Sessile droplets on deformable substrates

Wetting of deformable substrates has gained significant interest over the past decade due to a multiplicity of industrial and biological applications. Technological advances in the area of interfacial science have given rise to the ability to capture interfacial behavior between a liquid droplet and an elastic substrate. Researchers have developed several theories to explain the interaction between the two phases and describe the process of wetting of deformable/soft substrates. A summary of the most recent advances on static wetting of deformable substrates is given in this review. It is demonstrated that action of surface forces (disjoining/conjoining pressure) near the apparent three-phase contact line should be considered. Any consideration of equilibrium droplets on deformable (as well as on non-deformable) substrates should be based on consideration of the excess free energy of the system. The equilibrium shapes of both droplet and deformable substrate should correspond to the minimum of the excess free energy of the system. It has never been considered in the literature that the obtained equilibrium profiles must satisfy sufficient Jacobi’s condition. If Jacobi’s condition is not satisfied, it is impossible to claim that the obtained solution really corresponds to equilibrium. In recently published studies, equilibrium of droplets on deformable substrates: (1) provided a solution that corresponds to the minimum of the excess free energy; and (2) the obtained solution satisfies the Jacobi’s condition. Based on consideration of disjoining/conjoining pressure acting in the vicinity of the apparent three-phase contact line, the hysteresis of contact angle of sessile droplets on deformable substrates is considered. It is shown that both advancing and receding contact angles decrease as the elasticity of the substrate is increased and the effect of disjoining/conjoining pressure is discussed. Fluid inside the droplet partially wets the deformable substrate. It is shown that just these forces coupled with the surface elasticity determine the deformation of the deformable substrates

Wetting phenomena and foam drainage on complex substrates

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.</div

Supplementary information files for 'Drying of foam under microgravity conditions'

Supplementary information files for 'Drying of foam under microgravity conditions'Abstract:Foams have recently been characterised as ideal products for pharmaceutical and topical use applications for the delivery of topical active agents. Foams are usually produced in a wet form but in a number of applications moderately dry foams are required. Drying of foam under terrestrial conditions proceeds under the action of gravity, which is impossible under microgravity condition. Below a new method of drying foams under microgravity condition is suggested. According to this method foam should be placed on a porous support, which will absorb the liquid from foam based on capillary forces only. The final liquid content inside the foam can be achieved by a proper selection of the porous support. The suggested method allows drying foams under microgravity conditions. Interaction of foams with porous support under terrestrial conditions was developed only recently and theoretically investigated (Arjmandi-Tash, O.; Kovalchuk, N.; Trybala, A.; Starov, V. Foam Drainage Placed on a Porous Substrate. Soft Matter 2015, 11 (18), 3643–3652) followed by a theory of foam drainage on thin porous substrates (Koursari, N.; Arjmandi-Tash, O.; Johnson, P.; Trybala, A.; Starov, M. V. Foam Drainage Placed on Thin Porous Substrate. Soft Matter, 2019, (submitted)), where rate of drainage, radius of the wetted area inside the porous layer and other characteristics of the process were predicted. The latter model is modified below to investigate foam drying under microgravity conditions. Model predictions are compared with experimental observations for foam created using Triton X-100 at concentrations above CMC. Wetted radius inside the porous substrate was measured and results were compered to model predictions. Experimental observations for spreading area versus time show reasonable agreement with theoretical predictions for all investigated systems.</div

Modelling of foamed emulsion drainage

© 2020 Elsevier B.V. The drainage of foams created using emulsions has been investigated from both experimental and theoretical point of view. The drainage of emulsion foam is investigated using mixture of sodium dodecyl sulphate and oil, which were prepared using the double syringe method. For the preparation of each emulsion, SDS solution and oil are passed from one syringe into the other through a plastic tube leading to thorough mixing. In the course of drainage both the foam height and the thickness of the free liquid layer accumulated at the bottom of the foam were measured. A theoretical model was developed, taking into account both surface viscosity and non-Newtonian behaviour of the foamed emulsion to describe the time evolution of both the foam height and the thickness of the free liquid layer. The model is based on consideration of drainage of non-Newtonian liquid through the Plateau borders and the mobility of the gas/liquid interface is taken into account. Both experiments and theoretical predictions show no measurable change of the foam height while a free liquid layer starts to accumulate at the bottom boundary of the foam after an initial rapid increase of liquid volume fraction to the maximum value at the bottom of the foam. Theoretical predictions of rate of drainage, free liquid layer formation, foam height and liquid volume fraction for foamed emulsion systems of various oil volume fractions are compared with experimental observations. Comparison of the predicted and the experimentally measured time dependences showed a reasonable agreement