1. Microgravity Research Center, CP-165/62, Université libre de Bruxelles (ULB), 50, Ave. F.D. Roosevelt, B-1050 Brussels, Belgium
2. Department of Mathematics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
3. Mechanical and Manufacturing Department, Mondragon University, Loramendi 4, Apdo. 23, 20500 Mondragon, Spain
4. Basque Foundation for Science, Bilbao, Spain