Abstract
Conventionally, only two states are assumed to exist in water: well-dispersed gas monomers and gas bubbles. Rarely is this paradigm explored experimentally. To close this gap, here we used transmission electron microscopy (TEM) to study degassed water, deionized water, and gas-supersaturated water encapsulated in graphene liquid cells. While neither degassed water nor deionized water yielded specific features, two major microscopic structures were evident in gas-supersaturated water: (1) polycrystalline nanoparticles formed of gas molecules and (2) a high density of tiny cells. Dark-field TEM imaging revealed that water molecules surrounding each cell form crystalline structures-a surprising discovery of a clathrate state in gas-supersaturated water that may help resolve several long-standing puzzles. Overall, this study suggests that water may form a matrix that actively interacts with gas molecules in complex and subtle ways.