Stress development due to capillary condensation in powder compacts: A two-dimensional model study

A model experiment is presented to investigate the relationship between the humidity-dependent liquid distribution and the macroscopic stress in a par tially wet powder compact. Therefore, films of monosized spherical particle s were cast on silicon substrates. Using environmental SEM the geometry of the liquid necks trapped between particles was imaged as a function of rela tive humidity. Simultaneously the macroscopic stress in the substrate adher ed particle film was measured by capacitive deflection measurement. The exp erimentally found humidity dependence of the liquid neck size and the macro scopic film stress are compared with model predictions. The circle-circle a pproximation is used to predict the size of the liquid necks between touchi ng particles as a function of the capillary pressure. Using the modified Ke lvin relation between capillary pressure and relative humidity, we consider the effect of an additional solute which may be present in the capillary l iquid. The results of the stress measurement are compared with the model pr edictions for a film of touching particles in hexagonal symmetry. The contr ibution of the capillary interaction to the adhesion force between neighbor ing particles is calculated using the integrated Laplace equation. The resu lting film stress can be approximated relating this capillary force to an e ffective cross section per particle. The experimentally found humidity depe ndence of the liquid neck size is in good agreement with the model predicti ons for finite solute concentration. The film stress corresponds to the mod el predictions only for large relative humidities and shows an unexpected i ncrease at small values. As is shown with an atomic force microscope, the r eal structure of the particle-particle contact area changes during the wet/ dry cycle. A solution/reprecipitation process causes surface heterogeneitie s and solid bridging between the particles. It is claimed that the existenc e of a finite contact zone between the particles gives rise to the unexpect ed increase of the stress at small relative humidities.