# Relationships between Surface Properties and Snow Adhesion and Its Shedding Mechanisms
## 3 Results and Discussion
While it is not possible to directly measure the size of snow particles on the surface, we hypothesize that when the substrate is very smooth (RMS values of -7.17 um or lower), snow adhesion strength is relatively low (Figure 4b). An increase in shear adhesion strength of snow is observed when the surface roughness increases to a level comparable to the size of the snow particles. The highest snow adhesion strength is observed on the Al 6061 substrate with RMS roughness of -36.35 um, where snow has more contact area with the surface, when compared to very smooth surfaces. We believe that the enhanced surface area, while maintaining the apparent surface area constant (\(6\times 6\) in), might be responsible for the observed increase in the snow adhesion strength, given the considerable presence of liquid water in snow (LWC of -22%). As surface roughness increases further beyond the size scale of the snow particles, the surface roughness becomes filled with the snow particles, which are lubricating each other. During the adhesion measurements, cohesive failure happens locally between the bulk layer of snow and the snow particles that are filling the surface roughness. This results in reduced apparent snow adhesion strength on these very rough surfaces. However, this only can be claimed in the macroscopic level (large surface area) and not in the microscopic level. Figure 5a-c schematically demonstrates the impact of surface roughness on the adhesion strength of snow. Surface energy is another important parameter when studying snow adhesion strength. Since snow is composed of liquid water, ice, and air, it is a polar structure with a surface energy of 30-72 mN/m, depending on its LWC value [29]. To understand the impact of surface energy, aluminum 6061, stainless steel, HDPE, and copper were studied. It is important to note that the aluminum used in this experiment was pretreated to prevent oxidation of pure aluminum to form Al\({}_{2}\)O\({}_{3}\). This pretreatment of the metal results in lower surface energy than the pure element [30]. Each substrate was roughened using aluminum oxide sandpaper to obtain the same average roughness (RMS of -36.4 um) before adhesion measurements. In addition, for the adhesion measurements on all substrates, the LWC of snow  
Figure 5: Impact of surface roughness on the snow adhesion mechanism, hypothesized schematically. (**a**) On a very flat surface, snow does not become trapped within the roughness and shears at the interface. (**b**) When the size of snow particles and surface roughness are on the same range, the snow particle are sheared in half, leading to enhanced adhesion strength values. (**c**) When the snow particles are smaller than the surface roughness, shear adhesion strength is lower because some snow particles glide over others that are inside the roughness. and the substrate temperature were kept at \(\sim\)22% and 0 \({}^{\circ}\)C, respectively. A strong correlation was found between polar and dispersive surface energy and snow shear adhesion strength. Dispersive surface interactions are caused by temporary fluctuations of charges between atoms or molecules. These temporary interactions exist in every substance and are known as Van der Waals interactions with a shear adhesive strength of 0.4-4.0 kJ/mol. Polar interactions are comprised of Coulomb interactions between dipoles (e.g., hydrogen bonds). Hydrogen bonding has a shear adhesive strength range of 12-30 kJ/mol, which is much stronger, compared to the der Waals interactions [31]. Modulating surface roughness impacts the polar and dispersive surface energy while maintaining the same total surface energy (Table 1). Dispersive surface energies are naturally weak. Polar surface energy is related to hydrogen bonding and Coulombic forces, which are stronger than the Van der Waals forces. For highly nonpolar surfaces, variation in dispersive surface energy greatly impacts the adhesive strength.