AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
What Slows the Freezing of Pentane Nanodroplets?
Kehinde Ogunronbi, Sherwin Singer, BARBARA WYSLOUZIL, The Ohio State University
Abstract Number: 180 Working Group: Aerosol Physics
Abstract Freezing of straight chain n-alkanes has been extensively investigated in part because they are a ubiquitous component of bioorganic molecules and fuels, and they exhibit unique behavior, such as surface freezing above the bulk freezing point. Their high heat of fusion and chemical stability also make them desirable in energy storage applications. A well-known oddity is that the melt temperatures of the low carbon number (C4<Cn<C11) n-alkanes do not increase evenly with carbon number, an effect that reflects the ability of even alkanes to pack more efficiently than odd alkanes. The degree to which alkanes can be supercooled also varies with chain length. In bulk samples the degree of supercooling achieved by n-alkanes with 15≤n≤60, is close to zero. In contrast, emulsified droplets of n-alkanes can be supercooled by up to 32°C. The near-zero supercooling for bulk samples was attributed to the formation of a crystalline monolayer at the sample-air surface at temperatures up to ~3°C above the melt temperature, that then leads to surface-induced heterogeneous nucleation once the melt temperature is reached.
In our work, we study the freezing of n-alkane nanodroplets in a rapidly cooling flow. For 6≤n≤10 we can form these droplets and watch them freeze in the ~0.250 milliseconds available. Furthermore, freezing kinetics are consistent with surface freezing followed by crystallization within the interior of the droplet. We observe a strong even-odd alternation in the degree of supercooling reached before the rapid stage of crystallization started. In contrast, for n=5 we do not observe freezing on the timescale of the experiment, despite reaching temperatures that are more than 60K below the melt point. Molecular dynamics simulations of the n-alkane liquid-vapor interface, suggest that this behavior is consistent with a decreased ability to form the surface frozen layer.