1. Yusuke Minami* Tomoaki Iwai**, Yutaka Shoukaku**
30th Annual Conference on Tire Science and Technology
September 13-14, 2011
Akron, Ohio, USA
Observation of Water Behavior in the Contact Area between Porous Rubber and Mating Surface during Sliding
Thank you Mr. chairman, I’m Yusuke Minami from Kanazawa University.
I’d like to talk about Observation of Water Behavior in the Contact Area between Porous Rubber and Mating Surface during Sliding. Or
I’d like to talk about friction of Porous Rubber under wet condition.
Yusuke Minami* Tomoaki Iwai**, Yutaka Shoukaku**
* Graduate School of Natural Science and Technology
Kanazawa University
** College of Science and Engineering

2. 1. Introduction and objective 2. Apparatus and method
Table of contents
1. Introduction and objective
2. Apparatus and method
Friction experiment and condition
Observation method
Observation area
3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area
4. Conclusions
Here is my presentation outline
First I’m going to talk about introduction and objective.
Then, I will show you apparatus and method in this study.
In particular, I’ll describe how we observed the contact surface between the rubber specimen and mating prism.
After that I will talk about results and discussion.
Finally, I’d like to talk about conclusions.

3. 1. Introduction and objective 2. Apparatus and method
Table of contents
1. Introduction and objective
2. Apparatus and method
Friction experiment and condition
Observation method
Observation area
3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area
4. Conclusions
First, introduction and objective

4. Studless Tire
Studless tires are designed for use in winter conditions, such as snow and ice
Characteristics of studless tires
Soft tread compound
Increase the contact area
A lot of sipes in the tread pattern
Wipe and evacuation the water
I’d like to talk about the studless tire.
The studless tire is one of the snow tire. So, it is designed for use in winter conditions, such as snow and ice.
The tread compound of studless tire is usually softer than that used in tires for summer conditions
Therefore contact area with road increase.
Studless tire has a lot of sipes in the tread pattern.
The sipes are wiping and evacuating the water.
As the result, studless tire provides better traction at low temperatures.
FIG.1 Tread of studless tire

5. The tread rubber of studless tire has been devised in various ways.
Design of tread pattern and sipes
Various hard materials in tread rubber
glass fibers, ceramics, nut shell・・・
Development of tread compound
Porous rubber is tread compound that has numerious pores both surface and inside.
The tread rubber of studless tire has been devised various ways to grip on the icy surface.
・Design of tread pattern and sipes
･Various hard materials that is grass fiber, ceramics, nut shell, eggshell, and any more. These are mixed in tread rubber.
・Development of tread compound
And any ways.
Porous rubber is one of the development of tread compound.
In recently, the porous rubber is often used as the studdless tire.
Fig.1 shows studless tire and micrograph of porous rubber surface.
It has numerous pores both surface and inside.
FIG.2 Porous rubber surface

6. Effect of the porous rubber
・The decrease in elastic modulus of the rubber
・The water removal between tire tread and road surface by water absorption effect of the pores.
The porous rubber is consider to have two effects on friction under wet condition.
One is the decrease in elastic modulus of the rubber.
Another, the water removal between the tire tread and the wet road surface.
This is due to the pores on surface absorbs the water.
The water removal images is shown in Fig.2.
In generally, these effects result in increasing the real contact area between the tire and the wet road.
FIG.3 Water removal image
The real contact area　between the tire and the wet road is believed to be increased

7. The removal of the water for absorption by the pores on surface of porous rubber, as the details of the process was not clearly understood.
The water removal by absorption of pores on the surface of the porous rubber is few studied.
So, the detail of the water removal process was not clearly understood.
Here is the objective
The purpose of this study was to clarify the effect of water absorption by the pores in contact area during sliding under wet conditions.
Objective
The purpose of this study was to clarify the effect of water absorption by the pores in contact area during sliding under wet conditions.

8. 1. Introduction and objective 2. Apparatus and method
Table of contents
1. Introduction and objective
2. Apparatus and method
Friction experiment and condition
Observation method
Observation area
3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area
4. Conclusions
Then, apparatus and method in this study

9. Friction experiment and experimental condition
A rotating rubber specimen was rubbed against a mating prism.
➢The friction surface between rubber specimen and dove prism is observed through dove prism.
Fig.4 is shown Experimental apparatus.
A rotating rubber specimen was rubbed against a mating prism.
The prism is compressed by dead weight.
This apparatus can observe the contact surface through the dove prism simultaneous with the measurement of friction force by strain gauges were attached to the parallel leaf spring.
FIG. 4 Experimental apparatus:
1, weight; 2, rubber specimen;
3, dove prism; 4, parallel leaf spring;
5, strain gauge; 6, prism holder;
7, linear guide.
➢The friction force was measured by strain gauges were attached to the parallel leaf spring.

10. TABLE 1 Specification of the rubber specimen
12.5mm
Pore
TABLE 1 Specification of the rubber specimen
For the experiment, natural rubber filled with 2 phr carbon black was used as the rubber specimen.
Table 1 shows specification of the rubber specimen.
The pore generated on the circumference of the rubber specimen using a milling machine.
f 60mm
Formulation of
rubber specimen
Natural rubber filled
with carbon black
Pore diameter, mm
No pore, f 0.5, f 1, f 2
FIG.5 Rubber specimen

11. TABLE 2 Experimental condition Sliding speed v, mm/s 3-30
Syringe
TABLE 2 Experimental condition
Sliding speed v, mm/s
3-30
Normal load, N
14.7
Pure water
TABLE 3 Specification of the fine particles
The experimental condition is shown in table 2.
In this study, pure water was adopted as the lubricant. The water fell in drops from a syringe onto the prism.
When we observed and visualized the water flow, fine particles were mixed into the water.
Table 3 shows specification of the fine particles
material
calcium carbonate
diameter, mm
50-80
Rolling direction
Mating prism
Rubber　specimen
FIG.6 Cross section of contact surface between the prism and the rubber specimen

12. Observation method
Next, we will show the observation method.
Fig.7 shows the optical systems for the contact area measurement.
We used two methods to observe the friction surface.
(a)Total internal reflection method
(b) Orthographic method
FIG.7 Optical systems for the contact area measurement:
1, rubber specimen; 2, dove prism;
3, CCD camera; 4, light sources.

13. To distinguish the contact surface against rubber, water, and air.
To observe and visualize the water flow
One is the total internal reflection method.
This method conducted to distinguish the contact surface against rubber, the water, and air under wet condition.
(a)Total internal reflection method
(b) Orthographic method
FIG.7 Optical systems for the contact area measurement:
1, rubber specimen; 2, dove prism;
3, CCD camera; 4, light sources.

14. - The total internal reflection method -
n1 > n2
When incident light as passes from a medium of high refractive index n1 to a medium of lower refractive index n2,
θ1’ θ1 n1 n2
Incident light
Reflected light
Refraction light
Medium　1
・・・(1) θ2
I’ll explain about total internal reflection
When incident light as passes from a medium of high refractive index (n1) to a medium of lower refractive index (n2), the relationship between incident angle and reflected angle follow this Eq.1.
Medium 2
FIG. 8 Refraction of light as passes from a medium of high refractive index (n1) to a medium of lower refractive index (n2)

15. - The total internal reflection method -
n1 > n2
Medium　1
Incident angle is increasing, the reflected angle becomes right angle and the incident light completely reflected.
Now, the incident angle is called the critical angle. Based on Eq. (1), the critical angle q c was determined as follow:
θ1’ θ1 n1 n2
Incident light
Reflected light
When the incident angle is increasing, the reflected angle becomes right angle and the incident light completely reflected.
This phenomenon is defined the total internal reflection.
Now, the incident angle is called the critical angle. Based on Eq. (1), the critical angle q c was determined as follows Eq.2.
θ2=90°
Medium 2
FIG. 8 Refraction of light as passes from a medium of high refractive index (n1) to a medium of lower refractive index (n2)
・・・(2)

16. TABLE 4 Refractive index Prism 1.52 Rubber 1.51-1.52 Water 1.33 Air
1.0
Here, we considered about critical angle.
Table 4 describes the refractive index of each medium.
According to Eq.(2), we obtained the critical angle as the light passes from the prism.
The result is shown in Table 5.
TABLE 5 Critical angle as the light passes from the prism
Incident medium
Critical angle, °
Rubber
83-90
Water 61
Air 41

17. 41° < θ1 <61° - The total internal reflection method -
rubber
water
air
prism θ1 θ1 θ1
41° < θ1 <61°
Thus, the incident angle adjusted greater than critical angle of prism-air interface 41゜ and smaller than that of prism-water interface 61゜.
The total internal reflection image is described (b).
Prism- air interface is represented by the white area.
This is owing to the all incident light is reflected at the interface. So, here looks very bright.
Prism-rubber interface is represented by the black area in the total reflection image.
This is due to the greater part of the incident light passes to the rubber and no refracted light. There looks like very dark.
Prism-water interface is represented by the gray area because a part of the incident light is reflected, and another passes to the water.
(a) Cross section
(b) Total internal reflection image
FIG. 9 Reflected light and the refracted light at the interface of various refractive indexes

18. - The total internal reflection method -
rubber
water
air
prism θ1 θ1 θ1
41° < θ1 <61°
The differences of intensity of the reflected light allow distinction of contact surface variation
(a) Cross section
(b) Total internal reflection image
The differences of intensity of the reflected light allow distinction of contact surface variation
FIG. 9 The reflected light and the refracted light at the interface of various refractive indexes

19. To distinguish the contact surface against rubber, water, and air.
To observe and visualize the water flow
The second observation is the orthographic method.
This method conducted to observe and visualize the water flow.
We conducted the particle tracking velocimetry to investigate the water flow.
(a)Total internal reflection method
(b) Orthographic method
FIG.7 Optical systems for the contact area measurement:
1, rubber specimen; 2, dove prism;
3, CCD camera; 4, light sources.

20. - Visualized water flow-
(b) t2
Here, we describe the particle tracking velocimetry.
The fine particles are mixed into water and continuous images are taken.
Figure 6(a) and 6(b) represent continuous images of particles at time t1 and t2.
We considered the image (a) superimposed (b).
Now, we obtained the movements and directions of corresponding particles in the given time interval.
These are described by the vectors.
Each vector describes the water flow in the time differences between t1 and t2.
(c) Particles at t2 superimposed on the image at t1
(d) Movement direction
　　 of each particles from
t1 to t2
FIG. 10 Principle of the particle tracking velocimetry (PTV)

21. - Visualized water flow-
(x2. y2)
(x2. y2)
(x1. y1)
(x1. y1)
(x4. y4)
(x4. y4)
(x3. y3)
(x3. y3)
(a) t1
(b) t2
As with the fig.7, The image (a) superimposes (b).
But in this study, the object moved in the continuous images is not only the particles.
The pore moved simultaneous. Δy
(c) Movement direction of
each particles from t1 to t2
FIG. 11 PTV considered relative displace between pore and particles

22. - Visualized water flow-
(x2. y2)
(x1. y1)
(x4. y4)
(x3. y3)
(a) t1
(b) t2
Thus, we considered the relative distance between pore and particles.
The particles positions at t1 was displaced only the pores movement Δy to rubber sliding direction.
So, we obtained the relative movements and directions between corresponding particles and pore. In addition, we consider the vectors.
(x4. y4)
(x2. y2)
(x2. y2)
(x1. y1)
(x1. y1)
(x4. y4) Δy Δy
(x3. y3)
(x3. y3)
(x2. y2-Δy)
(x1. y1-Δy)
(c) Movement direction of
each particles from t1 to t2
(d) Superimposed image considering the relative distance between pore and particles
FIG. 11 PTV considered relative displace between pore and particles

23. Observation area Leading area Trailing area Mating prism
The surface transitioned from noncontact to contact with the mating prism.
The observation was conducted in each of two areas, leading area and trailing area.
The leading area is the surface transitioned from noncontact to contact with the prism.
The trailing area is the surface transitioned from contact to noncontact with the prism.
Trailing area
The surface of transitioned from contact to noncontact with the mating prism.
Rolling direction
Rubber　specimen
FIG. 12 Definition of the area of contact

24. 1. Introduction and objective 2. Apparatus and method
Table of contents
1. Introduction and objective
2. Apparatus and method
Friction experiment and condition
Observation method
Observation area
3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area
4. Conclusions
After that, results and discussions

25. Coefficient of friction
I’ll show the result about coefficient of friction.
Fig is Variation in coefficient of friction with the pore diameter under wet conditions
FIG. 13 Variation in coefficient of friction with the pore diameter under wet conditions

26. Coefficient of friction
The coefficient of friction of the rubber specimen with pores was larger than that of the rubber specimen without pores.
The coefficient of friction of the rubber specimen with pores was larger than that of the rubber specimen without pores.
Fig. 12 Variation in coefficient of friction with the pore diameter under wet conditions

27. Observation in leading area
Sliding direction of rubber
(a) 0.2s
(b) 0.4s
Then, I will show result of observation in leading area.
Fig.14 describes observation using the total internal reflection method.
The sliding direction of rubber is from the top to the bottom.
2mm
(c) 0.6s
(d) 0.8s
2mm
FIG. 14 Rubber surface of leading area observed by the total internal reflection method

28. Observation in leading area
Sliding direction of rubber
Front edge
(a) 0.2s
(b) 0.4s
In this study, the curvature of the pore in the sliding direction of rubber called the front edge of the pore,
2mm
(c) 0.6s
(d) 0.8s
2mm
FIG. 14 Rubber surface of leading area observed by the total internal reflection method

29. Observation in leading area
Rear edge
Sliding direction of rubber
(a) 0.2s
(b) 0.4s
and opposite edge was called the rear edge of the pore.
2mm
(c) 0.6s
(d) 0.8s
2mm
FIG. 14 Rubber surface of leading area observed by the total internal reflection method

30. Observation in leading area
water
air
Sliding direction of rubber
rubber
(a) 0.2s
(b) 0.4s
The white area in the pore represents the air in contact with the prism surface. The gray area shows the contact area between the prism and the water. Moreover, the contact with the rubber is represented by the black area.
As the result, the air is contained in the pore in leading area.
In the case of (c) and (d), the water and air exist coincide in the pore.
2mm
(c) 0.6s
(d) 0.8s
2mm
water and air exist coincide in the pore
The pore contained an air bubble during the sliding.

31. Observation in leading area
water
air
Sliding direction of rubber
rubber
(a) 0.2s
(b) 0.4s
The front edge of the pore was not clearly seen black area when the rubber specimen rotated.
Thus the front edge of the pore is noncontact with prism.
2mm
(c) 0.6s
(d) 0.8s
2mm
The front edge became noncontact with the mating prism.

32. Observation in leading area
Sliding direction of rubber
(i) t2=0.2s
(ii) t2=0.4s
(iii) t2=0.6s
(iv) t2=0.8s
(a) Orthographic images of particles at time t2
FIG. 15 shows the orthographic image of particles and the flow results of PTV in leading area.
(a) Represents orthographic images of particles at time t2.
White particles are tracer particle.
(b) describes displacement of particles and pore from t1 to t2.
Solid circle shows the pore at t2. The dashed circles represent the pore position 0.2 s prior to the solid circles.
The outward flow from the pore was calculated using the particle movement at each interval time.
(i) From t1=0s
to t2=0.2s
(ii) From t1=0.2s
to t2=0.4s
(iii) From t1=0.4s
to t2=0.6s
(iv) From t1=0.6s
to t2=0.8s
(b) Displacement of particles and pore from t1 to t2
2mm
FIG. 15 Orthographic image of particles and the flow 　　　　　 results of PTV in leading area

33. Observation in leading area
The water did not intrude into the pore when the pore was rubbed.
The water flowing along the edge of pore was observed.
Fig.16 shows superimposed image considering the relative distance between pore and particles.
The water flow did not intrude into the pore when the pore was rubbed.
The water flowing along the edge of pore was observed.
FIG.16 Superimposed image considering the relative distance between pore and particles

34. Observation in leading area
water
air
rubber
2mm
2mm
In the leading are,
According to the total internal reflection method, the pore contained an air during the sliding.
In regard to the water flow, these flowing along the edge of pore were observed.
Thus, we considered the water flow detouring the pore is due to the air bubble in the pore. So, the air bubble in the pore pushed aside the water.
The pore contained the air bubble during the sliding.
The water flowing along the edge of pore was observed.
The water flow detouring the pore is due to the air bubble in the pore. The air bubble in the pore pushed aside the water.

35. Observation in trailing area
Sliding direction of rubber
(a) 0.2s
(b) 0.4s
Then, I will talk about the trailing are.
2mm
2mm
(c) 0.6s
(d) 0.8s
FIG. 17 Rubber surface of trailing area observed by the total internal reflection method

36. Observation in trailing area
Sliding direction of rubber
(a) 0.2s
(b) 0.4s
In the total internal reflection method,
The air in the pore still remained when the pore left the prism as with leading are.
2mm
2mm
(c) 0.6s
(d) 0.8s
The air in the pore remained even if the pore left the prism.

37. Observation in trailing area
Sliding direction of rubber
(a) 0.2s
(b) 0.4s
The front edge was not contact with the mating prism as with leading area, and the rear edge of the pore contacted with mating prism even if the pore left the mating prism.
2mm
2mm
(c) 0.6s
(d) 0.8s
The front edge was not contact with the mating prism as with leading area, and the rear edge of the pore contacted with mating prism even if the pore left the mating prism.

38. Observation in trailing area
Sliding direction of rubber
(i) t2=0.2s
(ii) t2=0.4s
(iii) t2=0.6s
(iv) t2=0.8s
(a) Orthographic images of particles at the time t2
In regard to the water flow, fig.18 shows the orthographic image of particles and the flow results of PTV in trailing area.
(i) from t1=0s
to t2=0.2s
(ii) from t1=0.2s
to t2=0.4s
(iii) from t1=0.4s
to t2=0.6s
(iv) from t1=0.6s
to t2=0.8s
(b) Displacement of particles and pore from t1 to t2
2mm
FIG. 18 Orthographic image of particles and the flow 　　　　　 results of PTV in trailing area

39. Observation in trailing area
The water flowed along the pore edge.
No particles were observed to cross the rear edge.
Fig .19 Superimposed image considering the relative distance between pore and particles.
The water flow was moving along the pore edge.
No particles were observed to cross the rear edge.
FIG. 19 Superimposed image considering the relative distance between pore and particles

40. Observation in trailing area
2mm
2mm
In the trailing are, we obtained The rear edge of the pore contacted with mating prism even if the pore left the mating prism
by total internal reflection method.
In regard to the water flow, the water flowed along the pore edge and didn’t cross the rear edge.
Thus, we considered the rear edge of the pore was probably rubbed strongly against the prism and most likely wiped the water.
Reason for the coefficient of friction of the rubber specimen with pores was larger than that of the rubber specimen without pores, we considered the frictional force made the rear edge pull back and form a small roll.
The rear edge of the pore contacted with mating prism even if the pore left the mating prism.
The water flowed along the pore edge and didn’t cross the rear edge.
The rear edge of the pore was probably rubbed strongly against the prism and wiped the water.

41. 1. Introduction and Objective 2. Apparatus and method
Table of Contents
1. Introduction and Objective
2. Apparatus and method
Friction experiment and condition
Observation method
Observation area
3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area
4. Conclusions
Finally, I will talk about conclusions.

42. Conclusions
1. The coefficient of friction of the rubber specimen with pores was larger than that of without pores under wet condition.
2. The pore contained an air bubble during sliding under wet condition.
3. The front edge of the pore was not contact with the mating prism. On the other hand, the rear edge of the pore contacted with mating prism even if the pore left the mating prism.
4. The water flow detouring the air bubble in the pore was also observed.
1. The coefficient of friction of the rubber specimen with pores was larger than that of without pores under lubricated condition.
2. The pore contained an air bubble during the sliding under wet condition.
3. The front edge of the pore was not contact with the mating prism. On the other hand, the rear edge of the pore contacted with mating prism even if the pore left the dove prism.
4. Water flow detouring the air bubble in the pore was also observed.

43. Thank you for your kind attention

45. -Observation of contact area (Leading area)-

47. -Observation of contact area (Trailing area)-

49. - Visualized water flow-
・Observation　method
- Visualized water flow-
(a) t1
(b) t2
(c) Particles at t2 superimposed on the image at t1
(d) Movement direction
　　 of each particles from
t1 to t2

50. - Visualized water flow-
・Observation　method
- Visualized water flow-
(x2. y2)
(x1. y1)
(x4. y4)
(x3. y3)
(a) t1
(b) t2
(x4. y4)
(x4. y4)
(x2. y2)
(x1. y1) Δy
(x3. y3)
(x3. y3)
(x2. y2-Δy)
(x1. y1-Δy)
(c) Movement direction of
each particles from t1 to t2
(d) Superimposed image considering the relative distance between pore and particles
FIG. 7 PTV considered relative displace between pore and particles.

51. Studded Tire
Studless tires are designed for use in winter conditions, such as snow and ice
Characteristics of studded tires
Roughening the ice
Providing better frivtion between the ice and the soft rubber
Increased the road wear by the studs
I’d like to talk about the studless tire.
The studless tire is one of the snow tire. So, it is tires designed for use in winter conditions, such as snow and ice.
The tread compound of studless tire is usually softer than that used in tires for summer conditions
Therefore contact area with road increase.
Studless tire has a lot of sipes in the tread pattern.
The sipes are wiping and evacuating the water.
As the result, studless tire provides better traction at low temperatures.
FIG Studded tire
Use of studs is regulated in most countries, and even prohibited in some located

52. Friction force of Studded Tire
Fig Concept of tread pattern design for snow and ice covered road

53. Fig Rate of frictional force under various road condition

54. Rubber friction force F F = FH + a ( FA + FD)
FH : Hysteresis Friction
FA : Adhesion Friction
Energy loss caused by adhesion between tread and road
Energy loss caused by deformation of tread derived from road roughness
Rubber
Rubber
Road surface
Road surface
a : Friction improving coefficient developed by displacement of water friction
FD : Digging Friction
Energy loss caused by scratching road surface and wearing of rubber itself
Rubber
Road surface

55. Fig. Variation in coefficient of friction with the pore diameter
(a)Aspect ratio AR=0.5
(b)Aspect ratio AR=1
Fig. Variation in coefficient of friction with the pore diameter

56. Vulcanization accelerator
Composition of rubber specimen NR
100
ISAF CB 2
ZnO 4
Stearic acid
Antioxidant
Oil 3
Vulcanization accelerator 1
Sulfur
1.5