Effect of the advancing contact angle on coal particle–bubble detachment at the mesoscopic scale

Improving particle–bubble adhesion stability and reducing particle–bubble detachment are crucial in froth flotation processes. In this study, a nonequilibrium molecular dynamics approach is applied to investigate the dynamic detachment process of low-rank coal particles adhering with/without a collector from bubbles under different pull forces at the mesoscopic scale. Results indicate that a collector oil film on the surface of a low-rank coal particle can considerably increase the advancing contact angle and counteract the contraction of the three-phase contact line, enabling the particle to withstand a greater detachment force. Conversely, the residual water clusters on the particle surface tend to cause contraction of the contact line. Force analysis of the liquid surrounding the particle and the contact line indicates that the maximum static capillary force between the particle and the bubble is equivalent to the capillary force when the bending angle of the liquid surface reaches the advancing contact angle. When the bending angle of the liquid surface exceeds the forward contact angle, the force on the liquid around the contact line becomes unbalanced, causing the contact line to move to decrease the bending angle of the liquid surface and restore balance to the surrounding liquid. Hence, on mesoscopic scales of ten to tens of nanometers, the advancing contact angle still determines particle–bubble detachment. The study of particle–bubble detachment mechanisms at the mesoscopic scale is expected to establish a relation between microscopic mechanisms at the molecular scale and macroscopic experimental studies at the micrometer to millimeter scale.