Observation of Single NaCl Particle Hygroscopic Characteristics on a Glass and Paraffin Plate

Abstract

Image processing is widely used as a method of analysis in many fields such as astronomy,climatology, TEM, SEM and semiconductor industry, etc. One of the advantages of the image processing is the analysis of a trend by visualizing an object. In this study, hysteretic properties of NaCl particles were analyzed by the image processing method. Figure 1 shows the contact angle of deionized water (DIW) droplet on a glass or a paraffin surface measured with time, at a constant humidity and temperature condition, i.e. relative humidity (RH) of 50% and temperature of 25 degree Celsius. As time elapsed, the contact angle of DIW droplet was changed from 45 degrees to 25 degrees on the glass surface, while it remained almost constant, namely 110 degrees to 106 degrees, on the paraffin surface. This is because the glass surface was hydrophilic and the paraffin surface was hydrophobic. Yook and Ahn (2003) found a new application of an inertial impactor for real time observation of particles collected on an impaction plate, along with a microscope and a CCD camera. With the first design of Yook and Ahn (2003), however, only transparent materials were allowed for the impaction plate. In the present study, we improved the impactor design to use not only the transparent but also the opaque materials for the impaction plate, in order to analyze the hygroscopic characteristics of individual particles according to various plate materials. Using this device, NaCl particles were collected on a glass and a paraffin plate separately. The relative humidity inside the impactor was varied at room temperature and the hysteretic properties of NaCl particles were observed in real time. Figure 2 shows the growth rate of NaCl particles on the glass and the paraffin plate according to the relative humidity, namely hysteresis curve. For NaCl particles on both plates, the deliquescent relative humidity (DRH) was approximately 77% and the efflorescence relative humidity (ERH) was about 47%. DRH and ERH curves almost overlapped with each other for RH > 77% when NaCl particles were sampled on the paraffin plate, while a large deviation was found for NaCl particles on the glass plate. This is due to the larger surface tension by the glass plate, namely hydrophilic plate-surface condition as shown in Figure 1. In the present study, a new technique to analyze the hysteretic properties of particles was introduced. It was found that the hysteresis of particles collected on a plate was affected by the plate material, i.e. hydrophilic or hydrophobic surface condition. More detailed experimental procedure and results will be discussed in the future.