How Airflow Accelerates the Evaporation of Liquids

Experiment number : 4352

• Goal of experiment

The aim of the experiment is to show that the evaporation process is faster when the vapour above the liquid surface is removed.

• Theory

Evaporation, i.e. the transition of liquid substance into gas, occurs at any temperature at which the substance is liquid. The rate of evaporation is generally influenced by many factors; at high school level, it is usually mentioned that lower surface tension, higher temperature of the liquid, greater surface area, and removing of vapours formed above the surface contribute to faster evaporation. In the following experiment we qualitatively focus on the latter effect, since the quantitative approach is described in the experiment Dependence of Evaporation Rate of Liquid on Removing Fumes from above Its Surface.

We often remove vapour of the evaporating liquid, albeit unwittingly – by blowing on a spoonful of hot soup or tea to cool it down faster. Evaporating liquids take away the heat needed to change their state (called the specific heat of vaporization) from the surroundings and thus decreasing its temperature. Therefore, if blowing the hot liquid leads to more intense evaporation, it also leads to a faster drop in the temperature of the liquid.

• Equipment

Thermal imaging camera, ethanol, two sheets of paper.

• Procedure

Take two sheets of paper. One remains dry and on the second pour a small amount of room-temperature ethanol. The paper soaks up the alcohol very quickly. Now blow on the dry paper and on the wet paper in turns and observe the temperature changes with a thermal imaging camera.

• Sample result

The successful execution of the experiment is illustrated in the video below. While the dry paper (in the video to the left) is obviously warmed by the air from our lungs, the temperature of the wet paper (in the video to the right) decreases. (This is especially true at the edges of the paper; directly at the place where we are blowing, the heating by the air from our lungs may be predominant.)

In this video, FLIR i7 thermal imaging camera was used. The temperature range of a colour scheme was set in the interval of 11 °C and 34 °C, the emissivity was ε = 0.95.

• Technical notes

• The alcohol passes through the paper quite easily, so make sure you don’t put anything under the paper that could be damaged by the alcohol.

• Performing this experiment with water is much less conclusive. It is recommended to use more volatile liquids (of which rubbing alcohol is one of the more available and safer ones).

• Pedagogical notes

• The explanation of the experiment can be set up as a problem task. Blowing on dry paper clearly shows that the air from our lungs is warmer than the surroundings. Therefore we can ask a question of how is it possible, that a wet surface is cooled by it? By discussion we should come to the conclusion that although the air from our lungs is indeed warmer than the surroundings, when it flows (i.e. the vapours are being removed), it causes intense evaporation of alcohol, thus dissipating the specific heat of evaporation resulting in temperature decrease. This effect is stronger than the heating by the air from our lungs.

• Since the alcohol evaporates quite intensely even without blowing, the alcohol-soaked paper will have a lower temperature than the dry paper at the beginning of the experiment. It is therefore necessary to emphasise that this temperature will drop even further when blowing; and vice versa, it increases back to its original value (even though it is still lower than the surroundings temperature) when we stop blowing.

• Wind-chill

The above experiment can be a model for a very real situation that we commonly experience in real life – just replace the blowing and alcohol-soaked paper with wind and human skin. More than one of us has had a nasty cold after going out sweaty and inadequately dressed for a windy day. As in the case of alcohol, the evaporation of human sweat is accelerated by airflow (removing vapours) and the uncovered areas of the body are intensely cooled by the removal of the specific heat of evaporation.

In everyday life, however, we encounter another effect that reduces our thermal comfort, regardless of whether we are sweaty or not; again, the wind is responsible for this effect, which is why it is often referred to as the wind-chill effect. The human body creates a thin “layer” of heated air really close to our skin which forms a kind of our thermal insulation, reducing the temperature difference between our body and the environment and thus partially preventing heat loss. In windy weather, this insulating layer is immediately blown away and we perceive the ambient temperature as lower than when there is no wind. The perceived temperature is called wind-chill and is very familiar to climbers, skiers, polar explorers, etc.

Tables of the wind-chill effect for different combinations of actual air temperature and wind speed can be easily found on the internet – as an example, see the table on the website Fyzweb. In addition, there are also online calculators that calculate wind-chill temperature – for example, the National Weather Service website can be recommended, that use the empirically observed relationship for the calculation

$\{t_\mathrm{w}\}\,=\,13.12\,+\,0.6215\{t_\mathrm{r}\}\,-\,11.37\{v\}^{0.16}\,+\,0.3965\{t_\mathrm{s}\}\{v\}^{0.16}$

where tw is wind-chill, tr real air temperature (both temperatures are in degrees centigrade) and v is the speed of wind (in kilometers per hour). This model is used for winds stronger than 5 km/h and actual temperature lower than 10 °C.

Together with wind speed, humidity also influences our perception of temperature – higher humidity means subjectively higher perceived temperature (so called heat index). This effect is usually evaluated at temperatures higher than 27 °C and relative humidity higher than 40%.

Both above mentioned temperatures (wind-chill and heat index) are combined in the term Feels Like Temperature and are becoming a standard part of a weather forecast, as can be seen in Fig. 1 taken from Weather Underground.

There are web apps (e.g. Meteopage), that calculate the Feels like temperature based on the wind speed and air humidity for a wide range of temperatures.

• Thermal imaging camera basics – link to PDF

In this experiment, a thermographic measurement is used. The theory of thermography and basic recommendation and procedures that can help you obtain more accurate and undistorted results can be found in Experiments with thermal imaging camera (in Czech only).

×Original source: Kácovský, P. (2016). Experimenty podporující výuku termodynamiky na středoškolské úrovni. (Disertační práce.) Matematicko-fyzikální fakulta UK, Praha.