particle size distribution

A particle size distribution describes not one size but the whole spread of sizes in a powder. Imagine sorting a jar of mixed gravel into piles by size and counting how much falls in each pile: some powders are tightly grouped around one size, others span a wide range. The distribution is that full picture rather than a single number.

It is usually shown as a histogram or a smooth curve. A frequency distribution shows how much of the powder lies in each size band; a cumulative distribution shows the percentage finer than each size. From the cumulative curve we read off values like the d10, d50, and d90, the sizes below which 10, 50, and 90 percent of the sample falls.

Distributions can be weighted by number or by mass (volume). A number distribution counts every particle equally, so it emphasises the many tiny ones; a volume distribution weights by how much material each represents, so it emphasises the few large ones. The same powder can look quite different depending on which weighting an instrument uses.

The width of the distribution matters as much as its centre. A narrow distribution gives predictable flow and uniform dissolution, while a broad one risks segregation, where coarse and fine fractions separate during handling and spoil dose uniformity.