## 2 Pulp Suspensions Imaging Overview
[PERSON] and his co-workers [34; 35]characterised velocity fluctuations and studied paper formation by fluidisation and refoloculation of wood pulp suspensions with concentrations up to 1% (w/w) using UVP. [PERSON] et al. [31; 36] obtained instantaneous velocity profiles in a steady laminar flow of 0.5-3% (w/w) and later up to 7.8% (w/w) cellulose pulp suspensions using UVP. [PERSON] and her co-workers [37] used UVP to characterise pulp fibre suspensions through a sudden expansion at two different concentrations, 1.8% and 2.8% (w/w), and at diverse velocities, from 1 up to 2.2 m/s: They were able to instantaneously obtain velocity profiles as well as axial real mean square velocity fluctuations in the evaluated semi-concentrated flows just after the expansion. To enable improved flow information for higher concentrations, [PERSON] et al. [38] tested a pulsed velocimetry-based system to measure velocity profiles of cellulose flows for two different concentrations, 6.9% and 8% (w/w), in three different pipe diameters (16, 22.5 and 52.8 mm). Also using a pulsed approach, [PERSON] and colleagues [39] obtained velocity profiles of fibre flows in a rectangular channel, and investigated the influence of fibre concentration and Reynolds number on the shape of the measured profiles; they used natural wood fibres (with an average length and diameter of 2.3 and 35 \(\upmu\)m, respectively), and the tests were conducted with varying average flow velocities of between 0.16 and 7.0 m/s, and different fibres concentrations between 0.05% and 1.0% (w/w). Optical Coherence Tomography (OCT) is a recent technique that allows the internal structure and the motion behaviour of opaque suspensions to be measured simultaneously and with high spatial resolution. Depending on the technology used in the OCT construction, the axial scan rates can go up to hundreds of kHz [7]. OCT operating modus is quite similar to ultrasound imaging, with the main difference being its operating frequency; while OCT operates in the light frequency band, ultrasound uses the sound band. The depth of the obtained images depends to a large extent on the optical properties of the domain; consequently, its distance from the wall can vary between micrometres and some millimetres [40]. This technique was used by [PERSON] et al. [41] to characterise the boundary layer of micro-fibrilated cellulose flowing in a straight pipe, for consistencies in the range 0.4-1.6% (w/w). However, in the case of ultrasound systems, which usually operate in the MHz frequency band, the waves are readily transmitted to the domain, making it easy to obtain images of significant depth: nevertheless, at those frequencies they can also suffer from high attenuation, limiting then the used frequencies. Furthermore, it is much more difficult to focus sound waves than light waves [42]. Particle Image Velocimetry (PIV) is a laser measuring technique, as is LDA, which measures the velocity by means of tracking tracers in the suspension. However, PIV measures the velocity by cross-correlation algorithms, allowing the velocity 3D-components or the velocity gradient tensor in the measuring plane to be obtained [43]. PIV has the advantage of not being a single point measurement technique, and consequently is attracting increasing interest in the evaluation of complex and turbulent flows. The main disadvantage of this technique is the limitation it presents when more opaque domains are the object of visualisation. [PERSON] and his co-researchers used PIV to estimate sets of 2D pulp flow velocity vectors [8]. With recent improvements in digital imaging, PIV also makes it possible to obtain information about the orientation of the fibres, as in [PERSON] et al. [44]. In the experiments, they used synthetic fibres with diverse aspect ratios (18.8, 25.0 and 37.5) in two different concentrations, 0.1% and 0.2%. They obtained important information about the influence of the impeller speed, fibre aspect ratio and concentration on the field velocity and orientation.