Adhesion

Type of physical science: Chemistry

Field of study: Chemical processes

Adhesion is a chemical process in which unlike molecules and/or compounds are attracted to each other, thereby increasing their chemical interactions and slowing their separation from each other (sticking). The phenomenon, which has many industrial applications, is found throughout nature in both living and nonliving processes.

Overview

Adhesion is the chemical attraction and sticking between the molecules of two surfaces that are in direct contact. The two surfaces must have a different chemical makeup. They must be composed of different molecules. This behavior differentiates adhesion from cohesion, the attraction between like molecules. Together and separately, adhesion and cohesion are important processes that occur throughout nature.

The universe consists of matter, and matter can exist in the form of any of ninety-two naturally occurring elements. Each element consists of unique atoms having specific physical and chemical properties. Atoms of single elements combine to produce molecules; atoms of different elements combine to produce compounds. Each molecule and compound has its own unique physical and chemical properties based upon the arrangement of atoms in its structure.

Among these physical and chemical properties is the behavior of the molecule or compound when it interacts with other atoms, molecules, and compounds.

When different substances combine, they form mixtures or solutions. The three principal mixture/solution types are homogeneous mixtures, heterogeneous mixtures, and colloids. A homogeneous mixture occurs when different substances are combined such that one cannot distinguish between the different substances. Numerous homogeneous mixtures exist, including coffee, seawater, glass, and other commonly used or encountered items. A heterogeneous mixture occurs when different substances combine, but one can tell them apart.

Examples of heterogeneous mixtures include oil-and-water, chocolate chip ice cream, and so forth. A colloid appears to be homogeneous, but closer analysis with a light beam reveals fine particles suspended in solution. An example of a colloid is air, which contains floating dust particles. Any of these three solution types can exist as solids, liquids, or gases.

Solutions of various substances have different adhesive properties. Strong adhesives stick tightly to surfaces. Weak adhesives stick loosely to surfaces if they stick at all. Examples of strong adhesives include tape, glue, plaster, paint, gum, oil, pine sap, and the like. These substances stick, or adhere, strongly to surfaces that they contact. Some adhesives adhere better to certain types of surfaces than to others. Such differential adhesive properties of various substances depend upon the inherent physical and chemical properties of each adhesive substance.

Adhesive properties depend upon interactions between molecules located upon the surfaces of materials that are in direct contact. These molecules have specific physical and chemical properties that are unique to each molecular type. Such properties include intermolecular forces such as hydrogen, ionic, and covalent bonding. These forces hold atoms and molecules together by the sharing of electrons, the exchange of electrons, or positive-negative charge interactions. Hydrogen bonds involve the sharing of hydrogen atoms between the atoms of different molecules. Ionic bonds involve the exchange of electrons between atoms and/or molecules, resulting in oppositely attracting positively and negatively charged atoms called ions. Covalent bonding involves the sharing of electrons between atoms. These intermolecular bonding forces contribute to the attractiveness of adhesive molecules for various substances.

Chief among these interactions is the oppositely charged polarity of ionic bonding. In adhesive interactions, the primary attractive force is the pull of opposite charges between the molecules located upon the contact surfaces. Positively charged molecules on one surface are attracted to negatively charged molecules on the contact surface, and vice versa. Certain adhesives will work only on specific types of surfaces (for example, wood, plastic, metal) because of the polarity of the surface molecules.

The practical and industrial applications of adhesives are considerable. Nevertheless, adhesion is a process found throughout nature. For example, water adheres to many types of surfaces, including glass, metal, and leaves. Capillary action, in which water or other liquids move up a tube, is caused by the adhesive interactions between a liquid and a solid surface surrounding the liquid.

The tires of an automobile must adhere to the road surface, at least to a certain degree.

Too much adhesion in the rubber of the tires would create considerable friction (resistance) between the tire and road, resulting in considerable wearing of the tires. Too little adhesion would cause the tires to slide on the road, especially if water is present. Consequently, tire manufacturers must take into consideration both of these extreme conditions. A little adhesion is essential for all processes, including the efficient functioning of machines.

Within the earth and throughout the universe, adhesion is an important process that contributes to aggregate formation, the accumulation of materials to produce larger structures.

The earth and other planets in the solar system formed from adhesive collisions between objects floating in space. On Earth, soils can be classified into several categories: sands, clays, silts, and loams. These soil types are based upon the adhesive properties of fine particles and minerals found within the soils of given areas.

Adhesion is principally a phenomenon of solid-solid and liquid-solid interactions. The viscosity, or turbulent resistance, of liquid solutions helps to strengthen their adhesiveness.

Liquid-liquid adhesion occurs. Yet, gas-liquid, gas-solid, and gas-gas adhesions do not occur.

Within living organisms, adhesion is a critical process for a variety of important processes, ranging from tissue connections to the circulation of fluids through body vessels to the movement of body parts in animals. Adhesion, cohesion, viscosity, and other related phenomena play major roles in the movement of bodily fluids and proper functioning of these fluids. Fluids compose 60 to 80 percent of all living organisms on Earth.

In vascular plants, water, minerals, sugar, and organic nutrients are transported through bundles of vessels called vascular cambia. These vessels allow long-distance transport through the plant tissues, thereby enabling the vascular plant to grow to a large size and away from water.

Examples of vascular plants, also called tracheophytes, include all trees, grasses, ferns, and flowering plants. Cross sections through any vascular plant tissue reveal vascular bundles containing two vessel types: xylem and phloem. Xylem vessels transport water and minerals from the soil through the roots to all other parts of the plant. Phloem vessels transport sugar and organic nutrients from leaves to all other parts of the plant. In both xylem and phloem, the processes of adhesion and cohesion enable fluid to move through the vessels. Xylem and phloem vessel walls are composed of the complex polysaccharide substance called cellulose. In xylem, water moves upward against gravity by adhesion and cohesion. Water molecules have great cohesiveness for one another. Simultaneously, water molecules adhere to cellulose, thereby assisting the upward movement through the plant. In a similar manner, sugar and organic nutrients, collectively called sap, flow through the phloem vessels by adhering to the phloem's cellulose walls. When sap falls from a tree onto a car and solidifies, one can appreciate the adhesive strength of sap. The most effective treatment for such a situation is the application of a synthetic adhesive known as car wax.

In vertebrate animals, such as fish, amphibians, reptiles, birds, and mammals, elaborate closed vessel circulatory systems conduct oxygen- and nutrient-rich blood to all the cells of the animal body. Blood, composed of cells and fluid plasma, must have strong cohesiveness plus adhesive attraction for the materials composing the blood vessel walls. Animal blood vessels are composed of epithelial, muscular, and nervous tissues, but it is connective tissue that gives the vessels their strength and adhesive attraction. A major connective tissue protein that is found in blood vessel walls and in the fluid plasma flowing through the vessels is fibrin. The fibrin-fibrin interactions between plasma and vessels is an adhesive event that facilitates efficient blood flow.

Adhesion between the water component of plasma and the blood vessel walls is also important.

Furthermore, fibrin-fibrin adhesion helps to clot the blood and seal damaged blood vessels after injuries.

In addition to blood, other connective tissues throughout the animal body contribute to adhesive interactions that connect and hold the body's tissues together so that they can function at maximum efficiency. Examples of adhesive connective tissues include adipose (fat), bone, cartilage, ligaments, and tendons. Throughout the body, they assist in the welding together of body parts, the support of body structure, and the control of movement between body segments.

In joints between bones, articular cartilage and synovial fluid help to allow adequate movement with little friction between bones. Friction is present and is needed in small amounts. Victims of rheumatoid arthritis have considerable adhesion problems in their joints. Modern medical science is attempting to address these and related problems.

Applications

Adhesive materials have tremendous uses in industry, medicine, and everyday life.

Adhesives, both natural and synthetic, include items such as tape, glue, cement, gum, paste, plaster, putty, solder, and mortar. All of these substances have the same purpose: They hold things together. Consequently, they serve the same uses as do the connective tissues within animal bodies. Furthermore, each adhesive comes in a number of varieties, with each variety having different bonding strength (surface affinity).

Adhesive tape consists of a plastic backing upon which is layered the adhesive material. The sticky adhesive is usually a synthetic polymer, itself a derivative of plastic and, therefore, of petroleum. The makeup of the polymer is modified for different surfaces and for higher or lower stickiness, thereby affecting the nature and purpose of each tape adhesive type.

Furthermore, some adhesives are modified for transparency (such as transparent tape) and for other special needs.

Other types of adhesives are also synthetic polymers in many cases. A synthetic polymer is an artificially produced polymer. A polymer is a long chain of repeating smaller molecules. Synthetic polymers are usually derived from petroleum by-products. Their importance in modern industrial society is emphasized by the fact that they comprise roughly half of the top fifty substances manufactured in the United States each year.

Glue, cement, and gum are substances which, like tape, hold things together. They adhere one surface to another. Based upon each specific adhesive's chemical makeup, it is designed to adhere one type of surface to another. Glues and cements can be used for adhering paper to paper, plastic to wood, wood to wood, metal to glass, and so forth. Special types of glues, such as epoxy glues, must be mixed together to stimulate chemical bonding and become adhesively active. Household uses of glues and cements are endless, including items such as floor tile, wallpaper, acoustic ceilings, paneling, and spackling for holes in walls, among others.

Paste is similar to glue. Young children learn to make this rudimentary adhesive simply by mixing together flour and water. Building materials such as plaster, mortar, and putty are more elaborate variations of this simple mixture. These adhesives are mixed together, usually with the addition of adhesive water, to produce a powerful chemical bond. They support walls, bricks, blocks, and glass. They also serve as heat insulators.

Among other adhesives, tar is a petroleum by-product that is used in roofing and road surfacing. Solder, which requires melting and refreezing, is an extremely useful and powerful adhesive for the electronics industry. Industry focuses intensive research on new adhesive polymers to produce better building and insulation materials.

In medicine, new synthetic polymers are being developed to remedy various ailments and to mimic natural tissues--for example, the chemicals used in wart removers are adhesives that essentially solidify and kill wart virus-infected cells. Synthetic blood clotting agents and artificial skin employ adhesives. Synthetic trachea and blood vessels employ adhesive polymers.

These advances are helping humans to live longer and more meaningful lives.

Context

Adhesion is a process that is essential to the interaction of matter throughout the universe. Without its occurrence, large-scale formation of structures throughout the universe would be difficult, if not impossible. Stars and galaxies would form; however, the processes of planetary formation around stars might be reduced drastically in the absence of adhesion.

Consequently, the evolution of life on Earth or elsewhere would be impossible. Even if life could evolve in such a scenario, only single-celled organisms could exist, and life would be very difficult for such life. The cells of multicellular animals and plants could not stay together without adhesion. Without adhesion, human bodies would disintegrate into a formless, slimy mass.

Adhesion is a physical process closely linked to friction, the resistance that occurs between two surfaces that are moving against each other. Adhesion is the polar (negative-positive charge) attractiveness between different substances whose surfaces are in contact. Adhesion greatly increases the friction between two surfaces so that they stick together.

Weak adhesives hold two surfaces together temporarily with some movement. Strong adhesives immobilize two surfaces together as one unit. The synthetic polymer industry manufactures adhesive materials of many types and strengths for the attachment of various materials to each other. The need for adhesives is extensive throughout society, ranging from such simple tasks as attaching a postage stamp to a letter to the stacking of bricks along the walls of a five-story building.

Water is an amazing chemical that exhibits adhesive properties. It also exhibits cohesion between its own molecules. Nevertheless, its adhesive properties enable it to stick to other substances. This property, along with several others (for example, water's highest density is at 4 degrees Celsius, slightly above its freezing point), makes life possible on Earth. Water comprises 60 to 80 percent of every living organism on Earth. It also covers 70 percent of the earth's surface. Most of the natural and synthetic adhesives that are used require water as a principal ingredient.

Humans use and depend on adhesives everyday. These substances are used in building materials, labels, desk memorandums, insulation, dentures, casts, molds, artwork, and so forth.

They hold things together. They are artificial connective tissue for artificial technological machines, houses, tools, and surroundings. Daily life would be very difficult without them.

Natural adhesives are found within the connective tissues of the body. Blood, bone, cartilage, and fat have adhesive properties. Collagen is a major protein that holds human skin together. This protein gives with age, thereby giving rise to wrinkles. Consequently, collagen is used in antiaging skin creams as a quick fix, albeit only a cover-up. Medicine utilizes adhesives in the treatment of various body conditions, in surgery, and in artificial tissues, organs and blood vessels. A visit to the dentist involves the removal of plaque from one's teeth; plaque is layers of bacteria that have adhered to the enamel of teeth. In the same manner, corals grow in the ocean.

As they die, their skeletons harden and adhere to the skeletons of their ancestors. Thousands of adhesive coral generations have actually produced artificial islands that project above the ocean surface. Living organisms use natural adhesives extensively.

Principal terms

ADHESION: the chemical attraction of unlike substances for each other, thereby causing resistance or sticking between the molecules composing each substance

AFFINITY: the degree of attractiveness or adhesion between unlike substances; high affinity indicates considerable attraction, whereas low affinity indicates very little attraction

CAPILLARY ACTION: a phenomenon in which a fluid moves upward through a tube partially resulting from the adhesive attraction between the fluid molecules and the molecules composing the tube walls

COHESION: the chemical attraction between like molecules or compounds, that is, molecules or compounds of the same substance

FRICTION: the resistance that occurs between the surfaces of two materials that are moving against each other

HETEROGENEOUS MIXTURE: a solution composed of two or more different substances that interact but are discernible to the naked eye

HOMOGENEOUS MIXTURE: a solution composed of two or more different substances that interact and appear to be a single substance

HYDROGEN BOND: a polar chemical interaction between several molecules which are sharing hydrogen atoms between them

INTERMOLECULAR FORCES: the physical and chemical interactions between atoms and molecules, such as charged polarity, ionic bonding, and covalent bonding

VISCOSITY: the degree of turbulence within a fluid caused by the affinities of the fluid's constituent molecules for each other

Bibliography

Ceyer, Sylvia T. "New Mechanisms for Chemistry at Surfaces." SCIENCE 249 (July 13, 1990): 133-139. This review research article describes various models depicting the various chemical and physical interactions between surface molecules in contact with each other. Ceyer, an MIT chemist, describes surface collisions, adhesion, adsorption, and chemisorption. The importance of energy and charge events during surface interactions is illustrated by using several critical examples.

Curtis, Helena. BIOLOGY. 3d ed. New York: Worth, 1979. This outstanding introduction to biology is detailed, but is clearly written for the average reader. All aspects of biology are described. Chapter 29, "Transport Systems in Plants," describes the importance of adhesion during fluid transport through the xylem vessels of vascular plants. Chapter 35, "Circulation of the Blood," describes the properties involved in blood transport through vertebrate animals. Excellent photographs and illustrations support the text.

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 students is clearly written and beautifully illustrated. All major subject areas within chemistry are described and supported by many useful examples, problems, and illustrations. Chapter 1, "Structure of Matter," and chapter 12, "Water, Liquids, Solutions, and Intermolecular Forces," discuss the structure of matter and interactions between molecules.

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. Although detailed, the book clearly presents major chemical topics and strongly emphasizes their applications to everyday life. Chapter 5, "Chemical Bonds: The Ultimate Glue," describes the intermolecular forces that hold molecules together.

Masterton, William L., and Emil J. Slowinski. CHEMICAL PRINCIPLES. 4th ed. Philadelphia: Saunders, 1977. This introduction to chemistry for the beginning student is concise and comprehensive. Important topics are described in detail and are supported by excellent illustrations and sample problems. Chapter 8, "Chemical Bonding," is a thorough presentation of the major types of intermolecular forces, including ionic and covalent bonding.

Serway, Raymond A., and Jerry S. Faughn. COLLEGE PHYSICS. Philadelphia: Saunders College Publishing, 1989. This strong introduction to physics is designed for the beginning student. All topics are discussed in great detail with many useful illustrations and examples. Numerous sample problems are provided; the mathematics is very manageable. Chapter 9, "Solids and Fluids," discusses bonding, surface tension, and other important interactions between surface molecules.

Tinoco, Ignacio, Jr., Kenneth Sauer, and James C. Wang. PHYSICAL CHEMISTRY: PRINCIPLES AND APPLICATIONS IN THE BIOLOGICAL SCIENCES. Englewood Cliffs, N.J.: Prentice-Hall, 1978. This detailed, very mathematical presentation of molecular interactions is intended for advanced chemistry students. Nevertheless, the discussions of key concepts are useful for nonscience majors, also. Chapter 5, "Physical Equilibria," discusses the principles behind adhesion and surface tension. It also discusses the applications of these phenomena to biological membranes.

The Formation of Aggregates

Electrical Properties of Solids