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Surface Wettability Change by Irradiation

Sample and Irradiation Facility

To investigate surface wettability change by irradiation, samples are irradiated by using an ultraviolet lamp, a 60Co γ-ray source, and a proton accelerator. In this study, a TiO2 sample, which is a typical photocatalyst [10], is used to compare the irradiation effects of ultraviolet, γ-ray, and proton beam. TiO2 is prepared through anodizing a 0.1-mm-thick titanium plate [11]. Details of the experimental procedure with TiO2 samples and irradiation facilities are described as follows.

10.2.1.1 Ultraviolet

Ultraviolet irradiates TiO2 by using a commercial UV lamp. Irradiation intensity is measured by an ultraviolet meter and is controlled by changing the distance between the lamp and the sample. The intensity is varied at a range from 0.01

10 Heat Transfer Study for ADS Solid Target: Surface Wettability... 97

Fig. 10.1 Water droplets on the TiO2 surface before and after ultraviolet irradiation. (a) Before ultraviolet irradiation. (b) After ultraviolet irradiation with 1 mW/cm2 for 1 h

to 5 mW/cm2. The center wavelength of the ultraviolet from this lamp is 365 nm. Figure 10.1 shows a typical irradiation effect on surface wettability change before and after ultraviolet irradiation.

10.2.1.2 Gamma Rays (γ-Rays)

The 60Co γ-ray irradiation facility in the Research Reactor Institute, Kyoto University, is utilized for γ-ray irradiation. The integrated irradiation dose is estimated by an irradiation time and a distance from the γ-ray source. The γ-ray energy of this facility is about 1 MeV (1.17 and 1.33 MeV) and the maximum dose rate is about 15 kGy/h.

10.2.1.3 Proton Beam

The FFAG (fixed-field alternating gradient) accelerator in the Research Reactor Institute, Kyoto University, is utilized for proton-beam irradiation. The energy of the proton beam is set at about 100 or 150 MeV. The maximum beam current of this facility is about 10 nA.

Contact Angle Measurement

The wettability of the sample before and after irradiation is evaluated by measuring the contact angle of a water droplet on a sample surface. The measurement system (Fig. 10.2) consists of a digital video camera, a stage (with a biaxial stage and a goniometer), a backlight, and a PC. Pure water of 2 μl is dropped onto the horizontal surface of the sample using a micropipette. The water droplet is imaged by the camera and the images are processed to obtain the contact angle. In the image processing, it is assumed that the droplet is a part of a sphere (Fig. 10.3), and the contact angle is estimated by the following equation:

98 D. Ito et al.

Fig. 10.2 Contact angle measurement system

Fig. 10.3 Estimation of contact angle

where r and h are obtained by using an image processing software (ImageJ).

Effect of Irradiations on Surface Wettability

Figure 10.4a–c shows the wettability change from ultraviolet, γ-ray, and proton-beam irradiation, respectively. The horizontal axis denotes an integrated irradiation dose or irradiation time and the vertical axis denotes the measured contact angle. As shown in these figures, the contact angle decreases with the irradiation dose. In these experiments, the ambient effect is also studied during the irradiations, which are performed in air or water. As shown in Fig. 10.4a, the ambient effect on the contact angle change is not obvious in the ultraviolet irradiation. However, the ambient effect is very distinct both in the γ-ray and the proton-beam irradiations. It is suggested that the wettability enhancement by the radiations may be attributed to the radiolysis of water.

 
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