The spreading of surfactant solutions on thin liquid films

Abstract

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.

Introduction

This review paper was inspired by the meeting held at Imperial College to recognise the sixty-fifth birthday of Tharwat Tadros. One of us, (PFL) has had the pleasure of collaborating with Tharwat Tadros for the last twenty five years. We have had great fun and I like to think made some contribution to the understanding of the rheology of colloidal systems. One of the main influences of Tharwat Tadros on my (PFL) scientific career was to be willing to move into new scientific areas. It is good to do this with support of course and so when approached by OKM to become involved in a collaboration investigating the spreading of surfactant solutions over this water films, this seemed a good opportunity to find out something new. PFL thought this would prove to be a simple problem, however, this is far from the case as this review and some preliminary observations of ours will reveal.

The presence of surface tension gradients across a thin liquid film of uniform height induces shear stresses at the air–liquid interface. These stresses distribute the liquid from areas of low surface tension to areas of high surface tension and in doing so also deform the interface resulting in height variations. This is termed Marongoni flow and can be generated by the presence of surface active material on a liquid film or by temperature gradients along a liquid film; the latter is referred to as thermo-capillary flow [1].

Marongoni flows can be found in many diverse settings and the ensuing deformation of the air–liquid interface may not always be desirable. Paint films drying by solvent evaporation, for example, are subject to Marongoni stresses when one component evaporates non-uniformly causing a local change in surface tension. These stresses cause deformation in the film leading to permanent defects on the paint surface [2], [3]. Turbine blades, drill bits and car components are thermally sprayed with metallic or ceramic films to prevent wear, abrasion and oxidation. This involves molten material being sprayed onto the substrate and may generate thermally induced Marongoni stresses, which in turn results in a non-uniform coating. This leads to some areas being more susceptible to corrosion than others [4]. Marongoni drying, however, relies on Marongoni stresses, created by drawing alcohol vapour across the surface of a wet substrate to dewet, or expel liquid from, the area of contact. This is an effective means of drying integrated circuits and liquid crystal displays [5], [6].

The scope of this paper is limited to the study of surfactant-induced Marongoni spreading only. Such flows can be found in coating processes and physiological applications. For instance, anti-icing films are sprayed onto the bodies of grounded aircraft to prevent ice-formation. They consist of water; ethylene or propylene glycol, which acts as a freezing point depressant; and a soluble surfactant, which enhances the fluid's spreading capability. As the fluid spreads and coats the aircraft it eradicates any existing ice while preventing further accumulation [7]. Also, surfactants are normally present in a healthy mammalian lung to reduce surface tension forces. This keeps the lungs compliant and prevents collapse of the small airways during exhalation. However, the lungs of prematurely born babies are insufficiently developed to produce adequate quantities of surfactant. This leads to the development of respiratory distress syndrome (RDS) which can be life threatening. The treatment for this condition is surfactant replacement therapy (SRT), which involves instilling surfactant (synthetic or animal derived) into the deficient lung. The surfactant spreads by gravitational forces in the large to medium sized pulmonary airways. When gravity becomes negligible in the small airways, surface tension gradients dominate and Marongoni flow distributes the surfactant to the distal regions of the lung [8]. The success of both these processes is dependant on the uniform and complete coverage of the substrate with surfactant.

The disturbances in film height that result from Marongoni stresses may give rise to instabilities, which hinder the spreading process. In the case of complete wetting, ‘fingers’ of surfactant-coated liquid may become apparent in the region thinned by Marongoni forces [9], [10]. Several groups have conducted experiments using surfactants on thin films in different geometrical configurations to ascertain the validity of this phenomenon [11], [12], [13], [14]. However, the origin of the fingering instability is yet to be determined. In addition, Marongoni stresses may thin the surfactant coated film to the extent that long-range intermolecular forces become significant enough to cause film rupture thus halting the spreading of surfactant [15], [16], [17], [18]. In the partial wetting case, Marongoni induced film rupture results in the formation of a dry patch that grows, expelling fluid from its path and compromising the uniformity of the liquid film [19]. As well as so called ‘dewetting’, surfactant solutions may also exhibit ‘autophobing’ and ‘stick-jump motion’ [20], [21].

In this paper these effects are reviewed together with some of our preliminary observations of ours concerning the spreading of sodium dodecyl sulfate solutions at various concentrations onto films of various thicknesses.