In studying the hygroscopic properties of levitated particles using an Electrodynamic Balance ( EDB), the relative humidity is usually adjusted in discrete steps. This static method, which takes several hours to obtain a set of mfs vs. aw data, may not be most suitable for studying semi-volatile compounds such as some carboxylic acids and ammonium salts.  Evacuated systems in which the actual partial vapor pressure of water is measured may not be easily adaptable to studying volatile species. To reduce the evaporation of the solute during the water activity measurement, we have developed a much faster technique developed in our laboratory (Liang and Chan, 1997). This experimental approach exploits the fact that the rate limiting step of all the associated heat and mass transfer processes in the evaporation of an aqueous droplet levitated in an EDB is the change of relative humidity (RH) inside the EDB.  Hence, the particle is in quasi equilibrium with its surrounding in the EDB. Instead of using multiple step changes of feed RH, only one step change in RH is required to measure a set of mfs vs. aw data, when the change of RH inside the EDB is well calibrated. The balancing voltage of an evaporating droplet is continuously monitored as RH changes. This “dynamic” method can reduce the experimental time for measuring a set of water activities at different solution concentrations from 5 – 10 hours to an hour.  Water activities of mixtures of Na+ salts (Chan et al., 1997), mixtures of Mg²⁺ salts (Ha and Chan, 1999), and mixtures of Na⁺/NH⁺ salts (Ha et al., 1999) were measured based on a step decrease of RH in the feed of the EDB.
 

Step 1:       A step change of RH of the stream of the feed to the EDB from RH1  to RH2.
Step 2:       Because of the relatively large residence time of the EDB, the RH at the center of the EDB will change gradually from RH1 to RH2.  This “calibration curve” can be determined using species with known mfs vs RH properties such as sulfuric acid.
Step 3:       Since the change of the RH inside the EDB is the rate limiting step, the particles will respond to the change of RH immediately.  mfs is determined from the balancing voltage measurements.
Step 4:       By combining the calibration curve (RH(t)) and the mfs data (mfs(t)), water activity data in the form of mfs as a function of a_w (=RH/100) can be obtained.