Study On Label-free Magnetic Separation Of Micron Particles In A Simple Straight Channel

Microfluidic chips with advantages such as low cost,small size,and small sample consumption are booming in the biomedical field,especially in particles/cells separation.Many scholars have proposed the magnetic field-driven microfluidic chip because the magnetic field is flexible and controllable,without heat and expensive external systems.Ferrofluids are stable colloidal suspensions of surfactant-coated magnetic nanoparticles in aqueous or organic solutions.A ferrofluid is widely used in particles/cells separation technology based on the principle of negative magnetophoresis due to its large magnetic susceptibility.Aiming at the emerging labelfree particles/cells separation technology,this paper proposes two methods of particles separation in the inclined magnetic field and multiphase flow field and carries out systematic theoretical and experimental research.The purpose is to inspire researchers in related fields to promote the development of label-free magnetic separation of particles,eventually to isolate and detect cancer cells.A model of particle motion in ferrofluids was established for inclined permanent magnets.The expression of the magnetic force acting on the particle was derived.The trajectory of the particle was simply dimensioned.The magnetic force is related to the particle size,the relative susceptibility of the particle-ferrofluid,and the inclined angle.Polystyrene particles are non-magnetic,their magnetic susceptibility is approximately equal to zero.The magnetic susceptibility of the ferrofluids is positive.The difference between the two magnetic susceptibilities is negative,and the magnetic force is negative,causing the particles to move toward a region with a lower magnetic flux density.When two permanent magnets are placed with the same poles facing each other,a zero field strength region is generated at the center of the channel,resulting in particles focusing.After the permanent magnet rotates for a certain angle,the magnetic field strength and magnetic force on the centerline of the channel will change abruptly.Based on the abrupt value and the actual blood flux,the range of the rotating angle was determined to be 0°?20°.The model introduces the viscosity resistance coefficient to consider the influence of the channel wall on the particle motion.Based on the balance between magnetic force and viscous resistance,the calculation equation of particle velocity was derived.The velocity distribution curve of the particle is consistent with the magnetic field strength.The Reynolds number was introduced to determine the flow state of the fluid in the rectangular microchannel,and the calculation equation of the fluid velocity in the rectangular channel was derived based on the relationship between the channel inlet flow rate and the pressure drop.The fluid velocity is parabolic in the cross-section of the channel.Due to the symmetrical distribution of the magnetic field strength of the permanent magnet,the trajectory of the particles in the channel depends on the relative magnitude between the magnetic force and the inertial force.Based on the maximum magnetic force,the critical flow rate was calculated to prevent the particles from forming a ring at the front of the channel.For the problem that the exposure time of the biological particles in the ferrofluids should not be too long,the fluid multiphase method was used to resuspend the biological particles in deionized water or buffer solution.The distribution of each fluid width in the channel under different channel inlet rate was studied,and the influence of the magnetic field on the interface distribution was discussed.According to the relationship between the stress and strain of the Newtonian fluid,the variation rule of the width of the deionized water and the ferrofluids in the channel was derived.The width ratio of the two is the product of the viscosity ratio and the velocity ratio.The surface tension between fluids is the main obstacle for particles to break through the interface.The critical magnetic field was calculated based on the maximum surface tension.The smaller the ferrofluid concentration and the larger the particle diameter,the smaller the required critical magnetic field and the easier it is for the particles to pass through the interface.Based on theoretical calculations,combined with COMSOL software,the magnetic field,flow field,particles trajectory,and phase interface were simulated.The simulation results of magnetic field strength and velocity are consistent with theoretical calculations.An experimental study of the effects of critical flow rate,inclined angle,and residual magnetization on particles focusing was carried out.The experimental results verify the theoretical prediction well.The improvement of the focusing efficiency can be considered from the following factors: ferrofluid concentration,particle size,the residual magnetization of the permanent magnet,the distance between the permanent magnet and the channel,and the channel inlet rate.The effects of the magnetic field,channel inlet angle and channel inlet flow ratio on the phase interface were explored.The width of the ferrofluids in the channel is affected by the magnetic field,and its variation rule is the same as the magnetic field strength over the length of the channel.When the channel inlet angle is 45° or 60°,and the inlet flow ratio of deionized water and ferrofluids exceeds 7:1,a clearer interface can be formed.This paper systematically studies the magnetic separation of particles through theoretical analysis,numerical simulation and experimental verification.The method of inclined permanent magnets has not been proposed in previous studies.It is expected to provide corresponding theoretical and experimental references for researchers in the field of particles/cells label-free magnetic separation.