Interaction and instability of air films between bituminous coal surfaces and surfactant droplets

Abstract While the suppression of airborne coal particulates with water and surfactant sprays has been widely used in underground coal mines, the microscopic mechanism involved during the attachment process between water droplets and dust particles has not been well understood. In this work, both the interaction forces and instability of air films between aqueous droplets and coal surfaces are studied by monitoring the evolution of spatial and temporal thickness profiles of the air films. The measurements are conducted with both non-ionic and anionic surfactants. The results of this work show that the air film between the water droplet and the bituminous coal surface collapses at a critical rupture thickness of 104 nm, which is followed by a rapid spreading of the three-phase contact line. It has been found that both the critical rupture thickness and kinetics of film thinning increase with increasing surfactant concentrations regardless of the types of surfactants. An analysis of the spatiotemporal thickness profiles using the Reynolds equation shows that an attractive force is present across an air film. This force is found to be an electrostatic force. The force becomes more attractive with increasing surfactant concentrations of the aqueous solutions. The present work sheds new light on the capture of dust particles by water and surfactant sprays at a molecular level.