The effects of the concentration are presented in Fig. 4a and b. Only results for the horizontal part are plotted. On the one hand, increasing the delivered concentration leads to an increase of the pressure drop, as expected. The relative increase of the pressure drop at the critical point with respect to C = 5% is roughly 40% for C = 10% and 70% for C = 15%. Very few points are available for the concentration C = 20% owing to the large power required; these points are nevertheless on both sides of the critical point. The rel-ative increase in pressure drop at the critical point for C = 20% is around 100%. On the other hand, changing the concentration seems to increase only very slightly the critical velocity (see Section 4.3 for a discussion of this point).