Anna Trybala

Fluoro- vs hydrocarbon surfactants: why do they differ in wetting performance?

Fluorosurfactants are the most effective compounds to lower the surface tension of aqueous solutions, but their wetting properties as related to low energy hydrocarbon solids are inferior to hydrocarbon trisiloxane surfactants, although the latter demonstrate higher surface tension in aqueous solutions. To explain this inconsistency available data on the adsorption of fluorosurfactants on liquid/vapour, solid/liquid and solid/vapour interfaces are discussed in comparison to those of hydrocarbon surfactants. The low free energy of adsorption of fluorosurfactants on hydrocarbon solid/water interface should be of a substantial importance for their wetting properties.

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 (superspreader) 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 until the contact angle, θ, 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 the next three stages. The next stage after the spreading, which is referred to herein as the first stage, takes place at constant perimeter and ends when θ reaches the static receding contact angle, θr. During the next, second stage, the perimeter decreases at constant contact angle θ = θr for surfactant concentration above the 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. In what follows, we consider only the longest stages I and II. 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.

Current applications of foams formed from mixed surfactant–polymer solutions

Foams cannot be generated without the use of special foaming agents, as pure liquids do not foam. The most common foaming agents are surfactants, however often for foam stability one active agent is not enough, it is necessary to add other component to increase foam lifetime. Foams on everyday use are mostly made from mixture of different components. Properly chosen combinations of two active ingredients lead to a faster foam formation and increased foam stability. During the last decade polymers (mainly polyelectrolytes and proteins) have become frequently used additives to foaming solutions. Mixtures of surfactants and polymers often demonstrate different foaming properties in comparison to surfactant only or polymer only solutions. The nature of surfactant-polymer interactions is complicated and prediction of resulting foaming properties of such formulations is not straightforward. Properties and foaming of surfactant-polymer mixtures are discussed as well as current applications of foams and foaming agents as foams are widely used in cosmetics, pharmaceutics, medicine and the food industry.

Evaporation kinetics of sessile droplets of aqueous suspensions of inorganic nanoparticles.

Evaporation kinetics of sessile droplets of aqueous suspension of inorganic nanoparticles on solid substrates of various wettabilities is investigated from both experimental and theoretical points of view. Experimental results on evaporation of various kinds of inorganic nanosuspensions on solid surfaces of different hydrophobicities/hydrophilicities are compared with our theoretical predictions of diffusion limited evaporation of sessile droplets in the presence of contact angle hysteresis. The theory describes two main stages of evaporation process: (I) evaporation with a constant radius of the droplet base when the contact angle decreases from static advancing contact angle down to static receding contact angle and (II) evaporation with constant contact angle equal to the static receding contact angle when the radius of the droplet base decreases. Theoretically predicted universal dependences for both evaporation stages are compared with experimental data, and a very good agreement is found.

Surfactant-enhanced spreading: Experimental achievements and possible mechanisms.

Surfactants are broadly used to improve wetting properties of aqueous formulations. The improvement is achieved by essential reduction of liquid/air and solid/liquid interfacial tensions resulting in the decrease of contact angle. For moderately hydrophobic substrates, there is a range of surfactants providing complete wetting of substrate. With the decrease of substrate surface energy, this range of surfactants reduces very quickly and only trisiloxane surfactant solutions are capable to wet completely such highly hydrophobic substrates as polypropylene and parafilm. That is why these surfactants are referred to as superspreaders. The most intriguing feature of wetting surfactant solutions is their ability to spread much faster than pure liquids with spread area, S, being proportional to time, t, S~t, as compared to S~t(0.2) for pure liquids, which wet completely the solid substrate. Trisiloxane surfactant solutions spread faster than other aqueous surfactant solutions, which also provide complete wetting, being superspreaders in the sense of spreading rate as well. The mechanism of fast spreading of surfactant solutions on hydrophobic substrates and much higher spreading rates for trisiloxane solutions are to be explained. Below the available experimental data on superspreading and surfactant-enhanced spreading are analysed/summarised, and possible mechanisms governing the fast spreading are discussed.

Kinetics of Wetting and Spreading of Droplets over Various Substrates

There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and spreading of non-Newtonian liquid (blood) over porous substrates. We showed that in spite of the enormous complexity of blood, the spreading over porous substrate can be described using a relatively simple model: a power low-shear-thinning non-Newtonian liquid. (iv) The kinetics of spreading of surfactant solutions. In this part, new results related to various surfactant solution mixtures (synergy and crystallization) are discussed, which shows some possible direction for the future revealing of superspreading phenomena. (v) The kinetics of spreading of surfactant solutions over hair. Fundamental problems to be solved are identified.

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

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.

Removal of micrometer size particles from surfaces using laser-induced thermocapillary flow: Experimental results.

HYPOTHESIS Reducing particle contaminations on solid and delicate surfaces is of great importance in a number of industries. A new non-destructive method is proposed, which is based on the laser-induced thermocapillary effect for the removal of micron size particles from surfaces. The cleaning mechanism is related to the surface-tension-driven flows produced by the laser heating of thin layer of a cleaning liquid deposited onto a surface contaminated with particles. EXPERIMENTS Focusing the laser irradiation into the line laser beam allowed using this method for a large-scale cleaning of surfaces. Hexadecane was used as a cleaning liquid to remove micron-sized polyethylene, Teflon, talc and Al2O3 particles from surfaces of welding glass, carbolite and soft magnetic disc using the line beam of the IR laser. FINDINGS A good cleaning efficiency was achieved for cases of polyethylene and Teflon particles on both the complete wettable welding glass and the low-wettable soft magnetic disc, while in case of oleophilic talc and Al2O3 particles the effectiveness of the cleaning method was lower on all three substrates investigated. The thermal influence of the laser irradiation on substrates used was measured with infrared camera. It was shown that temperature in the irradiated area during the long-time heating increases insignificantly and cannot cause any damage of the substrate.

Electroosmotic flow measurements in a freely suspended liquid film: Experimhents and numerical simulations

Fluid flow profiles in free liquid films stabilised by anionic and cationic surfactants under an external electric field were investigated. Depthwise velocity fields were measured at the mid region of the free liquid film by confocal micron‐resolution particle image velocimetry and corresponding numerical simulations were performed using Finite Element Method to model the system. Depthwise change in velocity profiles was observed with electroosmotic flow dominating in the vicinity of the gas–liquid and solid–liquid interfaces while backpressure drives fluid in the opposite direction at the core of the film. It was also found that the direction of the flow at various sections of the films depends on the type of surfactant used, but flow features remained the same. Numerical simulations predicted the flow profiles with reasonable accuracy; however, asymmetry of the actual film geometry caused deviations at the top half of the computational domain. Overall, electroosmotic flow profiles within a free liquid film are similar to that of the closed‐end solid microchannel. However, the flow direction and features of the velocity profiles can be changed by selecting various types of surfactants. The free liquid films thickness was selected to match dimensions of foam Plateau border. Hence, these findings will be useful in developing a separation system based on foam electrokinetics.