CHARACTERIZING OPTICAL TRANSMISSION THROUGH SESSILE WATER DROPLETS UNDERGOING SOLIDIFICATION

The current increase in the usage of solar panels in cold-weather climates requires a comprehensive understanding of droplet freezing. Freezing dynamics of a water droplet resting on a hydrophobic surface is well understood, but changes in optical transmission during this process have not received much attention. As the droplet freezes, its refractive index and the shape change simultaneously. Here, an attempt is made to characterize the change in optical transmission during the freezing process using a unique setup that allows for bidirectional imaging of a droplet as it undergoes freezing on coated glass slides. This allows for phase change dynamics to be decoupled from the change in droplet geometry. There is a 45% reduction in optical transmission during the freezing process, but this is primarily due to reflection and not attenuation. The droplet geometry is observed to influence a change in optical transmission due to changes in both reflection and attenuation. Common hydrophobic coatings are tested, and their influence on the optical transmission is characterized. The results of this study provide new insight in the development of optically transparent ice/water repellent coatings for solar panels.