Fiber-optic evaporation sensing: Monitoring environmental conditions and urinalysis

The evaporation dynamics of subnanoliter chained droplets from the cleaved facet of a standard fiber are monitored by reflectivity measurements of incident light. The rate of evaporation of doubly distilled water droplets is calibrated as a function of temperature and relative humidity (RH). The experimental uncertainty in the rate of evaporation corresponds to measurement errors of ±0.5 °C in temperature or ±1.5% in RH. The evaporation analysis distinguishes between binary mixtures of gasoline and methanol, used as alternative fuel blends, with mixing ratios that differ by 5%. Finally, the analysis properly estimates the concentration of synthetic samples, designed to replicate human urine. Knowledge of sample concentration is central to the correct interpretation of urinary sodium measurements and to provide accurate estimates of sodium intake by hypertension patients. This last result may form the basis for a potential consumer application of the sensor, in mobile point-of-care diagnostics.