The characterisation of irregularly-shaped particles: a re-consideration
  of finite-sized, porous and fractal grains
  Context. A porous and/or fractal description can generally be applied where
particles have undergone coagulation into aggregates. Aims. To characterise
finite-sized, porous and fractal particles and to understand the possible
limitations of these descriptions. Methods. We use simple structure, lattice
and network considerations to determine the structural properties of irregular
particles. Results. We find that, for finite-sized aggregates, the terms
porosity and fractal dimension may be of limited usefulness and show with some
critical and limiting assumptions, that highly-porous aggregates (porosity >
80%) may not be constructable. We also investigate their effective
cross-sections using a simple cubic model. Conclusions. In place of the terms
porosity and fractal dimension, for finite-sized aggregates, we propose the
readily-determinable quantities of inflation, I (a measure of the solid filling
factor and size), and dimensionality, D (a measure of the shape). These terms
can be applied to characterise any form of particle, be it an irregular,
homogeneous solid or a highly-extended aggregate.
