Effect of contact angle and contact angle hysteresis on the stability and detachment of floating particles

Abstract

Flotation is a significant separation process widely used in many industrial processes. Thefundamental principle of conventional flotation is the capture of hydrophobic particles bythe rising bubble where surface properties play an important role. With the development ofthe mining industry, coarse and composite ore particles of low grade face the biggestchallenge of high detachment efficiency. The key to understanding bubblenparticleaggregate stability is to determine whether or not the adhesive force, acting on the threephase contact line, is sufficient to prevent the destruction of the aggregates under thecomplex conditions in a flotation cell. The literature review shows that the current forceanalysis of the bubble-particle stability is all based on the ideal case of spherical particlesand constant contact angle. However, in reality, particles are usually non-spherical, andthe contact angle is not constant due to the surface roughness and chemical heterogeneity.This Ph.D. study aimed to investigate the effect of contact angle hysteresis on the stabilityand detachment of attached particles. Specifically, the research was conducted regardingparticles floating on gas-liquid interface.To analyze the stability and detachment of the floating particles at the air-water interface, itis essential to obtain the interface shape of the meniscus, which is governed by YoungLaplaceequation (YLE). It is a highly nonlinear differential equation involving manyimportant factors such as contact angle, contact radius, and surface tension. In Chapter 3,a novel algorithm for solving YLE is developed for the numerical fitting to match theexperimental results to quantify the meniscus deformation. The application of the algorithmis successfully demonstrated using the experimental data for the deformation of externalmenisci of the air-water interface around a sphere.Furthermore, the algorithm and parameter fitting method were applied to investigate thecontact angle hysteresis. The contact angle of single floating spheres at air-water interfacewas repeatedly determined. A normal distribution of contact angle for the single sphereswas discovered. It indicates that contact angle hysteresis occurs randomly due to thedispersion of surface roughness. However, contact angle calculated from the forcebalance method gave higher values than that from the meniscus-fitting method. Thereason is that the force balance method gives an average contact angle calculated basedon the ideal smooth surface without consideration of small-scale roughness which causesthe contact angle hysteresis.The contact angle hysteresis can also be affected by the geometry of sharp edges, knownas Gibbs Inequality Condition (GIC). The contact line is normally pinned at the edge.Chapter 5 investigated the effect of GIC on the forces governing the stability anddetachment of floating particles. Spheres were truncated with different angles. The forcepushing the truncated sphere into the water was measured. The experimental data agreedwith the theoretical results. The outcome shows the existence of a critical truncated angleover which the GIC is important. In this case, the conventional theory is no longer valid.Lateral interaction among floating particles can be important in supporting their floatability.Chapter 6 quantified the effect of the lateral interaction by experiments and theoreticaldevelopment. The force of supporting a pair of floating spheres was measured using aforce sensor. The floatabiltiy of multiple particles was determined by film flotation. 3D YLEwas numerically solved. The result was used to calculate the vertical and horizontal forceson the spheres. A good agreement between experimental data and theoretical models wasobtained. The results show the contact angle along the contact line on the pair spheres isnot constant and is the key factor in influencing the lateral attraction between spheres andthe floatability of multiple particles.In summary, this research is aimed to investigate the role of contact angle hysteresis indetermining stability and detachment of floating particles. Contact angle hysteresis causedby small-scale surface roughness was confirmed by measuring the contact angle of thesingle floating spheres. The GIC was verified through theoretical modeling andexperimental results. The floatability of a pair of floating spheres was examined bymeasuring the force pushing the spheres into the water and numerically solving the3DYLE as well as calculating the vertical forces and lateral interactions. To furtherunderstand the effect of contact angle hysteresis on stability and detachment in practice,more research efforts are demanded into the surface chemistry of flotation. Specifically,the quantification of surface roughness, the recognition of particle shapes, and thecollector adsorption are required to be considered in related to the contact anglehysteresis in the future study.