Dispersion of particles on liquid surfaces
Department of Mechanical and Industrial Engineering
Doctor of Philosophy
Fischer, Ian Sanford
Rosato, Anthony D.
Rao, I. Joga
Blackmore, Denis L.
When small particles (e.g., flour, pollen, etc.) come in contact with a liquid surface, they immediately disperse. The dispersion can occur so quickly that it appears explosive, especially for small particles on the surface of mobile liquids like water. This explosive dispersion is the consequence of capillary force pulling particles into the interface causing them to accelerate to a relatively large velocity. The maximum velocity increases with decreasing particle size; for nanometer-sized particles (e.g., viruses and proteins), the velocity on an air-water interface can be as large as 47 m/s. They also oscillate at a relatively high frequency about their floating equilibrium before coming to stop under viscous drag. The observed dispersion is a result of strong repulsive hydrodynamic forces that arise because of these oscillations. Experiments were conducted to validate the Direct Numerical Simulation results which were available already.
This dispersion of particles was also witnessed on the liquid-liquid interface. The dispersion on a liquid-liquid interface was relatively weaker than on an air-liquid interface, and occurred over a longer period of time. This was a consequence of the fact that particles became separated while sedimenting through the upper liquid and reached the interface over a time interval that lasted for several seconds. The rate of dispersion depended on the size of particles, the particle and liquids densities, the viscosities of the liquids involved, and the contact angle. The frequency of oscillation of particles about their floating equilibrium increased with decreasing particle size on both air-water and liquid-liquid interfaces, and the time taken to reach equilibrium decreased with decreasing particle size. These results are in agreement with the analysis.
Although it is known that a clump of powder floating on a liquid surface breaks up to form a particle monolayer on the surface, the mechanism that causes this break up remains abstruse. It is shown that a floating clump breaks up because when particles on its outer periphery of a floating clump come into contact with the liquid surface they are pulled into the interface by the vertical component of capillary force overcoming the cohesive forces which keep them attached and move away from the clump. The latter is a consequence of the fact that when a particle is adsorbed on to a liquid surface it causes a flow away from itself on the interface. This flow causes the newly-adsorbed particles to move away from the clump, and thus the clump size decreases with time and this exposes a new layer of particles that are then adsorbed onto the liquid surface. Interestingly, when many particles are asymmetrically broken apart from a clump’s periphery the clump itself is pushed away in the opposite direction by the newly adsorbed particles.
njit-etd2011-094 (119 pages ~ 2,414 KB pdf)
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Created March 9, 2012