Complex Fluids

Pinch-off of complex fluid drops

The fragmentation of a liquid in droplets is an ubiquitous process both in nature and in industry. Whether one considers a dog that shakes its dripping hair, a man who sneezes, an inkjet printer or an atomizer, there is a point when the liquid breaks up into tiny droplets. By observing a single drop hanging off a nozzle, we can see the capillary bridge that binds it to the rest of the liquid get thinner and thinner until it eventually breaks. The last moment before the breakup is characterized by a self-similar dynamic, which follows different scaling laws depending on the liquid properties. The drop detachment is further complicated by presence in the liquid of dispersed objects, such as polymer chains, solid particles or bubbles. However, although the pinch-off processes for simple Newtonian liquids are understood, the case of such complex fluids remains little studied. In the case of a dilute solution of polymer, the rate of strain of the liquid becomes sufficiently high to unwind then stretch the polymer chains. This process makes the fluid visco-elastic. We showed that the addition of sold particles to such a polymer solution lowers the strain rate threshold beyond which the fluid becomes visco-elastic. This phenomenon is explained by the increase of the local viscous stress due to the particles. Furthermore, this pinch-off experiment of visco-elastic suspensions could enable the measurement of local stresses in dense suspensions.