## 3 Results and Discussion
### Influence of Working Pressure around the Cooling Surface on Frost Thickness  
The growth habit of the frost layer is mainly reflected by the frost thickness, which is the most important and most intuitive parameter among the frost characteristics. The frost thickness under different working pressures around the cooling surface is shown in Figure 6. The frost thickness increases with time. Furthermore, the frost thickness decreases with rising working pressure around the cooling surface. The initial growth rate of frost thickness is slower when the working pressure around the cooling surface is higher, indicating that crystallization of frost is more difficult. It can be attributed to the higher pressure that leads to a stronger nucleation barrier of frost crystals, so the nucleation rate of frost crystals lessens. Furthermore, from the perspective of molecular diffusion, the growth rate of frost thickness is related to the concentration of water molecules around the frost crystals. The concentration of water molecules is proportional to the molecular diffusion coefficient [17]. In frosting, water vapor is continuously consumed, so a water vapor concentration gradient is formed near the frost crystal. With the decrease in pressure, the molecular diffusion coefficient augments. The water vapor diffuses to the frost crystal surfaces more easily. Therefore, the concentration difference of water vapor around the frost crystal surface is higher, which leads to faster growth of the frost crystals.  
Influence of Working Pressure around the Cooling Surface on the Frost Accumulation and Frost Density  
The frost accumulation and frost density with different working pressures around the cooling surface at 60 min are shown in Figure 7. The frost accumulation reduces with a higher working pressure around the cooling surface. On the one hand, it can be attributed to the diminishing diffusion coefficient of water vapor, which means that less water vapor condenses into frost crystals. On the other hand, the thermodynamic potential energy driving any crystallization process needs to exceed the change in chemical potential from one phase to another. This potential energy can be either supercooling in the case of freezing or supersaturation in the case of condensation from water vapor [7]. When the temperatures of air and cooling surface keep constant, the surface temperature of the frost crystal is invariant. Therefore, its corresponding saturation vapor pressure is the same. When the working pressure near the frost crystal reduces, excess concentration of water vapor leads to a larger driving potential. The phase transition rate is in direct proportion to the driving potential. Therefore, more water vapor is deposited and condensed into frost crystals, which leads to an increase in frost accumulation. In addition, the growth rate of frost accumulation declines more rapidly than frost thickness with higher pressure, and the frost density reduces with pressure, as shown in Figure 7. Under conditions where other parameters are constant, the lateral growth of frost crystals is notably promoted when the working pressure around the cooling surface lessens (from Figure 5), resulting in more narrow gaps between the frost crystals, while the growth in the vertical direction is not significant. Therefore, the reduction of density is reflected through the pressure effect on the growth habit of frost crystal.  
Figure 6: Frost thickness under different working pressures around the cooling surface with time: (**a**) Photos; (**b**) diagram of frost thickness.