  We present the first direct measurement of three-body interactions in a
colloidal system comprised of three charged colloidal particles. Two of the
particles have been confined by means of a scanned laser tweezers to a
line-shaped optical trap where they diffused due to thermal fluctuations. Upon
the approach of a third particle, attractive three-body interactions have been
observed. The results are in qualitative agreement with additionally performed
nonlinear Poissson-Boltzmann calculations, which also allow us to investigate
the microionic density distributions in the neighborhood of the interacting
colloidal particles.
