Thermophoretic Levitation of Solid Particles at Atmospheric Pressure

TL;DR

This study demonstrates thermophoretic levitation of silica nanoparticle clusters at atmospheric pressure, revealing threshold temperatures for spreading and levitation, and explains the phenomena through thermophoretic forces and particle properties.

Contribution

It introduces the first observation of nanoparticle cluster levitation at atmospheric pressure driven by thermophoresis, with analysis of underlying physical mechanisms.

Findings

Threshold temperature for spreading: 403K

Threshold temperature for levitation: 373K

Levitation explained by thermophoretic force and particle properties

Abstract

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.