Going with the flow

European researchers say they can overcome the problems of modelling flow in fluids like mayonnaise, engine oil and snow. 

Fluids that do not flow unless they are subjected to a certain load, or stress, are called yield stress fluids. Everyday examples of yield stress fluids include; mayonnaise, shaving foam, engine oil and snow.

Daniel Bonn at ?cole Normale Sup?rieure in Paris and colleagues report that the difficulties in measuring the yield stress of a material can be overcome by taking thixotropy into account. Thixotropy is a phenomenon shown by some materials, where they flow more easily upon shaking or stirring. 

These materials have an internal structure, such as entangled polymer chains, which holds them together and stops them from flowing. Applying force breaks down this structure, making it easier for the material to flow; in the same way that yoghurt becomes liquid when stirred. 

Understanding how and when yield stress fluids will flow is important for everything from manufacturing cosmetics to predicting landslides. 

According to Bonn, taking thixotropy into account solves many of the problems associated with predicting how yield stress fluids will behave. Bonn said he hopes to use this insight to model fluid flow in avalanches and landslides.

Clare E Boothby