*2.1. Grease Worker*

The grease was degraded by shearing in the grease worker rig shown in Figure 1. This rig contained a 2 hp gear reduction motor, plunger assembly, a grease cup, cover, and an electrical counter. The plunger assembly consisted of a handle, a shaft, and a perforated plate. The handle had an oval shape slot to convert the rotary motion of the motor to linear-reciprocating motion. The number of strokes was counted using an electric counter. The shearing action was induced in grease by reciprocating the handle and shaft inside the grease cup and forcing grease to pass through a series of holes in the plunger.

**Figure 1.** Modified grease worker.

Grease samples of 30 g by weight were sheared in the grease cup at 1 s<sup>−</sup><sup>1</sup> shear rate at room temperature (25 ◦C). The grease from the cup was then used for evaluation under three cases: Case 1: 10,000 strokes, Case 2: after 86,400 strokes (i.e., after 24 h), and Case 3: after 172,800 strokes (i.e., after 48 h). Measurements of grease samples were performed using a digital scale with an accuracy of 0.1 mg. The testing conditions were selected such that the grease sheared enough to show considerable degradation within the cases.

## *2.2. Water Droplet Analyzer*

The drop shape analyzer (Krüss, Hamburg, Germany) shown in Figure 2 was used to determine the contact angle of the water droplets on the grease surface. This setup consisted of a camera (IDS UI-5480CP-M-GL GigE camera) and adjustable lens (Thorlabs AC254-075-A-ML Lens) through which the water droplet on grease sample was analyzed. Using an adjustable screw, the height of the sample stage was adjusted such that the water droplet was in line with the lens height. The angle of the lens was further adjusted by an alignment screw, as needed. The apparatus provided a monochromatic blue light that helped in obtaining a clear and distinguishable image of the droplet from the background. The apparatus used the captured image to calculate the contact angle *θ* via the builtin software.

**Figure 2.** Drop shape analyzer.

A mold made of polymer with rhombus shapes was used in the present work to achieve consistent thickness and a uniform surface of grease. To remove the trapped air, the grease in the mold slots was completely compressed. The grease sample was refilled again if trapped air was observed. The slot containing grease to be tested was kept in front of the camera and a 5 μL water drop was placed on the grease surface using a 10 μL syringe. Due to the semi-solid nature and complex structure of grease, the dimensions of the droplets on the grease surface changed with time, making it difficult to capture the image instantaneously. To address this issue, a video of the droplet was recorded for more than 5 min at 3 frames/s. Images of the water droplet from the video after 60 s were considered for measuring the contact angles. The standard operating procedure is explained in Appendix A and also an explanation of the methodology is provided elsewhere [19]. It was made sure that the grease sample considered for testing was at 25 ◦C.

## *2.3. Rheometeric Tests*

The yield stress results for grease sheared for different numbers of strokes were measured using a rheometer (Anton Paar MCR 301, Graz, Austria) shown in Figure 3. The details of the specification of the rheometer are provided elsewhere [18].

 **Figure 3.** Rheometer for measuring yield stress and penetration. 80

To determine the yield stress, a grease sample of 2 mm thick and 15 mm diameter was placed onto the stationary surface (see Figure 4a). The plate was moved to the desired gap thickness of 1 mm (Figure 4b) and the excess grease was trimmed off (Figure 4c). To remove the deformation history due to the squeezing of grease, sufficient rest time was provided to relax.

**Figure 4.** Procedure for placing the grease samples between the plate and stationary surface: (**a**) place the grease sample; (**b**) squeezing the grease to the required gap; (**c**) trimming of the excess grease.

The yield stress values were determined from the shear stress-strain plot. The plot was obtained by oscillate-sweeping the plate from 0.001% to 1% at a fixed frequency of 1 Hz. The oscillatory strain sweep approach was adopted due to its robustness and reliability. Further, the results obtained were insensitive to the geometry of the plates, surface roughness, the gap between the plates, and the frequency of shearing [22,23]. The yield stress is the point on the stress-strain curve where the coefficient of determination (*R*2) between the third-order polynomial fit of the experimental values and a linear fit is found to be greater than 99.5% [23].
