Forces At Work: Adhesion & Cohesion
Before any glue comes into the equation, there are much larger, less sticky, and less visible powers at work; forces. Adhesion and Cohesion, to be exact. For those of you who have only a hazy memory of their science O-level/GCSE, these are the two of the forces that work together to help things stick together.
Adhesion is the force that brings two separate materials together – much like when a water droplet sticks to a shower curtain or a windowpane. Water is highly adhesive, so sticks to the window or plastic curtain.
Cohesion is the force that ‘sticks together particles of the same substance’ – which is the force behind water droplets forming together to create a bigger droplet. Eventually, they become so large and heavy that gravity gets the better of them, and they fall to the ground, causing the adhesion stop them from sticking. The Cohesion has prevented the Adhesion from working.
All properties have an element of Adhesion and cohesion, but the more adhesive and the less cohesive a property is; the more likely it is to stick to another property.
An alternative example of adhesion and cohesion at work is to replace the water droplets with tomato ketchup. It has adhesive properties, so, like the water, it sticks to the shower curtain. However its cohesive forces aren’t as strong as water, meaning the ketchup isn’t trying as hard to conjoin with other ketchup on the curtain. This means it’s more likely to stay put on the shower curtain, because gravity can’t drag it down.
Sandwich the tomato ketchup between two shower curtains, and the adhesive nature of the sauce will cause them to stick together, causing the tomato ketchup to act as an adhesive.