Viscoelastic Focusing Mechanisms Of Particles In Microchannels

In recent years, the technology of utilizing the fluid properties to viscoelastic manipulation of the particles has begun to attract attention of scholars. Due to their significant advantages, such as simple channel structure, without applying external fields and easy operation, the technology of viscoelastic manipulation of the particles are expected to open a new way to sample pretreatment for point-of-care diagnostic instruments. Besides, the utilizing viscoelastic effects can enhance the manipulation of the particles, which is expected to reduce the size of microfluidic chip, and to improve the efficiency of the manipulation. And in the process of commercialization of microfluidic technology, viscoelasticity is widespread in the practical application of solution. Therefore, the technology of viscoelastic manipulation of the particles is an indispensable component in the research of microfluidic. In this paper, according to micro-nano manufacturing technology, we explored the mechanism of viscouselastic migration of particles in microchannels, involved in a variety of flow rates, multi-sized particles, and different characteristics of the solution. Our major results are detailed as follows.(1)Based on a variety of the technology for fabrication of the micro-prototype device, we prepared diversified micro-devices of different cross-sectional dimensions to explore viscoelastic focusing mechanism of particles. Capillary was utilized to manufacture the rigid circular cross-section microchannel devices with high-pressure drainage and waste collection modules. The method of wire removal was also utilized to manufacture circular cross-section microchannels made of PDMS. A variety of rectangular cross-sectional microchannel with different dimensions were fabricated, such as straight microchannels, expansion-contraction microchannel, curved microchannel, using mask-based lithography and PDMS micromolding techniques.(2) Exploration of the viscoelastic focusing mechanisms of microparticles in circular cross-sectional microchannels. Firstly, the lateral migration behaviors of particles suspended in Newtonian fluids and viscouselastic fluids were compared. It is found that particles suspended in non-Newtonian fluids would migrate towards the channel centerline and form a single-line particles array under the action of elastic force, while particles suspended in Newtonian fluids would migrate to form a famous Segre-Silberberg particle annular ring under the effects of inertial lift forces. Secondly, the effects of the size of particles and driving flow rate on particles viscoelastic focusing were quantitatively analyzed. The results showed that the focusing degree increased and finally stabilized at a certain value with increasing flow rates, and the larger 10μm particles have better focusing quality than the smaller 5μm particles. The dynamic focusing process of particles along the channel length was investigated. The mathematical model for safe length of channel to achieve particles focusing was derived and validated by our experiments.Finally, the viscoelastic focusing of microparticles in circular cross-sectional microchannels made of PDMS (polydimethylsiloxane) were explored. We found special progress of viscoelastic focusing of particles with flow rates increasing. Viscoelastic effects were applied to focusing of blood cells in circular microchannels made of PDMS, to verify the feasibility of the technology of viscoelastic focusing used in biological particles.(3) Exploration of the viscoelastic focusing mechanisms of microparticles in straight microchannels and expansion-contraction microchannel of rectangular cross-sectional.The effects of the driving flow rate of PVP solution on particles focusing were explored, in straight microchannels with rectangular cross-sectional. And the process of manipulation of particles was similar to the straight microchannels with circular cross-section. How the cross-sectional shape and size of the microchannels affecting on viscoelastic migration of particles was analyzed. And the result show that a rectangular cross-section was more difficult to realize particles focusing than the circular cross-section. By quantitatively investigating the influence of blockage ratio on viscoelastic focusing of particles, we proposed the optimum blockage ratios exist, to achieve viscoelastic focusing of particles at channel center. Experimented with different viscoelastic migration process of particles in a variety of sample solution, the special migration of the particles in the PEO sample solution was observed and single focusing was unrealized. The effect of flow rates and cross-section dimensions on viscoelastic migration of particles in expansion-contraction microchannel was explored. A better result of viscoelastic focusing of particles on the center of channel was obtained in expansion-contraction microchannel. To optimize expansion-contraction microchannel, designing gradually rounded on the corner of sudden expansion structure, the results of experimental showed that the optimized structure was conducive to promoting viscoelastic focus of particles. Finally, we got 2.5 times enrichment of 10μm particles in a rectangular cross-section microchannel, and 2μm,10μm mixed particles sorted.(4) Exploration of the viscoelastic migration mechanisms of microparticles in curved microchannels. Firstly, the effects of the driving flow rate of PVP solution on particles focusing were explored in spiral microchannels with rectangular cross-sectional. The experimental results demonstrated that the particles distribution in outer side of the curved channls, and the results still indicated that the greater the flow rate was, the closer position of the particles distribution to the center of the channle, in the 8wt% PVP sample solution. It is realized that particles form a ideal single-line array in the same sample solution. The different concentration and type of polymer were used, and it is found that particles were easier to focus in 6.8wt% PVP sample solution than in 2wt% PVP sample solution. But after improvement of the concentration of PVP to 8wt%, the degree of viscoelastic focusing of particles were not promoted. Particles in the PEO sample solution with a small elastic coefficient could not form a single beam focusing, different from the single-line particles array in PVP solution. Finally, the progress of particles viscoelastic migration was analyzed in multi-width of the channl and multi-radius of curvature. Finally, we achieved 3.1 times enrichment of 10μm particles in the spiral microchannel.