2.3.6. Wettability

Contact angle measurement (θ) was used to determine the wettability of composite sheets which can be determined by the standard ASTM D 2578–17 test. This test assumes that the surface energy of a film (γ) is equal to the surface tension of a liquid that wets the film, without merging, when a drop is placed on the surface via contact angle measurements. Each composite sheet was placed in the Teclis tracker tensiometer equipment setup, and a droplet of liquid (water) was ejected out of the micrometer syringe onto the sample using a gauge needle. Images of the droplet on the surface of the sample were taken, and, using the Teclis tracker software, the angle between a tangent drawn on the liquid droplet and the surface of the composite sheet was calculated as reported in the literature [13].

Figure 7 shows samples for the tangent to the surface drawn on a sheet surface for non-polar, extremely non-polar, and polar surfaces, respectively. The same test was also performed using a non-polar solvent, benzene, a mixture of water-non-polar solvent, as well as pure printing ink (Epson ink 664, black, Carrefour, Al Ain, UAE) to test for the wettability characteristics of the prepared sheets for ink-based applications. The wettability tests were also performed on stone paper and regular A4 paper for comparative study.

**Figure 7.** Contact angle measurements. (**a**) Extremely polar surface: (θ) = 90◦, (**b**) Extremely non-polar surface (θ) => 90◦, (**c**) Non-polar surface (θ) =< 90◦ taken via drawing tangent to the liquid droplet and the surface.
