Luminescence Based Measurements in Micro Cavitating Flow
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Microchannels are often used to study fluid dynamics. For steady state problems like cavitating flow, fluorescent microscopy, with the addition of temperature sensitive nanoprobes into the observed fluid, can be used to determine the temperature at a chosen point, averaged over the integration time. Coupled with a confocal microscope setup, we are able to produce two and three dimensional temperature maps of the flow in the microchannel by the use of ratiometric intensity measurements. The nanometric scale of the probes assures fast thermalization of the probes and below certain concentrations does not modify the properties of the studied liquid. Since the probes are not present in the vapor phase, the relative intensity map also corresponds to the average void fraction in the flow. These nanoprobes are composed of a gold core and a polysiloxane shell containing fluorescent dyes (FITC, RBITC). Organic dyes were chosen due to their compatibility with the shell and primarily for the fast luminescence lifetime, which is essential due to the rapid flow in the microchannel and the consequent short dwell time of an individual nanoprobe in the excitation volume. The temperature information in each measured point is obtained from the temperature sensitive spectrum of the dye. The shell protects the dye from the environment and allows for the functionalization of the surface to prevent agglomeration, while the gold core mitigates photo bleaching. The technic allowed us to observe temperature gradients in microfluidic two-phase flow and observe the thermal effect associated with phase transition. Typically, a region of decreased temperature is observed downstream the orifice in the liquid-vapor stream, attributed to the cooling of the liquid due to the latent heat of the phase change. However, small changes in the diaphragm geometry can induce recirculating vortices, where the vapor bubbles condensate and induce a region of increased temperature.