Why does adhesion occur

Product category. Mechanical interlocking Mechanical interlocking happens when two dissimilar phases attach to one another by mechanical forces only. Physical bonding Physical bonding consists of van der Waals forces and is always present. Chemical bonding Chemical bonding includes covalent, ionic and metallic bonding which are much stronger than physical bonds. A low contact angle is needed for good adhesion Based on the above mention bonding mechanisms, it is evident that the coating should spread on the substrate to achieve good adhesion.

To read more about contact angle and how it can be utilized, read the overview below. Topics: Adhesion. Explore the blog. You have only scratched the surface. Robin Ras 1 Prof. Zhenghe Xu 1 View all. Visit our other sites. Thus the pressure inside a bubble is greatest when the bubble is the smallest. Another bit of evidence for this is illustrated in Figure 4. When air is allowed to flow between two balloons of unequal size, the smaller balloon tends to collapse, filling the larger balloon.

Figure 4. With the valve closed, two balloons of different sizes are attached to each end of a tube. Upon opening the valve, the smaller balloon decreases in size with the air moving to fill the larger balloon. The pressure in a spherical balloon is inversely proportional to its radius, so that the smaller balloon has a greater internal pressure than the larger balloon, resulting in this flow. Calculate the gauge pressure inside a soap bubble 2. Convert this pressure to mm Hg.

Note that if a hole were to be made in the bubble, the air would be forced out, the bubble would decrease in radius, and the pressure inside would increase to atmospheric pressure mm Hg. Our lungs contain hundreds of millions of mucus-lined sacs called alveoli , which are very similar in size, and about 0. See Figure 5. You can exhale without muscle action by allowing surface tension to contract these sacs.

Medical patients whose breathing is aided by a positive pressure respirator have air blown into the lungs, but are generally allowed to exhale on their own. Even if there is paralysis, surface tension in the alveoli will expel air from the lungs. Since pressure increases as the radii of the alveoli decrease, an occasional deep cleansing breath is needed to fully reinflate the alveoli.

Respirators are programmed to do this and we find it natural, as do our companion dogs and cats, to take a cleansing breath before settling into a nap. Figure 5. Bronchial tubes in the lungs branch into ever-smaller structures, finally ending in alveoli. The alveoli act like tiny bubbles. The surface tension of their mucous lining aids in exhalation and can prevent inhalation if too great. The tension in the walls of the alveoli results from the membrane tissue and a liquid on the walls of the alveoli containing a long lipoprotein that acts as a surfactant a surface-tension reducing substance.

The need for the surfactant results from the tendency of small alveoli to collapse and the air to fill into the larger alveoli making them even larger as demonstrated in Figure 4. During inhalation, the lipoprotein molecules are pulled apart and the wall tension increases as the radius increases increased surface tension. During exhalation, the molecules slide back together and the surface tension decreases, helping to prevent a collapse of the alveoli. This tension change is a unique property of these surfactants, and is not shared by detergents which simply lower surface tension.

See Figure 6. Figure 6. Surface tension as a function of surface area. The surface tension for lung surfactant decreases with decreasing area.

If water gets into the lungs, the surface tension is too great and you cannot inhale. This is a severe problem in resuscitating drowning victims. A similar problem occurs in newborn infants who are born without this surfactant—their lungs are very difficult to inflate. This condition is known as hyaline membrane disease and is a leading cause of death for infants, particularly in premature births. Emphysema produces the opposite problem with alveoli.

Alveolar walls of emphysema victims deteriorate, and the sacs combine to form larger sacs. Because pressure produced by surface tension decreases with increasing radius, these larger sacs produce smaller pressure, reducing the ability of emphysema victims to exhale. A common test for emphysema is to measure the pressure and volume of air that can be exhaled. Why is it that water beads up on a waxed car but does not on bare paint?

The answer is that the adhesive forces between water and wax are much smaller than those between water and paint. Competition between the forces of adhesion and cohesion are important in the macroscopic behavior of liquids.

See Figure 7. Table 2 lists contact angles for several combinations of liquids and solids. Figure 7. Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion.

In cohesion, water molecules are attracted to each other because of hydrogen bonding , keeping the molecules together at the liquid-gas water-air interface, although there is no more room in the glass. Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress.

Like the narrow tube, plants contain narrow vessels called xylem which are composed of polar cellulose. They carry water up through capillary action against the force of gravity.

The diameter of these vessels decrease towards the tops of taller trees, allowing the water to rise even higher, from the roots to the leaves. Adhesion occurs when one type of molecule is attracted to a different kind of molecule. Water exhibits adhesive properties in the presence of polar surfaces—like glass or cellulose in plants.

Regarding glass, the positively charged hydrogen molecules in water are more attracted to the negatively charged oxygen molecules in the silica than to the oxygen in neighboring water molecules. When a narrow glass tube is inserted into water, the water molecules bind to the tube surface, and the water level inside the tube rises.

The smaller the tube diameter, the farther the water rises, because more water molecules are exposed to the glass surface. Capillary action continues as long as the adhesive force is greater than the pull of gravity. Plants use the adhesion of capillary action and cohesion between water molecules to move water up from the roots to the leaves. In plants, xylem vessels consist of long, narrow cells called tracheary elements, which transport water.

Because water molecules have an attraction to cellulose, they cling to the xylem cell wall. Cohesive forces between water molecules also attract the water molecules to each other. Together, these forces of adhesion and cohesion create a column of water molecules that gradually moves up the xylem vessels. To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian.

If you have any questions, please do not hesitate to reach out to our customer success team. Login processing Chapter 2: Chemistry of Life. Chapter 1: Scientific Inquiry. Chapter 3: Macromolecules.