Unique Field Measurement Detects Mpemba Effect during the Natural Ice Thickening
Abstract:Gaseous phase often displays ubiquitous bubble textures within the ice body from natural lakes and ponds. An occurrence of bubbles (enclosures filled with gas) within ice relates to a content of the dissolved gas in the lake/pond water prior to freezing over the surface. When water freezes, dissolved gases are rejected and redistributed at the ice-water interface, depending on the saturation ratio between the gas and water. Therefore, the content of dissolved gases within the water along the ice interface increases as the ice gets thicker. When the concentration of dissolved gases surpasses a critical value (as freezing progresses with variable speed), the water at the interface becomes supersaturated, and gas bubbles nucleate and grow to a visible size along the interface. The bubbles generated at the ice-water interface are either incorporated into the ice crystal as the ice-water interface advances, thus forming gas pores in the ice, or migrated along the interface.
We provide temperature measurements of the ice formed under natural winter conditions over the pond Dolní Tušimy in Mokrovraty, Czech Republic. Data show decrease in ice growth rates, while freezing rates remain constant or increase. We interpret this observation as Mpemba effect detected for the first time in natural lake/pond environment. Additionally, distinct layers of gas bubbles were captured after the ice samples were retrieved. Their presence indicates fast growth rates of ice. Our measurements and calculation showed for these layers the maximum growth rates were about 1 μm/s, much lower than when produced in laboratory condition. This comparison emphasizes that the presence of impurities in natural waters lowers the limit of growth rate, required for bubble occurrence, more than order of magnitude compared to distilled water in laboratory setting (from over 20 μm/s down to 1 μm/s).