This is a past event.

Please join physics graduate students, Elise Rosky, and Samuel Groetsch, for their in-person presentations Thursday, February 23rd at 4 PM - Fisher Hall 139.

Molecular Simulations Reveal That Heterogeneous Ice Nucleation Occurs at Higher Temperatures in Water Under Capillary Tension

Elise Rosky [Advisors: Dr. Will Cantrell/Dr. Raymond Shaw]

Gaps in our understanding of ice formation in atmospheric cloud droplets can be traced all the way down to the molecular scale, where the physical processes governing the phase transition from liquid water to ice remain unresolved. Thermodynamics dictates that homogeneous ice nucleation (freezing in pure water) occurs at higher temperatures when water is under tension, also known as negative pressure. If also true for heterogeneous ice nucleation (freezing on a substrate material), then this phenomenon can result in higher heterogeneous freezing temperatures in water capillary bridges, pores, and other geometries where water is subjected to negative Laplace pressure. Using a molecular model of water freezing on a hydrophilic substrate, we find that heterogeneous ice nucleation exhibits a similar temperature increase at negative pressures as homogeneous nucleation. A linear function is found to be a reasonable approximation of the temperature increase as a function of pressure, with the slope determined primarily by the water density anomaly and the enthalpy of fusion (also known as latent heat). Simulations of water in capillary bridges confirm that the negative Laplace pressure within the water corresponds to an increase in heterogeneous freezing temperature. Varying the height and width of the capillary bridge reveals the role of geometrical factors in heterogeneous ice nucleation. One example is the absence of ice nucleation in the region within 10 Angstroms of the air-water interface and a preference for nucleation in the region just beyond 10 Angstroms.

Constructing a Very High and Ultra High Energy 𝜸-ray Catalog Using HAWC Data

Samuel Groetsch [Advisor: Dr. Petra Huentemeyer]

Catalogs of astronomical objects have been made by astronomers throughout history, with some of the earliest originating from China and Sumeria in 1500 BCE. Astronomers use both generalized and specialized catalogs to gain insight into numerous aspects of our galaxy and the local universe as a whole. This presentation will discuss the creation of a catalog of sources that produce VHE/UHE (100+ GeV/100+ TeV) 𝜸-rays using HAWC data. There are several unique challenges that arise when using the VHE/UHE data from HAWC, such as the reduced angular resolution compared to pointing-type telescopes/observatories. This proposed catalog uses a multi-source fit algorithm to help produce the most comprehensive catalog of HAWC sources possible and mitigate the unique challenges when using HAWC data.