Bubble size and air content of wet fibre foams in axial mixing with macro-instabilities

The air content and bubble size distribution in pure foams and in fibre foams were determined in axial mixing for varied rotation speed, liquid surface tension and wood fibre concentration. Onset of flow macro-instabilities was observed at high rotation speeds both with and without fibres. Before the instability, the bubble size decreased and air content increased with increasing rotation speed. After the instability, the air content was not only strongly affected by rotation speed but, interestingly, both the air content and bubble size increased with reduced surface tension. In both regions, the average bubble size could be explained based on the capillary number after the air content was explicitly included in the model. Added fibres reduced the bubble size in the stable mixing region and for low surface tension levels in the unstable mixing region. This was probably caused by increased shear forces due to inertia of fibres breaking up the large bubbles in the foam.