Every substance evaporates, but how exactly?

Computer simulations carried out by scientists from the Institute of Physical Chemistry of the Polish Academy of Sciences proved that the theoretical models that exist today regarding the evaporation phenomenon are based on incorrect assumptions

Evaporation is a common phenomenon in nature and for about one hundred and thirty years it seems that its mechanism has been correctly understood. However, computer simulations performed by scientists from the Institute of Physical Chemistry of the Polish Academy of Sciences proved that the existing theoretical models are based on incorrect assumptions. Thanks to the simulations, it was possible to determine the exact mechanisms responsible for the evaporation of droplets into a vacuum or into an environment saturated with the vapors of the liquid being tested. At the same time, the mechanism that plays the decisive role in cases of evaporation into a mixture of gases, such as the air we breathe, is still unknown.

Evaporation occurs frequently everywhere in our environment. The phenomenon plays an important role in the formation of the ecological systems on Earth, and in the life processes of many organisms - including humans, who, like many other organisms, use this process to stabilize their body temperature.

"The first scientific publication discussing the mechanism of evaporation was written by the well-known physicist James Clerk Maxwell. We were able to show that there was a mistake in this publication that continued and took hold even for the following 130 years," says Professor Robert Hołyst from the Institute of Physical Chemistry at the Polish Academy of Sciences. Computer simulations, completed long ago, provided a solution to some of the information that was missing regarding the evaporation of a liquid into a vacuum or into the vapor of the liquid itself. Now, the researchers are planning a series of experiments that will verify the corrections introduced following the simulations to the model describing the mechanism of evaporation of water droplets into the air.

About seventy percent of the Earth is covered by oceans and seas that are constantly evaporating. Since the heat of evaporation of water is very high, the evaporation largely determines the climate according to the Earth. Moreover, the content of water vapor - the main greenhouse gas - in the atmosphere changes as a result of this evaporation. The concentration of water vapor in the air may reach a level of about four percent - a value that is a hundred times higher than the value of the notorious greenhouse gas carbon dioxide. According to several estimates, if there was no evaporation of water into the air at all, the temperature on Earth would be twenty to thirty degrees Celsius lower.

Although the evaporation process is common and plays an important role in the environment, only little attention has been paid to the phenomenon. "Our studies also grew by chance, as often happens in science," says the lead researcher. "Several years ago, our institute had to test new software for calculations related to fluid flow. We decided to test the simulator using a known issue. We chose evaporation because we believed that such a common and well-known phenomenon would lead to clear results. However, after we performed the calculations using familiar formulas, we realized that there is a discrepancy."

The Polish scientists developed their own theoretical model that explains the phenomenon and in the next step performed computer simulations referring to the process of nanodroplet evaporation into a vacuum or into their own vapors. The starting point was a drop of liquid trapped in a closed vessel, which is in equilibrium with its vapor. In some of the simulations the walls of the vessel were heated, in another part the steam was removed and in another part the steam was removed but the temperature remained constant.

During evaporation, the most fascinating events occur at the boundary between the liquid and the vapor. The thickness of this interface is equal to the diameter of a single atom. Simulating evaporation in a relatively small cube with metre-long edges would require the calculation of tens of billions of points along each of the three dimensional axes. At this point, the total number of calculation points will reach billions of billions, a number that exceeds the calculation capabilities of existing or future computers. In order to face this challenge, the institute's scientists examined a system that is only one centimeter in size, where a droplet with a diameter of about seventy micrometers is evaporated. In addition, due to symmetry considerations, it was possible to reduce the theoretical description from three dimensions to only one dimension. The simulation results were in complete agreement with the available information obtained from the measurements.

"Maxwell assumed that evaporation occurs at a constant temperature. This is indeed so, if we examine the initial state, i.e. the liquid, and the final state, i.e. the vapor. Indeed, the temperatures of the two states are equal. However, during the evaporation process, nature works in a completely different way," explains one of the team of researchers.

The existing description assumes that the heat transfer in the system is stable and that the evaporation rate is limited by the efficiency of the process in which the particles are detached from the vapor surface, that is, the diffusion process. However, based on the simulations performed at the institute, it was discovered that during the evaporation into a vacuum or into the vapor of the liquid itself, the system reaches a mechanical equilibrium very quickly. Particles are detached from the surface of the liquid and their mechanical repulsion allows comparison of internal pressures inside the droplet. If the evaporation rate of the surface reached its maximum value and the system was still unable to balance the pressures, then the drop would begin to boil. On the other hand, the researchers discovered that the mechanical equilibrium of the pressures may be insufficient and then the temperature of the liquid surface decreases: the drop tends to maintain the equilibrium of the pressures at the expense of its internal energy. Hence, the researchers state, the findings imply that the decisive factor during the evaporation process is not the diffusion of the particles into the environment, but rather the transfer of heat and the equalization of pressures. In the next step, the researchers plan to examine the system in which evaporation occurs within a mixture of gases, and in particular - the air.

In conclusion - a deeper understanding of the physical mechanisms responsible for the evaporation process will affect many areas of human activities - better climate models will allow for more accurate weather forecasts, both in the short term and in the longer term, the development of more efficient devices for cooling processors will be possible - computer, and for the development of new lasers. Since the evaporation of microfluids of fuel in vehicle engines injected into the interior of the combustion chamber must occur after ignition, the insights into the exact mechanisms of evaporation will greatly improve the efficiency of the vehicles in the future from the aspect of saving fuel.

"Our study proves that it is still worth re-examining old formulas," concludes the lead researcher.

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