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Electrowetting Actuation: Electrowetting is a phenomenon in which the contact angle of a polarizable/conductive liquid drop is manipulated by the means of applying an electric potential between the drop and the electrode on which the drop is placed. In this manner, the electrode must be coated with an insulator (dielectric) covered subsequently by a hydrophobic coating. This phenomenon can be used to manipulate and move liquid droplets in drop-based microfluidics. It can also be used to move the droplets to a hot-spot and transfer heat to avoid the concentration of heat in a very small area on an electronic device. Using a PCB-based electrowetting device, the contact angle of a droplet is manipulated under various values of applied AC voltages. Also an in-house numerical code is developed to simulate electrowetting using VOF method. The images taken from the experiments and simulations are compared. Experiments are performed both in air and silicone oil environments.
Calculated images (a) and experimental results for (b) air and (c) silicone oil environments for a 20 μL mercury droplet under the effect of various applied 50 Hz AC voltages. Numerical results involve the electric potential contours and the black line shows the free surface of the droplet.
Thermal Shrinkage: Because of their thermal expansion characteristics and changes in density, metals shrink or contract during solidification and cooling. Shrinkage causes dimensional changes and sometimes cracking and is a result of:
a) Contraction of a molten metal as it cools prior to its solidification. b) Contraction of the metal during phase change from liquid to solid (release of the latent heat of fusion). c) Contraction of the solidified metal (casting) as its temperature drops to the ambient temperature.
In thermal spraying processes, coatings such as various metals and alloys, carbides, and ceramics are applied to metal surfaces by a spray gun with a stream of oxyfuel flame, electric arc, or plasma arc. These coatings are formed by spraying and deposition of molten droplets on the substrate. The properties and quality of these coatings depend on many factors such as flame/laser/arc, material used for spraying, substrate and spray characteristics. As an effective phenomenon, thermal shrinkage affects the formation and integrity of the coating. Using FLUENT software, it has been found that as the molten metal droplets impinge and solidify onto a cold substrate, in comparison to the case of constant density, thermal shrinkage causes the formation of a void at the center of the solidified splat.
The Impact and Solidification of a Semi-Molten Particle: Thermal spraying technology includes the coating of surfaces using different techniques to serve in several industries such as automotive industries, chemical and electronic industries, etc. In these techniques, molten particles are generated from raw material which is fed into a thermal spray torch, in the form of wire, rod or powder. For example in plasma spraying, a high-temperature plasma flame melts and accelerates particles, usually provided as powder, to form the coating on the substrate. The properties and microstructure of the coatings strongly depend on the phenomena happening during the particle flight time between the injection of the particles into the flame and their impact on the substrate. Depending on the size and properties of the powder particles and the plasma operating conditions, such as temperature and velocity, the particles may be completely molten, completely solid or semi-molten at the impact. The presence of semi-molten particles highly increases the porosity of the final coating; as a result, the existence of these particles in a coating process is undesirable. Consequently, a numerical model is proposed based on the method of High Viscosity to simulate the impact and solidification of semi-molten particles. In this model, the solid core is treated as a liquid with a high viscosity. Because of the solid core, a bump remains in the final solidified splat.
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