Surfactant induced flows in thin liquid films : an experimental study

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

Self-propelling surfactant droplets in chemically-confined microfluidics – cargo transport, drop-splitting and trajectory control

We demonstrate the applicability of self-propulsion as a passive driving mechanism for droplets in chemically-confined microfluidics. The droplets can be used to transport considerably sized solid cargo particles. We implemented thermal actuation as a steering mechanism for the droplets at fluidic junctions

Soluble surfactant spreading on spatially confined thin liquid films

We studied the spreading of soluble surfactants on spatially confined thin liquid films by means of comprehensive experiments and numerical simulations. We determined the time evolution of the liquid film thickness both from interference microscopy measurements and finite element calculations. A characteristic rim develops ahead of the spreading surfactant front. Within certain time intervals, the rim position can be well represented by a power-law relation xrim z ta. The corresponding spreading exponent a depends on the method of surfactant deposition and the numerical values deduced from experiments and simulations quantitatively agree. Depth-resolved simulations that account for domain deformability using the Arbitrary Lagrangian–Eulerian method show that shear-induced concentration non-uniformities across the rim film thickness tend to reduce the rim height. Fingering instabilities that are frequently observed in experiments were qualitatively reproduced in the simulations

Immiscible surfactant droplets on thin liquid films : spreading dynamics, subphase expulsion and oscillatory instabilities

After deposition of immiscible, surface-active liquids on thin liquid films of higher surface tension, Marangoni stresses thin the liquid film around the surfactant droplet and induce a radially outward flow. We observed an oscillatory instability, caused by temporary trapping and subsequent release of subphase liquid from underneath the surfactant droplet. Height profiles of the thin liquid films were monitored using optical interferometry and fluorescence microscopy, both in the vicinity of the deposited surfactant droplet and at larger distances. Numerical calculations based on the lubrication approximation are compared to the experimental results. Good agreement between the experimental and calculated far-field dynamics and values of the spreading exponents was found. (C) 2011 Elsevier Inc. All rights reserved

Soluble surfactant spreading on spatially confined thin liquid films

We studied the spreading of soluble surfactants on spatially confined thin liquid films by means of comprehensive experiments and numerical simulations. We determined the time evolution of the liquid film thickness both from interference microscopy measurements and finite element calculations. A characteristic rim develops ahead of the spreading surfactant front. Within certain time intervals, the rim position can be well represented by a power-law relation xrim z ta. The corresponding spreading exponent a depends on the method of surfactant deposition and the numerical values deduced from experiments and simulations quantitatively agree. Depth-resolved simulations that account for domain deformability using the Arbitrary Lagrangian–Eulerian method show that shear-induced concentration non-uniformities across the rim film thickness tend to reduce the rim height. Fingering instabilities that are frequently observed in experiments were qualitatively reproduced in the simulations

Insoluble surfactant spreading along thin liquid films confined by chemical surface patters

We conducted a combined experimental and numerical study of the spreading of insoluble surfactants on spatially confined thin liquid films. We found that the spreading dynamics can locally be represented by a power-law relation x B ta. We determine the time evolution of the liquid film thickness and the corresponding spreading exponents a both from experiments using interference microscopy and numerical finite element simulations. The lateral confinement induces non-uniform height- and surface velocity profiles, which manifest themselves in a pronounced transition of the evolving rivulet morphology. Excellent agreement between experimental and simulation results has been achieved

Self-propulsion of surfactant droplets on inclined liquid films

Other presentation title : Mobilization and self-propulsion of surfactant droplets on thin liquid films. We present a systematic investigation of the self-propulsion of insoluble surfactant droplets on spatially confined thin liquid films, deposited on chemically patterned substrates. We studied the dependence of the propelled distance of the droplets on the initial height of the deposited film as well as the initial spreading pressure of the surfactant. We observed an emulsification process of subphase liquid inside the surfactant droplets which is coupled to their self-propelling motion. Using the thin-film approximation, a numerical model is developed for an insoluble surfactant drop moving at constant velocity at the liquid-air interface, the predictions of which compare favorably with the experimental data

Surfactant spreading on thin liquid films on chemically patterned surfaces

The injection of surfactant solutions is a promising technique for enhanced oil recovery from sub-surface reservoirs. Towards a quantitative understanding of the fluid-mechanical aspects of the process, we conducted a combined experimental and numerical study of the spreading dynamics of soluble and insoluble surfactants on confined thin liquid films. To good approximation the surfactant front propagates according to a power-law relation [formula]. We determined the spreading exponents alfa for different film thicknesses and initial conditions and found a favorable agreement between experiments and simulations

Self-propulsion of surfactant droplets on inclined liquid films

Other presentation title : Mobilization and self-propulsion of surfactant droplets on thin liquid films. We present a systematic investigation of the self-propulsion of insoluble surfactant droplets on spatially confined thin liquid films, deposited on chemically patterned substrates. We studied the dependence of the propelled distance of the droplets on the initial height of the deposited film as well as the initial spreading pressure of the surfactant. We observed an emulsification process of subphase liquid inside the surfactant droplets which is coupled to their self-propelling motion. Using the thin-film approximation, a numerical model is developed for an insoluble surfactant drop moving at constant velocity at the liquid-air interface, the predictions of which compare favorably with the experimental data

Sliding droplets of Xanthan solutions: A joint experimental and numerical study

We have investigated the sliding of droplets made of solutions of Xanthan, a stiff rodlike polysaccharide exhibiting a non-Newtonian behavior, notably characterized by a shear thinning viscosity accompanied by the emergence of normal stress difference as the polymer concentration is increased. These experimental results are quantitatively compared with those of Newtonian fluids (water). The impact of the non-Newtonian behavior on the sliding process was shown through the relation between the average dimensionless velocity (i.e. the capillary number) and the dimensionless volume forces (i.e. the Bond number). To this aim, it is needed to define operative strategies to compute the capillary number for the shear thinning fluids and compare with the corresponding Newtonian case. The resulting capillary number for the Xanthan solutions scales linearly with the Bond number at small inclinations, as well known for Newtonian fluids, while it shows a plateau as the Bond number is increased. Experimental data were complemented with lattice Boltzmann numerical simulations of sliding droplets, aimed to disentangle the specific contribution of shear thinning and elastic effects on the sliding behavior. In particular the deviation from the linear (Newtonian) trend is more likely attributed to the emergence of normal stresses inside the non-Newtonian droplet