Electric Charge Measurement on Micro-Particles Suspending in Electrokinetic Microfluidic Devices

When microcolloid particles are used as seeds in biologic and chemical experiments, they are often considered to be neutral. However, our experiments have proved that the charge on seeds can be significant in electrickinetic flow even if micro- particle´s diameter is less than 500nm. A combination method of particle tracking velocimetry (PTV) and current method velocimetry has been proposed to measure the charge on polystyrene microparticles suspending in electrolyte. An equation between electrostatic force and Stokes-Einstein viscosity force applied on particles has been built in simply form of Basset-Boussinesq-Oseen equation. Observed mobility of particles is measured by PTV and electroosmotic mobility of the electrolyte is measured by current method velocimetry. Then the particles´ electrophoretic mobility is obtained. The charge on the particles can be estimated statistically from the electrophoretic mobility considering electroosmotic mobility. Experiments indicate that the charge on polystyrene latex particles with diameter of 520 nm dispersing in the 20mM borax buffer is 10.1 times 10^-17 C. In the pure grain alcohol, it is 2.1 times 10^-17 C. The relatively deviation is analyzed to be less than 20 percent of the measurement value. This method is very important to investigate quantificationally electrokinetic microfluidic devices and microparticles dispersing in electrolyte