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 65^o and 94^o 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 experimental observation is that the internal flow moves down along the surface and up in the center to start with, and switches to down in the center and up along the surface after a while. Along the centerline, the increase of contact angle presented an increase in velocity magnitude, freezing time, and time until directional change. Interestingly, substrate temperature (at -8 ^oC and -12 ^oC) had little impact on the time of the directional change, in comparison to the influence from the contact angle.

When the ice was removed from the contact area and only aluminium was in contact with the droplet, heat transfer naturally increased, reducing freezing time and slightly shortening the time until directional change. In experiments where solidification was prevented, i.e. causing the droplet to become supercooled, a similar flow pattern was observed, though the directional change occurred much later. This indicates that while phase change affects velocity magnitude and time of the directional change, it is not the main driver of internal flow.

Complementing the experiments, numerical methods using Computational Fluid Dynamics (CFD) to further analyse the effect of heat transfer on internal flow were used. Specifically, the effects of external heat transfer (i.e conduction, natural convection and evaporation) on the internal flow and temperature was examined. Although comparison with experiments show an underestimation of the internal velocities, natural convection as the driving force of the internal flow give comparable results in terms of time of the directional change.  For this set-up, simulations show that the directional change is closely related to the density inversion of water. 

In summary, the time from droplet impact until the internal flow is approaching zero, a phenomenon closely related to the time of the directional velocity change, is strongly influenced by both phase change and the contact angle of the droplet. The two variables are dependent on the substrate, suggesting the flow can be controlled by manipulating the substrate material and possible used for anti-icing purposes. The magnitude and direction of the internal flow may in its turn be further controlled by heat fluxes and surface tension effects, i.e. of importance for mixing, deposition or orientation of particles in a droplet. Future research should focus on clarifying heat transfer effects, the temperature field, and the impact of surrounding air with forced convection.

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.

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.

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. 

A water droplet that impacts on a cold surface will start to freeze and in time ice will accumulate. To exemplify, effects of ice accretion is important in areas such as power generation e.g. wind power and vehicles located in a cold climate e.g. aircraft, cars, and boats. The common denominator for these examples is that ice accumulation can lead to a loss of efficiency and in some cases danger.

Most studies have so far focused on investigating freezing water droplets visually in experiments or numerically in regards to how the freezing process behaves in terms of shape or freezing time for either a sessile or impacting droplet. It has been observed that the surface material and structures of the substrate is of importance. One part of the freezing process that has been less investigated is the internal flow and how it affects the freezing process.

In this thesis, the internal flow in a freezing water droplet has been investigated experimentally. The internal flow inside a droplet is calculated by using Particle Image Velocimetry. A metal plate with a groove filled with ice was used to generate an area for the nucleation to start and to be able to control the shape of the droplet. 

Previous work indicate that the substrate is of importance for the freezing process. The influence of the substrate material on the internal flow for similar shaped droplets is therefore investigated in Paper A, for a substrate temperature of -8°C. The results show that the substrate material, here in terms of metals such as aluminum, copper and steel, affect the magnitude of the internal velocity. In paper B it is investigated how the contact angle influence the internal flow. The vector field is examined at 9% of the total freezing time for water droplets at five different contact angles. A droplet with a higher contact angle will have a higher internal velocity in the center. A lower contact angle will barely show any movement in the center, however a higher velocity magnitude is observed close to the free surface compared to a droplet with a higher contact angle. Paper C studies the time until the directional change of the internal flow in a water droplet. Experiments at -8°C as in Paper B are used as well as experiments at -12°C for the five different contact angles. The time until the directional change is similar in time for both -8°C and -12°C while the total freezing time and also the time of the directional change varies with contact angles. A droplet with a lower contact angle will have a shorter time until the directional change occure while an increase in contact angle prolongs both freezing time and the time until the directional change.

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.

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.