### Rheological Characterization
#### 4.2.2 Viscoelastic Energy Characterization  
Energy storage modulus (G\({}^{\prime}\)), which is essentially [PERSON]'s modulus and the real part of the complex modulus, represents the energy stored by elastic deformation of viscoelastic  
Figure 8: Thixotropic ring after grease freeze–thaw for 7 times: (**a**) PTFE; (**b**) Graphene; (**c**) CNTs; (**d**) WS\({}_{2}\). materials during deformation. It is an index of material rebound after deformation and represents the ability of materials to store elastic deformation energy. The loss modulus (G\({}^{\prime\prime}\)) is also known as the viscous modulus, and the imaginary part of the complex modulus refers to the energy lost due to the viscous deformation (irreversible) when the material is deformed, which reflects the viscosity of the material. When the storage modulus is greater than the loss modulus, the elastic deformation of the grease can be recovered. When the storage modulus is equal to the loss modulus, the grease is semi-solid. When the loss modulus is greater than the storage modulus, the grease will undergo viscosity deformation and has irreversible viscous energy. It can be seen from Figure 9 that the four materials all enter the non-linear viscoelastic region after experiencing the linear viscoelastic region, and the intersection of the storage modulus and the loss modulus appears to be the phase transition point, where shear stress represents the structural strength of grease. It can be seen from the figure that the intersecting strains of PTFE, graphene, CNTs and WS\({}_{2}\) are 8.9, 12.7, 13.9 and 12.2, respectively. This indicates that PTFE has the lowest structural strength and the worst elastic recovery, while CNTs have the highest structural strength and the best elastic recovery. As can be seen from Figure 10, after seven freezing and thawing energy-storage loss modulus tests, all four arterials intersect, and the intersection points of polytetrafluoroethylene, graphene, CNTs and WS\({}_{2}\) are at 8.9, 14.6, 12.1 and 14.4, respectively. It can be seen that among the four additives, PTFE has the earliest phase transition point and the worst elastic recovery, while graphene has the best elastic recovery and the largest structural strength. The greater the structure strength of grease, the more difficult grease is to lose. Figure 9: Grease modulus curve: (**a**) PTFE; (**b**) Graphene; (**c**) CNTs; (**d**) WS\({}_{2}\). Even in the vertical direction or without sealing the friction parts, it still can maintain enough thickness, even under the action of centrifugal force, to ensure reliable lubrication. Only high structural strength is suitable for the construction of a rotating bridge. Therefore, the high-strength performance of graphene is most suitable for the construction of rotating bridge grease additive. The molecules of grease are mainly connected by hydrogen bonds and van der Waals forces to form a stable spatial structure. Different molecular groups form intermolecular forces with different strengths and spatial structures with different strengths. In this paper, the apparent viscosity was used to determine the fluidity of grease. Figure 10 shows the viscosity change curve of grease with PTFE powder, graphene, WS\({}_{2}\) powder and CNTs at constant shear rate at room temperature. It can be seen from Figure 11 that the viscosity of the four greases decreases significantly with the increase in shear time. At 300 s shear, the viscosity of PTFE grease is 1.67 Pa-s, the viscosity of WS\({}_{2}\) grease is 1.85 Pa-s--10.8% higher than that of PTFE--and the viscosity of graphene grease is 1.78 Pa-s, which is 6.6% higher than that of PTFE. The viscosity of CNTs grease is 1.76 Pa-s, which is 5.4% higher than that of PTFE. This shows that the properties of the three replacement materials are better than that of PTFE, among which WS\({}_{2}\) is the best. Figure 10: Modulus curve of grease after 7 rounds of freeze–thawing (**a**) PTFE; (**b**) Graphene; (**c**) CNTs; (**d**) WS\({}_{2}\).