In a wall-bounded shear flow, deformable particles form a crystal-like structure [Phys. Rev. Lett. 120, 268102 (2018)]. To expand the applicability of microfluidic devices to particle sorting, we performed a lattice Boltzmann simulation. In this study, the crystal structure of capsules can be utilized to segregate capsules of different sizes. We add small capsules into a system where large capsules form a crystal structure, and find that the movement of the small capsules changes significantly with size ratio $R_a$. When the sizes of the small capsules were comparable to those of the large capsules $(R_a\sim 1)$, the small capsules are trapped in the crystal structure. In addition, they composed the crystal with the large capsules and traveled in a diffusive manner. However, when $R_a\sim 0.6$ or lower, the small capsules were depleted from the crystal layer due to the low lift velocity, and they continuously traveled in an advective manner. Furthermore, we found that the stiffness of the small capsules had only a slight influence on this phenomenon. The results suggest that the crystal structure of capsules can be used to trap capsules of comparable sizes or to exclude capsules of smaller sizes. The findings of this study can serve as practical basis for microfluidic applications for segregating and sorting different types of soft particles.

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• Fluid-particle interactions
• Microfluidics
• Complex fluids