Forces generated in solidifying liquid bridges between two small particles

The formation of strong, solidified binder bridges is essential in powder granulation where small particles are held together and large permanent granules are formed during the process. Solid bridges can also be detrimental by binding particles into lumps during moisture-induced caking of an otherwise free flowing powder. Two kinds of solidifying bridges, one due to drying of a concentrated polymer solution and the other due to crystallization of a saturated salt solution, were studied experimentally in this work and some rather intriguing results are presented here. It was found that large attractive forces, of the order of hundreds of times the weight of the polymeric bridge, develop in the solid neck during formation. Successive ruptures of polymer strands within the bridge occur during drying of the binder, thereby partially releasing some internal stress. The salt bridge, however, tends to push the particles apart with an ever increasing force that is not released in the dry bridge. The implication of these findings on granule formation during binder granulation and during powder caking are briefly discussed.