Five Things We Still Don’t Know About Water
There is something remarkable about the mist surrounding Niagara Falls: The individual droplets move as if they are negatively charged.
Together with his colleagues, David Chandler, of the University of California, Berkeley, used a theory capable of describing such rare events, called transition path sampling, to calculate the water evaporation coefficient. They arrived at a value near one. This corresponds fairly well to recent liquid microjet experiments that produce a value of 0.6 for both normal water and heavy water.
However, there are a couple of wrinkles. For one thing, it remains unclear why experiments performed under more atmospherically relevant conditions yield much lower values. Also, the transition path sampling simulations suggest that evaporation relies on an anomalously large capillary wave running along the liquid’s surface, which strains and weakens the hydrogen bonds holding on to an evaporating water molecule. The addition of salts to water raises the surface tension and suppresses the capillary wave amplitude, and so should reduce the evaporation rate. But experimental studies show little or no effect when salts are added. More.
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