Water Status on Micro/Nano Gap Between Galvanic Arrays and Flowing Current Around 100% in Relative Humidity
Authors: Terada Eiji , Sakamoto Yukihiro , Kawakita Jin 
Published: December 2020

Introduction
Dew condensation might cause various phenomena like metal corrosion, crop disease, etc. They could be avoided properly if dew condensation is detected at early and initial stages or ideally in advance and followed by dehumidification. Conventional hygrometer cannot detect dew condensation accurately and quickly. Therefore, a new sensor to detect moisture has been developed. It has two interdigit structures made of different metal that intercalates each other. When water droplet connects the adjacent metals, galvanic current flows between these metals. Decreasing a gap between the adjacent metal lines enables smaller water droplet to be detected [1]. It was recently found that the sensor with 0.5 µm in metal gap showed possible detection of adsorbed water molecules as well as droplets under 100% in relative humidity [2]. In addition, temperature and wettability of this sensor could be controlled to imitate target where dew condensation occurs [3]. It means that this sensor might indicate moisture status on target surface. In order to quantify the moisture of the sensor surface, it is important to correlate the current response and the amount of water on the sensor surface. Especially the relationship around 100% in relative humidity was still unclear and studied by measuring the current and observing the sensor surface under stepwise increase and decrease in relative humidity.
Experimental
Arrays of Al and Au with 1 μm in width were alternately arranged with 0.5 μm in gap between the adjacent arrays on the insulating substrate and used as the electrodes. Electric current between the electrodes were measured by a precise amperemeter. Platinum wires was also placed on the substrate and their temperature was estimated from calibrated straight line of its electric resistance as a function of changing temperature. This sensor chip was attached with the measurement device and put inside a chamber, in which relative humidity was kept constant at 50% by using humidifier and hygrometer. The temperature of chip surface was changed by 1°C every 20 minutes with a Peltier device. The chip surface was observed by an optical microscope while the response current was measured.
Result and Discussion
Relative humidity (RH) on the sensor surface and response current with experimental time are shown in Fig. 1. The microscopic images at certain times are indicated as the inset of the figure. This RH was calculated from both vapor pressure in the chamber and the temperature of the chip surface. RH more than 100% in this study was nominal and the excess over 100% corresponds to condensed water on the sensor surface. When the current increased stepwise with RH at 100% or below while liquid water could not be observed on the sensor surface below 100% and it started to appear at 100%, as indicated in Fig. 1a. This behaviour was determined by equilibrium of water molecules between vapor in the chamber and adsorbed state on the sensor surface. When RH exceeded 100%, the current increased continuously. Simultaneously, water droplets appeared and became large to cross over the gaps between the arrays, as seen from Fig. 1b. Thereafter, while RH was lowered stepwise to 100%, the response current decreased slightly and kept almost the same level. During this period, the obvious area and the number of droplets on the sensor surface were reduced and most of liquid water seemed to exist between the electrodes, as seen in Fig. 1c. Above 100%, liquid water continued to be formed on the sensor surface and droplet on the gaps turn to infiltrate it by a kinetic reason. When RH fell below 100%, the current decreased continuously and was comparatively higher than that observed during the increasing RH instead of its stepwise change. In this region, liquid water became thin film in the gap, as shown in Fig. 1d. The liquid water tended to remain in the gap kinetically.
Conclusion
The relation between water status on micro/nano gap between the galvanic arrays around 100% in relative humidity (RH) and flowing current was clarified in this study. When RH increases up to 100%, the response current depended on RH owing to equilibrium of water molecules between vapor and adsorbed state on the sensor surface. Once liquid water was formed on the sensor surface, current depended on presence of water determined kinetically.
References
[1] J. Kawakita et al., ECS Trans. 75 (2017) 51.
[2] Y. Kubota et al., Sensors and Actuators A: Physical 303 (2020) 111838.
[3] R.G. Shrestha et al., Sensors 19 (2019) 4500.

Figure 1