Thomas B. Jones

Dielectrophoretic liquid actuation and nanodroplet formation

Water, like any polarizable medium, responds to a nonuniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis (DEP) makes possible a variety of microelectromechanical liquid actuation schemes. In particular, we demonstrate a new class of high-speed DEP actuators, including “wall-less” flow structures, siphons, and nanodroplet dispensers that operate with water. Liquid in these microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on these substrates from a micropipette, are manipulated, transported, and subdivided into droplets as small as ∼7 nl by sequences of voltage application and appropriate changes of electrode connections. The finite conductivity of the water and the capacitance of the dielectric layer covering the electrodes necessitate use of rf voltage above ∼60 kHz. A simple RC circuit model explains this frequency-dependent behavior. ...

Basic theory of dielectrophoresis and electrorotation

This article presents a concise, unifying treatment of the electromechanics of small particles under the influence of electroquasistatic fields and offers a set of models useful in calculating electrical forces and torques on biological particles in the size range from /spl sim/1 to /spl sim/100 /spl mu/m. The theory is used to consider DEP trapping, electrorotation, traveling-wave induced motion, and orientational effects. The effective dipole method, and its generalization to effective multipoles, makes it possible to treat multilayered concentric shells and particles exhibiting ohmic and dielectric loss. This method may be extended further to the case of nonspherical particles, where alignment torques can be considered. These capabilities are well suited to modeling DEP behavior of biological particles including cells. The models and methods presented in this review are sufficiently general to be of use in a broad range of applications for biological dielectrophoresis and particle electrokinetics. The range of validity can be stated confidently to cover particles having diameters approximately 1/spl mu/m and larger.

An electromechanical interpretation of electrowetting

Electrowetting on dielectric-coated electrodes involves two independently observable phenomena: (i) the well-known voltage dependence of the apparent contact angle and (ii) a central electromechanical force that can be exploited to move and manipulate small liquid volumes on a substrate. The electromechanical force does not depend upon field-mediated changes of the contact angle; it is operative even if the liquid surface is constrained. Forces on liquid volumes may be determined using capacitance or the Maxwell stress tensor with no reference made to liquid surface profiles. According to this interpretation, a nonlinear mechanism manifesting a voltage threshold is responsible for both contact angle saturation and the observed clamping of the electromechanical force.

Multipolar dielectrophoretic and electrorotation theory

Abstract A new dyadic tensor representation for multipolar moments is used to formulate expressions for the dielectrophoretic force and electrorotational torque exerted on lossy spherical particles by an arbitrary electric field. The new tensor expressions for the moments satisfy the requirement of symmetry. When combined with previously derived expressions for the induced moments in terms of the electric field and its derivatives, the theory accommodates electrically linear spherical particles with any type of ohmic or dielectric loss in any sinusoidally-varying electric field. The special case of a plant protoplast suspended in an aqueous medium and subjected to a traveling electric field wave is chosen to demonstrate the theorys easy adaptation to the practical planar or multi-planar microelectrode geometries now used to move, rotate, or position small particles such as biological cells.

Electro-orientation of ellipsoidal erythrocytes : theory and experiment

The frequency-dependent orientation of human and llama erythrocytes suspended in isotonic solutions and subjected to linearly polarized electric fields is examined. Human erythrocytes may be represented as oblate spheroids (3.9:3.9:1.1 microns) with two distinguishable orientations, while the llama cells are approximated as ellipsoids with three distinct axes (4.0:2.0:1.1 microns). Under appropriate experimental conditions, both orientations of the human cells and all three orientations of the llama cells are observed. A theoretical cell model which accounts for the membrane as a thin confocal layer of ideal capacitance is used to predict the orientational spectra. The predicted spectra compare favorably in frequency range and orientational sequence with experimental data. Estimates for cell internal conductivity and permittivity are obtained by adjusting the values of these important parameters to achieve the closet fit of the theoretical curves to the data. By the use of this method, the internal conductivity of llama erythrocytes is estimated to be 0.26 S/m (+/- 20%), while the effective internal dielectric constant and conductivity of Euglena gracilis are estimated to be 120 (+/- 10%) and 0.43 S/m (+/- 20%), respectively.

Multipolar dielectrophoretic force calculation

Abstract The conventional theory of dielectrophoresis, based on the approximation that only the dipole is induced, fails to predict observed behavior for a particle situated near a null or in a highly non-uniform field because of the neglect of induced higher-order moments. A recently published equivalent multipolar moment theory, while it enables calculation of these higher-order DEP force contributions, is not easily interpreted or used because it is expressed in terms of a Legendre function expansion of the field. The new formulation of the multipolar DEP theory provided here expresses the net force as the gradient of a series of scalar electromechanical potential functions. This result, valuable in its own right because it enables calculation of the higher-order DEP force terms without reliance upon Legendre function expansions, also leads to a generalized formalism for the complete ponderomotive force on a particle using multipolar moment tensors. The tensor form, readily recognized as a generalization of the original dipole-based DEP theory, provides clear physical insight about the magnitude and the direction of the net force on an uncharged particle in a non-uniform electric field.

Active feedback‐controlled dielectrophoretic levitation

Dielectrophoretic levitation of small metallic particles in insulating dielectric liquids is achieved using a microprocessor‐controlled digital feedback system. An optical detector senses vertical particle position and the error signal generated controls the voltage applied to the electrodes of the levitator cell. Experiments with single particles confirm a theoretical expression for the axial profile of the electric field. The method of images is used to calculate the effective dipole moments for chains of conducting particles. Experimental levitation data obtained using two‐ and three‐particle chains provide preliminary confirmation for these effective dipole moment expressions.

Dual-frequency dielectrophoretic levitation of Canola protoplasts

A novel dual-frequency excitation technique is introduced which permits investigation of the low-frequency dispersion of Canola plant protoplasts using feedback-controlled dielectrophoretic levitation. The upper and intermediate frequency spectra obtained using the new technique are generally consistent with previous work. However, below some cross-over frequency f(OL), the protoplasts exhibit an apparent positive dielectrophoretic response that is not predicted by conventional theory. This cross-over frequency is linearly related to suspension conductivity, virtually independent of the suspension pH, and inversely proportional to the square of the cell radius. Examination of the complex Clausius-Mossotti polarization coefficient reveals that the observed positive dielectrophoretic response can not be accounted for in terms of Maxwell-Wagner polarization associated with a conventional layered model for the protoplast. The failure of straightforward enhancements to the protoplast model in explaining the low frequency behavior may indicate the presence of an electrophoretic contribution to the net observable force on the particle. To account for such fluid mechanical effects, it will be necessary to modify the existing dielectrophoretic force formulation.

Nonlinear interactions of particles in chains

Measurements of the effective electric dipole moments of conducting particle chains suspended in insulating dielectric liquid reveal the influence of dielectric breakdown between particles. This breakdown can lead to reversal of the sign of the interparticle force from attractive to repulsive. Other measurements with a vibrating sample magnetometer of the effective magnetic dipole moments of ferromagnetic particle chains provide clear evidence that chaining enhances nonlinear magnetization at a field intensity about a factor of 10 below the value at which saturation is observed for single particles. Both of these physical situations are examples of how nonlinear effects can influence the electromechanical interactions of closely spaced particles. The nonlinearity is caused by the very strong field intensification that occurs in the gaps between particles for chains parallel to an applied electric or magnetic field. A correspondingly strong nonlinear effect is predicted for the forces between particles in chains.

Dielectrophoretic interaction of two spherical particles calculated by equivalent multipole-moment method

A generalized equivalent multipole-moment theory is developed for the analysis of the interaction between two spherical dielectric particles in an external field. The method is based on the re-expansion technique: the potential distortion caused by the existence of a dielectric sphere is first expressed as a series of spherical harmonics with r/sup -n-1/ dependence. This potential, having no singularity except at the center of the sphere, is then re-expanded around the center of another sphere as a series of spherical harmonics with r/sup n/ dependence. Once this re-expansion is done, the effect of neighboring spheres can be incorporated as an externally applied potential, and the field problem can be solved. In this paper, the authors present the principle of the method, together with calculated results for the case of two spherical particles in a uniform external field.<<ETX>>