  Optical tweezers are employed to measure the forces of interaction within a
single pair of DNA-grafted colloids in dependence of the molecular weight of
the DNA-chains, and the concentration and valence of the surrounding ionic
medium. The resulting forces are short-range and set in as the
surface-to-surface distance between the colloidal cores reaches the value of
the brush height. The measured force-distance dependence is analyzed by means
of a theoretical treatment based on the compression of the chains on the
surface of the opposite-lying colloid. Quantitative agreement with the
experiment is obtained for all parameter combinations.
