The three activities in this edition of WonderNet focus mostly on the characteristics of different substances that make them work well as adhesives. Another closely related issue is how the characteristics of the substance that the adhesive is used on effects the degree to which the adhesive sticks. The characteristics of some substances that make them work well as adhesives has to do with the structure of the molecules that make up the substance. Many glues are made from proteins. Milk, egg yolk, and animal cartilage are all natural sources of protein for making glue.
Proteins work well because they are often long, branching, stringy chains of molecules.
These molecules are mixed into certain liquids to make different kinds of glue. When the liquid glue is applied to a surface, it seeps into the tiny spaces of the material. The proteins connect to molecules of the material or flow into the pores and spaces. When the liquid evaporates, the proteins are left behind in a hardened state connected to the material. If a material such as polished metal or glass has very few pores into which glue can seep, the glue will not stick well to that material.