The second stage of water condensation was investigated in more detail by measuring the NaCl surface curvature as a function of time and RH (Figure , See Supporting Movie 4 for full in situ movie). Here the surface curvature (r) is defined as the reciprocal of the radius of the incircle for a given set of points along the surface of the NaCl particle. When determining and comparing cube corner curvature, we note that the terminology, "high" or "low," for corner curvature is relative to the curvature of that corner at a different time or relative to a neighboring corner of the same cube. Figure shows time lapsed in situ BF STEM images of an NaCl cube with the water layer removed by digital background subtraction to isolate the NaCl particle for analysis. Here we observed the top right cube corner curvature fluctuated every few seconds between a low and high curvature state via dissolution and crystallization. There was nearly no change to the particle area during this dynamic process. The curvature change dynamics can be observed very clearly in the in situ STEM movies (Supporting Movies 4-6). We note that all corners of a given cube experienced curvature fluctuations with one or two corners typically showing the largest fluctuations. The corner with the largest curvature fluctuations was different for different particles (Supporting Movies 4-6). Mapping the surface curvature of adjacent cube corners showed that the curvature fluctuations were anticorrelated in time ( ). For instance, the righthand cube corner in Figure showed an increase in curvature that was synchronized in time with a decrease in the left-hand corner curvature (Figure ). The synchronized increase/decrease in adjacent corner curvature was further evidenced by the negative Pearson's correlation coefficient over these time ranges (inset Figure ). The curvature of non-adjacent corners showed nearly no time correlation (Figure ). The corner curvature time fluctuations can be explained in terms of curvature-dependent solubility and NaCl concentration fluctuations in the droplet (Figure ). According to the Gibbs-Thomson effect, the solubility of a solid increases with increasing (positive) surface curvature: 41