## 2 Materials and Methods
### COF Testing  
For the COF experimental testing, the device shown in Figure 3 was developed using the pulling force method. The pulling force method is one of the most common COF test methods and it was chosen for this device.  
The designed device provided a wide range of speed, different surface conditions (dry, wet, lubricated), different material testing, possibility of normal force change and control, and testing with constant speed. The device consisted of a support for rubber sliders, which also acted as a weight that defined the normal force, a force sensor registering the friction force, an electric linear actuator with a servo drive, and a base (skater) on which different floor samples were placed. The device itself was based on the measurement of the coefficient of friction according to the EN 13893 standard [26].  
The device, shown on Figure 3,consisted of the following elements: (1) frame, (2) support construction, (3) AC servo motor, (4) actuator, (5) actuator rod, (6) force sensor, (7) slider with rubber samples (8), (9) normal load knob regulator with foot (12), (10) linear guides, and (11) normal load regulating frame. The device could reach maximum testing speeds of up to 400 mm/s and could work at a very low speed of 0.2 mm/s. The device could also reach a testing speed of 0.1 s, while the maximum stroke of the device was 600 mm. The measured travel distance during testing was 300 mm according to the EN 13893 standard. The real travel distance was about 500 mm because of the distances needed for accelerating and decelerating to 300 mm/s and 0 mm/s, respectively. The maximum load that the linear actuator could withstand was 200 N. Three rubber sliders were attached to the skater. The sliders for testing were made of rubber that was tested for its hysteresis properties. These rubber sliders were cut from rubber plates with dimensions of 40 \(\times\) 10 \(\times\) 5 mm, with a lateral cut of 35\({}^{\circ}\) at the beginning of the sample to prevent sample snagging during movement in the direction of travel, according to the EN 13893 standard. The rubber samples were flat, without texture. Before testing the rubber, both the samples and the granite tiles were cleaned with a brush and cotton to remove dust. Consequently, the influence of dust or nano dust was not included in this research. The normal force was 100 N, which corresponded to a contact pressure of 83 kPa (these values are in line with the EN 13893 standard). This research did not include the influence of different normal loads, which could be a direction for future research. The granite tiles had different surface roughnesses (the smooth tile with Ra = 0.03 \(\upmu\)m and the anti-slip tile with Ra = 4.7 \(\upmu\)m). Surface roughness was measured using a Mitutoyo SurfTest SJ-301 device, with the stylus profilometer in accordance with the ISO 4287:1997 standard [27] (profile R). The measuring distance was 4 mm and the load was 0.75 mN. The cut length was 0.8 mm. For each tile, there were three surface roughness measurements. The testing conditions for the COF  
Figure 3: Friction coefficient testing device.  
were dry, wet (presence of water), and covered with a soap solution, while the sliding speeds were 50 mm/s and 300 mm/s. The latter speed was accepted according to the EN 13893 standard (where recommended speeds range from 200 to 300 mm/s), while the former speed was chosen as the lower speed. There were five measurements for each condition, for a total of 240 COF measurements. No contact angles were used for the COF measurements in the experiments. Table 1 presents the COF experiment input data.