The pair distribution function of 3.18 mm diameter particles was measured in the plane of shear of a simple shear flow at concentrations of 5%, 10% and 15% by volume. A new direct flow-visualization procedure and a new pattern recognition algorithm were used in the investigation. The measurements show a depletion of bound pairs of particles in the direction of flow. A simple model which includes the effect of particle surface roughness on the particle interactions and the pair distribution function is presented. An important effect of surface roughness is that the particles in a suspension can experience irreversible interactions in the presence of an externally imposed simple shear flow. The model shows that such irreversibilities eliminate all bound pairs of particles in the plane of shear by displacing particles out of the closed orbit trajectory region. Surface roughness is found to induce significant asymmetry in the fore and aft region of a two-particle interaction. The measurements and predictions are in qualitative agreement with these conclusions.