# In Situ Rubber-Wheel Contact on Road Surface Using Ultraviolet-Induced Fluorescence Method
## 1 Introduction  
The study of contact mechanics between tire and road surface is a tribological application science in automobile. The science in which friction is produced arises from the contact between rubber tire and road surface in order to transmit the fractional force. Many studies established the effect of road-tire contact interaction on grip performance, rolling resistance, wear, fuel efficiency, and energy loss, which correlates with each other [1; 2; 3; 4; 5; 6]. Fuel efficiency can be improved by reducing rolling resistance of vehicles. One way to reduce rolling resistance is by optimizing the tractional force between tire and road surface. In order to optimize the tractional force, [PERSON] evaluated the tire and surfacing contact stresses and depth texture [2]. [PERSON] and [PERSON] also proved in their experiment that contact patch between tire and road affect the rolling resistance of vehicles [3]. [PERSON] has produced a lot of articles related to contact mechanic of rubber material since around two decades ago. In 2008, [PERSON] and [PERSON] had reported an article about rubber friction, tread deformation and tire traction in sliding situation [4]. Their experiment intended to contribute data for tire dynamic contact during cornering and braking for vehicles with Anti-Braking System (ABS). In general, road-tire contact is highly affected by external factors such as the applied load and moving speed of vehicles. In rainy seasons for example, the water on the road surface significantly affects the grip performance of tire rubber. According to the Michelin corporation report, a tire's grip performance is reduced to 50% on a wet surface when the car runs at 60 km/h. Similarly, the tire grip coefficient, which is considered as 1 on a dry surface, is reduced to 0.6 on a wet surface with 1.5 mm depth of water when the vehicle runs at 60 km/h. The grip coefficient was 0.4 when thevehicle runs at 100 km/h, showing the effect of running speed to tire grip performance of vehicles [7]. According to the Michelin report, the tire grip coefficient is highly influenced by indentation and road surface roughness. It has an undeniable relationship between tire rubber contact area and grip performance. The information regarding the contact area is significant in designing and improving the tire's performance. Specifically, more information helps design superior tires. This is achieved when the mechanisms of tire-road contact conditions are well-understood. Moreover, a lot of data are related to the contact between tire and road surface in various conditions, including internal and external factors are needed. For that reason, it is essential to observe and analyze the contact condition between tire and road surface in various conditions. The contact areas of tire and road surface is part of contacted-soft-material, same as rubber seal and wiper. Various approaches have been used to observe the contact condition and measure film thickness of the contacting parts, including optical interferometry [8], Raman spectroscopy [9], fluorescence microscopy [10; 11], and some others [12; 13]. In 1996, a group study established an experimental technique that provides high-resolution thickness measurement in the horizontal and vertical directions at 1.4 \(\upmu\)m, and 0.5 nm, respectively [14]. This technique was called relative optical interference intensity (ROII). [PERSON] et al. [9], conducted an experiment using Raman microscopy to measure fluid film thickness between rotated and pressed polydimethylsiloxane sphere on a quartz flat disc. The results showed that the contact condition varied due to sliding speed. [PERSON] et al. [10] used fluorescence microscopy on soft materials to identify the central film thickness of the contact area at a different speed. Fluorescence was first applied to tribology in the 1970s when [PERSON] and a co-worker measured the surface film thickness of rotating cylinders and raceway using various oils [15].