Determining Dielectrophoretic Mobility in DC Insulator-based Systems

Mark A. Hayes, Noah S. Weiss, Prasun Mahanti, and Paul Jones
Chemistry and Biochemistry Department, Arizona State University
Email: MHayes@asu.edu
Web: http://www.public.asu.edu/~mhayes/

Particles are ubiquitous in our bodies and our environment. This class of materials includes cells, organelles, nanoparticles, aerosols, large proteins and DNA strands, bacteria, and viruses—among other organic and inorganic debris. As the members of AES know, dielectrophoresis (DEP), specifically insulator-based DEP (iDEP), is continuing to emerge as an important technique for manipulating these micro- to nano-scale particles. Within these important works, comparison of theoretical constructs to real-world data is only beginning. One limitation in connecting the properties of particles with dielectrophoretic (and electrokinetic) actions is a lack of valid physical descriptions based on first principles. Currently, assessments are made that ignore any subtle particle properties, or limit the discussion to polarizability and/or the Clausius-Mossoti factor. In fact, most discussions of real data rarely advance beyond an analysis of positive (towards stronger fields) versus negative (towards weaker fields) DEP.

Particles are ubiquitous in our bodies and our environment. This class of materials includes cells, organelles, nanoparticles, aerosols, large proteins and DNA strands, bacteria, and viruses—among other organic and inorganic debris. As the members of AES know, dielectrophoresis (DEP), specifically insulator-based DEP (iDEP), is continuing to emerge as an important technique for manipulating these micro- to nano-scale particles. Within these important works, comparison of theoretical constructs to real-world data is only beginning. One limitation in connecting the properties of particles with dielectrophoretic (and electrokinetic) actions is a lack of valid physical descriptions based on first principles. Currently, assessments are made that ignore any subtle particle properties, or limit the discussion to polarizability and/or the Clausius-Mossoti factor. In fact, most discussions of real data rarely advance beyond an analysis of positive (towards stronger fields) versus negative (towards weaker fields) DEP.

Another approach is to define a DC-iDEP mobility according the form of electroosmotic and electrophoretic mobilities. The DEP mobility (µDEP) is defined independent of the electric field gradient and thus becomes a universal parameter. In other words this definition of DEP mobility is intrinsic to the particle and represents the relative DEP velocity per unit electric field gradient squared as shown: and, where vDEP is the DEP velocity, E is the electric field, εf is the permittivity, rp is the particle radius, Re(fcm) is the real part of the Clausius-Mossoti factor defined by the particle and medium conductivities (σ) at low frequency (f) and η is the fluid viscosity. Furthermore, an ideal method for quantifying iDEP ought to simultaneously quantify other electrokinetic effects (electroosmosis and electrophoresis).

We have initiated a strategy to quantitatively determine dielectrophoretic mobility in a converging microfluidic channel where dielectrophoretic and electrokinetic mobilities of polystyrene particles are determined simultaneously using a unique streak-based velocimetry to generate the velocity profile of particles (see Figure 1). The electrokinetic mobilities are determined in the non-converging zone and the funnel-shaped section has a constant gradient—allowing for trivial differentiation of DC-iDEP actions. An automated algorithm detects, processes, and determines the velocities traveled by the particles at all pixels across the image sequence. Streak-based velocimetry operates by associating the length of a particle trajectory (streak) with the exposure time to estimate the velocity field.

Using 1 µm polystyrene particles the electrokinetic mobility was estimated to be 3.5 x 10-4 cm2/(V·s) using the velocity data from non-converging zone and the dielectrophoretic mobility was -2 x 10-8 ± 0.4 x 10-8 cm4/(V2·s) (n=3) was determined from the converging zone using the slope of the velocity profile. This result agrees with the general finding that polymeric particles exhibit negative dielectrophoresis under similar conditions. Under these particular conditions the electrokinetic velocity is about twice the magnitude of the dielectrophoretic velocity. The relative DEP velocity could be increased by increasing the applied voltage, reducing electroosmotic flow, or utilizing steeper field gradients. However, particle motion rapidly changes direction in these regimes complicating velocity associations. For optimal quantitative analysis, the DEP motion must be observable but not predominant.

Quantitative approaches like this one enable an unprecedented evaluation of iDEP and provide a metric for standardization. Ideally, discussions will eventually evolve from subjective descriptions of particle behaviors to more objective quantitative responses.

Figure 1. Schematic showing A)-C) various forms of iDEP patterns, D) 3-D representation of the converging channel design, E) calculated electric field in channel, and F) velocity profiles for no, positive and negative DEP forces with the channel. Design allows for determination of particle properties for use as probes for A)-C) systems.


Dr. Mark Hayes
Arizona State University
mhayes@asu.edu

Noah Weiss
Arizona State University
ngweiss@asu.edu

Prasun Mahanti
Arizona State University
pmahanti@asu.edu

Paul Jones
Arizona State University
pjones6@asu.edu