1. Department of Biochemical Engineering, University College London, Gower Street, WC1E 6BT London, U.K.
2. Department of Physics, University of Cagliari, Cittadella Universitaria, I-09042 Monserrato, Cagliari, Italy
3. Department of Chemical Engineering, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece