Optical particle characterization in flows

The purpose of granulometry is to measure the statistical distribution of the size and shape of – particles – a generic term used here to designate drops, bubbles, solid particles and aggregates. When applied optically to flows with dispersed phase(s), it must characterize these particles in situ, without disturbing the flow, which may be reactive and distant, and with an adequate temporal response from the point of view of the dynamics of the system studied. Determining the local particle concentration and other standardized statistics requires determining the effective dimensions of the technique's optical measurement volume. This measurement volume is certainly one of the most difficult parameters to obtain with precision, as it is constantly evolving with the characteristics of the particles and the measurement conditions. The existence of size-velocity correlations necessitates the determination of particle dynamics, not only to obtain flows, but also to ensure the reliability of the statistics.

Most optical granulometry techniques, with the exception of certain imaging techniques, require a precise description of the scattering and absorption properties of electromagnetic waves by particles. Indeed, from a simple scattered light intensity, an inter-fringe or an extinction spectrum, it is necessary to be able to deduce, in a few micro or milliseconds, sometimes complex intrinsic or extrinsic properties. This explains why many granulometric techniques only really became operational in the 1990s, with the progress made in light scattering models and regularization methods, and the increased capabilities of personal computers, detection electronics and laser sources.

Optical particle sizing techniques are all limited to a given range of particle sizes, concentrations and morphologies. There are three main reasons for this. Firstly, if characterizing nano to millimetric particles means covering six orders of magnitude dimensionally, this corresponds to approximately twenty-four orders of magnitude in terms of scattered intensity – an impossible-to-measure dynamic. Secondly, the light-scattering modes of particles are highly dependent on their size and morphology. Thirdly, the optical density of the particle flow disturbs the propagation of laser beams, but also the flux scattered by the particles and our ability to interpret signals blurred by multiple scattering.

For all these reasons, the optical granulometry of flows is a complex discipline, with numerous ramifications and special cases. This article focuses on the main techniques used to characterize optically diluted flows where the particles are essentially nano to micrometric, or even millimetric. The first introductory section covers the characteristics of the particles to be analyzed,...