Efflorescence
At RH below ∼50%, the relationship between IAV decay rates and NaCl concentrations is no longer valid. NaCl reaches its solubility in a droplet (310 g L−1) at 75% RH, and evaporation produces a supersaturated solution at RH<75%. The NaCl concentration can rise as high as 580 g L−1 before the droplet crystallizes [25], [26]. The critical RH at which a droplet crystallizes is termed the efflorescence RH (ERH), and it depends on the composition and size of the droplet [27].
We observed the droplets of each medium under a microscope immediately after incubation at different RHs and found that no crystals formed at RH>60%, while droplets of all media crystallized at an RH just below 50% (Figure 3). Crystallization occurred within ∼10 min. Crystallization in DMEM, possibly at a lower NaCl concentration (corresponding to higher RH) than in PBS, due to a different ERH of the media, could explain the lower decay rate in DMEM at a concentration of 510 g L−1 (50% RH). Knowing the ERH of each media would lend further support to this idea, but the ERH of a solution is difficult to calculate and can only be measured accurately through specialized laboratory experiments [27].
Figure 3  Crystals of the four media: (A) PBS, (B) PBS+FCS, (C) DMEM, (D) DMEM+FCS. Light microscope, 100X magnified; scale bar = 20 µm.