kinetics of blood spreading on thin porous medium during dried blood spot sampling

Dried blood spotting (DBS) is a convenient blood collecting and sampling method which is widely applied in newborn screening and blood analysis. At the moment, the practice is to try to keep the blood within a marked circle in a thin porous filter paper. However, it is not always possible to predict exactly how the blood spot spreads inside the filter papers and it depends on many factors including the properties of the filter papers, blood properties and how the blood is deposited on the filter paper. In this paper, we aim to identify the relationships between the physical properties and the spreading behaviour of blood on a typical DBS filter paper (Whatman 903). Pig’s blood was used to mimic the behaviour of human blood and investigate the spreading/imbibition processes of blood drops on the filter paper. Both top and side views were used to analyse the spreading/imbibition behaviour. The experimental data present the haematocrit effect on the spreading dynamics of blood for dried blood spot sampling. The results obtained prove that the spreading/imbibition time dependences of droplet height, droplet base radius and contact angle are universal function of dimensionless time

Mixtures of catanionic surfactants can be superspreaders: Comparison with trisiloxane superspreader

© 2015 Elsevier Inc.Hypothesis: Mixed solutions of cationic and anionic surfactants show considerable synergism in their wetting behaviour, but their spreading is affected considerably by the phase separation processes. The valuable information about wetting properties of synergetic mixtures can be obtained by using mixtures in which phase separation occurs at concentrations above cmc. Experiments: Spreading properties of mixed solutions of cationic and anionic surfactants over highly hydrophobic substrate such as polyethylene are investigated and compared with those for trisiloxane superspreader. Experiments are performed at relative humidity of 40% and 80%. Interfacial tension at water/air and water/alkane interfaces is measured to explain spreading performance. Findings: Catanionic solutions can wet hydrophobic substrates nearly as effective as solutions of trisiloxane superspreader. The spreading factor reaches 70% of that of superspreader for the most effective mixed solution. The spreading slows down earlier at high surfactant concentrations. At room humidity (40%) spread area has a maximum vs concentration. However, the maximum was not observed at higher humidity 80%. Humidity does not affect the short-time spreading rate, but it influences considerably the time when spreading slows down. The spreading rate of mixed solutions is smaller than that of superspreader despite the same spreading exponent α= 0.5

Electroosmotic flow in free liquid films: Understanding flow in foam plateau borders

Liquid flow in foams mostly proceeds through Plateau borders where liquid content is the highest. A sufficiently thick (~180 µm) free liquid film is a reasonable model for understanding of electrokinetic phenomena in foam Plateau borders. For this purpose, a flow cell with a suspended free liquid film has been designed for measurement of electrokinetic flow under an imposed electric potential difference. The free liquid film was stabilised by either anionic (sodium lauryl sulfate (NaDS)) or cationic (trimethyl(tetradecyl) ammonium bromide (TTAB)) surfactants. Fluid flow profiles in a stabilised free liquid film were measured by micron-resolution particle image velocimetry (µ-PIV) combined with a confocal laser scanning microscopy (CLSM) setup. Numerical simulations of electroosmotic flow in the same system were performed using the Finite Element Method. The computational geometry was generated by CLSM. A reasonably good agreement was found between the computed and experimentally measured velocity profiles. The features of the flow profiles and the velocity magnitude were mainly determined by the type of surfactant used. Irrespective of the surfactants used, electroosmotic flow dominated in the midfilm region, where the film is thinnest, while backflow due to pressure build-up developed near the glass rods, where the film is thickest