Simultaneous,ensemble-averaged measurement of near-wall temperature and velocity in steady micro-flows using single quantum dot tracking

We present results from a series of experiments demonstrating the use of single quantum dots (QDs) as simultaneous temperature and velocity probes at the micro-scale. The fluorescence intensity of QDs varies predictably with temperature due to changes in quantum efficiency. We use total internal reflection fluorescence microscopy to study the region within 200 nm of a fluid-solid interface. A two-color, time-averaged temperature sensing technique based on the ensemble intensity changes of single QDs as compared to a reference dye (rhodamine 110) is presented. Many single QD intensity measurements are used to build intensity distributions which can be mapped to fluid temperature. Simultaneously, we track the motion of individual QDs, building a distribution of particle displacements, where the mean displacement yields the local fluid velocity. We also show that the width of the displacement distribution (or the diffusion coefficient) captures the scaling of the temperature to viscosity ratio, which may allow for independent viscosity measurement.