1. Fibre and Particle Engineering Research Unit, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland
2. Department of Chemical and Biological Engineering, The University of Sheffield, Sheffield S1 3JD, UK
3. Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, UK
4. Department of Physics, The University of Warwick, Coventry CV4 7AL, UK