Aqueous foam as the carrier medium for producing tailored fiber materials

Use of foam instead of water as a material carrier in the manufacturing of novel paper-like structures has recently been studied intensively [1]. The technology can provide materials with striking new features such as tailored microporous structures. Here the control of the foam properties including air content, bubble size, rheology and stability becomes important. To address these problems, we have investigated aqueous foams and their interaction with cellulose fibers both in axial mixing [2] and in pipe flow [3]. In axial mixing, the bubble size is affected by several factors such as rotational speed, air content and surface tension. The link between bubble size and air content can be described by a simple formula based on considering the effect of total interface area on the surface excess concentration of surfactant. At high shear rates, the bubble size becomes small and the importance of surface interactions increases affecting the rheological behaviour described by the Herschel-Bulkley equation. Response of wet foam to inclusion of natural and regenerated cellulose fibers is quite different with smaller mean bubble size for natural fibres than for regenerated fibres. The likely reason behind this behaviour is the rough surfaces of the natural fibers and their fine particle fraction. In pipe flow, the strongest impact on viscosity was created by hardwood fibers with rich surface texture and their mean length exceeding the average bubble diameter by an order of magnitude. The observed effect on viscosity was still much smaller than in the case of plain aqueous fiber suspension. Fibers do not interact or flocculate in foams to the same extent as in water suspensions. Thus, the fiber networks produced using foam forming are generally very homogeneous. 1) A. M. Al-Qararah, A. Ekman, T. Hjelt, J. A. Ketoja, H. Kiiskinen, A. Koponen, J. Timonen, A unique microstructure of the fiber networks deposited from foam-fiber suspensions. Colloids and Surfaces A: Physicochem. Eng. Aspects 482, 544-553 (2015). 2) A. M. Al-Qararah, T. Hjelt, A. Koponen, A. Harlin, J. A. Ketoja, Response of wet foam to fibre mixing. Colloids and Surfaces A: Physicochem. Eng. Aspects 467, 97-106 (2015). 3) A. Jäsberg, P. Selenius, A. Koponen, Experimental results on the flow rheology of fiber-laden aqueous foams. Colloids and Surfaces A: Physicochem. Eng. Aspects 473, 147-155 (2015).