  We report on the construction of colloidal stars: 1 micrometer polystyrene
beads grafted with a dense brush of 1 micrometer long and 10 nm wide
semi-flexible filamentous viruses. The pair interaction potentials of colloidal
stars are measured using an experimental implementation of umbrella sampling, a
technique originally developed in computer simulations in order to probe rare
events. The influence of ionic strength and grafting density on the interaction
is measured. Good agreements are found between the measured interactions and
theoretical predictions based upon the osmotic pressure of counterions.
