Different freezing behavior of millimeter- and micrometer-scaled(NH4)2SO4/H2O droplets

Abstract

Although the freezing of aqueous solutions is important for nature and differentbranches of science and freeze-applications, our understanding of the freezingprocess is not complete. For example, numerous measurements of micrometer-scaled(NH4)2SO4/H2O droplets report one freezing event below the eutectic point. However, measurements oflarger millimeter-scaled droplets reveal two freezing events: the freezing out of ice andsubsequent freezing of a residual freeze-concentrated solution. To resolve thisapparent contradiction we performed numerous calorimetric measurements whichindicate that the freezing of a residual solution of millimeter-scaled 5–38 wt%(NH4)2SO4 droplets occursmainly between ∼ 210 and 225 K. We also find that micrometer-scaled droplets produceone freezing event which is within or in the vicinity of the ∼ 210–225 K region. This fact and the analysis of thermograms suggest that the residual solution ofmicrometer-scaled droplets may partly crystallize simultaneously with ice and partly transform to glassat Tg≈172 K. Our results suggest for the first time that the size of(NH4)2SO4/H2O droplets may affect the number of freezing events below the eutectic point.