## 4 Wettability
### The Effect of Surface Roughness on Wettability
For example, [PERSON] [83] observed that for natural hydrophilic surfaces, increasing the surface roughness of ultra-low-ash coal particles from 0.2 \(\upmu\)m to 1.6 \(\upmu\)m increased the contact angle from about 77\({}^{\circ}\) to 83\({}^{\circ}\). They also found that the contact angles for anthracite particles were 88\({}^{\circ}\), 87\({}^{\circ}\), 86\({}^{\circ}\), and 80\({}^{\circ}\) when the surface roughness was 2.68 \(\upmu\)m, 2.49 \(\upmu\)m, 1.66 \(\upmu\)m, and 1.12 \(\upmu\)m, respectively [91]. They further reported that variations in surface roughness did not directly influence wettability. The wettability of natural and gold-coated ultra-low-ash coal surfaces initially increased with the surface roughness, then decreased, and finally experienced a slight increase again (Figure 8a,b) [60]. [PERSON] et al. [122] demonstrated that as the surface roughness increased, both the advancing and receding contact angles continued to rise until reaching their maximum values, after which they began to decrease within the studied range of the surface roughness. In studies involving natural hydrophilic (\(\theta<90^{\circ}\)) particle surfaces treated with collectors, [PERSON] and [PERSON] et al. [33; 34; 48; 49; 62; 63] reported that smoother surfaces improved the hydrophobicity of minerals such as quartz, calcite, barite, talc, and chalcopyrite with the addition of collectors. [PERSON] et al. [98] found that the contact angle of chalcopyrite increased from 79.8\({}^{\circ}\) to 100.9\({}^{\circ}\) and attributed this change to the presence of smoother surfaces. Most research has primarily focused on the relationship between the surface roughness of single mineral surfaces and their wettability. However, limited research has paid much attention to achieving the selective flotation separation of different minerals by adjusting the surface roughness. [PERSON] et al. [59] observed that the contact angle of hematite decreased from 58\({}^{\circ}\) to 44\({}^{\circ}\) with the addition of starch and DDA as the Rq value increased from 3 nm to 145 nm. In contrast, the contact angle of quartz increased from 79\({}^{\circ}\) to 87\({}^{\circ}\) as the Rq  
Figure 7: Effect of different surface roughness of natural minerals on contact angle with addition of collectors. (**a**) The water droplet on hematite surfaces after interacting with 125 mg/L NaCl at pH 9 [39]; (**b**) natural hematite and quartz in absence and presence of 4 \(\times\) 10\({}^{-4}\) mol/L dodecyl amine (DDA) [59]; (**c,d**) natural calcite in absence and presence of NaCl [1]; (**e**) natural graphite in absence and presence of kerosene [38]; and (**f**) malachite surface after treatment with 5 \(\times\) 10\({}^{-5}\) mol/L sodium oleate at pH 9 [2]. value increased from 4.8 nm to 182 nm. This increase in surface roughness enhanced the contact angle (wettability) difference between hematite and quartz, resulting in a significant difference in floatability and allowing for the effective flotation separation of hematite from quartz. [PERSON] et al. [24] reported similar results, that the apatite was hydrophilic with the addition of NaOL after the acid pretreatment while the dolomite was hydrophobic, and the difference in the wettability between apatite and dolomite increased as the surface roughness of the minerals was enhanced. This finding suggested that the selective separation of dolomite from apatite under acidic conditions through reverse flotation could be improved by increasing the surface roughness of the minerals during comminution (Figure 9). Most researchers have focused on proving that surface roughness is beneficial or detrimental to the wettability of mineral surfaces by comparing the contact angles of several different surface roughness values. But few investigations have been conducted on the wettability variation over a wide range of surface roughness values, which may make the results unreliable.