Fundamentals of Viscoelastic Particle Focusing in Suddenly Expanded Microfluidic Channels.

Particles focusing in suddenly expanded microfluidic channels with viscoelastic biofluids have attracted increased interests due to the potential biomedical applications. In this work, three types of suddenly expanded channels were designed and employed for experiments. These suddenly expanded channels contained a certain number of triangular expansion structures on the sides of the straight channel to investigate the effect of secondary flow on particle viscoelastic focusing and we investigated particle focusing behavior in those three channels with 0.1% hyaluronic acid (HA) solution. The effects of various suddenly expanded channel structures on particle focusing were discussed. The results indicated that the viscoelastic focusing effect was enhanced by using the sudden expansion structures. Consequently, particles with multi-peak distributions were successfully focused into a focusing stream by using the secondary flow induced by the suddenly expanded structure. Finally, the viscoelastic focusing of leukocytes was explored using suddenly expanded channels. This work offers a deep understanding on the fundamentals of particle focusing in suddenly expanded channels and the results will enable the design of particles focusing microfluidic devices with suddenly expanded channels for critical applications in biomedicine.