| In recent years,microfluidic chips have made great progress in both processing precision and control methods.Because of its low sample consumption,low response time,refinement,easy handling and versatility,microfluidics has attracted more and more attention from researchers.In addition,magnetic fluids have always been the focus of research in the field of microfluidics.As a kind of intelligent fluid,magnetic fluids can be generally divided into ferrofluids or magnetic nanofluids and magnetorheological fluids(MRF).The magnetic force can directly act on the magnetic materials in the magnetic fluids,or control the non-magnetic materials in the micro-channel by means of the magnetic fluids.Magnetic valves,magnetic pumps,and magnetic plugs,etc.based on ferrofluids are not uncommon in the field of microfluidics.However,magnetic fluids with a particle size of a few hundred nanometers have been neglected by microfluidic researchers for a long time.These magnetic fluids have stronger magnetic properties than general ferrofluids,have better stability than magnetorheological fluids,and plays an important role in drug delivery,cell labeling,and material monitoring.Under the action of the external magnetic field,the research on the generation of particle chains in magnetic fluids,the separation of magnetic or non-magnetic particles,and the monitoring and characterization of micromagnetic droplets are of great significance for analyzing the magnetorheological mechanism and further expanding their application.Combined with the significant advantages of microfluidic chips,this paper used conventional soft lithography microfluidic chips and 3D printed microfluidic chips to conduct a variety of research on magnetic fluids,especially magnetic fluids with particle sizes ranging from a few nanometers to hundreds of nanometers.The details are as follows:1.Particle size dependent rheological property in magnetic fluid.Ferrofluids with good sedimentation stability are the most commonly used magnetic matrix in microfluidic chips,but their nanoscale particles exhibit a small magnetic force,and the insignificant magnetorheological effects limit their application range.Fe3O4 particles of different particle sizes were prepared based on solvothermal method.Although their particle sizes vary widely(40 nm to 200 nm),the magnetic properties are almost the same.The magnetic fluids were prepared by dispersing Fe3O4 nanospheres in the solution as MRF-40,MRF-100,and MRF-200.Magnetic fluids exhibit good sedimentation stability.The rheological properties were investigated by a rheometer and it was found that the relative magnetorheological effects increased with increasing the particle size.Particle dynamics simulates the particle arrangement of three magnetic fluids under the action of a magnetic field.The particle chains formed are dense and slender to slightly thicker,then sparse and short.Such particle chain structures exhibit a consistent trend with the observation of the injection of a magnetic liquid into a transparent microfluidic chip,which is a good explanation for the data difference between the three magnetic liquids obtained by the rheometer test.And it was found that the higher MR(magnetorheological)effect of the large particle based magnetic fluid was originated from the stronger assembling microstructure under the applying magnetic field.2.Size-selective separation of magnetic nanospheres in a microfluidic channel.Magnetic particles have a very wide range of applications in drug delivery and magnetic labeling,especially magnetic particles of 100 nanometers,which poses an urgent need for obtaining magnetic particles with uniform particle size.Since the obtained magnetic particles are distributed over a wide range,the magnetic nanospheres are dimensionally selectively separated using a self-focusing microfluidic chip equipped with a permanent magnet.Under an external magnetic field,the magnetic field force exerted on the particles causes them to undergo dimensionally related deflections in the laminar flow path and thereby complete effective particle separation.By adjusting the distance between magnet and channel's main path,two monodisperse nanospheres samples(Ca.90 nm,Ca.160 nm)were obtained from polydispersing particles solution whose diameters varied from 40 nm to 280 nm.Based on the magnetostatic and laminar flow models,numerical simulations were also used to predict and optimize the nanospheres migrations.Two thresholds of particles diameters were obtained by the simulations and diverse at each position of magnet.Therefore,appropriate position of the magnet could be determined at a certain particles sizes range when the flow rate of the two inlets remains unchanged.3.3D printed microfluidic manipulation device integrated with magnetic array.Non-magnetic particle manipulation has always been the focus of microfluidics research on magnetic fluids.Conventional soft lithography chips greatly limit the efficiency of the application of complex magnetic fields and magnetic separation.This paper made a transparent,high-precision 3D printed microfluidic device integrated with magnet array for magnetic manipulation.A reserved groove in the device can well constrain the Halbach array or conventional alternating array.First,the magnetic field parameters of Halbach array in long range situation are better than the traditional alternating array.Then,Halbach array is superior to other arrays in both theoretical and experimental analysis.The results of numerical simulations were verified by particle trajectories observed by fluorescence microscopy.The deflection angle of the particle trajectory under the Halbach array is greater than the deflection angle of the alternating array,while the deflection angle decreases as the volumetric flow rate increases.The difference in deflection angle explains the dependence of particle size in magnetic manipulation.Here,the 3D printing material in this work is transparent,the processing precision is high,and the residual resin in the sample is easily removed.These results show that the 3D printing device integrated with the Halbach array has excellent magnetic processing capabilities and is widely used in particle sorting,separation,and mixing.4.Magnetic droplet co-location monitoring in an integrated microfluidic giant magnetoresistive system.The micro-magnetic droplets in the flow-focusing device can be recorded using a high speed camera.In addition,magnetoresistive sensors are also an effective tool for co-location monitoring.However,for paramagnetic materials,excessive external magnetic field excitation may directly lead to saturation of the sensor.Therefore,it is very important to study the magnetization process of magnetic droplets under the action of weak magnetic fields.First,a flow-focusing device was used to generate magnetic droplets.Changing the flow rate ratio of the dispersed phase and continuous phase could result in droplets of different modes,which were mainly embodied in the squeezing to dripping in this paper.In particular,within a certain two-phase flow ratio range,the droplets enter a special "bifurcation" mode,which appeared as a large droplet followed by a small "satellite" droplet.Secondly,a giant magnetoresistance(GMR)sensor was fabricated on the surface of a silicon wafer.The process of making a GMR sensor in a clean room was described in detail,which included at least two lift-off processes.The resulting sensor was characterized by a magnetic field response,and the prepared sensor was bonded to the flow-focusing chip.Molecular dynamics was used to simulate the structural changes of microdroplets under the action of a 100 Oe magnetic field.The magnetic field generated by droplets with a diameter of 50?m or 60?m is sufficient to be detected by the GMR sensor.This is of great significance for the study of the monitoring and characterization of superparamagnetic materials in magnetoresistive microfluidic systems. |
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