1. Department of Optics and Fluid Dynamics, Risø National Laboratory, P.O. Box 49, 4000 Roskilde, Denmark, Department of Manufacturing Engineering and Management, Technical University of Denmark, Bygning 424, 2800 Kgs. Lyngby, Denmark, Danish Polymer Centre, Risø National Laboratory, P.O. Box 49, 4000 Roskilde, Denmark, and Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061