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Electrostatic charge (also known as triboelectric charge) on microparticles and microspheres have been of interest to scientists in chemical, pharmaceutical, cosmetic, drug delivery, displays and other industries for many years. Until recently there were no reliable techniques to measure or quantify electrostatic charge on microparticles, with too many uncontrolled variables the measurements were inconsistent. As a result scientists were treating charging as a black box process, performing the experiments blindly as trial and error.
With recent advances in microsphere manufacturing, techniques have been developed that promise not only to quantify the charge on microspheres, but control it in the manufacturing process.
The most basic and most critical factor that needs to be controlled
in order to obtain a reliable charge measurement is environmental.
Since moisture from the air will form static dissipative coatings on all
surfaces, charges will always be lower at high humidity. As an example,
walking across carpet at 20% RH will generate 35kV compared to only
1.5kV at 80% RH.
Proper techniques for handling microparticles are also very
important. Electrostatic surface charge is acquired every time two
materials come together: when microparticles are being collected,
sifted, sampled, put in a bag, touched with a spatula, put on a glass
microscope slide. Each of these processes adds or changes electrostatic
charges, resulting in inconsistent and not repeatable charge
measurements. These steps of sample preparation are necessary and
impossible to control. The solution is to neutralize all those acquired
surface charges after the sample has been prepared and ready for
testing. Depending on the situation, the charges can be neutralized just
by letting the sample sit in controlled environment for a number of
hours, using de-ionizing blowers, putting the sample on a conductive
surface, or in a dielectric fluid environment.
The most difficult to control factor comes from the fact that even
properly sampled and conditioned microspheres will interact and transfer
charges to each other. The challenge is finding a way to separate and
measure individual microspheres while still getting data representative
of billions of microparticles.
Co-founders of Cospheric, a microtechnology company based in Santa
Barbara, CA, developed real-time reliable technique for measuring
electrostatic charge on microspheres. The technique involves creating a
monolayer of microspheres contained in microstructures with each sphere
being separated from its neighbors. This technique allows precise
measurement of electrostatic charge of a large quantity of microspheres
without allowing sphere-sphere interactions. The microstructure consisting of channels or cavities is
filled with dielectric fluid and placed between two transparent
electrodes under a microscope. The spheres move in response to
direction and strength of electric field, which is observed and recorded
under the microscope. Polarity of each individual sphere is immediately
obvious by direction of its movement. Electrostatic charge is
calculated from strength of electric field, dielectric constant of
surrounding materials, and sphere velocity.
In this technique, the critical variables are carefully selected
materials for microstructure and dielectric fluid, as well as precise
assembly of the device. Since effective electric field is inversely
proportional to thickness, any variation in thickness of the device will
produce inaccurate data.
Armed with a fast and reliable technique for measuring
electrostatic charge, engineers at Cospheric are able to do controlled
experiments and tune their manufacturing process to produce microspheres
and coatings of desired charge based on customer’s needs and
“For the past seven years we have been manufacturing high-tolerance
microspheres for digital display industry, where polarity and quantity
of charge are critical for the display performance. We are very excited
to have a real-time reliable solution to measure charges on our spheres,
which allows us to bring even better value to our customers,” says
Brian Gobrogge, CEO and co-founder of Cospheric.
Bipolar and bichromal janus particles were originally developed for very high tolerance electronic
digital displays where functionalized microspheres are used to create an
image that appears to the viewer. Read more about Bipolar and Bichromal Microspheres in Electrophoretic Reflective Displays.
Our neutrally-buoyant highly charged yellow microspheres have a strong negative charge and are used by scientists in medical technology, biotechnology, applied physics, diagnostics and research. Precise negatively charged microparticles with known density that behave in a known way are useful as a model particles in simulation experiments. The particles are available in the selection of size ranges from 5 to 500microns (0.5mm).
to see how we can help you design electrophoretic microspheres with dual or single
functionality and charge characteristics that meet your needs.