Air content and bubble size are the key parameters in describing aqueous wet foam. These two parameters respond differently to added fibres in axially agitated mixing. Both air content and bubble size depend on the process characteristics such as mixing geometry, rotation speed and surface tension. However, for moderate rotation speeds, air content is rather insensitive to the added cellulose-based fibres, whereas the mean bubble size varies strongly with the fibre type. Even though the bubble size is usually reduced by the added fibres, short fibres can also increase the bubble size. The mean bubble size clearly becomes smaller for wood fibres than for viscose fibres even if these two types of fibres have similar geometric dimensions. Moreover, the bubble size distribution becomes narrower with wood fibres. This suggests a bubble breaking mechanism induced both by the rough surfaces of wood fibres and by the associated fine particle fraction that are both absent with smooth viscose fibres. At high rotation speeds, the air content becomes extremely sensitive to the fibres due to macro-instabilities causing an irregular flow of the wet foam. (C) 2014 Elsevier B.V. All rights reserved.
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