Freezing water droplets are a natural phenomenon that occurs regularly in the Arctic climate. It affects areas such as aircrafts, wind turbine blades and roads, where it can be a safety issue. To further scrutinize the freezing process, the main objective of this paper is to experimentally examine the influence of substrate material on the internal flow of a water droplet. The secondary goal is to reduce uncertainties in the freezing process by decreasing the randomness of the droplet size and form by introducing a groove in the substrate material. Copper, aluminium and steel was chosen due to their differences in thermal conductivities. Measurements were performed with Particle Image Velociometry (PIV) to be able to analyse the velocity field inside the droplet during the freezing process. During the investigation for the secondary goal, it could be seen that by introducing a groove in the substrate material, the contact radius could be controlled with a standard deviation of 0.85%. For the main objective, the velocity profile was investigated during different stages of the freezing process. Five points along the symmetry line of the droplet were compared and copper, which also has the highest thermal conductivity, showed the highest internal velocity. The difference between aluminium and steel was in their turn more difficult to distinguish, since the maximum velocity switched between the two materials along the symmetry line.

Ice accretion upon a surface is of interest in areas such as wind power, electric power transmission and vehicles in cold climate. Ice assimilation appears when humid air or water droplets impacts and freezes on a cold surface. In the study presented in this paper, droplets are deposited onto aluminium plates constructed to generate specific a contact angle between the droplet and substrate. Five contact angles are investigated and Particle Image Velocimetry (PIV) is used to analyse the internal flow. The droplets are studied along the vertical centerline and at horizontal lines at distances of 50% and 75% of the total height of the droplet. From the results it is found that a lower contact angle will increase the magnitude of the internal flow close to the edges. A larger contact angle will instead increase the magnitude of the flow in the center of the droplet. For a droplet with lower contact angle it was furthermore found that there is a triangular area inside the droplet with close to zero velocity. 

Freezing of water droplets are of interest in areas such as de-icing and anti-icing of wind turbine blades, aircrafts and cars. On part of the ice build-up that has been less studied is the internal flow in water droplets and how it affects the freezing process. In this paper the aim is to investigate how the contact angle, substrate composition and temperature influences the internal flow. Particle Image Velocimetry (PIV) is used to determine the magnitude and direction of the internal flow, with specific emphasis on directional changes. Results show that a larger contact angle will increase the internal velocity, freezing time and time until the directional change. Cooler substrate temperature increase the internal velocity while reducing the freezing time, but the dependence on the time until the directional change is not as pronounced. The result thus indicate differences in the driving forces between freezing time, internal velocity and directional velocity change. Difference due to substrate composition, i.e. mixture of ice and metal versus only metal is furthermore compared.

By comparing freezing and non-freezing water droplets with Particle Image Velocimetry (PIV), this paper aims to further advance the knowledge of the internal movement in deposited water droplets. The experiments are carried out at substrate temperatures below 0 °C, and the onset of the freezing process is controlled by the structure of the plate. Results show similar flow patterns for the freezing and non-freezing case, indicating that the phase change is not the initial driving force of the internal movement or the directional change phenomena. The phase change will, however, decrease the volume of the liquid and generate a difference in heat transfer and temperature, hence introducing a faster decrease in velocity and sooner directional velocity change. In the non-freezing case, a stagnated period at the beginning of the internal movement is observed before the decrease in velocity.

During the freezing process of a water droplet, a two-stage internal flow has been experimentally observed. As the droplet is deposited on a cold substrate the internal flow is initiated and first moves downwards along the surface and upward in the center. The flow is then reversed, moving up along the surface and down in the middle, before eventually settling with minimal movement. Using ANSYS Fluent this paper aims to numerically investigate the influence of the heat transfer mechanisms and its effect on the internal temperature and velocity. With e.g. natural convection to drive the flow, the internal flow and the directional change phenomena is investigated and related to the temperature field inside the droplet. Dimensionless numbers such as Marangoni number, Rayleigh number and Bond number are also used to correlate the temperature gradients with the directional change.