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Flotation of liquid droplets on pool surfaces, in the presence of temperature differences, is studied experimentally and numerically. Coalescence or sinking of the droplet is prevented by the thermal Marangoni motion, owing to the surface tension imbalance at the pool surface. The mechanism is the same as that investigated in previous works on coalescence and wetting prevention in the presence of temperature differences. If the droplet is colder than the liquid surface, the flow is directed radially towards the drop; this radial flow field drags the ambient air under the drop, thus creating an air film and avoiding a direct contact between the droplet and the pool molecules. The surface velocities are measured visually with a CCD camera to image the motion of tracers floating on the pool surface; the surface temperature distributions along the pool and the droplet surfaces are measured by an infrared thermocamera. The experimental results are correlated by numerical results obtained under the assumption of spherical drop and axisymmetric flow regime. Different liquids are considered and the influence of evaporation is discussed, showing a good agreement between the experiments and the numerical simulations.

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In this study, Marangoni flows effects on heat pipe performances are explored. Recent results dealing with computations of the meniscus shape in a wedge are extended to take into account a nonzero flow rate, caused by the liquid mass lost at the evaporator and supplied by the vapor condensation at the opposite side. Moreover “inverse Marangoni effects” (prevailing in some binary mixtures) are investigated. Dry-out conditions have been computed for different flow rates and surface tension gradients. Results show that while dry-out occurrence is anticipated by the usual Marangoni effect, as illustrated by Yang and Homsy [Phys. Fluids18, 042107 (2006)], an inverse Marangoni effect is beneficial and dry-out conditions are delayed at much larger heating rates.

A convective transport model is developed to study the role of thermal diffusion, or the Ludwig–Soret effect, in nanofluid systems with temperature gradients. The study deals with a fluid suspension of nanoparticles enclosed between two differentially heated horizontal, relatively closely spaced plates (Bénard configuration). An order-of-magnitude analysis is performed to identify the relevant parameters of the problem. Three-dimensional simulations are performed taking into account different conditions, including normal or microgravity conditions, gravity orientation, and positive or negative Soret effect. Different modes of convective instabilities are shown to be present in the system, which are associated with the gravity force and the density differences induced by concentration gradients. The characteristic flow patterns and instability developments are in agreement with the experimental findings obtained by independent investigators on colloidal suspensions. The onset of instabilities, their characteristic time scales, and flow patterns corresponding with different geometrical configurations, gravity levels, and gravity orientation are shown.

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