Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-01-25
2003-09-02
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S600000, C356S344000
Reexamination Certificate
active
06613209
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electrophoresis apparatus, and more specifically a method and apparatus for the measurement of characteristics such as electrophoretic mobility of particles suspended in a low-permittivity non-aqueous liquid medium. The invention will be discussed in relation to its relevance to electrostatographic printing or copying process parameters but is not so restricted as it also relates to other situations where characteristics such as mobility of particles under the influence of an electric field are of interest.
BACKGROUND OF THE INVENTION
It is well known that the electrical properties of liquid toners used in electostatography significantly influence the quality of a printed image; the most important electrical characteristics of conventional liquid toners are known to be conductivity, electrophoretic mobility and charge to mass ratio. Although the physical background of these characteristics is understood, accurate and reliable methods for measuring these properties are limited.
The term zeta potential is understood in colloidal chemistry as an indicator of the electrophoretic mobility of particulate matter colloidally suspended or dispersed in a fluid medium. The electrophoretic mobility, in turn, is an indication of the velocity of the particles through the solution due to the effect of an applied electric field.
The stability of a particle suspended in a bulk medium is related to the zeta potential of the particle. Stable particles remain dispersed whereas unstable particles tend to agglomerate and eventually precipitate out of the solution. The higher the zeta potential, the more stable the system is since highly charged particles repel one another and remain dispersed.
In the field of liquid electrostatographic printing and copying, development of a latent image takes place at high speeds, which requires, in one form, that a large amount of uniformly characteristic liquid developer marking particles be supplied to the latent image surface as uniformly as possible to produce a high quality image without any variations in the development thereof.
Therefore, the measurement of the mobility of electrostatic particles dispersed in a dielectric medium under the influence of externally applied electric fields is useful for allowing the assessment of liquid toner sensitivity and at the same time providing a means for analysis of the behaviour of liquid toners as well as their individual constituents.
Various prior art apparatus and means have been utilised to measure the zeta potentials of colloidal particles suspended in a liquid medium. U.S. Pat. No. 3,454,487 to Riddick discloses an electrophoresis apparatus, wherein the electrophoretic mobility, i.e. the velocity of the particles per unit field strength, is measured in an electrophoretic cell which consists of sample receiving chambers connected by a liquid-flow communication passageway.
This measurement is performed utilising an ocular micrometer or distance scale which is inserted into the eyepiece of a microscope. The particles are timed as they cross a fixed distance in the observation chamber of the electrophoretic cell under a D.C. electric field of known strength.
In U.S. Pat. No. 3,764,512 to Greenwood et al. discloses an apparatus utilising a coherent light beam from a laser which is caused to intermittently scan a path located on the stationary layer of an electrophoresis chamber by means of a mirror galvanometer at a rate equal to the migration rate of the particles in the chamber. The operator views the migrating particles in the chamber through a microscope and simultaneously adjusts the scanning rate of the mirror galvanometer by adjusting a potentiometer in the galvanometer control circuit until the scanning laser beam appears to visually track the migrating particles as viewed through the microscope. Via appropriate scaling circuitry interacting with the galvanometer drive circuit and the circuit supplying the voltage drop across the chamber, a value for electrophoretic mobility or zeta potential may automatically be displayed through suitable means, such as an electronically operated digital readout.
In U.S. Pat. No. 3,793,180 to Flower et al. discloses a system capable of measuring zeta potential, particle size distribution, total charge density and other distribution functions of aqueous suspensions. A laser beam is provided in the instrument and is focused on the particles in the sample solution which are contained in an electrophoretic cell. A reticle or grating is positioned such that the reflection of the laser beam from the particles passes through the reticle to a photo-tube placed on the other side of the reticle. Then, as the particles move through the solution the photo-tube is intermittently illuminated through the reticle and, as a result, generates a train of electric pulses whose frequency is directly proportional to the velocity of the particles. Since the frequency of the electrical signal is a measurement of the particle velocity, it is also a measure of zeta potential.
In U.S. Pat. No. 3,909,380 to Day, a television camera or other equivalent image sensor, such as a photosensitive array utilising a suitable lens system, observes the fluid within an electrophoresis cell which is illuminated by the cold light of a fiberoptic source in order to prevent the production of convection currents in the medium. The suspended particles are magnified by a microscope and the image is projected onto the monitor screen. A reference pattern is superimposed onto the monitor and the sweep speed of the reference pattern is then manually adjusted to match the speed of any single particle or group of particles on the monitor screen. The sweep speed of the reference pattern may then be converted to a zeta potential signal which must be corrected for the temperature of the sample.
In U.S. Pat. No. 4,046,667 to Goetz describes an electrophoresis chamber, a circuit for impressing a voltage across the chamber, a light beam to illuminate a portion of the chamber, and a microscope including an objective lens system and an eyepiece for viewing illuminated particles migrating relative to a suspending medium within the chamber under the influence of the applied voltage. Within the microscope, between the objective lens and the eyepiece, is a movable optical prism driven by a galvanometer, the drive circuit of which includes an adjustable potentiometer for controlling the rate and direction of movement of the optical prism.
A circuit connected to the galvanometer drive circuit and the circuit applying the voltage potential across the chamber are adapted to develop a signal proportional to the electrophoretic mobility or zeta potential of the migrating particles when the rate of movement of the optical prism is adjusted such that it cancels the transfer velocity of the migrating particles. The particles then appear stationary when observed through the eyepiece of the microscope.
The hereto disclosed prior art apparatus and means can be characterised as utilising direct methods in which to determine electrophoretic mobility, that is, electrophoretic mobility being a linear function of particle velocity, is therefore derived from direct measurement of actual said velocity; this requiring the liquid toner sample to be transparent or semi-transparent to the light beam of the sensing device. In contrast, there also exist techniques whereby indirect methods can be used in determining electrophoretic mobility. In these indirect methods, secondary effects caused by particle motion within the dispersion are used, processed and converted into mobility measurements, as taught by the following prior art examples.
In U.S. Pat. No. 4,679,439 to Culkin, there is described a method and apparatus for measuring the unsteady sedimentation potential of particles in a suspension comprising the insertion of a portion of the suspension of particles in a cell, the cell having a first and a second electrode. A speaker motor means is used to vibrate the cell and to accelerate the part
Nguyen Nam
Noguerola Alex
Research Laboratories of Australia Pty Ltd.
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