Experimental Investigation on Near Wall Natural Convection With Fluorescent Nanoparticles

The mechanisms of microscale natural convection are yet to be understood for its fundamental characteristics differing from those at the macroscale. The spatial distribution of the fluid velocity in the near wall region, which is crucial to the mechanisms of microscale natural convection, is especially not well understood. An experimental investigation of the spatial velocity and the Brownian diffusion coefficient in the near wall region of a platinum heating wire, 20 micrometers in diameter, was conducted using a multilayer nano-particle image velocimetry (MnPIV) technique. The in-plane velocity of deionized water in the near wall region between a micro heating wire and a glass wall was measured using MnPIV technique with fluorescently tagged polystyrene nanoparticles, 200 nm in diameter. The results indicate that both the magnitude and the deviation of the fluid motion increases with increasing heat flux. The Brownian diffusion coefficient was also calculated and indicates that the Brownian motion in microscale natural convection is important, especially for nanoscale colloidal tracers. The thermophoresis was found to be negligible, even at low heating powers. To enhance the resolution of the fluid motion in the near wall region, it is necessary to use smaller fluorescent nanoparticles as seed tracers.