We investigated the relationship between capture-bead aggregation and VIF driving conditions. To measure the flow field around the pillars, we used a micro-particle-image velocimetry (micro-PIV) technique. Figure shows a two-dimensional (2D) vector plot of the net flow velocity at the center plane of the pillar (h = 25 µm). Swirling flow occurred around the pillar, with slightly lower velocities at the top, bottom, left, and right of the pillar. This deceleration was due to the interference with adjacent pillars that were separated by an interval of 200 µm.