*3.6. Wettability*

Photo-induced hydrophilicity is often associated with photocatalytic TiO2 coatings [31]. Large differences in the wettability of TiO2 coatings after irradiation by light or after storage in the dark are believed to be due to the generation of hydrophilic radicals on the TiO2 surface by the action of light. Measurement of contact angle had been found to be an effective and easy method of detecting the presence of the TiO2 sol coating, MC. In addition, it is expected that the contact of contaminants with the surface is enhanced in the case of hydrophilic surfaces, resulting in an increase in the effect of the photocatalyst. Thus water contact angle measurements were made on each test coupon listed in Table 3 to help determine the presence and activity of each coating.

Contact angles were firstly measured for the coupons immediately on unpacking (dark), and then after 20 hours irradiation. It was noticeable that many of the coupons were heavily soiled so a portion of each sample was cleaned by wiping with 2-propanol on a soft cloth and then with water. Contact angles were re-measured after 20 h. under UVA light, and again after 6–7 days in the dark. The results are shown in Figures 9–11.

Figures 9–11 show that for most coatings, the effect of light on the wettability was more pronounced in the case of the reference surfaces than those having undergone the processing conditions. This may indicate changes in the coating activity resulting from the exposure to the cleaning chemicals *etc.* used during the processing.

**Figure 9.** Water contact angle measurements for (**a**) TiO2 (T1); and (**b**) TiO2-Ag (low) (T2) coupons after three-month Brewery Trial.

**Figure 10.** Water contact angle measurements for (**a**) TiO2-Ag (high) (T3) and (**b**) TiO2 (U1) coupons after three month Brewery Trial.

For the TiO2 sol coating, MC, the two coupons sited at the Brewery C showed similar wettability to the control sample (MC-R2), after cleaning, both in the dark and the light. Visual inspection of the coupons sited at the other two breweries showed that the coating was wholly or partly removed from these coupons, and the contact angle measurements reflect this loss of coating (Figure 11b). Contact angle values on blank stainless steel surfaces after cleaning were 70°–80°.

**Figure 11.** Water contact angle measurements for (**a**) TiO2-Mo (U2); and (**b**) TiO2 (MC) coupons after three month Brewery Trial (Coated area of MC-R1 was too small to test).
