Inter-particle forces in cohesive powders studied by AFM: effects of relative humidity, particle size and wall adhesion

Abstract

A range of commercially important powders (hydrated alumina, a silica aerogel, limestone, titania and zeolite) was attached to atomic force microscopy (AFM) cantilevers, and force–distance curves were studied for particle–particle and particle–wall contacts as a function of relative humidity (10–90%RH). The single-particle behaviour was compared with observations from bulk cohesion testers. Topographic images of particles were also acquired to compare surface morphology and roughness. The particle–particle pull-off forces with weak cantilevers (spring constant 0.032–0.064 N/m) were reproducible for a given material and rather similar (5–15 nN) for all materials in dry air. Alumina and limestone showed simple force curves with no significant RH dependence. No particle size effects were apparent in alumina (6 and 60 μm), in contrast to the behaviour in cohesion testers. The other materials showed more complex force curve behaviour. Zeolite showed adhesion increasing strongly with RH and evidence for stable liquid bridges, possibly associated with this material's special pore structure. Coated silica aerogel showed long-range charging effects. Most materials showed much larger particle–wall (steel) adhesion than particle–particle adhesion, with the exception of titania. This behaviour may be linked with particle size and wall roughness and is relevant to cohesion testers. The adhesion studies, together with parallel work on friction described elsewhere, form the basis of an ongoing study linking the single-particle and bulk behaviour.