Micron

2009, Jonathan Amos, (bbc.co.uk) Satellites weigh California water As one spacecraft lurches and drags through the Earth's uneven gravity field, the second follows 210 km behind, measuring changes in their separation to the nearest micron (a thousandth of a millimetre).

The microcannon was seen moving at about a micron per second, it was about 15 microns long, and the high speed photograph was done in water (density of 1 kg/L, viscosity of about 0.001 pascal seconds).

Little pozzolanic reaction is evident for glass above 300 microns but the pozzolanic activity increases as the particle size is reduced below this size with good pozzolanic properties generally being achieved below about 45 microns (Federico & Chidiac 2009). […] In a second mix 20% of the Portland cement was replaced with the same Vycor glass which was ground to pass 45-micron (median particle size ~ 10 micron).