Abstract
Forces acting between individual grains in a powder can have a critical and controlling effect on powder bulk behaviour. Operations such as powder flow, fluidisation, compaction, agglomeration and mixing are all influenced significantly by the intensity of interparticle forces. This is especially true when the particle size falls below around 100 mum at which point the surface forces outweigh the force due to gravity acting on a single particle. Studies of cohesion using bulk powder samples are of limited use because it is difficult to decouple the fundamental mechanisms of interparticle force from other contributions to cohesion such as variations in the powder microstructure, or geometric interlocking of individual particles. A review of the relevant literature has unearthed conflicting evidence associated with the influence of relative humidity (RH) on both bulk powder cohesion and interparticle force. Therefore there is a need for experimental force studies at the scale of the individual particle to identify the fundamental mechanisms that prevail and resolve some of the apparent uncertainty that currently exists. A custom built force instrument, incorporating Atomic Force Microscope (AFM) technology, was designed, constructed and commissioned. This instrument was used to quantify the interactions between particles of around 40 mum in diameter and flat surfaces as a function of the relative humidity of the surrounding air. Interactions between soda-lime glass surfaces, gold surfaces and amorphous quartz surfaces were studied. Striking results were obtained on soda-lime glass surfaces upon decreasing the RH from > 70% to around 40%. At this point the glass surfaces suddenly exhibited a strong repulsion upon approach. The range of this repulsion was observed at separation distances as great as 250 nm. Once the surfaces were brought into contact the strong repulsion was accompanied by a very large force of adhesion. This strong repulsion and associated peak value of adhesion was not observed at other RH values and was specific to desorption rather than adsorption. Force curves for gold and quartz surfaces showed no such repulsion and peak adhesion. It is thought that the critical humidity coincides with the formation of a complete monolayer of adsorbed water molecules. A number of possible explanations have been offered for the effect and its uniqueness to soda-lime glass in the present experiments. Theoretical calculations of adhesion force have been performed based on the concept of capillary meniscus formation. Calculations give values of around 17000 nN for a sphere 40 mum in diameter and a contact angle of 20°. These values are somewhat larger than measured values in all cases apart from peak adhesion. It is thought that at low humidities there is insufficient water adsorbed to overcome the effect of surface roughness. Contact occurs at asperities, which reduces the expected contact area and hence leads to an adhesive force that is lower than predicted. At humidities > 80% the experiments show evidence of capillary elongation upon surface separation. This implies that the surface adsorbed film is mobile with bulk liquid being drawn into the bridge under the action of the surface tension force. The associated increase in bridge volume and the change in bridge curvature with I elongation will tend to equalise the Laplace pressure inside the bridge and therefore give a value of adhesion that is lower than predicted.