We employed an avidin-biotin affinity binding pair, which is well known for its high specificity and binding strength (Kd < 10 -15 M), as a model NP-capture system. Avidin-coated capture beads (3 µm nominal diameter) and biotin-coated NPs (diameter measured using an imaging analyzer was 152 ± 55 nm; see Figure , Supporting Information) were mixed by gentle pipetting and placed on the substrate with an array of micropillars. Subsequently, a cover glass was carefully placed on the substrate to define the boundary conditions of the fluid volume (Figure ). /ml NPs and 3.0 × 10 7 /ml capture beads. The vibration condition was fixed as circular vibrations with a frequency f = 500 Hz and an amplitude A = 3.3 µm. Before the application of vibrations (0 min), the capture beads were evenly dispersed in the observed field under all conditions, although NPs could not be observed because of their smaller size. This indicates that pre-mixing by pipetting in a microtube did not cause aggregation of capture beads. After 15 min of applying the vibrations, several relatively large aggregates of capture beads (above 25 µm in diameter in the image) were formed in the mixture containing NPs in addition to the capture beads (Figure , lower panels). We conducted a control experiment without VIF and NPs. In the absence of VIF, no aggregate of the capture beads was observed (Figure , lower panels). This result reflects the fact that Brownian diffusion alone is not sufficient to induce collision and aggregation of capture beads. In the absence of NPs, relatively small aggregates (diameter less than 10 µm) were generated (Figure , lower panels). This result may be due to some intermolecular forces between the capturing beads that caused nonspecific adsorption; however, the average area of aggregate was significantly smaller than that in the experiment where NPs were present in the sample. When the same microscopic observation was performed for the sample subjected to vortex mixing in a test tube at 500 rpm instead of VIF, no obvious aggregates were observed even after 1 h (see Figure , Supporting Information). The actual stirring of capture beads and NPs with VIF is shown in Movie S1, Supporting Information. These results visualized local vortices generated between four adjacent pillars in the VIF device. On the other hand, we expect that vortices in a microtube are large in the case of conventional vortex mixing, meaning NPs and affinity capture beads tend to move in parallel so that efficient collision does not occur. Thus, we suppose vortices induced by local flow in the VIF lead to the efficient aggregate formation of affinity capture beads, rather than conventional vortex mixing often employed in commercial kits for NP detection using capture beads. This initial set of experiments provided the basis that only when there was an affinity between the target NPs and the capture beads, the localized flow around the pillar (Figure ) effectively induced aggregation of the capture beads, and this phenomenon can be used for NP detection. Specifically, it is considered that microscale local swirling promoted the collision of capture beads and NPs, and that the aggregates further encountered each other at the interference of the neighboring vortices, forming larger aggregates.