We examined the dependence of the average area of bead aggregates on the concentration of NPs to test the capability of the proposed system for quantitative measurements. The time variation of the average area of the capture-bead aggregates is shown in Figure , where the vibration conditions were: f = 500 Hz and A = 3.3 µm. In the presence of NPs, the average area of the capture-bead aggregates increased almost linearly with time in all cases (NP conc. = 5.7 × 10 9 /mL, 1.1 × 10 9 /mL, 5.7 × 10 8 , /mL, and 5.7 × 10 7 /mL). Microscopic images and beeswarm plots of the area of the capture-bead aggregates after 15 min for each NPs concentration are shown in Figures and, Supporting Information, respectively. These results support the hypothesis that the amount of NPs adsorbed onto the capture beads increases with the concentration of NPs, promoting the formation of aggregates. In contrast, in the control experiment without NPs (gray line) and without VIF (black line), the increase in the average area of capture-bead aggregates remained small. These results quantitatively show that the concentration of NPs in the sample correlates with the average area of aggregates in the VIF field. Figure shows the relationship between the NP concentration and the initial slope of the time-dependent increase in the area of aggregates. We chose this parameter rather than the size at the plateau, longer time mixing (~40 min) did not produce a clear and reproducible plateau (see Figure , Supporting Information). It is likely that, as the number of aggregates decreases, and statistically significant average was not obtained within a single frame. In addition, the total projected area of capture beads remained nearly constant over time. This result indicates that the aggregations grew in a two-dimensional fashion. From this plot, NP concentration in the sample can be estimated. The NP concentration of 5.7 × 10 7 /mL showed a significantly greater slope compared to the case without NPs, suggesting that the NP concentration can be quantified by determining the slope of the time-dependent increase in aggregate size above this concentration.