The cells do not actually bind to the polysucrose molecules. Rather, the centrifuge process forces cells with different densities to travel at different speeds through the one (or two ) gradient(s) like Histopaque-1077 and Histopaque-1119 that have fixed densities. Because red cells and granulocytes are most dense, they travel faster toward the bottom of the tube whereas the less dense platelets and monocytes travel slower and end up closer to the top of the tube. A simplified diagrammatic explanation is available on pages 10-11 in the Sigma Aldrich "Centrifugation Basics" publication available in full text at:

http://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma-Aldrich/Brochure/1/biofiles_v6_n5.pdf