Scientists have worked out why it is so difficult to mix ingredients into a cake. Emmanuelle Gouillart and a team from CEA Saclay near Paris have been studying how things mix together in a bowl. He added black dye to a clear syrup and mixed it automatically with a rod and studied the results.
He found that in the centre it mixed by a process of stretching and folding a bit like how a baker kneads bread, which is a very effective means of mixing. However at the edges of the container the material sticks to the walls so well that it doesn't join in with this process, so it keeps adding lumps of unmixed mixture to the centre slowing down the whole process.
This means that if you are mixing a cake or paint it is important to scrape the mixture from the sides of the bowl and into the centre. Though this sounds obvious having a thorough understanding of how things mix is important to predict how a machine you are building will behave especially if it is on a tiny or huge scale where things don't allways behave intuitively.