The concentration of bioactive nanoparticles in a small sample is the critical processes in many applications such as the rapid and sensitive disease diagnosis and environmental monitoring. Although many microfluidic devices have been developed that realize the collection of target nanoparticles from a sample containing miscellaneous molecular components, there remains a problem in reducing the amount of sample due to the use of an external pump (H. Xu, et al., 2018).
Here we aim to develop a microfluidic system to realize efficient nanoparticle capture using the vibration-induced flow, in which the net flow is induced around micro objects by the periodic vibrating motion. This fluid control technique requires no external pump, producing no dead volume (Hayakawa et al., 2015). In this study, we performed a numerical simulation of particle capturing process with the vibration-induced flow to predict the fluidic phenomena occurring in this unsteady flow field. The simulation was conducted for the micro object (pillar) with the asymmetric shape and for switching the rotation direction to expect the chaotic mixing.
The net flow field was obtained by the Lagrangian averaging method based on the instantaneous flow (Kaneko et al., 2018). Then, trajectories of the capturing particles and nanoparticles were calculated. Particle capture took place when these particles come closer than the threshold distance (Fig.1 (a)). Because the vector plots and streamlines around a pillar (Fig.1 (b)) were dissimilar depending on rotational directions, we found it is feasible to obtain the high capturing rate by switching rotational directions. Unexpectedly, however, capturing rate with switching did not exceed that without switching (Fig.1 (c)). Currently, detailed relationship between the flow field around the pillar and capturing rate is under investigation.
We will search for an optimal condition to realize a high capturing rate by utilizing this numerical simulation.

Fig 1. (a) Schematics of the simulation procedures for the nanoparticle capturing process in the vibration-induced micromixer. (i) Small particles and capture beads are arranged around a pillar. (ii) They are convected around the pillar by the mean flow induced by vibration. (iii) Small particles are determined to be captured by large capture beads when the distance between centers of two particles get smaller than the threshold.
(b) Averaged velocity field and streamlines around the pillar with the bending cross shape, with the rotational directions of (i, ii) CW and (ii, iv) CCW.
(c) (i) Initial distribution of particles; (red) large capture beads and (blue) small nanoparticles, with radii equivalent to Rc = 1.25 µm and Rs = 0.625 µm, respectively. Small particles were determined to be captured when the distance between centers of large and small particles becomes smaller than 1.875 µm. (ii) Timecourse of the capturing rate of small particles.

Kanji Kaneko

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