Formation of Aggregates

Type of physical science: Chemistry

Field of study: Environmental chemistry

Aggregates are accumulations of particles that have adhered or crystallized by chemical attraction and bonding. An aggregation begins with a central dust particle, which serves as a central, attracting vortex nucleus. Many natural phenomena, such as snowflakes, comets, and galaxies, behave as aggregates.

Overview

Aggregates are accumulations of materials that have stuck themselves together over time. Aggregate formation is a very fundamental process that occurs throughout the universe.

Examples of aggregates include snowflakes, hail, soil, building materials (such as concrete), metallic ores, aerosols, the earth, other planets, stars, and galaxies. Virtually everything is some form of aggregate. All aggregates form in a somewhat orderly, somewhat disorderly fashion.

Nevertheless, the formation of aggregates is based upon several important physical and chemical principles.

Among these principles is chaos theory, a view of the universe's integral makeup that has been developed from the 1960's to the present by a host of scientists, most notably Edward Lorenz, Stephen Smale, Mitchell Feigenbaum, and Benoit B. Mandelbrot. Chaos hinges upon the continuous cause-and-effect relationships that occur throughout the universe and that are responsible for all events that have occurred or will occur in the universe. Chaos deals with the emergence of order from disorder and the emergence of disorder from order in discrete dynamical systems.

Thermodynamics, another principle, is the study of energy and its interconversions within nature. Systems can become ordered because of an energy input, whereas other systems become disordered, or entropic, as they lose energy. Organizing and disorderly systems orient around a single point or set of points in space and time. These points are called strange attractors.

In an organizing thermodynamic system, the strange attractor may be called a vortex nucleus because all components of the system orbit and concentrate (that is, accrete) upon a central point, a nucleus. The vortex nucleus that eventually becomes an aggregate can be any type of matter ranging from an atom to a dust particle to a rock to a drop of water. Whatever the composition of this nucleus, it serves as the focusing point of attraction for all particles that it encounters. These particles, termed "particulates" because of their tiny structure, come within the vicinity of the nucleus by a variety of means, usually by wind, water currents, or gravity. The particles contact the nuclear matter and become part of the growing structure, a process that can be called aggregate formation, accretion, condensation, snowballing, and the like. As more and more particles contact the growing nucleus, the aggregate becomes larger and larger.

The chemical properties that contribute to nuclear growth in aggregate formation include adhesion, absorption, adsorption, and various forms of chemical bonding. Particulates adhere or adsorb to the growing aggregate nucleus by sticking to it. Particulates are absorbed by the aggregate nucleus when they stick and subsequently are attracted inside the nucleus.

Absorption usually occurs in a liquid or semisolid aggregate. Chemical bonding occurs when subatomic interactions occur between atoms and molecules of the aggregate mass and the accreting particulates. These subatomic interactions usually involve the sharing or exchange of electrons and positive-negative charge interactions (that is, ionization, polarity). While there are several types of chemical bonds, the three major types are covalent bonds, the strongest bond where atoms share available electrons; ionic bonds, the attraction between positively charged atoms called cations and negatively charged atoms called anions resulting from the exchange of electrons between the atoms; and hydrogen bonds, the sharing of hydrogen atoms between different molecules.

The accreting particles may form a disorderly aggregate lump or a very patterned, orderly crystal, depending upon the environmental conditions of temperature and pressure acting upon the aggregate-forming system. In either case, the disorderly or orderly structures each can be shown to consist of microscopic disorderly and orderly patterns. Some of these patterns repeat themselves; they are called fractals, reiterations of common patterns at microscopic and macroscopic scales. Fractals are recurrent themes in the orderly/disorderly interplay of aggregate formation.

Aggregates form in a variety of ways based upon these physical and chemical principles. In the case of a snowball, a handful of snow is compressed, in the process melting some of the snow that is lost as liquid water, while the compressed ice/snow aggregate assumes a stable rocklike state. Likewise, a snowball or even a clump of dirt being pushed down a long, snowy incline will accumulate particles of snow, ice, and dirt to generate a larger aggregate snowball.

On a different note, snowflakes begin as liquid water drops high up in the stratosphere.

As the water drop falls through cold air, it freezes and begins to accumulate moisture and dust particles. The added freezing water and dust will accrete to the frozen water drop to construct a snowflake. The beautiful geometric patterns unique to each snowflake are fractal patterns generated by the chaotic disturbance of the snowflake caused by variable wind currents during the snowflake's random fall to earth. Collisions with other snowflakes also contribute to this pattern. The crystallization of liquid water into the frozen snowflake relies upon another chemical phenomenon: The phase change from liquid to solid as energy (that is, heat) is lost from the system.

On a grander scale, there are billions of comets that orbit the sun in a huge mass called the Oort cloud, named for its discoverer, astronomer Jan Hendrik Oort. Some of these comets, such as Comet Halley or Comet Kohoutek, periodically pass through the inner solar system during their orbits around the sun. Each comet is an aggregate of water ice and interstellar dust, including some organic molecules. Comets appropriately have been called "dirty snowballs."

Each comet can range in size from a few to several tens of kilometers in diameter. As a comet moves through space far from a nearby star, it accumulates frozen water and other interstellar molecules, thereby increasing the comet's size. These particles are attracted to the comet by its gravitational pull and accrete to the cometary nucleus by freezing, adhesion, and so forth. When a comet moves close to a star such as the sun, heating of the outer layers of the cometary aggregate nucleus vaporizes some of the water into the semivacuum of space, thereby generating the comet's tail and loss of cometary mass.

The evolution of the solar system has involved violent aggregate formation, especially during the first 1 billion years of planetary formation. According to most theories of solar system formation, the sun, planets, moons, asteroids, and comets condensed from a gaseous nebula, a cloud of interstellar dust that was spewed out from a supernova explosion. Most of the matter from this nebula, principally the lighter elements hydrogen and helium, concentrated as an aggregate in the center of this spiraling presolar nebula. This central aggregate accumulated enough mass to condense with the occurrence of thermonuclear fusion, thereby generating a star that is known as the sun. Meanwhile, the remaining orbiting dust particles condensed into rocky aggregates that were so numerous that they began hitting one another. Some of these collisions were elastic, with the boulders bouncing off one another. Yet, most collisions were inelastic, the boulders combining to form one mass. These collisions continue to the present day, although most collisions occurred during a period between 4.6 billion and 3.8 billion years ago, a time referred to as the heavy bombardment. By the end of this period, most of the planets had formed, along with their orbiting moons. Between the orbits of Mars and Jupiter lies a chaotic belt of millions of asteroids that failed to accrete into a planet. The planets are aggregates. Earth, Mercury, Venus, Mars, and Pluto are primarily aggregates of iron and silicates. Jupiter, Saturn, Uranus, and Neptune are aggregates of hydrogen, helium, and nitrogen gases. Every now and then, an asteroid or comet pummels a planet, thereby adding to the planet's aggregate mass. On Earth, meteorites add to the planet's mass every few minutes.

In the atmosphere of Earth, aerosols are aggregates that form by the absorption and adsorption of particles to water droplets. A naturally formed aerosol will consist of a liquid drop of water surrounding a central nucleus of carbon, metal (such as iron and lead), salt, ash, and dust. Additional materials in the water droplet include organic hydrocarbons (for example, alcohol and aldehydes), nitrates, sulfates, ammonium, oxygen, ozone, and peroxides. Some of these materials are toxic to life. The absorption of sunlight energy can stimulate chemical reactions within the components of the aerosol droplet, thereby producing more toxic substances, such as acid and cyanide. Air pollution, primarily caused by heavy industry and automobiles, generates tremendous quantities of aerosols that produce smog in large cities and that rain downwind from the pollution source. Acid rain is the result of pollution-generated aerosols. Acid rain causes considerable property damage, destruction of forests, and severe damage to freshwater plant and animal life.

Aggregates form throughout nature. Even living organisms are prepatterned aggregates based upon the genetic code of deoxyribonucleic acid (DNA). Proteins and other structures within living organisms behave as aggregates. Clouds are vapor aggregates. The accumulation of leaves in doorways during autumn follows the principles of aggregate formation, helped in part by the air currents created by opening and closing doors. Household dust (that is, mostly dead skin) and cat hairs aggregate on floors by the same principles. Aggregate formation is central to the self-organization of the universe.

Applications

The study of aggregates and the mechanisms of their formation is of prime concern to many physicists and chemists. The structure of the universe, the mechanisms of geological change on and within the earth, the manufacture of useful materials for industry and the consumer, and even events and processes within human bodies involve the formation of aggregates. The overall process of accretion for many different aggregate systems has been adequately modeled. Nevertheless, the intricate details of aggregate formation remain to be deciphered fully.

Aggregate formation on the universal scale explains the formation of stars, quasars, galaxies, and planetary systems around stars. From chaos theory, the ever recurrent spiral pattern is seen, the condensation of cosmic matter into a spiraling vortex centered around a vortex nucleus. This phenomenon repeats itself even unto microscopic scales, where aggregate formation explains the structures of biological molecules such as DNA and protein.

The aggregate formation of the planetary system is of utmost concern to humanity. The earth is an aggregate of materials that have collided over the past 4.6 billion years. While much of this bombardment occurred during the first 1 billion years of planetary formation, the possibility of further collisions still exists, albeit at lower levels because of the progressive reduction of randomly moving objects within the solar system. Still, there are roughly 1 million asteroids, most less than 2 kilometers in diameter, that chaotically move through the solar system. Several asteroids stray close to the earth each year, some passing within a few hundred thousand kilometers of a direct collision with Earth. Asteroid collisions with Earth have happened in the past and will happen in the future, in an instant and without warning.

Approximately 65 million years ago, the dinosaurs and many other species went extinct, possibly resulting from massive asteroid collisions with the earth, according to a theory proposed by Walter and Luis W. Alvarez. The future of human civilization could be altered forever, in an instant, by such an impact. Unfortunately, many scientists have been ridiculed because of their efforts to set up asteroid-tracking projects designed to help prevent such a disaster from occurring.

On Earth, geological processes dynamically shape and reshape the planet, rearranging substances under the extreme pressures and temperatures of the deep interior core and mantle to generate a variety of aggregate rocks, minerals, and geological formations. The four principal geological processes that cause aggregate formation on Earth are erosion (wind, water), plate tectonics, volcanism, and meteorite/asteroid impacts. An understanding of these events can be of great use in identifying the locations of specific mineral deposits (such as copper, gold, and titanium), "fossil" fuels (such as coal and natural gas), and geologically stable areas for the storage of hazardous and radioactive wastes. Such knowledge can be useful in protecting the public from natural disasters (such as earthquakes, floods, volcanic lava flows, and avalanches).

In industry, the production of aggregates is used in the manufacture of thousands of composite materials, items composed of different substances. Such aggregate items range from clothing to food to cosmetics to soda containers to automobiles. The mist from a spray can, whether it be paint or deodorant, consists of aerosol aggregates. The production of useful and safe aggregate consumer items is a major focus of modern industry. In order to produce many of these items, aggregates from the earth must be broken down into their components (for example, mining iron and gold from their respective ores). The renewed interest in and immense need for recycling will involve the manipulation of aggregates.

In the biological sciences, the understanding of aggregate formation will help scientists to comprehend the nature of life itself. All animals, including humans, begin as hollow aggregates of cells, each aggregate individual differentiating into the various tissues and organs of the fully developed adult body. Understanding how this living aggregate formation occurs and how it is controlled will enable scientists to treat disease, to extend the length and quality of life, to control cancer, and to clone individuals.

Context

The universe, from microscale to macroscale, exhibits self-organizing properties.

Superstrings aggregate into quarks; quarks aggregate into protons, neutrons, and electrons, which together aggregate to form atoms. Atoms aggregate to form molecules; molecules combine to form aggregate substances and solutions. More massive aggregates produce entities such as planets and stars. Aggregates of stars form galaxies, aggregates of galaxies form universes, and so on. The universe's orderly self-organization involves accretion and aggregate formation at all levels from the infinitesimally small to the infinitely large.

The origin of the universe was roughly 15 billion years ago--the big bang. While there are many competing ideas and theories concerning the nature of the big bang, most theoretical astrophysicists agree that everything in existence today began 15 billion years ago as a singularity, a single point in space-time, an infinitely large vortex occupying zero volume and containing the matter of a trillion galaxies. For some reason, matter in the form of energy was released from this singularity (that is, the big bang) at supraluminal (faster than light) velocities.

The universe expanded and continues to expand today, although the current expansion rate is considerably slower because of the accumulated gravitational attraction of the universe for itself.

As the universe cooled during the first 1 billion years following the big bang, some energy transformed into matter. Once matter formed in the early universe, the process of aggregate formation was set in motion. The gravitational attraction of matter within the early universe led to the formation of stars, a type of aggregate formation. Stars accreted to form galaxies. Galaxies accreted to form superclusters of galaxies, and so forth. Futhermore, planetary systems accreted around some stars. Consequently, aggregate formation is a fundamental process in the emergence of order throughout the matter-dominated universe.

The accretion of matter to form aggregates centers around strange attractors or vortex nuclei. On a macroscopic scale, gravity serves as the strange attractor. This phenomenon is well-illustrated in the solar system, where the center of the spiral solar aggregate is the largest object: the sun. Likewise, galaxies spiral around dense starry vortex nuclei that may contain the ultimate strange attractors: black holes. On the microscale, processes such as air or sea eddy currents, the erosion of soil, and molecular arrangements within living organisms display aggregate formation. A common theme in many aggregate formations is the spiral pattern exhibited by galaxies, seashells, and DNA.

In everyday life, aggregates play important roles. Virtually all the materials that people use are aggregates. Natural aggregates are encountered in food, sand, rocks, diamonds, and wood, to name a few. Synthetic aggregates are used in soda cans, clothes, televisions, and automobiles. The list of such items is endless. Modern industry is becoming more and more efficient at the mining of aggregate mineral ores, the physical and chemical removal of pure minerals from these aggregate ores, and the reaggregation of pure minerals to produce the thousands of materials, items, tools, instruments, and machines that everyone takes for granted everyday. These materials save lives and assist people in work and recreation.

Principal terms

ABSORPTION: the attraction and engulfment of small particles by a substance

ADSORPTION: the attraction of particles to a particular substance such that the particles stick to the surface of the substance without being absorbed

AEROSOL: a substance such as a water droplet that absorbs particulate matter

AGGREGATE: a steadily increasing accumulation of matter in which contacted particles are absorbed or adsorbed to the existing accumulated matter

CHEMICAL BOND: an attractive force between atoms and/or molecules, usually involving the sharing or transfer of electrons and variable electromagnetic charges

CRYSTAL: an orderly, semipatterned growth of a substance as it accumulates and solidifies more particulate matter in space and time

PARTICULATE: a term referring to small dust particles or even molecules, compounds, and atoms that interact in various solutions or colloids

POLARITY: a phenomenon in which particles, molecules, compounds, atoms, or subatomic particles become positively or negatively charged, thereby affecting their interactions with other substances

STRANGE ATTRACTOR: an entity that serves as a concentrating point for dynamic phenomena such as the accretion of smaller particles to form an aggregate

VORTEX NUCLEUS: a strange attractor; the central focusing point for an accreting aggregate system of accumulating small particles

Bibliography

Baugher, Joseph F. THE SPACE AGE SOLAR SYSTEM. New York: John Wiley & Sons, 1988. This reference book describes the nature and structure of the solar system. Suitable for a general audience and contains beautiful photographs and illustrations. The sun, planets, moons, asteroids, and comets are described in detail. Chapter 17, "Comets--The Primordial Snowballs," describes the structure and evolution of comets as aggregate masses. Chapter 20, "The Origin of the Solar System," describes major theories of aggregate formation in the development of the sun and planets.

Gillespie, Ronald J., David A. Humphreys, N. Colin Baird, and Edward A. Robinson. CHEMISTRY. 2d ed. Boston: Allyn & Bacon, 1989. This lengthy introductory chemistry book for undergraduate chemistry majors is clearly written and beautifully illustrated. All major subject areas within chemistry are described and supported by many good examples, problems, and illustrations. Chapter 3, "The Atmosphere and the Gas Laws," and chapter 11, "The Solid State," describe the behaviors of these matter phases during their respective evolutions.

Gleick, James. CHAOS: MAKING A NEW SCIENCE. New York: Viking Press, 1987. Gleick's popular and entertaining book is the most thorough, exciting survey of chaos theory and fractal geometry that is available for the general public. He describes the development of research in these fields since the 1950's and cites numerous important experiments. Also, the lives of leading scientists in this field of work are chronicled. Strange attractors and vortex nuclei are discussed throughout the text.

Joesten, Melvin D., David O. Johnston, John T. Netterville, and James L. Wood. THE WORLD OF CHEMISTRY. Philadelphia: Saunders College Publishing, 1991. This outstanding chemistry and physical science textbook is a wonderful introduction to chemistry for a general audience. While it is detailed, the book clearly presents major chemical topics and strongly emphasizes their applications to everyday life. Chapter 18, "Clean Air--Should It Be Taken for Granted?," contains an excellent presentation of aerosol structure and formation.

Mason, B. J. "The Growth of Snow Crystals." In THE PHYSICS OF EVERYDAY PHENOMENA: READINGS FROM SCIENTIFIC AMERICAN, edited by Jearl Walker. San Francisco: W. H. Freeman, 1979. This fascinating 1961 SCIENTIFIC AMERICAN review article brilliantly describes crystalline aggregate formation for a variety of substances. Water ice and snowflakes are described in detail. Several beautiful photographs and illustrations supplement the well-written text. The physical conditions and pattern formation for several different substance aggregates are summarized in a very informative data table.

Sagan, Carl, and Ann Druyan. COMET. New York: Random House, 1985. In this outstanding book for a general audience, the eminent astronomer Carl Sagan and his wife describe the history, properties, and evolution of comets. Extensively illustrated with beautiful photographs and art. Chapter 12, "Mementos of Creation," describes the origin and evolution of comets. Other chapters are devoted to the chemical makeup and aggregate formation of comets.

Soderblom, Laurence A., and Torrence V. Johnson. "The Moons of Saturn." SCIENTIFIC AMERICAN 246 (January, 1982): 100-113. This very informative astronomy review article, updated for this special edition of SCIENTIFIC AMERICAN, is a description of the seventeen known moons orbiting the planet Saturn. Relevant astronomical, geological, and chemical data on each of these moons are provided from the Voyager missions. Many beautiful photographs and a diagrammatic scheme for the aggregate formation of these moons are also vividly presented.

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