Electrostatic Dispensing of Dry Dielectric Materials
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Electrostatic Dispensing of Dry Dielectric Materials Malinda M. Tupper, Marjorie E. Chopinaud, Takamichi Ogawa1, Michael J. Cima Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A. 1 NGK Spark Plug Co., Ltd., Nagoya, Japan ABSTRACT Dispensing micron-scale dielectric materials can be achieved through the use of dielectrophoresis. Electrodes are designed to create a nonuniform electric field. This method is expected to be applicable for transfer of a wide range of dielectric powders as well as small, shaped components. Small, 150 µm diameter silica spheres, as well as sodium fluorescein powder have been dispensed by this method. Selecting the appropriate electrode geometry and electric field intensity controls the amount collected. As little as 1.0 µg of sodium fluorescein powder, and as much as 16 mg of silica beads have been collected, and repeatability within 10 % of the total amount dispensed has been achieved. INTRODUCTION There are many applications that require precise dispensing of small quantities of dry particulate materials. These substances are often solubilized or dispersed in liquid, or dispensed as a solution followed by lyophilization to deliver such small amounts. Such methods are generally difficult to employ since each powder must be optimized for dispersion in the liquid vehicle. Solubilization methods do not completely determine the solid form produced upon precipitation. The dispensing limit of commercially available dry powder pipettes, such as the DryPette produced by Zinsser Analytic, is usually greater than tens of milligrams, and the accuracy is limited to ± 1 mg. Most pharmaceutical applications require greater accuracy, and dry pipettes are subject to clogging. Whether for dispensing in manufacturing or for use in high throughput materials discovery, appropriate powder delivery methods must be highly parallelizable or must be performed at high rates. The technique described in this paper exploits spatially non-uniform electric fields to manipulate dry powder particles. Spatially non-uniform electric fields have been used to manipulate micron or nanometer scale objects such as dielectric particles, or biological materials such as cells. This method has been used to aid in precise placement of millimeter-sized surface-mount capacitors[1]. Recently non-uniform electric fields have also been used to manipulate submicron metal particles to create microwires [2]. THEORY A spatially non-uniform electric field causes polarization of an uncharged dielectric body, resulting in a net dielectrophoretic force. The magnitude of the dielectrophoretic force on a spherical dielectric particle, FDEP, is a function of the particle size, a, the gradient of the square of the electric field intensity, ∇E2, and the relative permittivities, ε1 and ε2, of the particle and the medium as described by equations 1-3 [3,4]. FDEP = 2πε 1 a 3 K∇( E 2 ) S6.3.1
(1)
K=
ε 2 − ε1 ε 2 + 2ε 1
(2)
This phenomenon is illustrated for a spherical dielectric particle between concentric
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