Preparing and Depositing Particles on a Surface

Production of Spherical Particles. In carrying out experiments on adhesive forces, it is important to eliminate the effect of particle shape on adhesion. For this purpose it is most convenient to use spherical particles. Usually, quartz and glass 

The particles are heated and melted in the flame of a gas burner or an electric furnace, becoming spherical under the influence of surface tension, and then cooled. 

For melting the particles in the flame of a gas burner, the original dust is passed through a fimnel into the air line and sprayed into the burner flame. In order to increase the flame temperature so as to melt the large particles (more than 50 pi in diameter), a mixture of oxygen and propane is used. However, even under these conditions not all the particles over 120 pi in size are melted. In order to determine the best melting conditions (air, oxygen, and propane flow, burner flame temperature) as well as the yield of the various resultant fractions, the apparatus illustrated schematically in Fig. n.21 was used [92]. 

Particles with diameters between 0.5 and 25 pi may be obtained in an electric furnace larger ones cannot be heated right through. 

In order to separate out a fraction of melted particles with constant particle size, one may use simple meshes, water or air separation (for example, in a column such as that shown in Fig. 11.22), or other devices. The suggestion of certain authors [77] that the adhesive properties of the powder should be used in order to effect fractionization, can hardly be regarded as applicable, owing to the indeterminacy of the adhesive forces for even a single size of particles under the same conditions. 

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