Study On Characteristics And Mechanism Of Fluid Flow And Particle Dynamics In Viscoelastic Fluids In Microchannels

Microfluidic technology has many advantages such as multidisciplinary intersecting,miniaturization,easy control,high integration and portability,etc.,and has been widely used in aerospace,information and communication,biochemistry,medical,pharmaceutical research,automatic control and other sciences field s.With the continuous research and development of the applic ation of microfluidic technology,it has been found that there is a lack of sufficient knowledge about the physical processes that occur in the flow of liquids and entrapped particles in the flow at microscale.Especially for biological fluids such as bloo d,saliva,or tissue fluids,because of their complex rheological properties as non-Newtonian fluids,the research related to them is far inferior to Newtonian fluids.Therefore,the purpose of this paper is to study the characteristics and mechanism of th e flow of viscoelastic fluid in microchannels,and to use viscoelastic fluid to entrain particles inside the fluid to propose a particle sorting and enrichment device suitable for microfluidics.At first,in order to study the cause of the secondary flows of the viscoelastic fluids in the straight channels,a numerical simulation study was used.Using the generation term E and the dissipation term ? of the secondary flows in the quasi-vortex energy equation,the necessary and sufficient cond itions for the secondary flow generation are proposed: E>0.With the help of E and ?,the influence of the physical parameters of the viscoelastic fluid and the geometric parameters of the channel on the secondary flow intensity and flow pattern is analyzed.Then,In order to further understand the characteristics of viscoelastic fluid flow at the microscale,this paper used the laser-induced fluorescence bleaching velocimetry(LIFPA)with high spatial and temporal resolution to measure the flow of Newtonian fluids and viscoelastic fluids near the inner wall of the microchannel.We foound that the slip velocity will also be greater in the flow with a larger contact angle;slip will also occur in the flow of Newtonian fluid;the slip produced by viscoelastic fluids is generally greater than the slip produced by Newtonian fluids.By experimentally studying the relationship between the fluorescence signal and the velocity near the wall and theoretically analyzing the effects of wall effects such as total reflection(TIRF),it is concluded that there will be a critical slip flow or critical shear rate in the Newtonian fluid,and by comparing the polymer solution,it is found that as the concentration of the polymer increases or the molecular weight of the polymer increases,the increase rate of the slip rate is smaller than the increase rate of the specific flow rate.Next,in order to study how the viscoelastic fluids affect the movement of particles in the microfluidic device,in the deterministic lateral displacement(DLD)microfluidic particle separation device,the effects of shear thinning and elasticity of the viscoelastic fluid on the separation threshold are analyzed separately.It is found in experiments that as shear thinning and elasticity increase,the s eparation threshold of DLD also decreases.Taking advantage of the influence of elasticity on separation,this paper proposes the use of regulated flow to achieve dynamic threshold separation control based on viscoelastic fluid DLD devices.The maximum thr eshold adjustment range can reach 40%.At last,applying the particle focusing effect based on viscoelastic fluids,a microfluidic separation usedviscoelastic fluid prefocusing and dielectrophoresis(DEP)separation techniques.An optimal focusing condition will appear.Then use the calibration device and numerical calculation of the DEP frequency spectrum to obtain the appropriate voltage and frequency suitable for the DEP separation of the two cells.Finally,the best focusing conditions,electric field v oltage and frequency are applied to the separation device of two cells,and the microfluidic separation device with separation efficiency exceeding 95% and cell activity exceeding 90% is realized.To sum up,this paper has a deep understanding of the sliding effects of viscoelastic secondary flow and continuous flow of Newtonian fluid and viscoelastic fluid in straight channels.By studying the characteristics and mechanism of viscoelastic fluid's movement to particles in microchannels,two sorts of microfluidic particle sorting and enrichment devices based on viscoelastic fluid with broad application prospects are proposed.The research results provide theoretical basis and experimental data support for the study of viscoelastic fluid flow in microchannels,which helps people better understand the physical mechanism of viscoelastic fluid flow and also promotes the application of microfluidic devices based on viscoelastic fluid.