介電泳力主要根據粒子和溶液間不同的介電特性(導電度和介電常數)，同時利用非均勻交流電場使粒子產生非對稱的誘發極化能力，粒子受電場作用力後會往高或低電場強度處移動而分離。介電泳力的大小和方向跟粒子及溶液之間的介電特性、外加交流電場強度、頻率、粒子大小有關。在低頻中，粒子和溶液間的極化值(即Clausius-Mossotti因子，CM因子)可近似為僅由導電度決定，溶液的導電度可由導電度計測得，若能決定粒子導電度，即可知道介電泳力的方向(正或負介電泳)。利用微影製程技術完成微小電極晶片來縮小電極間距，僅提供± 10 V的電壓，可完成介電泳力所需的強電場趨動。目前介電泳分離晶片形狀主要採取星形電極(polynomial electrode)利用模擬找出該形狀的強弱電場區和電場強度(視不同電極大小及外加電壓大小而有所不同)。

　　本研究中以兩種方法決定粒子導電度: (1)粒子在溶液中其介面產成電雙層，造成表面離子電荷的聚集，而粒子的導電度僅由此層電荷所生成的電導值有關(假設粒子本體導電度為零)，此層表面電導值又可細分為Stern電導和擴散層電導。表面電導為zeta電位的函數，測出zeta電位即可得知粒子導電度大小。(2)利用導電度正比於濃度的關係，固定粒子的濃度，改變溶液的濃度，當粒子導電度高於溶液時，整體導電度會上升;低於溶液時，整體會下降，當整體導電度值不變時，此即粒子導電度。最後用實際介電泳分離實驗，捉取正負介電泳過渡區域的頻段(臨界頻率)，反推出粒子導電度，且視為標準值，來比較兩個方法的量測結果，發現以測量zeta電位以有修飾官能基的latex較符合其值(含修正結果)，而導電度差值的方法在不同的樣本(不同修飾)中導電度很相似，推測此法較不適合表面改質或導電度較小的樣本。

　　Dielectrophoretic forces are the forces created on a polarizable particles (e.g. a biological cell) when exposed to non-uniform electric field. A particle can be translated towards the regions of high field intensity (positive dielectrophoresis) or low field intensity (negative dielectrophoresis) depending on the electrical properties (conductivity and permittivity) of the particles and suspending medium. The magnitude and direction of dielectrophoresis depend on polarisability between particles and medium (Clausius-Mossotti factor), strength of applied AC electric field, frequency and the size of particles. The real part of the CM factor reaches a low frequency limiting value, it depends solely on the conductivity of the particles and medium. If we could decide conductivity of the particles, we can know the direction of dielectrophresis.

　　The microelectrode structures provide the route by which sufficient field strengths can be generated in order to move sub-micrometer particles without requiring high voltage signal generates. In order to understand the relationship between field geometries and particle behavior, we restrict ourselves to two widely-used electrode design, namely the polynomial electrode, and we use the numerical solutions for the electric field have been calculated using commercial finite element method software.

　　There is two ways for deciding conductivity of particles, (1) when a particle is immersed in an electrolyte, the region of liquid near to the interface has a higher density of counter ions (electric double layer), and conductivity of particles depends on surface conductance which produced by EDL. (Surface conductance is the function of zeta potential) (2) because the conductivity is proportional to the concentration of medium, so we fix the concentration of particles, and vary the one of medium. Trying to find the difference of conductivity of suspension and medium which equals to zero, this point is the conductivity of particles. Final we use DEP to separate particles, find the crossover frequency (translational area of positive and negative DEP), get conductivity of particles to compare that result of two way. For modified latex beads, measurement of zeta potential for conductivity of particles is corresponding to result, and the way of difference of suspension and medium has similar result for different sample, last way may not be suitable for surface modified or small conductivity sample.

[1] H. A. Pohl, “Dielectrophoresis”, Cambridge Uni. Press, 1978.
[2] H. Morgan, N. G. Green, “Ac electrokinetics: colloids and nanoparticles”, Research Studies Press, 2003.
[3] K. C. David, “Field and wave electromagnetics”, Addison-Wesley Press, 1989.
[4] T. B. Jones, “Electromechanics of particles”, Cambridge University Press, 1995.
[5] X-B. Wang, Y. Huang, R. Hölzel, R. Pethig, “Theoretical and experimental investigations of the interdependence of the dielectric, dielectrophoretic and electrorotational behaviour of colloidal particles”, Journal of Physics D: Applied Physics, 1993, 26, 312-322.
[6] M. P. Hughes, X-B. Wang, F. F. Becker, “Computer-aided analyses of electruc fields used in electrorotation studies”, Journal of Physics D: Applied Physics, 1994, 27, 1564-1570.
[7] G. Fuhr, C. Reichle, T. Müller, “Processing of micro-particles by UV laser irradiation in a field cage”, Applied Physics A, 1999, 69, 611-616.
[8] T. Schnelle, T. Müller, G. Fuhr, “The influence of higher moments on particle behaviour in dielectrophoretic field cage”, Journal of Electrostatics, 1999, 46, 13-28.
[9] T. Schnelle, T. Müller, G. Fuhr, “Trapping in AC octode field cages”, Journal of Electrostatics, 2000, 50, 17-29.
[10] Y. Huang, X-B. Wang, R. Pethig, “Electrokinetic behaviour of colloidal particles in traveling electric fields: studies using yeast cells”, Journal of Physics D: Applied Physics, 1993, 26, 1528-1535.
[11] M. P. Hughes, R. Pethig, X-B. Wang, “Dielectrophoretic forces on particles in traveling electric fields”, Journal of Physics D: Applied Physics, 1996, 29, 474-482.
[12] M. S. Talary, J. P. H. Burt, R. Pethig, “Electromanipulation and separation of cells using traveling electric fields”, Journal of Physics D: Applied Physics, 1996, 29, 2189-2203.
[13] Polystyrene latex beads and latex beads amine- and carboxylate- modified (fluorescent), Product information, SIGMA, http://sigma-alrich.com
[14] FEMLAB Model library, version 2.3, COMSOL, (2001).
[15] FEMLAB Reference Manual, version 2.3, COMSOL, (2001).
[16] CFD-ACE(U) User Manual, Version 2002, CFD research corporation (2002).
[17] CFD-ACE(U) Module Manual, Version 2002, CFD research corporation (2002).
[18] J. P. H. Burt, R. Pethig, “An optical dielectrophoresis spectrometer for low-frequency measurements on colloidal suspension”, Journal of Physics E: Scientific Instrument, 1989, 22, 952-957.
[19] CDM230 conductivity meter and 2-pole conductivity cells of operating instructions, Radiometer analytical.
[20] Y. Zalini, M. Vincent, E. V. L. Cynthia, “A simple method for the measurement of bacterial particle conductivities”, Journal of Microbiological Method, 2002, 51, 401-406.
[21] N. G. Green, H. Morgan, “Dielectrophoretic separation of nano-particles”, Journal of Physics D: Applied Physics, 1997, 30, L41-L42.
[22] Bockris J.O’M., Reddy A.K.N., “Modern electrochemistry”, Plenum Press, 1973.
[23] D. L. Chapman, “A contribution to the theory of electrocapillary”, Philosophical Magazine Letters, 1913, 25, 475-481.
[24] D. C. Grahame, “The electrical double layer and the theory of electrocapillary”, Chemical Reviews, 1947, 41, 441-501.
[25] J. Lyklema, “Fundamentals of interface and colloid science Vol II”, Academic Press, 1995.
[26] C.T. O’Konski, “Electric properties of macromolecules V: theory of ionic polarization in ployelectrolytes”, Journal of Physical Chemistry, 1960, 64, 605-619.
[27] D.C. Henry, “The cataphoresis of suspended particles. I. The equation of cataphoresis”, Proceedings of the Royal Society of London Series A, 1931, 133, 106-129.
[28] Zeta potential measurement and theory of operation, Malvern Ltd., 2000.
[29] T. Hanai, “Electric properties of emulsions. In: Sherman, P.(Ed.),
Emulsion Science”, Academic Press, 1968, pp. 353- 475.
[30] Y. Huang, R. Pething, “Electrode design for negative dielectrophresis”, Measurement Science & Technology, 1978, 2, 1142-1146.
[31] T. Heida, W. L. C. Rutten, E. Marani, “Understanding dielectrophoretic trapping of neuronal cells: modeling electric field, electrode-liquid interface and fluid flow”, Journal of Physics D: Applied Physics, 2002, 35, 1592-1602.
[32] M. P. Hughes, H. Morgan, “Dielectrophoretic trapping of single sub-micrometre scale bioparticles”, Journal of Physics D: Applied Physics, 1998, 31, 2205-2210.
[33] R. Pethig, Y. Huang, X-B. Wang, J. P. H. Burt, “Positive and negative dielectrophoretic collection of colloidal particles using interdigitated castellated microelectrodes”, Journal of Physics D: Applied Physics, 1992, 25, 881-888.
[34] M. Frénéa, H. Lhermite, B. L. Pioufle, “Design of biochip microelectrode arrays for cell arrangement”, IEEE-EMBS special topic conference on microtechnologies in medicine & Biology, 2002.
[35] X-B. Wang, Y. Huang, J. P. H. Burt, G. H. Markx, R. Pethig, “Selective dielectrophoretic confinement of bioparticles in potential energy wells”, Journal of Physics D: Applied Physics, 1993, 26, 1278-1285.
[36] M. P. Hughes, H. Morgan, F. F. Mary, “The dielectrophoresis behavior of submicron latex spheres: influence of surface conductance”, Journal of Colloid and Interface Science, 1999, 220,454-457.
[37] M. P. Hughes, H. Morgan, F. F. Mary, “The influence of Stern layer conductance on the dielectrophoretic behavior of latex nanospheres”, Journal of Colloid and Interface Science, 2002, 250,266-268.
[38] N. G. Green, A. Ramos, H. Morgan, “Ac electrokinetics: a survey of sub-micrometre particle dynamics”, Journal of Physics D: Applied Physics, 2000, 33, 632-641.
[39] J. Jin, “The finite element method in electromagnetics”, Wiley Press, 1993.
[40] P. P. Silvester, R. L. Ferrari, “Finite elements for electrical
engineers”, Cambridge University Press, 1996.
[41] Paul T Sharpe, “Method of cell separation”, Elsevier, 1988, 23-26.
[42] 魏正舒、宋金丹, “醫學細胞生物學”, 合記圖書出版社, 87年, 31-34.
[43] M. Berger, J. Castelino, R. Huang, M. Shah, R. H. Austin, “Design
of a microfabricated magnetic cell separator”, Electrophoresis,
2001, 22, 3838-3892.