(71a) The Influence of Nanobubbles on the Interaction Forces between Alumina Particles and Ceramic Foam Filters in Water

For various industrial processes as for example filtration and flotation, the knowledge about interaction forces and thus influencing parameters is of great interest. For instance poor wetted foam filters made of ceramics like alumina, spinel or titan dioxide are industrially used to remove oxide particles.

As it is seen for many investigations of water-based hydrophobic systems, there exists besides van-der-Waals and electrostatic forces another force, that is of adhesive nature and up to tenfold stronger than van-der-Waals. The origin of this “long-ranged hydrophobic force” was in discussion for a long time period until small cap-shaped objects have been detected via atomic force microscopy. These objects are so-called nanobubbles, which are formed during immersion, local temperature gradients or perturbation, have diameters up to some µm and can exist up to days. If two hydrophobic surfaces approach each other and a nanobubble is located between them, a capillary bridge develops that increases the adhesive forces between the surfaces. The long-ranged hydrophobic force is therefore of capillary nature. If no nanobubbles are present between the surfaces, only van-der-Waals and short hydrophobic forces due to an orientation of water molecules act as adhesive forces. This adhesive forces might be further increased by nanobubbles formed due to perturbation after contact. Finally, a force distribution including van-der-Waals, hydrophobic and capillary forces is received.

Most researchers investigate nanobubbles and their properties on very smooth surfaces like mica, graphite or silicon wafers. The main drawback of these experiments is the poor accordance with real systems because of the rough surface. The roughness will influence the force distribution in a way that low adhesive forces are decreased due to the reduced contact area. Large adhesive forces will be further increased because of the pinning of nanobubbles inside of asperities.

In this study, a variation of technical rough ceramic foam filter materials is investigated through the application of force spectroscopy using an atomic force microscope. An alumina particle is used as colloidal probe and glued on a tipless cantilever. The wetting behaviour of both filter and particle surface is changed by a silanization process. The filter materials are characterized by a differing roughness due to the manufacturing process, which influences the generation and stability of nanobubbles. Also the colloidal probes are not ideally smooth so that nanobubbles can sit inside of pores and enhance adhesive forces, too.