Microscale microsecond calorimetric sensing

Simple platinum metal lines are used for fast calorimetric measurements through the use of microsecond voltage pulses and measurement of the transient resistance changes of the metal line. When the metal line is immersed in water, microsecond voltage pulses may be used to superheat water and initiate microboiling on the heater. The nucleation of bubbles in superheated water produces an inflection point in the resistance-time transient that is shown to be sensitive to the presence of dilute species in the water and to the surface treatment of the metal line. We demonstrate effects of surfactant on the nucleation of bubbles in water in a microchannel. The effect of surface treatment is compared using hydrophobic and hydrophilic self-assembled monolayers formed on a thin gold plated coating on the Pt heater. Hydrophobic surfaces show a lower nucleation temperature. Selective coatings that change hydrophobicity in the presence of an analyte have potential for a new sensing approach. A final application is a humidity sensor, which uses fast voltage pulses to detect the calorimetric vaporization of water for dewpoint hygrometry.