title: Direct measurements of the frequency-dependent dielectrophoresis force.
abstract: Dielectrophoresis (DEP), the phenomenon of directed motion of electrically polarizable particles in a nonuniform electric field, is promising for applications in biochemical separation and filtration. For colloidal particles in suspension, the relaxation of the ionic species in the shear layer gives rise to a frequency-dependent, bidirectional DEP force in the radio frequency range. However, quantification methods of the DEP force on individual particles with the pico-Newton resolution required for the development of theories and design of device applications are lacking. We report the use of optical tweezers as a force sensor and a lock-in phase-sensitive technique for analysis of the particle motion in an amplitude modulated DEP force. The coherent detection and sensing scheme yielded not only unprecedented sensitivity for DEP force measurements, but also provided a selectivity that clearly distinguishes the pure DEP force from all the other forces in the system, including electrophoresis, electro-osmosis, heat-induced convection, and Brownian forces, all of which can hamper accurate measurements through other existing methods. Using optical tweezers-based force transducers already developed in our laboratory, we have results that quantify the frequency-dependent DEP force and the crossover frequency of individual particles with this new experimental method.
authors.affiliation: Physics Department, Lehigh University, Bethlehem, Pennsylvania 18015, USA.
authors.forename: Ming-Tzo
authors.initials: MT
authors.lastname: Wei
authors.forename: Joseph
authors.initials: J
authors.lastname: Junio
authors.forename: H Daniel
authors.initials: HD
authors.lastname: Ou-Yang
references.citation: Langmuir. 2008 Nov 4;24(21):12164-71
references.pmid: 18842062
references.citation: Electrophoresis. 2002 Jul;23(13):1973-83
references.pmid: 12210248
references.citation: Biomicrofluidics. 2007 Nov 27;1(4):44102
references.pmid: 19693401
references.citation: Opt Express. 2008 Jun 9;16(12):8594-603
references.pmid: 18545572
references.citation: Phys Rev E Stat Nonlin Soft Matter Phys. 2006 Mar;73(3 Pt 1):031802
references.pmid: 16605549
references.citation: Electrophoresis. 2005 Oct;26(19):3725-37
references.pmid: 16136529
references.citation: Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Feb;65(2 Pt 1):021906
references.pmid: 11863562
references.citation: Biophys J. 1999 Jul;77(1):516-25
references.pmid: 10388776
references.citation: Proc Natl Acad Sci U S A. 1995 Jan 31;92(3):860-4
references.pmid: 7846067