Dielectrophoresis in Concentrated Suspensions

Boris Khusid, New Jersey Institute of Technology

Compared to other available methods, dielectrophoresis is becoming one of the major techniques for micro- and nano-scale systems. The main advantages of dielectrophoresis are: the use of an electric field which requires no moving parts, employs polarization forces acting on a fine particle that are insensitive to its charge which is difficult to control, offers easy adaptability to electronics and, therefore, can be incorporated more favorably into micro-systems. A wide variety of dielectrophoretic micro-devices have been developed to address challenges in life science and analytical chemistry, such as separating a heterogeneous population of particles into homogeneous subpopulations, manipulating and concentrating biologically relevant molecules, and distinguishing between ill and healthy cells. Dielectrophoresis is now considered a viable tool for lab-on-a-chip systems that can integrate multiple analytical methods down to chip-format. The majority of studies on dielectrophoresis have considered dilute suspensions [1-4]; however, since one of its major features is its capacity to concentrate particles, it is essential to consider dielectrophoresis in concentrated solutions.

Figure 1. Concentration front in an aqueous suspension of polystyrene beads flowing from left to right through an AC dielectrophoretic gate [5]; red arrow shows a growing highly-concentrated bolus at different times: (a) 10, (b) 70, (c) 120, (d) 180 s. The blue line indicates the channel and electrodes, black line the energized electrodes.

Involved mechanisms: When exposed to a non-uniform electric field, a polarizable particle experiences a superposition of the electrophoretic force (product of the net particle charge and the field strength) and the dielectrophoretic force (product of the particle dipole moment and the field strength gradient) [1]. Electrophoretic force vanishes in an alternating current (AC) field due to the zero time average over the field oscillations. In contrast, dielectrophoretic force operates in AC fields as its averaging yields a nonzero value whose magnitude is the product of the particle volume, the gradient of the time average squared field strength, and the relative particle polarization at the field frequency. Depending on whether the particle is more or less polarizable than the suspending fluid, it migrates toward the regions of high field strength (positive dielectrophoresis) or low field strength (negative dielectrophoresis) [1-4]. This trend provides the ability to separate particles based on their polarizability. However, the separation efficiency is controlled not only by the interaction of particles with an externally applied field, but also by the dipole-dipole forces between the particles undergoing dielectrophoresis, as they acquire dipole moments. The main effect of the interparticle dipolar interactions is that they, if sufficiently strong, cause a suspension to undergo reversible phase transitions from a random arrangement of particles into a variety of ordered aggregation patterns. This trend has been long recognized and exploited in electrorheology to initiate the fluid-solid transition by applying a sufficiently strong field to a suspension. Control of multi-particle interactions driven by the interplay of dielectrophoretic and dipolar forces paves new ways to manipulate suspensions: form a sharp boundary between regions enriched with and depleted of particles (Fig. 1) [5], collect particles near the three-phase contact lines of the electrically formed fluid patterns [6], trap particles into predetermined locations and then translate them collectively [7], discriminate between biological and nonbiological particles [8], and remove particles from engine oil [9].

Modeling: A conventional linear model considers dielectrophoresis in dilute suspensions by including the electric, viscous and gravitational forces and torques exerted on a single particle and ignoring any interparticle interactions [1-4]. A nonlinear model [5, 10] describes the effects of the interparticle dipolar and hydrodynamic interactions on dielectrophoresis in concentrated suspensions. It encompasses the coupled equations for an electric field and polarization of a suspension plus the momentum and continuity balance equations for a suspension, both averaged over the field oscillations, with the suspension being viewed as an effective Newtonian fluid with a concentration dependent effective viscosity. These equations reduce to a linear single-particle model in the limiting case of a dilute suspension.

Future trends: Dielectrophoresis based microdevices will continue in an accelerated development in the years to come. The next step is successful commercialization of dielectrophoresis that requires a collaborative research relationship with industrial partners.

References
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