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 x rim $ t a . 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.
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∼t(α). We determine the time evolution of the liquid film thickness and the corresponding spreading exponents α 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.
Due to its flexibility, inkjet printing has become a widespread technique for the non-contact deposition of liquids, solutions and melts on a variety of substrates with a lateral resolution down to about 10 μm. Because the patterns are formed via coalescence of many individual droplets, ripples and undulations can appear in the deposited layers, which gradually disappear if sufficient time before ink solidification is given. In this manuscript, we study this spontaneous leveling process of inkjet printed lines that is driven by surface tension and hydrostatic pressure gradients. We show that the process can be significantly retarded if the ink contains soluble or insoluble surfactants, which are common additives to improve print quality. We present qualitative experiments as well as theoretical and numerical models that allow estimation of the leveling time for arbitrary ripple amplitudes and realistic surfactant properties.
While the Marangoni-stress-driven spreading of surfactants along continuous fluid interfaces is a well-studied problem, we demonstrate experimentally that swift and efficient surfactant transport can also occur along discontinuous interfaces. We used chemical surface patterning to create arrays of discrete drops and liquid bridges immersed inside a second immiscible liquid. Surface-active compounds introduced at one end of the linear array are transported along the array via surfactant-induced interfacial convection at a rate by far exceeding diffusion. We believe this mechanism to be relevant to the application of surfactants in enhanced oil recovery, where oil-water interfaces are likely to be discontinuous. Marangoni flows can provide access to dead-end pores and low-permeability regions that are otherwise bypassed by conventional pressure-driven waterfloods.
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