What is Diffusion?

Diffusion can be classified into many different types of diffusion, and their needs and states are generally different. Some diffusions require media, while others require energy. Therefore, we cannot generalize different kinds of diffusion. There are biological diffusion, chemical diffusion, physical diffusion, and so on.

Chinese name: diffusion
Foreign name: diffusion

explain in detail: Diffusion effect
Pinyin: kuò sàn

Introduction to Diffusion

Basic information

[Title] Diffusion

Pinyin kuò sàn

Part of speech

[Explanation] Expand and spread out.

Detailed explanation of diffusion

[Example]

Diffusion effect.
The toxin has spread throughout the body.

Atoms that rapidly vibrate at equilibrium positions in a crystal can gain energy by thermal excitation, overcome potential barriers, and migrate to neighboring positions. Such an atom migration phenomenon is called atomic diffusion. Because the local fluctuation of thermal energy is random, the atomic migration caused by thermal excitation is also random Brownian motion. Diffusion is the only mass transfer process in a solid. Most high-temperature solid-state reactions, such as solid solution, precipitation, phase change, recrystallization, grain growth, creep, sintering, pressure welding, etc., are completed by solid-state diffusion processes. Atoms in a complete crystal cannot diffuse, and the diffusion process must be accompanied by the transport of point defects (including lattice vacancies, self-interstitial atoms, and interstitial impurity atoms). Vacancy and self-interstitial atoms can be generated by thermal excitation, so they are often referred to as thermal defects. They also occur when irradiated or deformed at a lower temperature and freeze in the crystal ^[1] .

Diffusion scientific term

Diffusion: A phenomenon in which molecules of a substance are transferred from a high-concentration region to a low-concentration region until they are evenly distributed. The rate of diffusion is proportional to the concentration gradient of the substance.

Matter migration due to the thermal motion of molecules (atoms, etc.). It usually occurs in one or several substances in the same state or between different states, which is caused by the concentration difference or temperature difference between different regions, the former is mostly. Generally, diffusion is carried out from a region with a higher concentration to a region with a lower concentration, until the concentration of various substances in each part of the same state of matter reaches uniformity or the concentration of various substances between the two states of state reaches equilibrium. Obviously, due to the thermal movement of molecules, this "uniform" and "equilibrium" are all "dynamic equilibrium", that is, the number of particles exchanged on both sides of the interface is equal at the same time, such as red-brown nitrogen dioxide in still air The diffusion in blue, the infiltration of blue copper sulfate solution and still water, the carburization of the surface of steel parts and the process of making pure semiconductor materials into N-type or P-type semiconductor doping are all concrete manifestations of diffusion phenomena; During the formation of a semiconductor PN junction in electricity, the diffusion of free electrons and holes is the basic basis. The diffusion rate is the largest in the gas, the second in the liquid and the smallest in the solid, and the larger the concentration difference, the higher the temperature, and the smaller the mass of the particles involved, the larger the diffusion rate.

The diffusion process is a process in which molecules break away from each other's molecular gravity. In this process, molecules need energy to convert into kinetic energy, and they also need to absorb heat from the outside.

In crystallography, diffusion is the basic way of moving particles in matter. When the temperature is higher than absolute zero, particles in any system are moving thermally. When there is a gradient (chemical potential, concentration, stress gradient, etc.) in the substance, the directional migration of the particle due to thermal movement is called so-called diffusion. Therefore, diffusion is a mass transfer process, showing a directional migration of matter on a macro scale. In gas and liquid, in addition to diffusion, the method of material transfer can also be carried out by convection and other methods; in solids, diffusion is often the only method of material transfer. The essence of diffusion is the random movement of particles. The generation and recombination of defects in crystals is a kind of macroscopic disordered diffusion with no directional migration of particles. The main feature of a crystal structure is its regular arrangement of atoms or ions. However, the arrangement of atoms or ions in actual crystals always deviates more or less from strict periodicity. In the process of thermal fluctuations, some atoms or ions of the crystal break away from the lattice point into the interstitial position or crystal surface of the crystal due to the severe vibration, while leaving vacancies inside the crystal. Obviously, the vacancies left on the atoms or the original lattice points in the interstitial positions will not be permanently fixed. They will be able to regain energy from the process of thermal fluctuations, and constantly change positions in the crystal structure. Irregular movement from one place to another. The air pollution, liquid leakage, oxygen tank leakage and other phenomena encountered in daily life and production process are the directional migration of gas in gaseous medium, liquid in solid medium, and gas in solid medium in the presence of gradients. That is the diffusion process. It can be seen that the phenomenon of diffusion is widespread.

The diffusion of atoms or ions in the crystal is the basis of solid-state mass transfer and reactions. Many important physical and chemical processes in the preparation and use of inorganic materials, such as doping of semiconductors, formation of solid solutions, coating of metal materials or sealing with ceramic and glass materials, and erosion of refractory materials are all closely related to diffusion and are affected by Control of the diffusion process. Through the study of diffusion, these processes can be quantitatively or semi-quantitatively calculated and theoretically analyzed. The high-temperature kinetics of inorganic materialsphase transitions, solid-phase reactions, sintering, and so ondepend on the speed and progress of diffusion. In addition, many properties of inorganic materials, such as electrical conductivity and thermal conductivity, also depend directly on the migration behavior of micro-charged particles or carriers under the action of an external electric field or temperature field. Therefore, studying the diffusion phenomenon and the laws of diffusion dynamics can not only theoretically understand and analyze the structure of solids, the bonding state of atoms, and the mechanism of solid-state phase transitions; but also many dynamic processes in the preparation, processing, and application of inorganic materials Effective control has important theoretical and practical significance.

Diffusion process

Dispersal. The process by which an individual organism or its dissemination body (such as spores, seeds) spread to other regions. Also known as dissemination. Various organisms can expand their habitats and distribution areas through different diffusion methods; diffusion also promotes the succession of biological communities and increases species diversity; large-scale diffusion combined with geographical isolation is an important condition for species differentiation; a new environment It means that different natural selection mechanisms can change the gene frequency of the population, so diffusion is an important factor to promote biological evolution.

Diffusion phenomenon

It is a common natural phenomenon for organisms to spread from one region to another. This exchange of species between different places has promoted the succession of biological communities in various places and increased the diversity of species in each region. Large-scale diffusion combined with geographical isolation is species differentiation

diffusion Important conditions; for species, the new environment means different natural selection mechanisms, which will change the gene frequency of the population, so diffusion is one of the important factors that promote the evolution of biological germline. After the volcanic eruption destroys the native organisms, wind and ocean currents will bring new life, and various microorganisms, plants and animals will settle here. Pioneer species with high tolerance to abnormal environments spread first, followed by species with strong competitiveness in stable environments. They established stable communities on the basis of the former development.

Diffusion type

Active self-reliance

Biological diffusion includes two types: active (self-supported) and passive (by external force). Microorganisms and plants that lack mobility are mostly passively transmitted (see Transmission (Plant)), while animals actively spread in multiple lines.

Active Diffusion Cricket creatures need space, food, and spouse to survive and reproduce. When organisms are too dense, the need for space is not easy to meet, and insufficient food sources and environmental degradation will limit the development of the population. Compared with plants, animals have the ability to actively find living space, food sources and avoid danger.

The active spread of animals mostly occurs during the reproductive period. Many birds and beasts are looking for mating and fertility sites during the mating season. When the space near their birth place is occupied, they are forced to spread long distances. Most of these individuals often die due to adverse environments, but as long as a few individuals encounter suitable habitats with abundant food sources and few natural enemies, new populations may be established to expand the distribution area of species.

Diffusion performance

Many insects also have a large ability to spread, and some species also show polymorphism, that is, there are two or more types in the same species with different forms and behaviors. One of them specializes in diffusion, with wings or other Structure that facilitates diffusion. Some polymorphisms are hereditary. For example, the gray line moth has two different genotypes. One type of larva is not dense and easily spreads, and the other is not. The two coexist and do not replace each other, but the number The ratio varies with population density. Non-hereditary polymorphisms have different foundations, and some are developmental, such as the wingless larvae of butterflies, dragonflies, and bees that feed without spreading, while the adults spread and rarely eat. Some insects, such as adult maggots, do not even feed at all, only spreading, mating, laying eggs until death. The situation is similar for ants and termites. In this kind of community insects, the structure and behavior of different community members (different classes) are specialized, each performing different functions, and only the wing type is responsible for spreading to new areas in search of new breeding bases. Both cases belong to the conventional division of labor phenomenon, one is the division of labor between larvae and adults, and the other is the division of labor among community members. However, the polymorphism of migratory locusts is unconventional. Only when the population density increases and food is scarce can they change from the diaspora type to the diaspora type (that is, the diffuse type), resulting in large-scale spread. Everywhere the grass is preyed by it.

There are also examples of active diffusion in plants. For example, many leguminous plants have their fruit pods split and scattered the seeds when they mature. After the impatiens are ripe, if they are touched by external forces, the peel will curl inward and the seeds will pop out. However, the ability to spread is not high, and the range of transmission is limited.

Passive diffusion mainly depends on air (wind), water or other organisms.

Microorganisms and tiny spores and seeds are easily transmitted by the wind, and even larger animals can be carried far by the wind. Over the middle of the ocean, insects and spiders have been collected. Marked recapture experiments confirmed that armyworms can migrate in groups with the southwest airstream prevailing in southern China in March and April, and spread from southern China to the Jianghuai Basin. It was also confirmed that the number of brown planthoppers in Japanese rice fields increased sharply each June, and they were introduced by wind from China.

The spread of wingless larvae is dominated by wind. After the moth larvae hatched in the female's sac, they swarmed out from the excretory holes under the sac, shed silk and drooped, spreading around with the wind. The seeds of some plants are as small as dust (such as Orchidaceae), and some of them have specialized structures such as wings (maple, elm, etc.), hairs (willow, dandelion, etc.) and balloon-shaped sacs (such as physalis). Conducive to spread with the wind.

Terrestrial organisms spread by water must be able to float on the water, be difficult to be penetrated by water, and maintain the internal osmotic balance. The surface of some plant fruits that are specially diffused by water flow has waxy or fibers that are not easily permeable to water. The tissue contains air, so the body is light and not sinking. Coconut has a well-developed fibrous layer on the surface, which can drift on the water surface for a long time.

Huge ocean currents have a fixed itinerary, so estuaries or freshwater aquatic animals, and even some non-flying land animals may spread from one continent to another. Although some animals can swim, their long-distance spread may be achieved with driftwood and ice.

Animal-borne parasites spread by hosts, and some parasites parasitize different hosts at various stages of their life history. The process of changing hosts is either active or passive. Some parasites (such as schistosomiasis) seek their next host by their own actions, while others (such as malaria parasites) spread completely passively. Plasmodium then infects new hosts as they feed on the blood of different host individuals.

Many organisms attach to the surface of other organisms and spread. There are many ways to attach: some plants have thorns or hooks on their fruits and seeds (such as the fruits of cocklebur), and some have viscous secretions (such as flax seeds); some small insects, mites, etc. often adhere to vertebrates. On the fur, some small animals adhere to the feet of migratory birds.

Some plant seeds are spread by animal feeding. For example, herbivores often eat a large number of seeds, which can be discharged to multiple places with feces. Many fleshy fruits attract animals to eat by virtue of their color, aroma and taste, and their seeds are either spit out or discharged with feces. These seeds are generally resistant to the erosion of animal digestive juices, and some seeds can germinate only after the seed coat acts through the digestive tract.

Humans are also important biological diffusion factors. The long-distance circulation of people and goods intentionally or unintentionally spreads a large number of organisms. Cotton's main pest, cotton bollworm, originated in India and was introduced into China's coastal areas in the 1930s. Now it has spread to all cotton regions in China except Xinjiang. In addition, human beings have consciously introduced or cultivated many excellent grains, forest trees, and poultry and livestock, and promoted them worldwide.

Evolution of Diffusion Force In nature, there are two distinct evolutionary trends in diffusion force. One is the increase in diffusion force, and the other is the weakening or disappearance. For example, the western sky caterpillar of Vancouver Island near Victoria, the capital of British Columbia, Canada. In its population, weakly diffusing individuals lay eggs near their habitats, and their offspring often die in harsh climates. Strongly diffusing individuals lay eggs far away. Offspring born in a more favorable environment may survive, and are mostly lively individuals. Choosing from generation to generation, this species's ability to spread has continued to increase. On the other hand, if the chance of finding an unexplored, suitable environment by diffusion is small, natural selection will counter proliferation. For example, there are more birds and insect species that can't fly in island and mountain environments than on flat continents. When Darwin inspected the Madeira Islands, he noticed that 200 of the 550 species of beetles on the island lacked wings, while 23 of the 29 genus beetles were hidden during high winds, and only went out when the weather was better. Darwin believes that this situation is mainly due to the fact that good flying individuals may be blown into the sea by high winds during flight, while individuals with underdeveloped wings or habitual laziness can survive more. Animal geographer PJ Darlington believes that wings help escape when encountering an enemy, but with little or no natural enemies on the island, wings become useless. Similarly, the diffusivity of the plants on the island has significantly decreased or almost completely disappeared. The fruit of some plants becomes larger, but the appendages (such as barbs) used for transmission have not increased correspondingly, and some seed appendages have even deteriorated, and the number of such seeds or fruits often decreases accordingly. This evolutionary variation can reduce the ineffective spread to the outside of the island, while the larger the individual can store more nutrients, it is conducive to seedling growth.

Manual selection can also change diffusion capacity. For example, human cultivation of crops requires high yields, not high diffusion. The seed scatter of grains such as wheat and rice currently planted has disappeared, and the cracking mechanism of flax and poppy capsule has also been degraded.

The mathematical model of the diffusion phenomenon is based on the observation of seed scattering and the insect diffusion test using the marker recapture method. It is found that as the diffusion distance increases, the number of spreaders that can be found decreases exponentially.

Seed scattering is passive diffusion; some small insects are not strong in flying, and their directions and distances are basically random, so the above rules are also shown.

Diffusion is an important factor in studying population changes. In production practice, the spread of pests and weeds can cause damage to human health and agriculture and animal husbandry. Therefore, understanding the characteristics of biological diffusion is of great significance for controlling pests. The control of many bacterial and viral human and animal diseases and crop diseases often depends on cutting off the transmission of pathogens. Rice leaf rollers and brown planthoppers are important rice pests that spread in large distances in southern China. The number of field populations often increases or decreases suddenly. Mastering its spread can predict its occurrence and period. When using predatory natural enemies to control pests, it is necessary to study the ability of natural enemies and pests to spread in various environments (including different terrains, plant growth, climate change, etc.), the direction of propagation, and the possible distance to spread in order to determine the artificial release of natural enemies. Quantity, field release point layout and release point height, etc.

Diffusion Communication Terminology

Diffusion ^[2] (diffusion) is the diffusion of innovation among members of a social group over a period of time and through specific channels. It is a special type of communication that contains information related to new ideas.

Diffusion is a special type of transmission. The message transmitted is about a new idea, and the novelty of the idea gives a special kind of spread. New means that there is some degree of uncertainty in the diffusion.

Diffusion is a social change that can be defined as the process by which the structure and function of a social system change. It includes both spontaneous transmission and conscious transmission.

Diffusion

The phenomenon that different objects enter each other when they contact each other is called diffusion ^[3] .

Brownian motion : refers to the irregular movement of pollen particles suspended in liquid. It is not the motion of the molecule itself. The irregular motion of liquid molecules is the cause of Brownian motion. Although Brownian motion is not a molecular motion, its irregularity reflects the irregularity of liquid molecular motion ^[3] .

1. The severity of Brownian motion is related to particle size and temperature. The smaller the particles, the higher the temperature and the more obvious Brownian motion ^[3] .

2. The line drawn in the position of a solid particle every 30 seconds is depicted in the experiment, not the movement trajectory of the particle ^[3] .

3. The cause of Brownian motion is caused by the imbalance of the impulse when liquid molecules collide with small particles ^[3] .

Intermolecular interaction forces There are gravitational and repulsive interactions between molecules. Both gravitational and repulsive forces decrease with increasing intermolecular distance, but the repulsive force changes faster than the gravitational force. The actual displayed molecular force is the combined force of gravitation and repulsion ^[3] .

Surface diffusion

Refers to the movement of atoms, ions, molecules and atomic groups on the surface of a solid along the surface. Surface diffusion occurs when there is a chemical potential gradient field on a solid surface, the concentration of a diffusing substance changes, or the topography of the sample surface changes. Surface atoms move on the plane where the periodic potential is interrupted. Therefore, although surface diffusion and body diffusion have many similarities, they also have their own characteristics. The main reason is the high diffusivity. In the whisker growth experiment, the observed surface diffusivity is as high as 1 cm 2 / sec, which is similar to the gas phase diffusion process. The surface diffusion process is related to the orientation of the surface. The heterogeneous surface diffusion activation energies of the atoms on the (111) and (113) platforms of iridium are 0.52eV and 1.17eV, respectively, which are very different. The surface state, such as the presence of adsorbed substances, will also strongly affect the surface diffusion. Fluctuations in thermal vibrational energy may allow surface atoms to gain enough energy to overcome the surface barrier and become adsorbed atoms at neighboring positions. This is the simplest self-diffusion of a complete crystal surface. There are various types of defects on the actual crystal surface. Depending on the surface conditions, surface atoms can move on the platform or along the steps; surface atoms can also fill the surface vacancies, causing the vacancies to migrate; or more complex diffusion processes occur ^{[ 4]} .

The experimental methods for studying surface diffusion are mainly radiotracer, material transfer, field ion microscope and field emission techniques. People use these experimental methods to measure the surface diffusion coefficient to study the surface dynamics and determine the surface potential. Surface diffusion is closely related to sintering, crystal growth, thin film technology, creep, and so on. However, the understanding of the mechanism of the surface diffusion process needs to be deepened ^[4] .