The surfaces of colloidal particles resulting from many new fabrication methods are not molecularly smooth, so understanding how surface roughness can affect the depletion attraction between the particles and their assembly is very important. We show that the depletion attraction between custom-shaped microscale platelets can be suppressed when the nanoscale surface asperity heights become larger than the depletion agent. In the opposite limit, the attraction reappears and columnar stacks of platelets are formed. Exploiting this, we selectively increase the site-specific roughness on only one side of the platelets to direct the mass production of a single desired assembly: a pure dimer phase.

An analysis of the effects of diffusion in differential mobility analyzers (DMAs) at high Peclet number (Pe) shows that the associated peak broadening may be greatly reduced when the axial separation between entrance and exit slits is comparable to the inter-electrode gap. This prediction is verified in a shortened version of Reischl's DMA using molecular ions from an electrospray source. Gaussian peaks with relative full width at half maximum as small as 0.066 are observed at a DMA Reynolds number of 1190 in the mobility spectra of (butyl)4N+ ions in air (Pe = 104). After correcting for the broadening effect of the aerosol flow rate and the sampling slit width, the predicted diffusive peak widths agree well with those observed, but only at the highest resolutions. The DMA described here is the first one able to measure with excellent resolution the mobilities of particles down to 1 nm in diameter, and even of molecular ions.

Tiny 'living' particles may just be lumps of limestone. They look like tiny bacteria, have been implicated in several diseases and have even been hailed as a completely overlooked branch of the tree of life. But are 'nanobacteria' genuinely alive? New research suggests that the answer is probably no. Ever since they were first described in the early 1980s, nanobacteria — which can be just 50 nanometres, or millionths of a millimetre, across — have captured the imagination of everyone from health experts to space biologists. A panel convened in 1998 by the US National Academy of Sciences concluded that the particles are too small to be alive, but that didn't stop people from being fascinated by them, and some companies even say that they can detect nanobacterial infections. But now the nanobacteria theory has taken a blow. New research suggests that, besides being too small to be alive, they may also be made of something not much more complex than simple chalk.