Lycopodium club moss spores are well suited for use as test particles in electrodynamic ion traps. They are a good size, nearly spherical, inexpensive, and they present essentially no health risks (although people with bad alergies may experience the same response they have with mild pollen particles).  The top images show a series of spores photographed while they were trapped in the Single Particle Trap. Each of the 12 images is 57 μm square, and the particles were measured to have diameters of 262.5 μm. These 12 particles were trapped consecutively, and were not otherwise selected.

The additional image on the right shows two spores stuck together while trapped, as is occasionally seen. Note that the built-in microscope in the Single Particle Trap has an intrinsic resolution of about 3 μm.

The lower image is a high-resolution electron micrograph of Lycopodium spores, with a scale bar of 25 μm.  Note the characteristic “sphere with a corner” shape of these particles.  The average material density in the spores was measured to be 51040 kg/m^3, about half the density of water. [The electron microscope image is from "Naturally occurring spore particles at planar fluid interfaces and in emulsions", B.P. Binks, J.H. Clint, G. Mackenzie, C. Simcock and C.P. Whitby, Langmuir, 21, 8161-8167 (2005), with permission from the authors.]

Electrodynamic Ion Traps