Why is aerosol called aerosol?

Classification: Science and Engineering Discipline >> Engineering Technology Science

Problem description:

-_-

Analysis:

aerosol

Aerosol is a colloidal dispersion system formed by small solid or liquid particles dispersed and suspended in gas medium, also known as gas dispersion system. Its dispersed phase is solid or liquid particles with the size of 10-3 cm ~ 10-7 cm, and the dispersion medium is gas. Clouds, fog, dust in the sky, smoke from unburned fuel in boilers and various engines used in industry and transportation, solid dust formed in mining, quarry grinding and grain processing, artificial masking smoke and toxic smoke are all concrete examples of aerosols.

The classification has not been unified. A more reasonable principle is to consider the characteristics and preparation conditions of dispersed phase. At present, aerosols are usually divided into three categories: ① fog, which refers to condensed aerosol and dispersed aerosol of liquid particles; ② Dust refers to the dispersed aerosol of solid particles; ③ Smoke refers to the condensed aerosol of solid particles.

Preparation methods can be divided into two categories: dispersion method and coagulation method. Dispersion method is to divide a solid or liquid into smaller parts by external force, which is divided into solid mechanical grinding method and liquid spraying method, and the dispersion degree of the obtained aerosol is often not high. Coagulation method is to split the dispersed phase into monomolecular substances (i.e. gas or vapor substances) and then condense them into colloidal particles, so it includes two stages: the formation of supersaturated vapor and the condensation of supersaturated vapor. The key is to obtain supersaturated steam, which can be achieved by steam cooling condensation and chemical reaction. For each substance, the maximum concentration of saturated vapor and its corresponding saturated vapor pressure are constant at a certain temperature and decrease with the decrease of temperature, so when the vapor cools, the saturated vapor condenses at the condensation center (or core) to form aerosol particles. Condensation centers can be dust particles, other atmospheric cores, ions and polar molecules. However, when the supersaturation is quite high, the steam molecules themselves can condense without nuclei, and substances with low vapor pressure can be produced through chemical reactions, which will condense when the supersaturation is reached. Generally, the prepared aerosol particles are usually polydisperse, and monodisperse aerosol can be obtained by using aerosol generator and controlling reaction conditions.

Characteristics Because the dispersion medium of aerosol is gas, the viscosity of gas is small, the density difference between dispersed phase and dispersion medium is large, particles are easy to stick together when colliding, and liquid particles evaporate, which makes aerosol have its unique regularity. Aerosol particles have considerable specific surface area and surface energy, which may

In order to make some chemical reactions that are quite slow under ordinary circumstances go very fast, and even cause explosions, such as finely divided sugar, starch and coal. Aerosol particles can scatter light, which is why the sky turns blue and the sun turns red at sunset. In dynamic properties, its Brownian motion is very intense, and it has diffusion properties when the particles are small; When the particles are large, the settlement is significant because of the large density difference with the medium. Because the medium is gas, these dynamic properties are related to the free path of gas molecules. In electrical properties, aerosol particles do not diffuse electric double layer, but can be charged, and their charge comes from the collision with gas ions in the atmosphere or the friction with the medium, which changes with time. Particles can be positively charged or negatively charged, indicating that their electrical properties depend on external conditions. In terms of stability, aerosol particles have no solvation layer and diffusion electric double layer like sol particles, and coalesce to form large droplets (fog) or aggregates (smoke) when colliding. This process progresses very fast, so aerosol is an extremely unstable colloidal dispersion system, but it also has certain relative stability due to Brownian motion.

Aerosol has been widely used in industry, agriculture, national defense and other aspects, such as accelerating combustion speed and making full use of fuel. Spray drying can improve product quality and has been widely used in pharmaceutical industry and the production of washing powder. In agriculture, spraying pesticides can improve the efficacy and reduce the dosage of drugs; Using aerosol for artificial rainfall can greatly improve the drought. In national defense, it is used to make signal flares and cover smoke screens.

At present, the smog over industrial cities, the smoke emitted by factories and mines are extremely harmful to human health (such as silicosis), as well as acid rain that destroys nature and dust that is easy to cause explosion, all of which are related to aerosols.