Fano Flow

Adding polymers to fluids can lead to strangely counter-intuitive behavior. Here two examples of bizarre extensional flow, sometimes called Fano flow, are shown. First, in the “tubeless siphon” fluid is drawn into a syringe from the level of the free fluid surface.  When the syringe is raised above the free surface of the fluid, the polymer-laden fluid continues to flow upward and into the syringe.  A similar effect is shown in the “open channel siphon” where, once initiated, the flow up and over the side of a beaker continues after the free surface of the fluid has fallen below the level of the beaker’s spout. In both of these cases, the cross-linking and entanglement of polymers within the fluid makes it capable of exerting normal stress when extensionally strained (e.g. stretching a rubber band). In other words, when the syringe is drawn out of the pool, the stretching of the fluid causes the polymers to exert a force that counteracts the weight of the fluid column, enabling the flow to continue upward despite gravity.