Abstract
The study of evaporation and freezing of droplets is important in, e.g., spray cooling, surface coating, ink-jet printing, and when dealing with icing on wind turbines, airplane wings, and roads. Due to the complex nature of the flow within droplets, a wide range of temperatures, from freezing temperatures to heating temperatures, have to be taken into account in order to increase the understanding of the flow behavior. This study aimed to reveal if natural convection and/or Marangoni convection influence the flow in freezing and evaporating droplets. Droplets were released on cold and warm surfaces using similar experimental techniques and setups, and the internal flow within freezing and evaporating water droplets were then investigated and compared to one another using Particle Image Velocimetry. It was shown that, for both freezing and evaporating droplets, a shift in flow direction occurs early in the processes. For the freezing droplets, this effect could be traced to the Marangoni convection, but this could not be concluded for the evaporating droplets. For both evaporating and freezing droplets, after the shift in flow direction, natural convection dominates the flow. In the end of the freezing process, conduction seems to be the only contributing factor for the flow.
Highlights
Evaporation and freezing of droplets are two interesting areas with many applications.Evaporation of droplets is important in, e.g., spray cooling [1], surface coating [2], ink-jet printing [3], and droplet based biosensing [4]
This indicates that depending on how the droplet impact the surface, different types of flow will be seen inside the droplets and there will be a seemingly random motion in each droplet directly after impact. This can be compared to the freezing droplet where the viscosity in the water is high as it reaches the surface and will only get larger as the temperature inside the droplet decreases. This means that the movement inside of the freezing droplet is not induced by the release of the droplet, but for the heated droplet the release may have a larger impact on the flow inside the droplet
Three heating and two cooling temperatures were studied where the aim was to reveal if natural convection and/or if Marangoni effects influences the flow within the droplets
Summary
Evaporation and freezing of droplets are two interesting areas with many applications.Evaporation of droplets is important in, e.g., spray cooling [1], surface coating [2], ink-jet printing [3], and droplet based biosensing [4]. Evaporation and freezing of droplets are two interesting areas with many applications. The applications are found mainly when dealing with icing on wind turbines [5], airplane wings [6], and roads [7]. Due to the complex nature of the flow within droplets, a wide range of temperatures, from freezing temperatures to heating temperatures, have to considered to increase the understanding of the flow behavior. This is the motivation for performing this comparative study between freezing and evaporating droplets
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