With this project, we propose an investigation into a variety of dynamics and effects produced by the interaction of a droplet or a group of droplets with a solid or liquid surface. Related technological applications abound in the fields of thermal, mechanical and chemical engineering. The project will concentrate on “new” mechanisms discovered recently in the framework of “microgravity” research (carried out in space or on Earth using “small-scale” devices). The research will involve the application of both numerical and experimental techniques (50%+50%).
PROJECT DETAILS [full details of the project]:
With this project, we propose an investigation into a variety of dynamics and effects produced by the interaction of a droplet or a group of droplets with a solid or liquid surface.
Such phenomena are encountered in several physical circumstances [1,2]. Related technological applications abound in the fields of thermal, mechanical and chemical engineering. Significant examples include (but are not limited to) coating processes, galvanization of steel, welding processes, ink-jet printing, 3D printers and a variety of industrial fluid mixing and separation systems. Additional relevance can be found in recent nanotechnologies for which droplet self-assembly is being used as one of the main methods for construction of heterogeneous systems consisting of multiple component types.
The project will concentrate on “new” mechanisms discovered recently in the framework of “microgravity” research (carried out in space, e.g., on the International Space Station, or on Earth using “small-scale” devices). Many of these mechanisms have important implications or effects in terrestrial (“everyday”) conditions. Different situations will be considered, these including: droplet gravitational “splashing”, droplet non-coalescing temporarily (e.g., droplets bouncing upon collision with a larger pool of liquid and/or sitting momentarily on its liquid-air interface) or non-coalescing permanently (due to the formation of a layer or film of immiscible liquid or gas which allows the droplet to “float” over the underlying surface as it was in the absence of gravity).
The elaboration of relevant strategies to “control” the interaction of the droplets with the underlying surface will be also a relevant part of the project. Two variants, in particular, will be attempted, one being based on the application of temperature gradients and the other on the application of high-frequency vibrations in isothermal conditions. Both strategies will be implemented to promote the formation of the aforementioned layer of immiscible fluid, which can alter splashing phenomena, retard coalescence or prevent it in a temporary or permanent way (leading to droplet levitation).
Different static or dynamic configurations will be examined: hanging droplets in quasi static conditions, free falling droplets with various levels of collisional kinetic energy, a range of different fluids (including Newtonian and complex fluids), different heating conditions, different kinds of imposed vibrations (in terms of amplitude, frequency and direction) and possible combinations of all these variants. For the case of droplets interacting with solid surfaces the use of hydrophobic materials will be also addressed.
The research will involve the application of both numerical and experimental techniques (50%+50%). For the experiments, in particular, the student will take advantage of the recently developed microscale facility, available at the James Weir Fluid Labs, by which it is possible to create hanging or sitting droplets in well-controlled thermal conditions. The student will also be trained to use laser-based and optical techniques for flow visualization. From a numerical-simulation standpoint, the student will be trained to use OpenFoam and other numerical codes available at the Department of Mechanical and Aerospace Engineering.
 M. Lappa (2004),"Fluids, Materials and Microgravity: Numerical Techniques and Insights into the Physics", 538 pages, Elsevier Science (2004, Oxford, England)
 R. Savino, D. Paterna, M. Lappa, (2003) “Marangoni flotation of liquid droplets”, J. Fluid Mech., 479: 307-326.
From a theoretical point of view, training will be provided with regard to 1) the general background (importance of this kind of research and potential practical applications), 2) governing parameters, 3) gravitational and capillary phenomena in multiphase flows, 4) typical droplet coalescence and non-coalescence mechanisms, 5) Marangoni thermal effects, 6) Vibrational effects in multiphase systems, 7) Multiscale modeling, 8) Moving Boundary Numerical Methods. From a practical standpoint, the student will be trained to use available numerical tools. From an experimental standpoint, the student will be trained to investigate droplet coalescence and non-coalescence phenomena in different circumstances (static and dynamic conditions, application of temperature gradients and/or vibrations, different liquids). It is expected that such a wide spectrum investigation will provide the student with the necessary skills to address in the future more complex problems of technological interest.