In a previous OFFSPEC experiment we showed that when a suspension of 100-nm-diameter, charge-stabilized silica particles suspended in a refractive-index-matched solvent is placed next to a silicon diffraction grating, the volume fraction of particles in the 650-nm-wide grooves of the grating is greater than in the bulk. We believe that the the driving force for this particle sequestration is the fact that an attraction exists between like-charged colloidal particles when they are close to a charged wall, as first observed 20 years ago by Fradin and coworkers. If this explanation is correct, we would not expect sequestration of hard sphere colloids between parallel walls. Our experiment is designed to test this hypothesis.