Sticky fingers

摘要

If you have ever given a bottle of tomato ketchup a good shake to make it pour more easily, then you have experimented with rheology. This is the study of how materials flow, and it considers the many elements which give a liquid its overall viscosity. Shaking ketchup invokes one of those elements: shear thinning. This reduces a liquid's reluctance to flow, or viscosity, by forcing the layers within it to separate, suddenly making it runny.