Adhesion, intermolecular attraction forces

As the science of adhesion has developed, various theories of adhesion have been advocated for one material or another. With wood as a substrate, mechanical interlocking, interdiffusion of polymers, intermolecular attractive forces, and covalent chemical bonding all have been proposed, either individually or collectively, to explain adhesion. In reality, no experiments reported to date have been able to disprove the existence of any one of these mechanisms, or to quantify their relative importance. A most exasperating feature of research on adhesion to wood is that factors presumed to be independent in experiments are never totally independent. 

The molecular level characteristics which create friction forces are the intermolecular attraction forces of adhesion. If the two materials that make up the sliding surfaces in contact have a high degree of attraction for each other, the coefficient of friction is generally high. This effect is modified by surface conditions and the mechanical... 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories quantum mechanical forces, pure electrostatic forces, and polarization forces. Quantum mechanical forces give rise both to covalent bonding and to the exchange interactions that balance tile attractive forces when matter is compressed to the point where outer electron orbits interpenetrate. Pure electrostatic interactions include Coulomb forces between charged ions, permanent dipoles, and quadrupoles. Polarization forces arise from the dipole moments induced in atoms and molecules by the electric fields of nearby charges and other permanent and induced dipoles. 

The intermolecular attraction between like molecules in the liquid state, such as the water-water attraction based on hydrogen bonds, is called cohesion. The attractive interaction between a liquid and a solid phase, such as water and the walls of a glass capillary (a cylindrical tube with a small internal diameter), is called adhesion. When the water-wall adhesion is appreciable compared with the water-water cohesion, the walls are said to be wettable, and water then rises in such a vertical capillary. At the opposite extreme, when the intermolecular cohesive forces within the liquid are substantially greater than is the adhesion between the liquid and the wall material, the upper level of the liquid in such a capillary is lower than the surface of the solution. Capillary depression occurs for liquid mercury in glass capillaries. For water in glass capillaries or in xylem vessels, the... 

Molecular Forces Between Adherend and Adhesive. The various theories of adhesion invoke the occurrence and interplay of physical and chemical interactions across the adherend-adhesive interface, as well as the deformation behavior of the adhesive (6, 7). Therefore, bond formation depends upon the development of intermolecular attraction, both within the bulk of the polymer and between adhesive and adherend. 

Of the different types of forces responsible for intermolecular attraction, the foremost are the London or dispersion forces that act between all atoms and account for virtually all of the molecular attraction or cohesion in all molecules except the very polar molecules (described later). Dispersion forces are short-range interactions, effective at about 4 A, and rapidly decrease with the sixth power of the distance between molecules. Therefore, the adhesive polymer molecule must be flexible enough to come within this range of interaction with the rigid adherent surface under the conditions of bond formation. 

Density functional theory (DPT) calculations [18] suggest the tangential pulling force Ft as a solid/liquid adhesion originated in long-range solid/liquid intermolecular attraction... 

Besides viscosity, the surface-wetting ability of underfills is critical to capillary flow. For capillary flow to occur, the underfill material must wet the surfaces so that the advancing contact angle is less than 90°. " Also, for capillary flow, the intramolecular forces of attraction among adhesive molecules must be weaker than the intermolecular attraction of the adhesive for the die, the substrate, and the solder surfaces.t This occurs when the surface tension of the underfill is lower than the surface energy of... 

From another point of view, atomic force microscopy is focused on the intermolec-ular interactions between proteins (coated on the atomic force microscopy probe) and PNIPAAm surfaces at different temperatures. Using this method, Cho et al. reported several studies on intermolecular force between BSA and PNIPAAm surfaces (Cho, Kim, Cho, 2004 Cho, Kim, Cho, 2005 Cho, Kim, Cho, 2008). They observed a temperature-dependent adhesion force (defined as the maximum attractive force during retraction of the surface from the tip), which is negligible at lower temperatures and suddenly increases to a maximum value as the temperature increases above a transition temperature (27-29 °C). The lower transition temperature compared with the LCST of PNIPAAm in water (32 °C) may be due to the addition of salts becanse the experiments were conducted in a phosphate bnffer solution. The change in adhesion force is consistent with the affinity between soluble proteins and a PNIPAAm surface, which, at room temperature, is approximately an order of magnitude lower than at 37 °C (Cheng et al., 2006). 

Molecules in contact with the surface of their container experience two sets of intermolecular forces. Cohesive forces attract molecules in the liquid to one another. In addition, adhesive forces attract molecules in the liquid to the molecules of the container walls. 

Intermolecular forces of attraction among the solute and the solvent (cohesion) must be broken while intermolecular forces of attraction between the solute and solvent (adhesion) must be required. Work must be done to randomly distribute the solute in the solution. As the difference between the cohesive and adhesive forces becomes larger, the solubility of the solute will be lower. The activity of a nonideal... 

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