Negative wake behind bubbles in non-newtonian liquids

Gas bubbles rising by gravity in non-Newtonian elastic liquids are different to gas bubbles in viscous Newtonian fluids in at least two ways. First, the bubbles in the non-Newtonian liquids often have a peculiar tip at the rear pole, and second, the terminal rise velocity versus volume curve often has a discontinuity at a certain `critical' volume. To investigate this unusual flow situation further laser-Doppler anemometry was used to measure the liquid velocity in the weak behind air bubbles in a non-Newtonian liquid. The measurements reveal the unexpected result that the liquid velocity behind the bubbles is in the downwards direction away from the rising bubbles (the velocities are referred to an observer at rest with respect to the liquid far from the bubbles). Thus the liquid velocity is the usual wake behind objects moving in viscous Newtonian fluids and the phenomenon is called `negative wake'.