Everything evaporates, but how?

Evaporation takes place all the time in our environment. The phenomenon plays an important role in the formation of Earth's ecosystem and the life functions of many organisms -- including humans, who like many other animals use it to stabilise their body temperature.

"The first scientific publication concerning the mechanism of evaporation was written by a famous physicist James Clerk Maxwell. We showed that it contained an error that has been repeated for the last 130 years," says Prof. Robert Hołyst from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw. The computer simulations that have just been completed allowed some of the puzzles connected with the evaporation of a liquid into vacuum or its own vapour to be solved. Currently, in cooperation with the Institute of Physics of the PAS, scientists from the IPC PAS are preparing a series of experiments that will allow them to verify the correctness of the model in the case of the evaporation of drops of water into air.

As much as 71% of Earth is covered by oceans and seas which evaporate continuously. Since the heat of evaporation of water is very high, the evaporation determines Earth's climate. What is more, the content of water vapour -- the main greenhouse gas -- in the atmosphere changes as a result of evaporation. Its concentration in air may reach as much as four per cent, that is the value over one hundred times higher than that of the infamous carbon dioxide. According to various estimates, if there was no water vapour in air, the temperature on Earth would fall by 20-30 degrees.

Although evaporation is so common and it plays a big role in the environment, little attention has been given to the phenomenon. "Our studies also originated accidentally, as it often happens in science," says Prof. Hołyst. "Several years ago, in the Institute of Physical Chemistry of the PAS, it was necessary to test a new program for calculations relating to fluid dynamics. We decided to check the simulator using a popular problem. We chose evaporation because we thought that since the phenomenon was so common and the subject was known for over one hundred years, everybody knew well what happened during the process. However, after we had made calculations using the existing formulas, it turned out that many things simply did not add up."

Polish scientists developed their own theoretical model of the phenomenon and then carried out computer simulations illustrating the process of evaporation of nanodrops into their own vapour or vacuum. The starting point was a drop of liquid closed in a vessel, and in equilibrium with its vapour. In some computer simulations the walls were heated, in some others the vapour was removed, and in the others not only was the vapour removed but the temperature of the system was maintained constant.

During evaporation the most interesting events take place on the border of a liquid and a vapour. The thickness of this interface is more or less equal to the diameter of an atom. The simulation of evaporation in a relatively small cube with faces one meter long would require the calculation of dozens of milliards of points along each of the three dimensional axes. The total number of points would increase to billion of trillions, which exceeds calculation abilities of modern and future computers. In order to deal with this obstacle, scientists from the Institute of Physical Chemistry of the PAS analysed the system of a size of only 1 cm, in which a drop of a diameter of approx. 70 micrometers evaporated. In addition, thanks to the use of symmetry, it was possible to reduce the theoretical description from three-dimensional to one-dimensional. The results of simulation agreed perfectly with the available measurement data.

"Maxwell assumed that evaporation took place at constant temperature. It is so, if we look at the initial state, that is a liquid, and the final state, that is a vapour. It is true that their temperatures are equal. But during the evaporation process itself, the nature acts in a completely different way," explains Ph.D. Marek Litniewski from IPC PAS.

The existing description assumed that the heat transfer in the system was stable and the rate of evaporation was limited by the efficiency of the process during which the particles break away from the surface of drops, i.e. diffusion. However, the simulation carried out in the IPC PAS showed that during the evaporation into vacuum or the liquid's own vapour the system gained mechanical equilibrium very quickly. Particles break away from the surface of a liquid and their mechanical recoil allows the equalisation of the pressure inside the drop. If the rate of evaporation on the surface achieved the maximum value and the system was still unable to equalise the pressures, spaces with new surfaces would open inside the drop and it would start to boil. However, it was observed that the mechanical equilibration of pressure can be insufficient and the temperature on the surface of the liquid decreases: the drop aims at maintaining the pressure equilibrium at the cost of its internal energy. This observation suggests that the factor that is crucial during evaporation is not the diffusion of particles into the environment but the heat transfer and the equality of pressures.

The studies will continue, this time from the point of view of the analysis of evaporation into the mixture of gases, in particular into air. The experimental part will be carried out by scientists from the Institute of Physics of the PAS (IP PAS), headed by Assoc. Prof. Krystyna Kolwas. Physicists from the IP PAS have already observed the evaporation of microdrops of a liquid into the liquid's own vapour or vacuum. Drops of micrometric sizes were used in the experiments. Since their surface was electrically charged, the drops could be caught by the electric field, lighted by a laser and, while recording changes in interference patterns, it could be observed how their size changed during the evaporation.

Currently, thanks to a new measurement chamber with precisely controlled pressure and chemical composition of the atmosphere, a series of experiments on evaporation into air can be conducted, and consequently, it will be possible to determine which factor has a decisive influence on evaporation in the situation where pressures are equalised from the beginning. The results of the experiments along with computer simulations will allow creating a comprehensive picture of the process of evaporation of water drops in the conditions maximally similar to those that exist in nature.

The deeper understanding of physical mechanisms responsible for evaporation will affect many areas of human activity. Better climate models will allow more precise forecast of weather changes in a short and long time perspective, and more efficient devices for cooling processors and lasers will be developed. Since in engines the evaporation of fuel microdrops injected into a combustion chamber must precede the ignition, the knowledge of evaporation will allow increasing car efficiency in future.

"Our research shows that old formulas are still worth being examined," sums up Prof. Hołyst.