## 4 Wettability
### The Effect of Surface Roughness on Wettability
\begin{table}
\begin{tabular}{l l} \hline \hline
**Findings** & **References** \\ \hline A smoother surface improved the hydrophobicity of talc particles. & [33] \\ The talc particles with a higher roundness and surface roughness caused & [69] \\ a decrease in hydrophobicity. & [79] \\ The contact angles of smooth and rough coking coal particles were about \(99.10^{\circ}\) and \(94.83^{\circ}\), respectively, without collector addition. & [79] \\ The advancing contact angle of anthracite coal steadily decreased from \(93^{\circ}\) to \(78^{\circ}\) as the Ra value increased from \(0.12\) μm to \(2.51\) μm. & [30] \\ The contact angle of the air bubble on coking coal surfaces with a surface roughness of \(0.053\) μm and \(0.648\) μm (polished by different sandpapers) was \(46^{\circ}\) and \(23^{\circ}\), respectively. & [116] \\ As the surface roughness of the coking coal particles decreased from \(2.79\) μm to \(0.98\) μm and \(0.23\) μm, the contact angle increased from \(66.0^{\circ}\) to \(75.3^{\circ}\) and then to \(82.5^{\circ}\). & [117] \\ The contact angle decreased from \(59^{\circ}\) to \(29^{\circ}\) when the Ra value of coal particles increased from \(2.5\) μm to \(6.02\) μm. & [118] \\ The advancing contact angle of dull coals increased from a range of \(20\)–\(50^{\circ}\) to \(30\)–\(65^{\circ}\) when the surface roughness was reduced & [119] \\ Through polishing. & [58] \\ The receding contact angle of air bubbles on particles decreased from about \(32^{\circ}\) to \(17^{\circ}\) for hematite and from about \(30^{\circ}\) to \(20^{\circ}\) for pyrite, when the Rq value of hematite (3 nm) and pyrite (4 nm) increased to about \(50\) nm and \(18\) nm. & [58] \\ The advancing contact angle (measured at point F) decreased from about \(41^{\circ}\) to \(29^{\circ}\) and then fell below \(13^{\circ}\) as the Rq value increased from \(2\) nm to \(42\) nm and then to \(240\) nm. & [120] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The effects of surface roughness on the wettability of natural hydrophilic surfaces. \begin{table}
\begin{tabular}{l l} \hline \hline
**Findings** & **References** \\ \hline The contact angle could be adjusted by altering the solid roughness and showed that both hydrophobicity and hydrophilicity could be enhanced through roughness. & [121] \\ The contact angle of hematite and quartz surfaces increased from 87\({}^{\circ}\) and 80\({}^{\circ}\) to 98\({}^{\circ}\) and 94\({}^{\circ}\) in the presence of dodecyl amine (DDA). & [59] \\ The particles with a nanoscale roughness were more hydrophobic than those with smooth surfaces. & [44] \\ The contact angle of natural calcite increased from 93.07\({}^{\circ}\) to 122.43\({}^{\circ}\) with the NaCl addition as the Ra value rose from 0.13 nm to 3.21 nm. & [1] \\ The angles increased from 38.08\({}^{\circ}\) to 69.81\({}^{\circ}\) and then to 81.01\({}^{\circ}\) with the hydrophobic treatment, as the Ra value of the glass beads increased & [84] \\ from 0.418 \(\upmu\)m to 0.846 \(\upmu\)m and 1.565 \(\upmu\)m, respectively. & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._  
Figure 6: Effect of different surface roughness of natural minerals on contact angle. (**a**) bubble contacting test device; (**b**) coking and anthracic coal particles; (**c**) bubble contacting with coking coal surfaces [116]; (**d**) coking coal particles [117]; (**e**) coal particles [118]; (**f**,**g**) schematic of contact angle measurement device for air bubbles and water droplets [120]; (**h**) contact angles of air bubbles measured on hematite and pyrite surfaces (receding, stable, and advancing contact angle measured at point C (spreading), point E (maximum adhesion), and point F (pull-off)) [58]; and (**i**) contact angle of water droplets measured on magnesite surfaces [120]. Numerous studies have confirmed that an increased surface roughness enhances the hydrophobicity of hydrophobic surfaces and the hydrophilicity of hydrophilic surfaces. However, some researchers have reported differing results.