Near‐monodisperse 0.50 μm and 1.0 μm silica particles are surface‐modified using 3‐(trimethoxysilyl)propyl methacrylate (MPS) and subsequently coated by aqueous deposition of an ultrathin polypyrrole (PPy) overlayer to produce PPy‐coated silica particles. The targeted degree of MPS modification and PPy mass loading are systematically varied to optimize the colloidal stability and PPy coating uniformity. MPS surface modification is characterized by contact angle goniometry and the PPy overlayer uniformity is assessed by scanning electron microscopy. HF etching of the silica cores produces hollow PPy shells, thus confirming the contiguous nature of the PPy overlayer and the core–shell morphology of the original particles. Four‐point probe measurements and XPS studies indicate that the electrical conductivity of pressed pellets of PPy‐coated silica particles increases with PPy surface coverage. Colloidal stabilities of the bare, MPS‐modified, and PPy‐coated silica particles in aqueous solution are assessed using disk centrifuge photosedimentometry. MPS surface modification results in weak flocculation, with subsequent PPy deposition causing further aggregation. In contrast, white light aerosol spectrometry indicates a relatively high degree of dispersion for PPy‐coated silica particles in the gas phase. Such PPy‐coated silica particles are expected to be useful mimics for silica‐rich micrometeorites and may also serve as a model highly absorbing aerosol.

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The lunar fines contain an appreciable proportion of spherical glass particles. They are formed by local melting and splash due to meteorite impact. The observed particle size of the spheres is discussed in relation to the physical processes controlling their formation. Detailed studies of the structure of some larger spheres have been made. Three aspects are reported and discussed, the secondary cratering on the surface, the porosity of the spheres and their chemical homogeneity.

A simple method of testing fibrous filters to obtain the grade efficiency curve in the particle size range 2-22 mum is described. Dust collected by the method has been used to measure the retention efficency of model filters and it is shown that there is a maximun in the grade efficency curve. The influence of relative humidity on filter behaviour has been investigated. It is shown that the efficiency of filtration in high particle inertia systems increases with increasing relative humidity. It is considered that the reason for this is an increase in particle-fiber adhesion force.