Water is weird. Unlike any simple liquid, it becomes easier to compress as it cools, and it reaches its maximum density at 4°C—not at its freezing point. This anomaly is why ice floats and lakes freeze from the top down. For decades, scientists have suspected that liquid water is actually a shifting mix of two distinct molecular arrangements: one dense and chaotic, the other loose and ordered.
Now, a new study in Nature Physics offers fresh evidence that this "two-structure" theory is real. The challenge? No one had ever cleanly separated these structures, even in simulations. So, a team led by Prof. Xiao Cheng Zeng of City University of Hong Kong turned to AI. They trained an unsupervised neural network on 74 million simulated water configurations, letting it hunt for hidden patterns without human bias.
"It is practically impossible for humans to intuitively guess or manually construct such complex, nonlinear parameters," Zeng said. "We need AI's help."
The AI identified two distinct clusters—Structure A (dense and disordered) and Structure B (light and ordered)—across a wide range of temperatures and pressures. The next step? Deciphering the physical meaning of these AI-discovered traits and confirming them in real-world experiments.
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