Materials that accumulate on outdoor surfaces are collectively known as environmental films. These films undergo physiochemical changes as they mature due to environmental factors such as high relative humidity. Previous work has shown that the films consist of various salts, minerals, waxes, and bio-organic materials. Under varying relative humidity conditions, solid-liquid phase transitions (deliquescence and efflorescence) can result in particles spreading across the surface. We have seen that isolated particles can combine and mix with other nearby particles, activating chemical transformations concomitant with physical transformations. To quantify these effects, we present single component model films comprised of pure salts to further study the effects of the angle of the substrate on particle maturation. We probed tilt angles ranging from 15 to 60 degrees from surface normal, and our data show that increasing tilt angle increases particle maturation within the film evidenced by a decrease in particle count accompanied by an increase in the average diameter of each particle. We used k-clustering methods to identify particle types (shape and size) before and after exposure to high relative humidity. This revealed differences in how susceptible various particle types are to the maturation process. Furthermore, we found that films composed of two different salts displayed greater ion exchange as a function of increasing tilt angle. This data serves as a basis for more complex studies of film behavior dependent on substrate orientation.

Dependence of particle maturation and mixing in environmental films on substrate tilt angle

ACS Spring Meeting 2021

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