Jun 21, 2011
Rediscovered trapping mechanism favours smaller particles
A long overlooked aspect of electric field effects on small particles and molecules in fluid environments, dubbed AC electrophoretic (ACEP) phenomena, is in the spotlight thanks to the work of scientists in the US. In the study, the researchers note that the assumption that EP effects will vanish is only true for a spatially homogeneous AC field. The team goes on to show (both experimentally and theoretically) the non-vanishing ponderomotive EP effect in high-frequency electric fields and how to apply the phenomenon for trapping charged particles in aqueous solutions.
The group, led by Mark A Reed (Yale University) and Predrag S Krstic (Oak Ridge National Laboratory), has discovered that particles, when placed in a liquid with a spatially non-uniform electric field, experience not only the well known dielectrophoretic (DEP) force due to the polarizability of the particle, but also an electrophoretic (EP) force due to the charge on the particle.
More efficient trapping
Until now, EP effects were not observed in any experiments due to the choice of experimental conditions, leading to a widely held notion that the EP effect is not important in high-frequency AC fields. The Yale-ORNL team realized that this common belief is only true for a spatially uniform AC fields (such as in the case of capillary zone electrophoresis). In fact, the researchers discovered that under the right conditions, the ACEP effect can dominate and lead to a more efficient trap, especially for smaller particles or molecules.
Various techniques (such as optical tweezers and DEP traps) have been developed to achieve particle trapping in liquids. The holding force (that is, trap strength) of these techniques, however, decreases as the size of the particle decreases. This makes it very difficult for DEP traps and optical tweezers to trap very small particles, or molecules.
If the particle is charged – almost always the case for particles in liquid – then the ACEP effect can be employed to trap particles. In the study, the team uses a planar quadrupole trapping device fabricated on an SiO2/Si wafer.
The advantage of the ACEP approach is that the trap stiffness actually increases with decreasing size, which makes it a very attractive candidate for single-molecule trapping.
A full description of the work can be found in the journal Nanotechnology.
About the author
Weihua Guan is a PhD student in the Department of Electrical Engineering at Yale University, Connecticut, US. He is currently exploring electrokinetics in micro- and nanofluidics in the group of Prof. Mark A Reed, the Harold Hodgkinson Professor of Engineering and Applied Science at Yale University. Other authors of the paper include Jae Hyun Park (Oak Ridge National Laboratory, US).