If you mix mascara or paint in water, and then look at this water under a microscope, you can see the rapid movement of the smallest particles of soot or paint in different directions. What triggers such movements?
Who discovered when
In 1827, an English biologist Robert Brown observed through a microscope a drop of water into which a small amount of flower pollen accidentally fell. He saw that the smallest particles of pollen dance, moving randomly in a liquid. So it was openly named after the name of this scientist Brownian motion - the movement of the smallest particles dissolved in a liquid or gas. After observing the various types of pollen that were in his collection, the biologist dissolved the powdered minerals in water.
As a result, Brown was convinced that such an erratic movement was not caused in any way by the liquid itself and not by external influence on the liquid, but directly by the internal movement of the smallest particle. This particle, by analogy with the observed motion, was called the “Brownian particle”.
The development of the theory, its followers
Subsequently, Brown's discovery was confirmed, expanded, and concretized, based on the molecular-kinetic theory, by A. Einstein and M. Smolukhovskiy. And the French physicist Perrin twenty years later, thanks to the improvement of microscopes in the process of studying the random motion of a Brownian particle, confirmed the existence of the molecules themselves. Observation of the Brownian motion allowed Perrin to calculate the number of molecules per 1 mol of any gas and derive a barometric formula.
The discovery of the motion of a Brownian particle served as evidence of the existence of much smaller particles that are not visible even under a microscope - liquid molecules and any other substance. It is the molecules that, with their constant movement, make particles of pollen, soot or paint move.
Definition and size
If you look under a microscope at carcass particles suspended in water, you will notice that grains of different sizes behave differently. Relatively bulky particles, experiencing the same number of shocks from all sides for a certain period of time, do not begin to move. And small particles in the same time interval receive unilateral uncompensated impacts, pushing them to the side, and move.
What is the size of a Brownian particle exposed to molecules? It has been experimentally proved that cytoplasmic pollen grains with a size of not more than 3 micrometers (microns), or 10 -6 meters, or 10 -3 millimeters, come into motion. Larger particles do not become participants in the continuous motion discovered by Brown.
So, we will answer the question "what is a Brownian particle". These are the smallest grains of a substance no larger than 3 microns in suspension in a liquid or gas, making constant chaotic movement under the influence of molecules of the medium in which they are located.
Molecular kinetic theory
Brownian motion does not stop, does not slow down in time. This explains the concept of molecular kinetic theory, which says that the molecules of any substance are in constant thermal motion. With an increase in the temperature of the medium, the velocity of the molecules increases, and the Brownian particle, which is subjected to molecular impacts, also accelerates.
In addition to the temperature of the substance, the Brownian motion velocity also depends on the viscosity of the medium and the size of the suspended particle. The movement will reach its maximum speed when the temperature of the substance surrounding the particle is high, the substance itself will not be viscous, and the dust particles will be the smallest.
Molecules of the substance in which the smallest particles are located, randomly colliding, apply a resultant force (produce a push), causing a change in the direction of motion of the pollen. But such fluctuations are very short in time, and almost immediately the direction of the applied force changes, which leads to a change in the direction of motion.
The simplest and most illustrative example, allowing us to understand what a Brownian particle is, is the motion of dust particles, visible in an oblique sunbeam. In 99-55 years. BC e. the ancient Roman poet Lucretius accurately explained the reason for the erratic movement in the philosophical poem "On the nature of things."
Look here: whenever sunlight enters
Into our homes and darkness cuts through its rays,
Many small bodies in the void, you will see, flickering,
Rushing back and forth in the radiant radiance of light.
From this you can understand how tirelessly
The initial things in a void of immense crumple.
So great things help make sense
Small things, outlining paths for their comprehension.
In addition, because you need to pay attention
To the clutter in bodies flickering in the sunlight
What do you know from it matter also movements
What is happening in it is hidden and hidden from view.
For you will see there how many dust particles change
His way from hidden tremors and again fly back,
Forever going back and forth in all directions.
Long before the advent of modern magnifying technology, Lucretius, observing an analogue of the movement seen by Brown, came to the conclusion that there are tiny particles of matter. Brown confirmed this by making one of the most important scientific discoveries.