How do shorter hardwood fibres improve paper smoothness and opacity?

In the context of papermaking, hardwood fibres are shorter than softwood fibres, typically ranging from 0.5 to 1.5 millimetres in length. Because of their smaller size, these fibres are highly effective at filling in the gaps within the randomly felted layer of fibres that forms a sheet of paper.12

By filling these voids, the shorter hardwood fibres create a more uniform and compact structure, which directly results in a smoother surface and increased opacity. While longer softwood fibres are primarily utilized to contribute strength to the paper, the inclusion of hardwood fibres is essential for achieving the desired surface quality and visual density required for various printing and writing applications.1