Fairbrother, Robert James; (1999) A microscopic investigation of particle-particle interactions in the presence of liquid binders in relation to the mechanisms of "wet" agglomeration processes. Doctoral thesis (Ph.D.), University College London.

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Abstract

Agglomeration of powders by the use of liquid binders is a size enlargement process, important in a range of process industries particularly pharmaceuticals and foods. Traditionally the industries most reliant on such agglomeration have taken system specific approaches to the control and development of their equipment and thus little work has been carried out to study the fundamental microscopic mechanisms. Thus this project has investigated the microlevel phenomena in relation to particle and binder properties with the overall aim of improving understanding and control of macroscopic agglomeration processes. A novel piece of experimental apparatus has been developed capable of measuring the force of adhesion between particles attached by liquid bridges in both the gas and the liquid phase. The apparatus consists of an optical microscope fitted with an adapted stage, on which two micromanipulators are employed to hold micropipettes, to the ends of which particles are attached. A third micropipette is then used to add bridging liquid between the particles. The force is calculated by measuring how much a pre-calibrated pipette bends as the particles are separated (under piezo-electric control), allowing forces down to 10 -9 N to be resolved. The rupture sequence is captured on videoand analysed by image analysis software. Results have been obtained using spherical glass ballotini particles and a range of liquid binders of various contact angles and viscosities. The behaviour of liquid bridges through their formation, separation and rupture have been observed. The results presented focus on; the relationship between the bridge volume and separation distance between the particles, the geometry liquid bridge menisci adopt depending on the contact angle they make with the solid surface and the experimental measurement of the variation of liquid bridge forces with separation distance from particle contact through to liquid bridge rupture. The total energy to rupture the bridges was then calculated and compared to the bridge volume. A dependence on the square root of the bridge volume was noted and developments were made to a previously derived model. The relationship between the critical separation distance at which liquid bridges rupture compared to the cube root of their volume was also considered and shown to be a reasonable approximation. The significance of the improvements in the understanding of microscopic interactions to agglomeration arising from this work are considered along with suggestions for future studies.

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