It is well known that superhydrophobicity can only be observed on rough surfaces, i.e., both chemical and physical effects contribute to superhydrophobicity. Classical theories by Wenzel [27] and Cassie and Baxter [28] have been used to explain observed contact angles on rough substrates: on rough, hydrophobic surfaces, the water droplet resides mostly on air and thus exhibits very high contact angles. PHA-848125 cell line Shibuichi et al. [29, 30] presented an elegant analysis of how apparent

contact Bortezomib solubility dmso angles are affected by the surface roughness compared to a smooth surface. Here, in our study, the bulk compressibility of the reference paperboard has a minor effect on water contact angles whereas superhydrophobic TiO2 nanoparticle-coated paperboard CA-4948 ic50 supports the analysis by Shibuichi et al. [29, 30]: increasing the number of calendering nips results in a decrease of the water contact angles on the hydrophobic side and increase on the hydrophilic side after the ultraviolet treatment in Figure 2. This is expected as adding the number

of successive calendering nips will reduce surface roughness. The water contact angle is approximately 130° and 25° after 15 calendering nips for TiO2 nanoparticle-coated samples without and with UV treatment, respectively. This indicates that the TiO2 nanoparticles do not adhere to the steel calender roll but rather remain on the paperboard surface. Removal of the nanoparticles from the surface would bring the contact angles closer to those values of the reference paperboard in which the water contact angles are almost independent of both the number of calendering nips and the UV treatment. The surface of the reference paperboard was imaged using an FE-SEM showing mineral pigment particles (kaolin and calcium carbonate) immersed in an organic binder with pigment particle sizes in the range of microns as shown in Figure 3a. The high-magnification reference image displays the platy-like kaolin particles used in the pigment coating. The LFS coating of TiO2 nanoparticles results in a surface fully covered with nanoparticles as presented in the

low-magnification image of Figure 3a, and the average nanoparticle diameter is approximately 20 to 40 nm as depicted from the high-resolution Carnitine palmitoyltransferase II image of the LFS-coated TiO2 sample in Figure 3a. Calendering evens both reference and nanoparticle-coated paperboard surfaces. However, there is a more significant change in the morphology of the nanoparticle-coated sample as clearly seen in Figure 3b,c. High-magnification images of TiO2 nanoparticle coating in Figure 3b,c show that under compression nanoparticles start to cluster together forming large smooth areas. The size of these areas increases with the number of calendering nips. It is known from the literature that the compressibility of nanoparticles increases with decreasing particle size [24]. Even some structural transformations can take place in nanoscale that do not exist in macroscale [31].