arXiv subjects

G. Q. Gu

Publications and source records attributed to G. Q. Gu.

12 recordsLinked to original sources

arXiv2005

Multipole polarizability of a graded spherical particle

We have studied the multipole polarizability of a graded spherical particle in a nonuniform electric field, in which the conductivity can vary radially inside the particle. The main objective of this work is to access the effects of multipole interactions at small interparticle separations, which can be important in non-dilute suspensions of functionally graded materials. The nonuniform electric field arises either from that applied on the particle or from the local field of all other particles. We developed a differential effective multipole moment approximation (DEMMA) to compute the multipole moment of a graded spherical particle in a nonuniform external field. Moreover, we compare the DEMMA results with the exact results of the power-law graded profile and the agreement is excellent. The extension to anisotropic DEMMA will be studied in an Appendix.

cond-mat.mtrl-sci↗

arXiv2005

Dielectric response of spherical particles of graded materials

We have studied the effective response of composites of spherical particles with a dielectric profile which varies along the radius of the particles. We developed a differential effective dipole theory to compute the dipole moment of individual spherical particle and hence the effective dielectric response of a dilute suspension. The theory has been applied to two model dielectric profiles. Moreover, we compare the approximate results with the exact results of the power-law profile and the agreement is excellent.

cond-mat.soft↗

arXiv2005

Effective conductivity of composites of graded spherical particles

We have employed the first-principles approach to compute the effective response of composites of graded spherical particles of arbitrary conductivity profiles. We solve the boundary-value problem for the polarizability of the graded particles and obtain the dipole moment as well as the multipole moments. We provide a rigorous proof of an {\em ad hoc} approximate method based on the differential effective multipole moment approximation (DEMMA) in which the differential effective dipole approximation (DEDA) is a special case. The method will be applied to an exactly solvable graded profile. We show that DEDA and DEMMA are indeed exact for graded spherical particles.

cond-mat.mtrl-sci↗

arXiv2003

Electrokinetic behavior of two touching inhomogeneous biological cells and colloidal particles: Effects of multipolar interactions

We present a theory to investigate electro-kinetic behavior, namely, electrorotation and dielectrophoresis under alternating current (AC) applied fields for a pair of touching inhomogeneous colloidal particles and biological cells. These inhomogeneous particles are treated as graded ones with physically motivated model dielectric and conductivity profiles. The mutual polarization interaction between the particles yields a change in their respective dipole moments, and hence in the AC electrokinetic spectra. The multipolar interactions between polarized particles are accurately captured by the multiple images method. In the point-dipole limit, our theory reproduces the known results. We find that the multipolar interactions as well as the spatial fluctuations inside the particles can affect the AC electrokinetic spectra significantly.

cond-mat.soft↗

arXiv2003

Nonlinear alternating current responses of graded materials

When a composite of nonlinear particles suspended in a host medium is subjected to a sinusoidal electric field, the electrical response in the composite will generally consist of alternating current (AC) fields at frequencies of higher-order harmonics. The situation becomes more interesting when the suspended particles are graded, with a spatial variation in the dielectric properties. The local electric field inside the graded particles can be calculated by the differential effective dipole approximation, which agrees very well with a first-principles approach. In this work, a nonlinear differential effective dipole approximation and a perturbation expansion method have been employed to investigate the effect of gradation on the nonlinear AC responses of these composites. The results showed that the fundamental and third-harmonic AC responses are sensitive to the dielectric-constant and/or nonlinear-susceptibility gradation profiles within the particles. Thus, by measuring the AC responses of the graded composites, it is possible to perform a real-time monitoring of the fabrication process of the gradation profiles within the graded particles.

cond-mat.soft↗

arXiv2003

First-principles approach to dielectric response of graded spherical particles

We have studied the effective response of composites of spherical particles each having a dielectric profile which varies along the radius of the particles. We developed a first-principles approach to compute the dipole moment of the individual spherical particle and hence the effective dielectric response of a dilute suspension. The approach has been applied to two model dielectric profiles, for which exact solutions are available. Moreover, we used the exact results to validate the results from the differential effective dipole approximation, recently developed to treat graded spherical particles of an arbitrary dielectric profile. Excellent agreement between the two approaches were obtained. While the focus of this work has been on dielectric responses, the approach is equally applicable to analogous systems such as the conductivity and elastic problems.

cond-mat.mtrl-sci↗

arXiv2002

Electrorotation in graded colloidal suspensions

Biological cells can be treated as composites of graded material inclusions. In addition to biomaterials, graded composites are important in more traditional materials science. In this article, we investigate the electrorotation (ER) spectrum of a graded colloidal suspension in an attempt to discuss its dielectric properties. For that, we use the recently obtained differential effective dipole approximation (DEDA) and generalize it for non-spherical particles. We find that variations in the conductivity profile may make the characteristic frequency red-shifted and have also an effect on the rotation peak. On the other hand, variations in the dielectric profile may enhance the rotation peak, but do not have any significant effect on the characteristic frequency. In the end, we apply our theory to fit experimental data obtained for yeast cells and find good agreement.

cond-mat.soft↗

arXiv2002

Electrorotation of colloidal suspensions

When a strong electric field is applied to a colloidal suspension, it may cause an aggregation of the suspended particles in response to the field. In the case of a rotating field, the electrorotation (ER) spectrum can be modified further due to the local field effects arising from the many-particle system. To capture the local field effect, we invoke the Maxwell-Garnett approximation for the dielectric response. The hydrodynamic interactions between the suspended particles can also modify the spin friction, which is a key to determine the angular velocity of ER. By invoking the spectral representation approach, we derive the analytic expressions for the characteristic frequency at which the maximum angular velocity of ER occurs. From the numerical caculation, we find that there exist two sub-dispersions in the ER spectrum. However, the two characteristic frequencies are so close that the two peaks actually overlap and become a single broad peak. We report a detailed investigation of the dependence of the characteristic frequency and the dispersion strength of ER on various material parameters.

cond-mat.soft↗

arXiv2001

Electrorotation of a pair of spherical particles

We present a theoretical study of electrorotation (ER) of two spherical particles under the action of a rotating electric field. When the two particles approach and finally touch, the mutual polarization interaction between the particles leads to a change in the dipole moment of the individual particle and hence the ER spectrum, as compared to that of the well-separated particles. The mutual polarization effects are captured by the method of multiple images. From the theoretical analysis, we find that the mutual polarization effects can change the characteristic frequency at which the maximum angular velocity of electrorotation occurs. The numerical results can be understood in the spectral representation theory.

cond-mat.soft↗

arXiv2001

Relaxation of surface charge on rotating dielectric spheres: Implications on dynamic electrorheological effects

We have examined the effect of an oscillatory rotation of a polarized dielectric particle. The rotational motion leads to a re-distribution of the polarization charge on the surface of the particle. We show that the time averaged steady-state dipole moment is along the field direction, but its magnitude is reduced by a factor which depends on the angular velocity of rotation. As a result, the rotational motion of the particle reduces the electrorheological effect. We further assume that the relaxation of polarized charge is arised from a finite conductivity of the particle or host medium. We calculate the relaxation time based on the Maxwell-Wagner theory, suitably generalized to include the rotational motion. Analytic expressions for the reduction factor and the relaxation time are given and their dependence on the angular velocity of rotation will be discussed.

cond-mat.soft↗

arXiv2001

Nonlinear ER effects in an ac applied field

The electric field used in most electrorheological (ER) experiments is usually quite high, and nonlinear ER effects have been theoretically predicted and experimentally measured recently. A direct method of measuring the nonlinear ER effects is to examine the frequency dependence of the same effects. For a sinusoidal applied field, we calculate the ac response which generally includes higher harmonics. In is work, we develop a multiple image formula, and calculate the total dipole moments of a pair of dielectric spheres, embedded in a nonlinear host. The higher harmonics due to the nonlinearity are calculated systematically.

cond-mat.soft↗

arXiv2001

Dynamic electrorheological effects and interparticle force between a pair of rotating spheres

We consider a two-particle system in which a particle is held fixed, and the other one rotates around the axis perpendicular to the line joining the particles centers. The rotating particle leads to a displacement of its polarization charge on the surface. Our results show that the rotational motion of the particles generally reduces the force between the particles. The dependence of interparticle force on the angular velocity of rotation will be discussed.

cond-mat.soft↗