![]() ![]() This observation is consistent with a concept of stagnant cup model, which assumes the free-slip boundary condition at the front of a rising bubble and no-slip boundary condition at the bubble rear. It has to be noted that the steady rise velocity of bubble approaching the solid is reduced in contaminated liquid, but the normalized bubble impact velocity (ratio of impact and steady rise velocity) is similar to that in pure liquid. However, the available experimental data show that impact velocities depend on the Reynolds number, but the effect of surfactant presence is minor even for high concentrated solutions. It might be expected that the presence of surfactants would affect the impact velocity of bubble hitting the surface. In the case of surfactant presence, the bubble deformation before the impact is suppressed. The deformation is caused by an increase of pressure in the liquid film separating the bubble and particle. In pure liquids, the bubble deforms from its initial shape before it collides with the solid surface. An example of bubble-particle collision in pure liquid (deionized water) and in surfactant solution (n-octanol) is shown in Figure 1. īubble approaching the particle surface starts to decelerate. For high concentrated surfactant solutions, the reduction of bubble velocity is so significant that drag coefficient corresponds to the drag coefficient of solid particles with no-slip boundary condition at the interface. Consequently, the drag coefficient increases, and the bubble velocity and distortion are reduced in comparison with clean bubbles. This leads to the formation of surface tension gradients and consequently the formation of Marangoni stresses, which reduce the mobility of a part of bubble interface. The liquid flow around the rising bubble causes the transport of surfactant molecules resulting to the uneven surfactant distribution along the bubble surface. In the case of surfactant presence, the surfactant molecules adsorb to the bubble surface. The drag coefficient depends on Reynolds number (related to the bubble size D b, bubble steady rise velocity U b, liquid density ρ, and viscosity η) and on bubble shape defined by aspect ratio χ. In pure liquids, the bubble surface is free of any contaminants or surfactants, and the whole bubble surface is mobile (free-slip boundary condition is valid at the bubble interface). The bubble shape and velocity follow from balance of forces acting on the bubble. Effect of surfactants on the collision processīefore the bubble and particle collide, the bubble rises in liquid. We focused especially on (i) the influence of surfactants on bubble behavior during the collision with the hydrophobic solid particle, (ii) the drainage and rupture of thin liquid film separating the bubble and the particle, (iii) the influence of surfactants on the three-phase contact line enlargement, and (iv) the influence of different types of surfactants and their purity on bubble stability.Ģ. In this chapter, the influence of surfactants on the collision and attachment process is discussed. ![]() A stable particle-bubble aggregate is thus formed. This sequence of liquid film drainage, rupture, and contact line movement constitutes the second process of attachment. The collision process is then followed by the creation and movement of the three-phase contact line (the boundary between the solid particle surface, receding liquid phase, and advancing gas phase) until a stable wetting perimeter is established. For efficient capture between the bubble and the hydrophobic particle, they must first undergo a sufficiently close encounter. In flotation, the capture of particles by rising bubbles is the central process. Also, many impurities in water are surface-active, and they affect flows even at trace concentrations. Sometimes, they are added intentionally to the system (e.g., in flotation as froth agents or detergents in cleaning applications). The presence of surfactants in two-phase systems is very common. Their presence has important consequences to the flow: for example, the size distribution of bubbles or drops changes, and the rise velocity of bubbles decreases. Molecules of these substances accumulate at the interface, and they decrease the surface tension. ![]() The degree of complexity is further increased if some surface-active agents are present in the liquid. The multiphase flows are in general more complex due to the presence of moving boundaries separating gas and liquid phases. Significant applications are found in the chemical and process industry (separation of coal, mineral ores, or plastics by flotation) or wastewater treatment. The interaction of air bubbles with solid particles is an important mechanism in many industrial processes. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |