Viscoelastic ordering of particle in straight microchannels: an alternative route towards enhanced diagnostic

For many applications in the field of biomedical engineering and material science, controlling the spacing between particles and cells at the micron-size level can greatly impact performance of microfluidic systems. Following a series of studies, particle trains (strings of ordered particles), as it has been observed, ought to develop on the channel centerline, where both particle or cell rotation and local shear gradient are minimal. In these conditions, cell damages are minimized, and optical readout of flowing particles or cells is improved. Unfortunately, the only existing method for the formation particle trains (inertial microfluidics) promote such train formation near the channel walls, where cell rotation and shear gradient are maximum.

Recent studies have established that addition of polymer to aqueous suspensions promotes transversal migration of suspended particles toward the centerline of a straight microchannel, due to internal viscoelastic forces. As such, majority of the published subsequent studies have only considered particles in viscoelastic fluids flowing in microchannels having very dilute suspensions, where train formation is prevented by the lack of particle-particle interactions.

Recently, Dr. Francesco Del Giudice at Swansea University in collaboration with scientists at University of Naples Federico II: Gaetano D’Avino, Francesco Greco, and Professor Pier Luca Maffettone and Professor Amy Q. Shen at Okinawa Institute of Science and Technology they showed that simple addition of a tiny amount of hyaluronic acid biopolymer to an aqueous suspension, had the potential to drive self-assembly of a particle train on the centerline of a square-shaped straight microchannel, with a throughput up to approximately 2400 particles/s. Their work is currently published in the research journal, Physical Review Applied.

The research team considered a polymethylmethacrylate-based inlet to the glass channel through which the preferred fluid was pumped at several volumetric flow rates. Fraction of equally spaced particles were increased by increasing the volumetric flow rate and the distance from the channel inlet. Numerical simulations were then undertaken in a bid to validate experimental observations and further shed insight on the underlying mechanism leading to particle ordering. A simple theoretical argument was also proposed to explain the formation of the observed trains.

The authors observed that shear-thinning of the suspending liquid was required so as to achieve ordering. Additionally, they found out that the predominant distance between ordered particles did not depend on the Deborah number (De) or on the distance from the inlet (Lz/H). On the other hand, an increase of De and Lz/H was seen to enhance the fraction of ordered particles.

In summary, Dr. Francesco Del Giudice and colleagues successfully discovered that fluid viscoelasticity was responsible for driving self-assembly of particle trains along the centerline of a straight microfluidic channel. Generally, good qualitative agreement between experiments and numerical simulations was reported. Remarkably, the results presented in their paper believed to be applicable to biomedical engineering and material science, where, for the case of the latter, the discoveries reported could prompt further efforts toward microfluidic fabrication of materials to enhance localized properties at the micrometer-size level.