Measurement Techniques for Electrostatic Charged Microspheres

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, classified, 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 microstructure 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 is filled with dielectric fluid and placed between two transparent electrodes under a microscope. The spheres will 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 specifications.

?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.