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One of the most important fluid dynamics challenges is to understand and model the flow behavior of liquids under complex boundary conditions. How fluids behave in the vicinity of a solid or another fluid surface can significantly influence their flow profile. In particular, spreading of liquids on different substrate can be greatly affected by boundary conditions and the interactions of the fluid with the neighbouring surfaces. In general, the parameter space in the spreading problem is very broad and in each case based on the boundary conditions and the relevant time scale of the system a particular set of parameters should be taken into account. Many of these parameters are coupled and cannot be addressed separately.
In this thesis, we employ a unique configuration of “a droplet on a soap film” to study the spreading behaviour of complex liquids. In this set-up since the droplet is suspended in the air and the contact area between the droplet and the soap film is very limited, the spreading behaviour of liquids can be studied in the absence of a solid or deep liquid boundary. We started our journey with this ambition to understand and model the governing dynamics of spreading of liquids under this boundary condition and even calibrate this set-up as a method to study the spreading behaviour for thin film formation of complex liquids. With 4 experimental chapters presented in this thesis, we succeeded to understand and model the role of some of the major governing parameters.
In Chapter 2 we studied how the surface tension gradient between the droplet and soap film governs the spreading. Additionally, we investigated the effect of droplet volume and the size of the soap film. In Chapter 3 we took a close look at the effect of surfactant-surfactant interactions on the spreading behavior of droplets on the soap film. In the next step, we studied the spreading behavior of more complex liquids consisting a mixture of polymer and surfactants in Chapter 4. For low concentrations of polymer, we observed a viscous spreading regime. However, We observed that when the concentration of polymer in the droplet surpasses the entanglement concentration of the polymers, the spreading front was no longer stable. We showed the enhanced elastic response of the polymeric network above the entanglement concentration triggers the instabilities. In Chapter 5, we thoroughly investigated the rheological properties of polymer solutions in the presence of various surfactants. In the last chapter, we provided an over-view of all the influential parameters on the spreading of a droplet on a soap film that we discussed earlier in this thesis.
|Doctor of Philosophy
|4 Jul 2022
|Place of Publication
|Published - 4 Jul 2022