How small can a water cluster be and still stabilize ice I, the macroscopic crystal polymorph? What is the nature of coexistence between ice and liquid in small clusters? In this work, we show that the smallest water clusters that can sustain ice contain only around 90 water molecules. Our experimental and theoretical analysis suggests that the melting transition in these small droplets is very different from macroscopic water and our everyday experience. When cooled slightly below their freezing temperature, water clusters with less than ∼150 molecules can oscillate in time between the liquid and crystalline state. These results can be significant to understand the state of water confined in proteins and other materials.

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