Measurement of entropic force from polymers attached to a pyramidal tip

The measurement of the boundary shape dependence of the entropic force from long polymers was attempted. The pyramidal cone-plate geometry was chosen. The polymer molecules were covalently bound to a well-defined Au patch at the apex of a pyramidal cantilever tip of the atomic force microscope (AFM). A smooth hydrophobic plate was used as the second boundary to confine the polymer molecules. The use of the hydrophobic plate allows neglect of polymer adhesion forces. The measurements were made in salt water solution to decrease the effect of electrostatic forces from any uncompensated charges on the boundary. As the functionalized AFM tip approaches the flat hydrophobic surface, the induced entropic forces were measured as a function of the separation distance. The measured force-distance curves are compared with a model of polymer-mediated entropic force between scale-free objects and the Alexander-de Gennes (AdG) theory for a polymer brush.