Measurement of the diffusion of molecular translation and rotation in a transitional boundary layer

The fluorescence-based measurement of the diffusion of molecular translation and rotation in a laminar and a transitional boundary layer is described. In the experiment, fluorescent and quencher molecules are dissolved in water. Due to the quencher molecules the decay of the fluorescent molecules is enhanced beyond its natural decay. Comparison between the natural decay and the enhanced decay gives the diffusion of molecular translation. The diffusion of molecular rotation is measured with polarized laser light. Due to the polarization, only those fluorescent molecules are excited whose dipole axes are parallel or partly parallel to the axis of polarization. This elapsed time is a direct measure of the diffusion of molecular rotation. The macroscopic velocity profile has been measured with laser Doppler anemometry. Both the critical Reynolds number and the transitional Reynolds number are determined from the analysis of the mean and the fluctuating velocity.