Visualization of convective mixing in microchannel by fluorescence imaging

A novel measurement technique using fluorescent dye in combination with microresolution particle image velocimetry (micro-PIV) has been devised to investigate convective mixing in microspace. Tris(bipyridine)ruthenium(II), whose fluorescent intensity when excited by ultraviolet light is strongly temperature dependent, was applied to the bottom surface of a cover glass, that served as the upper boundary surface of a flow channel. This set-up thus realized a two-dimensional temperature measurement of the microflow channel. A spatial resolution of 5 mum x 5 mum and a temperature resolution of 0.26 K were achieved by using a cooled CCD camera and a 10x objective lens of a microscope. Pure water at differing temperatures was injected into opposite inlets of a T-shaped microchannel bound by cover glass and PDMS. The mixing process in the junction area was visualized by the present temperature and the micro-PIV techniques. The convective heat flux was calculated from measurement of velocity and temperature and compared to the heat conduction. It is found that the heat flux due to conduction was larger than that due to convection, thus it is noted that heat conduction may be an important factor in the design process of microfluidic devices.