The drainage of a thin liquid film between a colliding bubble and a solid in a colloidal suspension

Daria Shubik, Chemical Engineering, Technion, Haifa, Israel
Sarit Roizman, Chemical Engineering, Technion, Haifa, Israel
Amihai Horesh, Chemical Engineering, Technion, Haifa, Israel
Ofer Manor, Chemical Engineering, Technion, Haifa, Israel

We investigate the drainage of a thin liquid film between an air bubble and a solid substrate in a suspension of micro-particles as a simple model system for realising the physics of floatation procedures, which contain highly distributed particulate systems, and of bi-phase flow in porous media in connection to bi-phase flow in porous catalytic reactors.

In the experiment, we generate an air bubble in a suspension of micro-particle near a solid substrate. A micrometer thick film of suspension appears between the bubble and the solid substrate at close proximity. Initially, viscous stresses deform the surface of the bubble inward (film thickening), as the bubble approaches the substrate. Then, the excess Laplace pressure in the bubble stresses the liquid film, supporting the drainage of the suspension out of the film (film thinning). The film thinning eventually results in the dewetting of the solid and the attachment of the bubble to the solid or the formation of a nanometer thick black film, which is stabilised by surface (molecular) forces. 

The liquid film contains micro-particles, which many times are arranged as an immobilised monolayer of particles, sandwiched between the solid substrate and the bubble. Hence, the drainage of the liquid between the particles translates to liquid flow through a two dimensional porous medium due to pressure gradient along the radial direction of the liquid film. The draining liquid is further the subject of local capillary stresses due to the varying thickness of the liquid between the immobilised particles. 

By changing the concentration of the suspension we were able to alter the porosity of the structure consisting the immobilised monolayer of particles in the film. Moreover, by capturing light diffraction patterns using monochromatic light microscopy we imaged the 3D thickness dynamics of the intermediate liquid film between the bubble and the substrate from the point of bubble approach to the solid substrate and until we observed the dewetting of the solid substrate or the steady formation of a black film. 

Our observations show that an increase in the concentration of the suspension results in an appreciably faster drainage of the liquid film. Thus, we observe that the presence of micro-particles in the micron thick film between the bubble and the solid increase the rate by which the bubble attach to the solid. Moreover, the local position of dewetting of the solid substrate under the film appears to be dependent on the random arrangement of the particles in the liquid film, unlike the organised attachment drainage process in the absence of particles in the liquid.