Frequency Effect On Dielectrophoretic Interaction Of Dense Particles

In the applications of particle manipulation by dielectrophoresis(DEP),when a large number of dense particles are immersed in an electrolyte subjected to an applied electric field,the particles will form particle chains or gather into groups due to dielectrophoretic interactions.The particle chains are detrimental to particle separation.On the other hand,particle interactions can be applied to particle assembly in fields of microfabrication,new materials and environmental health.An effective computation of dielectrophoretic interaction of dense particles is a challenging research subject at present.Maxwell stress tensor(MST)method is the most rigorous approach in theory.However,due to its mathematical complexity,huge calculation quantity and the uncertain numerical error,the application of the MST method is limited to the DEP interaction of a few number of particles in two-dimensional field.The newly proposed iterative dipole moment(IDM)method is an effective algorithm for studying the DEP interaction of large numbers of dense particles due to simple algorithm,less calculation,and high precision.This thesis focuses on numerical study of the frequency effect on the DEP interaction of dense particles in both two and three dimensional electrical field by MST and IDM method.This thesis has completed following works.This thesis studied particle DEP interaction in a uniform DC electric field.The coupled electric field,flow field and the particle motion are simultaneously solved using finite element method with an arbitrary Lagrangian-Eulerian(ALE)algorithm.The DEP interactions of similar particles,dissimilar particles and particles with different sizes are studied.The distribution of the electric field inside the particles is calculated and analyzed.The particle velocities and motion trajectories,the particle chain-forming time are investigated in cases of various permittivity ratios of the fluid and particles.It is found that similar particles always form a particle chain parallel to the electric field,while dissimilar particles always form a particle chain perpendicular to the electric field.This thesis studied the frequency effect on the DEP interaction of particles in a uniform AC electric field by both MST and IDM methods.A comparison of the particle trajectories is made between these two methods.The IDM method is validated to be accurate enough to calculate the particle interaction forces.In the MST method.The distribution of electric field inside the particles,the distribution of the stress on the particle surface,the particle trajectories,and the final particle chains are studied at different frequencies.It is found that the behaviors of the DEP interaction are strongly dependent on the frequency of the AC field.By tuning the frequency,the particles can be transformed between negative DEP particles and positive DEP particles,which results in different particle chains.The stress is tensile on the surface of positive DEP particle,and is compressed on the surface of negative DEP particles.The DEP interactions of four,five and twenty particles are studied by the IDM method.It is found that the particle trajectories and chain patterns can be various styles depending on the initial particle locations,the frequency,and the electric properties of the particles.An arbitrary small disturbance to the particle initial locations may lead to dramatically different motion trajectories and final particle chains.The essential characteristics of all particle chains are consistent.The similar particles form a particle chain parallel to the electric field,dissimilar particles always form a particle chain perpendicular to the electric field.This thesis studied three-dimensional DEP interaction of large numbers of dense particles on a plane perpendicular to a uniform AC electric field by IDM method.It is found that all similar particles repel each other,do not form particle chain,and finally form an equal-distanced distribution pattern.Dissimilar particles attract each other to form a number of discrete particle chains on the plane.The particle chain behaviors strongly depend on initial particle distributions,the particle sizes,and the frequencies.In addition,the motions of randomly distributed particles in a bounded chip are also studied at different frequencies.By tuning the frequency,particle chains can be transformed to almost uniformly distributed pattern.The effects of low frequency,high frequency and crossover frequency on particle interaction in a circular chip are also investigated,and the general characteristic of frequency-tuning particles interaction is discussed in this thesis.This thesis proposed a new Stokes resistance algorithm including interaction of hydrodynamic forces of slowly moving dense particles.When large numbers of dense particles slowly move in the fluid,particle motions will interact with the neighboring particles.The traditional Stokes formula loses its precision in this situation.A modified algorithm is proposed to calculate the hydrodynamic forces.The local flow velocities are iteratively corrected by surrounding particle motions to obtain a new Stokes resistance,which includes the effect of particles interaction without numerical computation to solve full Navier-Stokes equation.Numerical examples indicate that in comparison with traditional Stokes resistance,the modified Stokes resistance greatly increase the accuracy of hydrodynamic forces of particles,and can be safely applied to particles DEP interaction,without losing the essential behaviors of particle chains.This thesis proposed a new chip design realizing continuous separation of particles with different sizes based on combination of field-modulated electroosmotic vortex and dielectrophoresis(DEP).A numerical simulation on the particle separation process is performed and the pertinent parameters are optimized.The modulated electric field is solved by the Poisson-Boltzman equation,the flow field is solved by the Navier-Stokes equation,and the DEP forces of the particles are calculated by the equivalent dipole moment method(EDM).Negative DEP particles with different sizes are successfully separated by the combination of the vortexes and DEP forces.The effects of the driving voltage,modulating voltage,electrode length,and channel width on the particle separation efficiency and the flow flux are sufficiently analyzed and discussed.