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Adhesion is the tendency of dissimilar particles and/or surfaces to cling to one another (cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be divided into several different types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion. In addition to the cumulative magnitudes of these intermolecular forces, there are certain emergent mechanical effects that will also be discussed at the end of the article.
Surface energy is conventionally defined as the work that is required to build a unit area of a particular surface. Another way to view the surface energy is to relate it to the work required to cleave a bulk sample, creating two surfaces. If the new surfaces are identical, the surface energy \gamma\ of each surface is equal to half the work of cleavage, W:
\gamma\ = (1/2)W11
If the surfaces are unequal, the Young-DuprÃ© equation applies: W12 = \gamma\ 1 + \gamma\ 2 - \gamma\ 12 where \gamma\ 1 and \gamma\ 2 are the surface energies of the two new surfaces, and \gamma\ 12 is the interfacial tension.
We can also use this methodology to discuss cleavage that happens in another medium: \gamma\ 12 = (1/2)W121 = (1/2)W212. These two energy quantities refer to the energy that is needed to cleave one species into two pieces while it is contained in a medium of the other species. Likewise for a three species system: \gamma\ 13 + \gamma\ 23 - \gamma\ 12 = W12 + W33 - W13 - W23 = W132, where W132 is the energy of cleaving species 1 from species 2 in a medium of species 3.
A basic understanding of the terminology of cleavage energy, surface energy, and surface tension is very helpful for understanding the physical state and the events that happen at a given surface, but as discussed below, the theory of these variables also yields some interesting effects that concern the practicality of adhesive surfaces in relation to their surroundings.
Mechanisms of adhesion
Five mechanisms of adhesion have been proposed to explain why one material sticks to another:
Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale.
Two materials may form a compound at the join. The strongest joins are where atoms of the two materials swap (ionic bonding) or share (covalent bonding) outer electrons. A weaker bond is formed if a Hydrogen atom in one molecule is attracted to an atom of Nitrogen, Oxygen, or Fluorine in another molecule, a phenomenon called Hydrogen bonding.
Chemical adhesion occurs when the surface atoms of two separate surfaces form ionic, covalent, or hydrogen bonds. The engineering principle behind chemical adhesion in this sense is fairly straight forward: if surface molecules can bond, then the surfaces will be bonded together by a network of these bonds. It bears mentioning that these attractive ionic and covalent forces are effective over only very small distances â€“ less than a nanometer. This means in general not only that surfaces with the potential for chemical bonding need to be brought very close together, but also that these bonds are fairly brittle, since the surfaces then need to be kept close together..
In dispersive adhesion, also known as physisorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a regions of slight positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density, although in larger or more complex molecules, there may be multiple "poles" or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule (Keesom forces) or a transient effect which can occur in any molecule, as the random movement of electrons within the molecules may result in a temporary concentration of electrons in one region (London forces).
In surface science, the term "adhesion" almost always refers to dispersive adhesion. In a typical solid-liquid-gas system (such as a drop of liquid on a solid surrounded by air) the contact angle is used to quantify adhesiveness. In the cases where the contact angle is low, more adhesion is present. This is due to a larger surface area between the liquid and solid and results in higher surface energy. The work of adhesion explains the interactive force between the liquid and solid phases and the Young-Dupre equation is used to calculate the Work of Adhesion. The contact angle of the three-phase system is a function not only of dispersive adhesion (interaction between the molecules in the liquid and the molecules in the solid) but also cohesion (interaction between the liquid molecules themselves). Strong adhesion and weak cohesion results in a high degree of wetting, a lyophilic condition with low measured contact angles. Conversely, weak adhesion and strong cohesion results in lyophobic conditions with high measured contact angles and poor wetting.
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials. Adhesives cure (harden) by either evaporating a solvent or by chemical reactions that occur between two or more constituents.
Adhesives are advantageous for joining thin or dissimilar materials, minimizing weight, and when a vibration dampening joint is needed. A disadvantage to adhesives is that they do not form an instantaneous joint, unlike most other joining processes, because the adhesive needs time to cure.
The earliest known date for a simple glue is 200,000 BC and for a compound glue 70,000 BC.
The oldest known adhesive, dated to approximately 200,000 BC, is from spear stone flakes glued to a wood with birch-bark-tar, which was found in central Italy. The use of compound glues to haft stone spears into wood dates back to round 70,000 BC. Evidence for this has been found in Sibudu Cave, South Africa and the compound glues used were made from plant gum and red ochre. The Tyrolean Iceman had weapons fixed together with the aid of birch-bark-tar glue.
6000-year-old ceramics show evidence of adhesives based upon animal glues made by rendering animal products such as horse teeth. During the times of Babylonia, tar-like glue was used for gluing statues. The Egyptians made much use of animal glues to adhere furniture, ivory, and papyrus. The Mongols also used adhesives to make their short bows, and the Native Americans of the eastern United States used a mixture of spruce gum and fat as adhesives to fashion waterproof seams in their birchbarkcanoes.
In medieval Europe, egg whites were used as glue to decorate parchments with gold leaf. The first actual glue factory was founded in Holland in the early 18th century. In the 1750s, the English introduced fish glue. As the modern world evolved, several other patented materials, such as bones, starch, fish skins and isinglass, and casein, were introduced as alternative materials for glue manufacture. Modern glues have improved flexibility, toughness, curing rate, and chemical resistance.
Adhesives are typically organized by the method of adhesion. These are then organized into reactive and non-reactive adhesives, which refers to if the adhesive chemically reacts to harden. Alternatively they can be organized by whether the raw stock is of natural, or synthetic origin, or by their starting physical phase.
There are two types of adhesives that harden by drying: solvent based adhesives and polymer dispersion adhesives, also known as emulsion adhesives.
Solvent based adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent. White glue, contact adhesives and rubber cements are members of the drying adhesive family. As the solvent evaporates, the adhesive hardens. Depending on the chemical composition of the adhesive, they will adhere to different materials to greater or lesser degrees.
Polymer dispersion adhesives are milky-white dispersions often based on polyvinyl acetate (PVAc). Used extensively in the woodworking and packaging industries. Also used with fabrics and fabric-based components, and in engineered products such as loudspeaker cones.
Pressure sensitive adhesives
Pressure sensitive adhesives (PSA) form a bond by the application of light pressure to marry the adhesive with the adherend. They are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow (i.e. "wet") the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in close proximity, molecular interactions, such as van der Waals forces, become involved in the bond, contributing significantly to its ultimate strength.
PSAs are designed for either permanent or removable applications. Examples of permanent applications include safety labels for power equipment, foil tape for HVAC duct work, automotive interior trim assembly, and sound/vibration damping films. Some high performance permanent PSAs exhibit high adhesion values and can support kilograms of weight per square centimeter of contact area, even at elevated temperature. Permanent PSAs may be initially removable (for example to recover mislabeled goods) and build adhesion to a permanent bond after several hours or days.
Removable adhesives are designed to form a temporary bond, and ideally can be removed after months or years without leaving residue on the adherend. Removable adhesives are used in applications such as surface protection films, masking tapes, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact (wound care dressings, EKG electrodes, athletic tape, analgesic and transdermal drug patches, etc.). Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally can not support much weight.
Pressure sensitive adhesives are manufactured with either a liquid carrier or in 100% solid form. Articles are made from liquid PSAs by coat
adhesion and cohesion attractive forces between material bodies. A distinction is usually made between an adhesive force, which acts to hold two separate bodies together (or to stick one body to another) and a cohesive force, which acts to hold together the like or unlike atoms, ions, or molecules of a single body. However, both forces result from the same basic properties of matter . A number of phenomena can be explained in terms of adhesion and cohesion. For example, surface tension in liquids results from cohesion, and capillarity results from a combination of adhesion and cohesion. The hardness of a diamond is due to the strong cohesive forces between the carbon atoms of which it is made. Friction between two solid bodies depends in part upon adhesion.
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Answers:Adhesion (Lat. ad, to, and haerere, to stick), the force by which the particles of different bodies stick together, distinguished from cohesion, which is the force that holds the molecules of the same body together. Water is attracted to other water. This is called cohesion. Water can also be attracted to other materials. This is called adhesion. so i would call this adhesion ..
Answers:Cohesive - water drops stick to other water drops Adhesive - water drops stick to the side of a glass
Answers:adhesion is when something sticks to something else. water molecules stick to other molecules, because of hydrogen bonding. cohesion is when something sticks to itself. Water molecules stick to surrounding molecules because the polarity of water results in hydrogen bonding.
Answers:adhesion and cohesion both refer to "two things sticking to each other", but the difference lies in the nature of the stickiness. In the word cohesion, by virtue of its prefix "co-", should evoke the notion of "cooperation" -- both surfaces provide the impetus to stick together. In contrast, adhesion implies only one of the surfaces wants to do the sticking. Sticking the sticky sides of two pieces of sticky tape together is cohesion. But sticking the sticky side of one piece of sticky tape to a non-sticky, painted classroom wall is adhesion -- the wall doesn't want to stick to anything, but doesn't reject the stickiness of the tape. surface tension is a small-scale force that appears between a fluid interface and another surface. It uses the surface area of the interface to exploit the free energy resulting from the contact of the components of the liquid-liquid or liquid-solid interface. For example, the surface tension between water and glass is responsible for capillary action -- the water climbing a short distance up the interior surface of a thin glass pipet. The free energy resulting from the electrostatics/chemistry of the water/glass interface is what provides the energy required to lift the small mass of water up against the force of gravity. Surface tension is the reason that some surfaces like to be wetted and some resist wetting. If you take an eyedropper of liquid and deposit a drop of it onto a surface, you get one of two results: (1) the drop spreads out to form a thin layer that attempts to cover the entire surface (2) the drop tends to retain its round shape and there is a distinct, fixed angle between the surface and the angle of the drop/air interface. In case (1), the surface likes to be wetted by the fluid. In case (2), the surface resists the wetting by the fluid. In both cases, the surface tension of the fluid/surface interface is used effect either result.