The Knudsen number of the particle define three different dynamical regimes that govern the behaviour of an aerosol:
where  is the mean free path of the suspending gas and  is the diameter of the particle. For particles in the free molecular regime, K >> 1; particles small compared to the mean free path of the suspending gas. In this regime, particles interact with the suspending gas through a series of "ballistic" collisions with gas molecules. As such, they behave similarly to gas molecules, tending to follow streamlines and diffusing rapidly through Brownian motion. The mass flux equation in the free molecular regime is:
where a is the particle radius, P and P are the pressures far from the droplet and at the surface of the droplet respectively, k is the Boltzmann constant, T is the temperature, C is mean thermal velocity and α is mass accommodation coefficient. The derivation of this equation assumes constant pressure and constant diffusion coefficient.
Particles are in the continuum regime when K << 1. In this regime, the particles are big compared to the mean free path of the suspending gas, meaning that the suspending gas acts as a continuous fluid flowing round the particle. The molecular flux in this regime is:
where a is the radius of the particle A, M is the molecular mass of the particle A, D is the diffusion coefficient between particles A and B, R is the ideal gas constant, T is the temperature (in absolute units like kelvin), and P and P are the pressures at infinite and at the surface respectively.
The transition regime contains all the particles in between the free molecular and continuum regimes or K ≈ 1. The forces experienced by a particle are a complex combination of interactions with individual gas molecules and macroscopic interactions. The semi-empirical equation describing mass flux is:
where I is the mass flux in the continuum regime. This formula is called the Fuchs-Sutugin interpolation formula. These equations do not take into account the heat release effect.