Variable-Capacitance Sensor to Measure the Evaporation Rate and Lifetime of Microliter Droplets

This article presents a variable-capacitance sensor (VCS) for sessile droplet evaporation rate and lifetime measurements. The sensor consists of a plane-parallel-electrode capacitor with a commercial glass substrate inserted within. The glass substrate has a concave cavity at its center to deposit a liquid droplet for its measurement. An air gap is left between the substrate and the upper electrode to allow for the droplet’s evaporation. The main advantage of this configuration is that it fixes the position and geometric shape of the droplet. An analytical model based on the local-height approximation (LHA) is developed to calculate the capacitance of the device, with and without a liquid droplet. Its validity is initially tested comparing with finite element calculations. Then, using the analytical model, we simulate the capacitance signal during the evaporation of a sessile droplet and compare it with experimental results finding very good correspondence. The differential capacitance signal closely follows the volumetric changes of the droplet. The evaporation kinetics and lifetime of
${3}.{5}\pm {0}.{1} \mu \text{L}$
droplets of ethanol and methanol mixtures are presented. We also show that the evaporation kinetics and lifetime of a droplet can be used to reveal the presence of a surfactant at very low concentrations. The results presented demonstrate the ability of the sensor to monitor the evaporating droplet from start to end.