Experimental Investigation of the Nucleation Rate of Supercooled Microscopic Water Droplets under Mpemba Conditions

Statistical analysis of homogeneously freezing micro-droplets in microfluidic devices is a powerful tool to investigate nucleation dynamics of supercooled liquids and solutions under closely controlled conditions with small internal temperature gradients. When investigating dynamic cooling of pure liquid droplets, specific challenges such as fast temperature transients, large absolute temperature differences and volumetric expansion of the droplets arise. We present a microfluidic setup, which leverages flowing micro-droplets to overcome those issues, while maintaining the ability to sample a large number of independent droplets.

We demonstrate its potential by investigating the rapid freezing of supercooled water droplets. Under those conditions, the Mpemba-effect, i.e. that water droplets at an initially higher temperature are freezing faster than at a lower temperature, is expected to occur. We present experiments with pure, gas free water at different initial temperatures, which show that certain nucleation characteristics of ice are retained under rapid freezing conditions, irrespective of the initial temperature. This has profound implications for the Mpemba-effect, which we demonstrate to exist only in certain supercooling regimes.