High Speed SPR Visualization of Frost Propagation Inside a Subcooled Water Droplet

A surface plasmon resonance (SPR) imaging microscopy coupled to a high-speed camera is used to visualize the frost propagation inside a subcooled liquid droplet. The SPR experimental setup consists of a 50 nm thick gold-coated cover glass placed on a BK7 dove prism and optically matched using index matching liquid. Collimated monochromatic light of 600 nm wavelength is incident on the gold-glass interface at 71.8°, which corresponds to the SPR minima angle for ice (RI 1.309). Images are captured using Photron APS-RS camera at 1000 fps with a shutter speed of 1 ms. The prism and the gold film are cooled using a thermo-electric cooler (TEC). A water droplet is placed on the gold film and the temperature of the droplet is decreased from room temperature (23.0 ± 1 °C) to below 0 °C. Adjacent to the water droplet, the vapor condensates to form tiny droplets. The tiny condensate droplets would freeze first and the frost propagates through the condensate region. During this period the central droplet is in a subcooled state. The speed of frost propagation through the condensates is slow and takes tens of seconds to cover the gold film with ice. Within a single condensate droplet, however, the frost propagation velocity is expected to be considerably higher. Eventually the frost line reaches the central droplet. There is a delay of few seconds between the frost line reaching the droplet and frost propagation inside the droplet. The point at which frost touches the subcooled droplet acts as a nucleation site for the droplet and the frost propagates in the droplet at high speed. The average velocities of frost propagations in the subcooled liquid droplet were calculated to be 5.2 ± 0.3 cm/s and 7.4 ± 0.5 cm/s, when the gold film temperature was -5.0 ± 1 °C and -7.8 ± 1 °C respectively.