Measurement of attractive forces between single aerogel powder particles and the correlation with powder flow

An atomic force microscope (AFM), which allows the measurement of forces as low as one tenth of a nano-Newton, was used to determine the attractive forces between single silica aerogel powder particles (100 μm diameter) under two different atmospheric conditions. Forces on the order of 10 nN were found to depend mainly on the contact geometry and thus on the surface structure of the powder particles. In a second series of experiments, the surface structure of the silica aerogel powder particles was characterized using the AFM. Smooth surface areas, several micrometers in size, are separated by cliffs and cracks. For comparison, the same measurements were made with precipitated silica powder. Here larger forces and a different surface structure were found. The structure of precipitated silica particles is spheroidal with bumps on it, which are some hundred nanometers in size. In addition to the AFM measurements, powder flow rate from a hopper was measured. The flow of powders depends on the surface structure but also on the atmospheric conditions (e.g. moist atmosphere or vacuum). In contrast to precipitated silica powder, silica aerogel powder showed a significantly higher flow rate under vacuum than in moist air.