Anyone who enjoys the air on the rising shores of the sea or on the graduation tower of a health resort breathes drops of salty water. Flirtatious champagne also sends small pearls to the room and thus reaches our nostrils as a tingling scent. These and many other liquids, in the form of suspended particles, become drams of air currents. It only works when they go down very slowly, and to do that they have to become very beautiful.
Each drop left for its own devices is accelerated towards the Earth due to its weight. In contrast, air exerts a force of resistance. It increases with the speed of degradation, until both forces have the same volume. As long as the mass of the drop remains the same, it will continue to fall at a constant speed.
There is amazing physics behind many everyday things. Have felt for many years Hans-Joachim Schlichting Pursues these events and explains them to readers of the “Spectrum of Science” in his column. Schlichting is a professor for the principles of physics and worked at Münster University until his retirement.
The drag force of a moving object depends not only on its speed, but also on the cross-sectional area: small droplets experience less resistance. The overall balance of power varies with size. Because with this the mass and thus weight also decrease, but not to the same extent. This is why the rate of descent is different for small drops.
If the radius of the falling radius decreases by a factor of 10, its area decreases by a factor of 100. However, with radius the volume and proportional mass decrease to a power of three, that is, a factor of 1000. Therefore the weight has decreased much faster than air resistance. Therefore, with small drops, both forces are balanced at very low speeds – the object sinks very slowly. So if you want to keep liquids or the substances that they transport in the air for a long time, you have to make the drops as small as possible.
that’s easier said than done. Because to make one to two balls, you need about a fifth of the surface. And this costs energy. How much is required to spray a certain amount of water into the droplets is calculated by multiplying the resulting total area by the characteristic surface tension of the water. Tiny drops are almost round. Their individual surface is multiplied by their number, which can be generated by the total amount of water, resulting in the total area. A brief calculation shows that there will be a total area of 300 square meters made up of one liter of water droplets with a radius of ten micrometers. For this, about 22 joules of energy will have to be used. This is quite remarkable: you can at least lift the liquid up to 2.2 meters.
The storm catches the splashing water and runs along the tidal line in a broad fog curtain
(Harry Mulisch, 1927–2010)
A simple atomization technique gives the impression of performance required for droplet production. To do this, a tube is lowered into a container with water, for example, and another one is moving against the upper opening of the first. A transparent straw, for example, shows how the water level in it rises. Even if you can’t see it, you can ever hear it by a high hissing sound – an expression of the changing vibrational frequency of a decreasing column of air.