Degree Name

Doctor of Philosophy


School of Mechanical, Materials and Mechatronic Engineering


I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids.

Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for biomedical and chemical analysis.

The main purpose of this research is to investigate particle migration in viscoelastic microfluidics, and develop microfluidic devices based on viscoelastic fluids to realize more efficient particle focusing, separation or solution exchange. These devices can be used as much easier particle manipulation methods in various biomedical or chemical II fields. Firstly, the concept of particle migration in viscoelastic fluid, and its applications were reviewed. Particle behaviours in viscoelastic fluid in a straight channel with asymmetrical expansion–contraction cavity arrays (ECCA channel) were investigated. Under the Dean-flow-coupled elasto-inertial effect, this device could offer a continuous, sheathless, and high throughput (>10000 s-1) 3D particle focusing performance. Additionally, based on the same principle, continuous plasma extraction with high purity was achieved in this channel by simply adding polymer to blood. After two series of filtration with the same ECCA channel, the purity of 3 μm, 4.8 μm and 10 μm diameter particles reached 100%, and the plasma purity reached 99.99%.

Further, a novel microfluidic device for sheathless particle focusing and separation in viscoelastic fluid is proposed. The device consists of two stages: straight channel section with asymmetrical expansion–contraction cavity arrays (ECCA section) for sheathless Dean-flow-coupled elasto-inertial particle focusing (1st stage), and straight channel section for viscoelastic particle separation (2nd stage). This work investigates the on-chip washing process of microparticles and cells using co-flow configuration of viscoelastic fluid and Newtonian fluid in a straight microchannel.

Additionally, particle lateral migrations in sample-sheath flow with different properties were experimentally investigated. By using viscoelastic sample flow and Newtonian sheath flow, a selective particle lateral migration can be achieved in a simple straight channel, without any external force fields. Furthermore, the on-chip washing process of microparticles and cells using this co-flow configuration in a straight microchannel was studied. This technique may be a safer, simpler, cheaper, and more efficient alternative to the tedious conventional centrifugation methods, and may open up a wide range of biomedical applications.

Overall, viscoelastic fluids show a lot of advantages, therefore, allowing fast growth of microfluidic applications based on viscoelastisity-induced particle migration. The proposed devices can realize particle migration in an easier way, which show potential to be better alternatives in various biomedical and chemical detection and analysis.