The Influence of Relative Humidity on Particle Adhesion – a Review of Previous Work and the Anomalous Behaviour of Soda-lime Glass

Abstract

The adhesion of fine particles plays a significant role in the performance of particulate processes and in the quality of particulate products. The extent of adsorption of water on the particles from the surrounding atmosphere is governed directly by the relative humidity of the air. Published evidence suggests that changes in relative humidity of the air can have a profound influence on the adhesion of individual particles. However, there are numerous conflicting reports in the literature suggesting that adhesion can increase, decrease or pass through a maximum as the relative humidity increases.A thorough review of the relevant literature is presented in which the experimental evidence relating to the influence of relative humidity on particle adhesion is gathered and discussed. Apart from the amount of water adsorbed on the surface, it is clear that the adhesion depends upon the surface roughness which prevents the formation of a complete capillary meniscus at the particle contact point until a critical relative humidity is reached. At this point, sufficient water is adsorbed to engulf the asperities and a marked increase in adhesion is noted. Many surfaces undergo some physical or chemical change in the presence of adsorbed water, e.g. solubility, softening, or phase change. This can lead to complex adhesion behaviour. Hysteresis of adhesion with increase/decrease of relative humidity is commonly observed. Theoretical approaches correctly recognise the role of the Laplace pressure in the capillary bridge as being dominant in controlling adhesion. However, evidence suggests that contributions from the solid-solid interaction, surface tension of the bridging film and dis-joining pressure can also be important under certain circumstances.As an illustration of complex behaviour, original data for the adhesion of a glass microsphere on a flat glass surface are presented. The data were obtained using a custom-built AFM instrument. Upon desorption, a critical relative humidity lying between 30% and 40% was observed. At this point, a singular peak in adhesion occurred which was accompanied by long-range repulsion between the surfaces prior to contact. The long-range repulsion was found to be an exponential function of separation distance. This phenomenon has been attributed to the spontaneous formation of glass corrosion products from the liquid layer on desorption when the two surfaces approach each other. According to published literature, these corrosion products can take the form of needle-like or dendritic structures. The resulting steric repulsion is observed to be exponential with respect to distance, in agreement with the established trend. Once contact is made between the surfaces, the strong adhesion may be due to either sintering or entanglement of the corrosion products. Further work is required to determine the precise chemistry of the process. At high humidities, the force-separation curves exhibit an extended pull-off region, suggesting that the adsorbed film is mobile. This effectively reduces the adhesion by increasing the volume of the liquid bridge as the surfaces are pulled apart. This may explain why some reports in the literature show decreases in adhesion at high humidity.

Keywords

• humidity
• particle adhesion
• inter-particle force
• afm
• soda-lime glass
• corrosion products