# Relationships between Surface Properties and Snow Adhesion and Its Shedding Mechanisms
## 3 Results and Discussion
However, when the surface is relatively polar, the component of the dispersive surface energy does not play a major role in determining the adhesion strength of the surface to another material, i.e., snow. For a relatively polar surface, by decreasing the polarity, the adhesion strength of snow decreases as well (Table 1). To decrease surface polarity the symmetry of the surface roughness must increase. These findings help in designing surfaces with reduced snow adhesion strength. While conducting snow adhesion measurements, three snow de-adhering/shedding mechanisms were identified (Figure S5). LWC was held constant at \(\sim\)22% for the observation of each failure mechanism. Slight adjustments in the compressed air pressure and water flow rate of the snow gun as well as the freezing room temperature enables modulating the LWC of the created artificial snow. Once the snow is on the substrate, the variation of liquid water movement in snow depends heavily on the freezing room temperature. This temperature was adjusted slightly while the LWC values were recorded using the sensor to ensure that the LWC was constant during the adhesion measurements. Figure 6b shows a complete compression which occurs when substrate temperatures are above 2 \({}^{\circ}\)C during snow de-adestion/shedding. At these temperatures there is minimal ice in snow and the snow-substrate interface contains considerable liquid water. Therefore, snow begins de-adhering/shedding by forcing each snow particle into the next, resulting in a rolling motion of the snow (see Movie S2). When temperature drops to below freezing (i.e., at \(-2\)\({}^{\circ}\)C) the snow de-adhering/shedding mechanism turns to bulk sliding (see Movie S3). A quasi-ice layer forms at the snow and substrate interface, shown in Figure 6a, allowing the snowpack to remain in one piece during shedding. In the sliding de-adhering/shedding mode, the snow shear adhesion strength is very high, when compared to the compression mode; this might be due to partial formation of ultrathin ice at the snow-substrate interface [27]. A combined compression-sliding snow de-adhering/shedding mode is observed when the substrate temperature is around 0 \({}^{\circ}\)C and is shown in Figure 6c. In this mode, snow compression occurs at the beginning of the adhesion test and then changes to a sliding mode, where the substrate temperature (\(\sim\)0 \({}^{\circ}\)C) is favorable for the combined presence of liquid water and ice at the snow-substrate interface [27]. In this mode, the snow may compress, then slide, and repeat, or vice versa (see Video S4 and Figure S6). Figure 6: **(a)** Depicts the snow–substrate interface in snow sliding de-adhering/shedding mode, in which the snowpack slides as a whole (one piece) without any deformation of the snowpack or rolling of snow particles. **(b)** Depicts the snow–substrate interface in snow compression de-adhering/shedding mode, in which snow particles roll and the snowpack deforms in shape. **(c)** A combined sliding-compression mode in which snow compression occurs at the beginning of the adhesion test and then changes to a sliding mode, where substrate temperature is 0 \({}^{\circ}\)C, favorable for the combined presence of liquid water and ice at the snow–substrate interface. To confirm the infiltration of liquid water in snow in relation to surface roughness, and to verify our hypothesis on the impact of temperature on snow adhesion strength at rough surfaces, we examined the surface roughness during the adhesion measurement [32]. Figure 7a-d shows the liquid infiltration progression, in which the free water completely fills all gaps in the roughness. In Figure 7a, water is just above the substrate roughness. As time progresses, water begins to infiltrate the surface roughness, shown with the white dashed line in Figure 7b-d. The arrow shows the front of the water progressively moving into the surface roughness. When temperatures are above freezing, this infiltrated liquid allows the snow to glide off.