The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Dielectric Properties of Colloidal Suspensions

Lei Dong

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium E at Helsinki University of Technology (Espoo, Finland) on the 19th of January, 2009, at 13 o'clock.

Overview in PDF format (ISBN 978-951-22-9716-0)   [1647 KB]
Dissertation is also available in print (ISBN 978-951-22-9715-3)

Abstract

This thesis focuses on theoretical research of dielectric properties of colloidal suspensions in which the suspended particles can be either homogeneous or graded. Colloids have enormous advantages of being experimentally accessible. Atomic size scale, time scale of diffusion and tunable interactions of colloids make them as ideal tools for fundamental investigations. Effective dielectric constant plays a key role in the investigation of the properties of colloidal suspension. Many applications such as dielectrophoresis (DEP) and electrorotation are useful to study the effective dielectric constant.

In the first part of this thesis, a theoretical study of the dielectrophoretic spectrum of a pair of touching colloidal particles is present. The multiple image method is employed to account for the effective dipole factor, and an analytical expression for the DEP force is obtained. It is found that, at low frequency, the DEP force can be enhanced (reduced) significantly for the longitudinal (transverse) field case due to the presence of multiple images.

The second part of the thesis investigates the dielectric properties of functionally graded materials using different methods. Analytical approaches such as Bergman-Milton spectral representation theory and first-principles approach have been generalized to study the dielectric properties of graded composite. Differential effective multipole moment approximation has been developed to study the multipole polarizability of a graded spherical particle in a nonuniform electric field. An anisotropic differential effective dipole approximation has been developed for calculating the dipole moment of anisotropic graded materials. We compared the approximative results with the analytical results and the agreement is excellent. Furthermore, the optical nonlinear response of graded films was studied in this thesis, and the result shows that the composition-dependent gradation can produce a broad resonant plasmon band in the optical region, resulting in a large enhancement of the optical nonlinearity.

This thesis consists of an overview and of the following 7 publications:

  1. L. Dong, J. P. Huang, and K. W. Yu. 2004. Theory of dielectrophoresis in colloidal suspensions. Journal of Applied Physics, volume 95, number 12, pages 8321-8326.
  2. L. Dong, J. P. Huang, K. W. Yu, and G. Q. Gu. 2004. Dielectric response of graded spherical particles of anisotropic materials. Journal of Applied Physics, volume 95, number 2, pages 621-624.
  3. J. P. Huang, L. Dong, and K. W. Yu. 2004. Optical nonlinearity enhancement of graded metal-dielectric composite films. Europhysics Letters, volume 67, number 5, pages 854-858.
  4. L. Dong, M. Karttunen, and K. W. Yu. 2005. Spectral representation of the effective dielectric constant of graded composites. Physical Review E, volume 72, 016613.
  5. L. Dong, J. P. Huang, K. W. Yu, and G. Q. Gu. 2005. Multipole polarizability of a graded spherical particle. The European Physical Journal B, volume 48, number 4, pages 439-444.
  6. J. P. Huang, L. Dong, and K. W. Yu. 2006. Giant enhancement of optical nonlinearity in multilayer metallic films. Journal of Applied Physics, volume 99, number 5, 053503.
  7. En-Bo Wei, L. Dong, and K. W. Yu. 2006. Effective ac response of graded colloidal suspensions. Journal of Applied Physics, volume 99, number 5, 054101.

Keywords: dielectric properties, functionally graded materials, colloid, suspension

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© 2009 Helsinki University of Technology

Last update 2011-05-26