Surfactant induced flows in thin liquid films : an experimental study

Abstract

The topic of the experimental work summarized in my thesis is the flow in thin liquid films induced by non-uniformly distributed surfactants. The flow dynamics as a consequence of the deposition of a droplet of an insoluble surfactant onto a thin liquid film covering a solid substrate where discussed as a starting point in Chapter 2. A strong focus in this context was on the effect of the conditions in the vicinity of the surfactant source. It was shown, by application of interference and fluorescence microscopy, that the radially outwards directed displacement of the subphase, induced by the surfactant induced Marangoni stresses, is strongly influenced by the conditions near the surfactant source, i.e. the supply of surfactant from the source as the spreading proceeds. A novel oscillatory contact line instability of the surfactant droplet was described that modulates the flow rate of liquid from under the droplet. Considering conditions relevant to surfactant spreading in an oil reservoir in Chapter 3 the influence of a chemically imposed confinement on the sub-phase, along the surface of which a surfactant spreads, was investigated. A pronounced transition in the morphology evolution of the flowing thin film was found to be induced by the spatial restriction imposed through chemical surface patterns. The experimental results are in excellent agreement with numerical simulations reported by Myroslava Hanyak. Considering conditions in an oil reservoir, results for the spreading of surfactants along liquid-air interfaces can only give a first order approximation. The studies of Chapter 3 were therefore extended to the interface between two thin liquid films in Chapter 4. Here the spreading of a surfactant, soluble in one phase, is studied along the liquid-liquid interface of thin films. Resembling reservoir conditions, the films were subject to both, physical confinement as well as confinement imposed by a wettability pattern. In the context of surfactant spreading, it is a conceptually entirely new discovery, that surfactant induced Marangoni flows cannot only transport surfactants along fluid interfaces but can also efficiently transport surfactants along interfaces exhibiting considerably sized discontinuities. All existing literature in the field of surfactant spreading exclusively regarded continuous fluid interfaces. This novel phenomenon was the topic of Chapter 5. The convective surfactant spreading along discontinuous interfaces is directly relevant to the spreading of surfactants in an oil reservoir. In these porous underground rock formations the oil-water interface is not necessarily connected, such that surfactant spreading through a reservoir involves transport over interface discontinuities. Besides the spreading of surfactants, I also studied the self-propulsion of surfactant droplets. The results of my experimental studies were presented in Chapter 6 and 7. Self-propulsion dynamics exhibited by insoluble surfactant droplets on thin liquid films are systematically investigated in Chapter 6. Several modes of motion were described from directed continuous propulsion over a meandering mode of propulsion, that can also be exhibited by a pair of droplets in a synchronized fashion, to an intermittent form of propagation. The systematic study of the various modes of propulsion is complemented with the outline of a potential application in microfluidic devices in Chapter 7. In this context I am describing the novel phenomenon of transporting solid cargo particles using these self-propelling droplets which can be routed across micro-fluidic networks by controlling the temperature field around the drop e.g. using an infrared laser. The independence from external power sources, integrated electrodes or heating elements to propel the droplets, makes the concept specifically interesting for applications in inexpensive, single-use-type devices. In this thesis surfactant induced flows are studied in a wide range of system configurations. Confinement effects on the spreading dynamics are investigated systematically. These studies are complemented by the presentation of novel phenomena such as the Marangoni driven convective transport of surfactants along discontinuous interfaces.