# Relationships between Surface Properties and Snow Adhesion and Its Shedding Mechanisms
## 2 Experimental Section  
A snow gun was designed for artificial snow formation year-round inside a walk-in freezing room that reached temperatures of \(-20\)\({}^{\circ}\)C (Figure S1) [26]. The snow gun has an internal mixing chamber and two ball valves for adjusting air and water flow (Figure 1). The addition of the ball valves allows the liquid water content to be controlled while producing artificial snow. When the snow gun began operating, the cold room would warm first. Next, the cold room temperature would remain constant between \(-5\) and \(0\)\({}^{\circ}\)C for approximately one hour for artificial snow to be produced. Compressed air of 1.5 to 3 Bar (depending on the desired LWC in snow) is mixed internally with room-temperature water flowing at a rate of approximately 1.51 L per minute. The mixture is forced through converging nozzles with end size of 1/16 inch. Fast moving liquid water droplets exit the nozzles and enter the walk-in freezing room, in which snow nucleation occurs. The spray pattern is conical to reduce the collision of particles within the air before they nucleate. The final mass ratio of water to air at the exit is approximately 98%. Reducing the inflow of water by the adjustment valve creates drier (low LWC) snow while increasing the opening of the adjustment valve results in a high density and high LWC artificial snow. Movie S1 shows snow formation using the snow gun. A high-speed camera (OLYMPUS, i-SPEED TR, 10,000 fps) was used for capturing the slow-motion side-view videos of artificial snow particles, before impacting the surface and possibly agglomerating on it. Images of the moving snow particles were extracted from the videos and analyzed with the ImageJ processing tool, obtaining snow particles with LWC of \(\sim\)22%, and with diameters of \(43.53\pm 24.99\)\(\upmu\)m.  
LWC plays a critical role in determining snow properties. A LWC sensor was used throughout all experiments to track any changes that could occur during testing. The sensor measures the temperature and the electrical conductivity of the medium (i.e., snow). It uses a database and a complex algorithm to determine whether air, water, or snow and/or ice are present on its surface. Further details about the sensor are available in our recent patent (Figure S2) [17]. Snow consists of air, water, and ice  
Figure 1: **(a)** Schematic of a snow gun with four nozzles that forms the snow by mixing air and water with controlled pressure. **(b)** The UToledo snow gun with four nozzles. Each nozzle has its own ball valve for controlling air and water flow. **(c)** The components of a single nozzle snow gun. The water entrance, compressed air entrance, and nozzle are depicted. The air and water entrance can be controlled by the corresponding ball valves, which in turn controls the LWC of snow. and based on their ratio it can be in the pendular or funicular regime. In the pendular regime, the water droplets are surrounded by air and graupel particles and the LWC is below \(\sim\)27% (for a density of 0.2 g/cm\({}^{3}\)) (Figure 2a). In the funicular regime, the air is surrounded by water and graupel particles and the LWC is high (Figure 2b). In both regimes water can migrate within the snow impacting its LWC values locally. It is hypothesized that the LWC of snow impacts its adhesion strength to any surfaces, regardless of the surface properties. Therefore, any setup to measure snow adhesion strength must prevent any water movement and change in the LWC within the snow, during the measurements. In a gravitational [27] or a centrifugal [28] setup the gradient of liquid water content occurs in snow (Figure 2c) during the adhesion measurement, leading to incorrect and unreliable correlation between LWC of snow and snow adhesion strength. Here, we utilized two LWC sensors, one on the top and another on the bottom of an inclined aluminum substrate, and visually observed migration of water to the bottom of the snowpack due to gravity (Figure 2d).