Overview: What Is an Adhesive?
Adhesives are substances like paste or glue that bond items together. Some are natural, such as tar made from bark and plants, glues made from animal parts, and egg whites. Many glues that are synthetic are made from polymers and other types of resin. They may form a weak bond between items that can be stuck or unstuck as it is needed, or they may form a very strong bond that reacts between both substances to be glued together.
How Do Substances Bond?
Molecules from one substance (for example, a piece of paper) interact with molecules from another substance (for example, a piece of cardboard) when adhesive is used between them. Sometimes the bond is temporary and relatively weak. For example, sticky notes use a type of adhesive that allows them to be positioned, removed, and repositioned without leaving a mark. Other adhesives cool or dry to form a strong bond. Many glues harden over time and when pressure is applied. During the cooling process. the molecules of liquid adhesive mix with the molecules of both solids that are being glued. When the adhesive is no longer liquid, a tight bond has been formed.
Chemical or Electrostatic Reactions
Some forms of very strong adhesives actually cause chemical reactions when the bonds are formed. The individual materials do not create an adhesive by themselves, but when they are mixed together they react with one another to create new compounds. Others form chemical reactions under light or heat. Electrostatic reactions can also be adhesive. For example, if a balloon is rubbed, creating static electricity, it will then stick to the surface of a wall for as long as the charge lasts.
How Does Water Fit In?
Insects are able to walk on vertical surfaces, as well as cling to them and hang upside down. They are able to use surface tension between sticky oils on their legs and the dry wall, similar to the capillary action between air, liquids, and solids. This is why a piece of paper while it is wet will stick to the wall temporarily until it dries. Scientists have applied the ability of some beetles to trap air bubbles and move underwater to develop a sticky material that forms bonds while it is underwater.
Using Electricity and Water as an Adhesive
Another application of surface tension is a new device that uses electricity and water together to form a temporary adhesive. The miniature device consists of a tiny battery that forces liquid through a screen to form water droplets. Those water droplets create surface tension, and can be used to power a small device through experiments with water as an adhesive.
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