  Separation and transportation of small particles are important processes in
various applications such as the food and pharmaceutical industries. Although
mechanical, chemical or electrical methods can provide possible solutions,
operational or environmental constraints may require alternative methods.
Spreading and levitation of clusters (aggregates) of fumed silica nanoparticles
placed under atmospheric pressure on a hot plate is reported. In a closed
chamber, the particles started to spread horizontally at the threshold
temperature of $T_c^*=403 \pm 1$K. The powder spreading in the chamber
continued until the temperature-dependent saturation value $r_{sat} (T)$, which
grew linearly with the temperature. Open space experiments clearly demonstrated
levitation of the powder clouds. The onset of levitation in the open space
corresponded to the minimal threshold temperature of $T_o^*=373 \pm 1$K.
Qualitative physical analysis of the observed phenomena is suggested. The
effect of levitation is explained by the lifting thermo-phoretic force emerging
in the Knudsen layer of air on the heater surface. The levitation of the powder
under atmospheric pressure becomes possible due to the combination of low
adhesion of the fluorinated fumed silica clusters built of nanoparticles to the
substrate, relatively low density of the particles and clusters, and their high
specific surface area. Ordering of the aggregates of nanoparticles within the
levitating powder cloud was quantified with Voronoi diagrams.
