More than temperature, it's changes in pressure that influence how quickly liquid turns into gas
The change in pressure between a liquid and its surface is more important than the difference between their temperature in the process of evaporation, the Massachusetts Institute of Technology (MIT) researchers have found.
From the coffeemaker to huge power plants, the process plays a pivotal role in everyday life. Yet, it was not analysed in detail at a molecular level, according to MIT.
“It turns out that for the process of liquid-to-vapour phase change, a fundamental understanding of that is still relatively limited,” said Evelyn Wang, department head at MIT.
“While there’s been a lot of theories developed, there actually has not been experimental evidence of the fundamental limits of evaporation physics,” Wang explained, in the study appearing in the journal Nature Communications.
To understand, the team conducted experiments using a very thin membrane riddled with small pores to confine the water. To measure its temperature, the membrane — just 200 nanometres (one nanometre is one billionth of a metre) thick, made of silicon nitride and coated with gold — was electrically heated up in a chamber that was isolated from the surrounding air.
Then, “we also use that membrane as the sensor, to sense the temperature of the evaporating surface in an accurate and non-invasive way”, said Zhengmao Lu, professor of mechanical engineering at MIT.
The findings demonstrated that the temperature difference between the surface and the liquid, did not determine how quickly the liquid could evaporate. Rather, it was regulated by the difference in pressure between the liquid surface and the ambient vapour.
“The actual driving force or driving potential in this process is not the difference in temperature, but actually the pressure difference,” Wang said.
The findings can act as a guideline to optimise the working conditions for certain kinds of applications. It can help engineers design new evaporation-based systems, providing information on both the selection of the best working fluids for a given situation, as well as the conditions of pressure and removal of ambient air from the system, Lu said.
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