Spreading Of Surfactant Solutions Research Articles

Spreading of Micrometer-Sized Droplets under the Influence of Insoluble and Soluble Surfactants: A Numerical Study

Wetting and spreading of surfactant solutions play an important role in many technical applications. In printing processes, the size of individual droplets is typically on the order of a few tens of microns. The purpose of this study is to develop a better understanding of the interaction between spreading and surfactant transport on these small length and related time scales. Therefore, numerical simulations based on the volume-of-fluid method including Marangoni stresses and transport of an insoluble or soluble surfactant are performed. The results for an insoluble surfactant show competing effects of Marangoni flow on the one hand, and a decreasing surfactant concentration as the droplet spreads on the other hand. Even in the case of a soluble surfactant, adsorption and desorption could only partly mitigate these effects, demonstrating the importance of the sorption kinetics for fast, small scale wetting processes.

Proteins as Surfactant Enhancers for Environmental and Industrial Applications

Data are presented on the effect of small proteins on the surface activity of synthetic surfactants and their blends as assessed by standard surface science methods. Surface tension of surfactant solutions, interfacial tension between those solutions and water insoluble substrates, and contact angle between these solutions and a solid surface were determined. It is shown that both small individual proteins, such as lysozyme and β-lactoglobulin, as well as proteins in a yeast extract, significantly and predictively reduce critical micelle concentration of multiple widely used surfactants: ionic, non-ionic and amphoteric. Typically, lower minimal interfacial tension could be achieved with a protein synergist, than with the same surfactant in the absence of protein. Examples of industrially important applications of a protein synergist are described. In one of them, the protein accelerates wetting and spreading of surfactant solutions on the hydrophobic surface of green leaves, with concomitant enhancement of the solution uptake by the leaf. In another example, the activity of bacterial lipase, an important industrial enzyme, towards a segregated oil substrate is enhanced by a surfactant-protein synergist blend, as compared to the same surfactant in the absence of a protein synergist

Influence of Drop Volume and Surfactant Concentration on the Marangoni Flow and Contact Line Instabilities during the Spreading of Surfactant Solutions on Hydrophilic Surface

The dynamics of drop spreading of glycerol-water mixtures in the presence of a nonionic surfactant on hydrophilic glass surfaces has been investigated. The influence of drop volume and alkyl (8)-(16) glucoside (Plantacare 2000) surfactant concentration on the number and the nature of the spreading regimes is systematically investigated. For each regime, the radius can be expressed by a power law of the form R˭Ktn. The number and the nature of the regimes are strongly affected by the drop volume. An analysis of contact line instabilities is presented. The occurrence of instabilities and the characteristics of the instability pattern are controlled by the drop volume and the surfactant concentration. For drop volumes less than or equal to 15 µL, no contact line instability is evident. A succession of two different regimes occurs in the following order: a molecular-kinetic regime followed by a hydrodynamic regime regardless of the surfactant concentration. For drop volumes larger than 15 µL, the spreading process goes through a combination of three different regimes. The succession of a hydrodynamic regime, followed by a Marangoni regime and lastly a gravity regime is demonstrated. In the second regime, the spreading is controlled by the Marangoni effect. Instabilities develop at the contact line for surfactant concentrations higher than the critical micelle concentration.

On the effect of pH on spreading of surfactant solutions on hydrophobic surfaces

Surfactants are invaluable in a number of agricultural applications in products such as pesticides and herbicides. In these products, surfactants are very often used in conjunction with acidifiers in order to improve their half-life. In this paper, we investigate how the change in pH affects surfactant wetting and spreading. We compare the performance of a conventional surfactant, Triton ® X-100, with that of a trisiloxane superspreader, Silwet ® L-77, on a number of polymer coated surfaces exhibiting various degrees of hydrophobicity. Silwet ® L-77 in water based solutions showed very good wetting capability on all surfaces. However, its wetting ability was drastically reduced with the addition of acetic acid. On the other hand, Triton ® X-100 was not affected by the addition of acid and exhibited the same spreading behaviour as in water-based solutions.

Spreading of surfactant solutions over thin aqueous layers at low concentrations: Influence of solubility

A small droplet of aqueous surfactant solution at concentration below CMC was deposited on a thin water layer. A moving circular wave in the centre was formed. The time evolution of the radius of the wave was monitored. Two surfactants of different solubility were used. It was shown that the time evolution of the moving front (i) proceeds in two stages: a fast first stage and slower second stage; (ii) the time evolution of the front motion substantially depends on the surfactant solubility. We suggest a qualitative explanation of the phenomenon, which reasonably agrees with our experimental observations.

Fingering phenomena during spreading of surfactant solutions

Fingering instabilities are observed at the contact line of drops of surfactant solutions spreading spontaneously on solid surfaces coated by a film of solvent. The occurrences of instabilities, and the characteristics of the instability pattern, are controlled by the surfactant concentration and the thickness of the film adsorbed or deposited on the substrate. This work provides experimental data as a basis for forthcoming theoretical analyses.

The spreading of surfactant solutions on thin liquid films

The spreading of a surfactant solution on a water film at first glance seems a trivial problem. However, in the last 30 years or so this has been shown to be anything like the case. There have been numerous studies which show that Marongoni driven fingering of the spreading surfactant front exists. In this paper this work has been reviewed and an attempt has been made to rationalise the results. The paper concludes with some recent observations of ours concerning the spreading of sodium dodecyl sulfate over relatively thick water films, 200 μm or less.

Unstable Spreading of Aqueous Anionic Surfactant Solutions on Liquid Films. 2. Highly Soluble Surfactant

The spreading of a surfactant solution across a thin water film may be accompanied by a fingering instability developing behind the spreading front. In this paper, the role of solubility on this instability is investigated by conducting spreading experiments using highly soluble surfactant solutions of sodium dodecyl sulfate over a range of concentrations on water films ranging from 25 to 100 μm in thickness. It is found that at surfactant concentrations up to and around the critical micelle concentration, spreading is largely accompanied by fingers upstream of the spreading front. In comparison with sparingly soluble AOT solutions (sodium di-2-ethylhexyl sulfosuccinate) studied in part 1 of this series, the instability becomes apparent sooner and the fingers are more pronounced and branched. Above the cmc, the instability takes a different form on thinner films, which was not noted in the sparingly soluble case, while fingering develops on thicker films.

Spreading of Surfactant Solutions over Hydrophobic Substrates

The spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid–vapor interfacial tension and hydrophilize the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by our experimental observations.