| Summary: | Freezing water is a common occurrence in Arctic climates and can pose hazards, for instance when water droplets impact surfaces. This is of specific interest in for example de-icing and anti-icing applications for wind turbine blades, aircraft, and roads. When a droplet hits a cold surface and begins to freeze, an internal flow is initiated. This thesis aims to study this internal flow, to determine the driving factors, and explore if it can be controlled for de-icing or anti-icing purposes. Such motions are also of generic interest and may be of importance for mixing on the micro-scale for medical purposes as one example. Since there are two possible driving mechanisms for the flow, temperature induced gradients in density and in surface tension, the flow direction within the droplet may change during the freezing process. Experimental work was carried out using Particle Image Velocimetry (PIV) to investigate droplets initially having room temperature being placed on metal surfaces cooled below 0 oC. Grooves were etched into the plates and filled with ice to control the contact area of the droplet and the material in contact (e.g., aluminium, or a combination of ice with aluminium, steel, or copper). The results show that the groove enabled consistent droplet shapes, with a deviation of around 0.85% in normalized contact radius. Among different substrate materials, copper (with the highest thermal conductivity) exhibited the highest velocity along the centerline of the droplet, while steel and aluminium showed similar magnitudes. For droplets on aluminium and ice, droplets with contact angle between 65o and 94o were compared and it was observed that smaller contact angles resulted in higher velocity magnitude along the free surface of the droplet and a lower velocity magnitude in the center as compared larger contact angles. The contact angle also affected freezing time and the time until directional change, meaning the time when the internal velocity changes direction before coming to a complete stop. The ...
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