Among the numerous phenomena in physics, the diffusion process is one of the simplest and most understandable. After all, every morning, while preparing himself fragrant tea or coffee, a person has the opportunity to observe this reaction in practice. Let's find out more about this process and the conditions of its occurrence in different states of aggregation.
What is diffusion
This word refers to the penetration of molecules or atoms of one substance between similar structural units of another. In this case, the concentration of penetrating compounds is leveled.
For the first time this process was described in detail by the German scientist Adolf Fick in 1855.
The name of this term was derived from the Latin verb noun diffusio (interaction, dispersion, distribution).
Fluid diffusion
The process under consideration can occur with substances in all three states of aggregation: gaseous, liquid, and solid. To find practical examples of this, you just need to look into the kitchen.
Borsch boiled on the stove is one of them. Under the influence of temperature, glucosin betanin molecules (a substance due to which beets have such a rich scarlet color) evenly react with water molecules, giving it a unique burgundy hue. This case is an example of diffusion in liquids.
In addition to borsch, this process can also be seen in a glass of tea or coffee. Both of these drinks have such a uniform saturated hue due to the fact that the tea leaves or coffee particles, dissolving in water, evenly spread between its molecules, staining it. The action of all popular instant drinks of the nineties is based on the same principle: Yupi, Invite, Zuko.
Gas interpenetration
Continuing to look further for manifestations of the process in question in the kitchen, you should sniff and enjoy the pleasant aroma coming from a bouquet of fresh flowers on the dining table. Why is this happening?
The atoms and molecules that carry the smell are in active motion and, as a result, are mixed with particles already contained in the air and are fairly evenly dispersed in the volume of the room.
This is a manifestation of diffusion in gases. It is worth noting that the very inhalation of air also belongs to the process under consideration, as well as the appetizing smell of freshly prepared borsch in the kitchen.
Solid Diffusion
The kitchen table, on which the flowers stand, is covered with a bright yellow tablecloth. She received a similar shade due to the ability of diffusion to pass in solids.
The process of giving the canvas some uniform shade takes place in several stages as follows.
- Particles of yellow pigment diffused in the dyeing container towards the fibrous material.
- Then they were absorbed by the outer surface of the fabric being dyed.
- The next step was again the diffusion of the dye, but this time inside the fabric fibers.
- In the final, the tissue fixed particles of pigment, thus staining.
The diffusion of gases in metals
Usually, speaking about this process, interactions of substances in identical aggregate states are considered. For example, diffusion in solids, solids. To prove this phenomenon, an experiment is conducted with two metal plates pressed to each other (gold and lead). The interpenetration of their molecules takes quite a long time (one millimeter in five years). This process is used to make unusual jewelry.
However, compounds in different states of aggregation are also capable of diffusing. For example, there is a diffusion of gases in solids.
During the experiments, it was proved that a similar process proceeds in an atomic state. To activate it, as a rule, you need a significant increase in temperature and pressure.
An example of such gas diffusion in solids is hydrogen corrosion. It manifests itself in situations where hydrogen atoms (H 2 ) that have arisen during a chemical reaction under the influence of high temperatures (from 200 to 650 degrees Celsius) penetrate between the structural particles of the metal.
In addition to hydrogen, diffusion of oxygen and other gases in solids is also able to occur. This process, invisible to the eye, brings a lot of harm, because metal structures can collapse because of it.
The diffusion of liquids in metals
However, not only gas molecules can penetrate solids, but also liquids. As in the case of hydrogen, most often this process leads to corrosion (if we are talking about metals).
A classic example of fluid diffusion in solids is metal corrosion under the influence of water (H
2 O) or electrolyte solutions. For most, this process is more familiar called rusting. Unlike hydrogen corrosion, in practice one has to deal with it much more often.
Conditions for accelerating diffusion. Diffusion coefficient
Having figured out in what substances the process in question can occur, it is worth learning about the conditions of its course.
First of all, the speed of diffusion depends on the state of aggregation of the interacting substances. The higher the density of the material in which the reaction occurs, the slower its speed.
In this regard, diffusion in liquids and gases will always take place more actively than in solids.
For example, if crystals of potassium permanganate KMnO 4 ( potassium permanganate ) thrown into the water, within a few minutes they will give it a beautiful crimson color. However, if sprinkled with KMnO 4 crystals piece of ice and put it all in the freezer, after a few hours, potassium permanganate will not be able to fully color the frozen H 2 O.
From the previous example, we can draw another conclusion about the diffusion conditions. In addition to the state of aggregation, the temperature also affects the rate of interpenetration of particles.
To consider the dependence of the process under consideration on it, it is worth learning about such a concept as the diffusion coefficient. This is the quantitative characteristic of its speed.
In most formulas, it is indicated using the large Latin letter D and in the SI system is measured in square meters per second (m² / s), sometimes in centimeters per second (cm 2 / m).
The diffusion coefficient is equal to the amount of substance scattered through a unit of surface over a unit of time, provided that the difference in densities on both surfaces (located at a distance equal to a unit of length) is equal to one. The criteria defining D are the properties of the substance in which the particle scattering process itself takes place, and their type.
The temperature dependence of the coefficient can be described using the Arrhenius equation: D = D 0exp (-E / TR).
In the considered formula, E is the minimum energy required to activate the process; T - temperature (measured by Kelvin, not Celsius); R is the constant gas characteristic of an ideal gas.
In addition to all of the above, pressure and radiation (induction or high-frequency) influence the diffusion rate in solids and liquids in gases. In addition, much depends on the presence of a catalytic substance, often it acts as a trigger for initiating active dispersion of particles.
Diffusion equation
This phenomenon is a particular form of the partial differential equation.
Its purpose is to find the dependence of the concentration of a substance on the dimensions and coordinates of the space (in which it diffuses), as well as time. In this case, a given coefficient characterizes the permeability of the medium for the reaction.
Most often, the diffusion equation is written as follows: ∂φ (r, t) / ∂t = ∇ x [D (φ, r) ∇ φ (r, t)].
In it, φ (t and r) is the density of the scattering substance at the point r during t. D (φ, r) is the diffusion generalized coefficient for the density φ at the point r.
∇ is a vector differential operator whose components with respect to coordinates belong to partial derivatives.
When the diffusion coefficient is density dependent, the equation is non-linear. When not, linear.
Having considered the definition of diffusion and the features of this process in different environments, it can be noted that it has both positive and negative sides.