Wetting phenomena and foam drainage on complex substrates
thesisposted on 25.11.2020, 15:27 authored by Nektaria Koursari
The interaction of foam and droplets with multiple substrates is investigated. This research is divided into the following areas: (1) equilibrium of droplets on deformable substrates—equilibrium conditions; (2) hysteresis of contact angle of sessile droplets on deformable substrates—influence of disjoining pressure; (3) foam drainage on thin porous layer; (4) drying of foam under microgravity conditions; and (5) modelling of foamed emulsion drainage. Equilibrium conditions of droplets on deformable substrates are investigated and it is shown for the first time that for equilibrium the Jacobi’s condition must be satisfied. It is shown that the deduced solution for both droplet and deformed substrate the Jacobi’s necessary equilibrium condition is satisfied. It is shown that the deduced profiles of the equilibrium droplet and deformable substrate satisfy the Jacobi’s condition and provide the minimum to the excess free energy of the system. A theory of contact angle hysteresis of sessile droplets on deformable substrates is developed in terms of the disjoining pressure isotherm. It is shown that calculated values of both advancing and receding contact angles for droplets on deformable substrate depend on droplet volume and decrease with increasing substrate elasticity. A theory of foam drainage placed on thin porous layer is developed by considering mobile upper foam surfaces and taking into account the presence of surface viscosity. The rates of drainage and imbibition into porous layer are predicted. Conditions and duration of free liquid layer formation on the foam-porous layer interface have been theoretically predicted. The theoretical predictions are compared with experimental observations and comparison showed a good agreement. The effect of model parameters on the kinetics of the drainage/imbibition process and the existence of three different imbibition regimes are evaluated and discussed. A new method of drying foams under microgravity conditions is suggested for the first time. It is known that gravity affects the foam formation, its evolution and stability by causing flow of liquid from higher to lower parts of the foam (drainage). However, under microgravity conditions the drainage is impossible because only capillary forces are involved. That is, drying of foams is impossible under microgravity conditions. A new method is suggested for drying of foams under microgravity conditions. According to the suggested method of foam drying the foam is placed on porous layer which will result in liquid absorbance from the foam driven by capillary suction. Model predictions are compared with experimentally obtained data with reasonable agreement. The drainage of foamed emulsions (also considered as complex multiphase systems) has been investigated both experimentally and theoretically. Foamed emulsions have been prepared using mixture of sodium dodecyl sulphate and oil, using the double syringe method. A theoretical model is developed, taking into account both surface viscosity and non-Newtonian behaviour of the foamed emulsion describing the kinetics of the drainage process. Theoretical predictions of rate of drainage, foam height and liquid volume fraction for foamed emulsion systems of various oil volume fractions are compared with experimental observations and comparison shows a reasonable agreement.
MAP EVAPORATION project, European Space Agency
- Aeronautical, Automotive, Chemical and Materials Engineering
- Chemical Engineering