# Custom-made spherical fluorescent tracer particles for laser Velocimetry Laboratory Experiments

Okba Mostefaoui, Diego Lopez, Emmanuel Mignot, Valérie

Massardier-Nageotte

► To cite this version:

Okba Mostefaoui, Diego Lopez, Emmanuel Mignot, Valérie Massardier-Nageotte. Custom-made spherical fluorescent tracer particles for laser Velocimetry Laboratory Experiments. Flow Measurement and Instrumentation, 2024, 99, pp.102665. 10.1016/j.flowmeasinst.2024.102665. hal-04669623

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Submitted on 15 Jan 2026

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# Custom-made spherical fluorescent tracer particles for laser Velocimetry Laboratory Experiments

Okba Mostefaoui<sup>a,b,*</sup>, Diego Lopez<sup>a</sup>, Emmanuel Mignot<sup>a</sup>, Valérie Massardier-Nageotte<sup>b</sup>

## ARTICLE INFO

**Keywords:**

Home-made particles

Spheres

Protocol

Fluorescent

PIV

PTV

## ABSTRACT

This article introduces a method for creating custom-made spherical fluorescent particles tailored for fluid mechanics optical velocimetry laboratory measurements techniques such as Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) measurement techniques. The incorporation of the fluorescent dye Rhodamine B into the particle bulk significantly reduces direct light reflections and enhances particle detection when enlightened by a laser and using a band-pass filter on the optical measurement device. Furthermore, the study offers a clear guide for adjusting particle diameter by modifying various parameters such as the stirrer used, its stirring speed, the surfactant concentration, and the type of polymer. This allows for the customization of particles best suited for water multiphase flow characterization and transport studies. The resulting particles exhibit excellent sphericity, making them suitable for a wide range of fluid mechanics investigations, from small to large scale. Experimental validations, including size measurements, density assessments, emission spectra analyses, and applicability in a confined flow affirm the effectiveness of the proposed methodology by exhibiting a signal-to-noise ratio four to five times higher than fluorescent surface-stained and non-fluorescent particles.

## 1. Introduction

Fluid mechanic's experiments, from environmental dynamics to industrial processes and passing by microfluidics are essential for understanding a wide range of phenomena. Among the various experimental techniques available for measuring a velocity field, optical approaches such as Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) have emerged as indispensable tools even in highly complex conditions, in particular thanks to their non intrusive nature [1,2]. These techniques require to seed the flow with tracers, i.e. non-inertial and almost neutrally buoyant particles that move with the surrounding fluid, and require a proper flow illumination to capture the tracers' motion by video recording and eventually compute accurate flow velocity fields. However, these techniques do come with certain drawbacks, especially in the context of experiments that require intense light sources and/or in the case of multi-phase experiments. Indeed, conventional light sources for velocimetry techniques, like lasers, often cause intense reflections while illuminating seeded particles, in particular in the presence of free surfaces, bubbles, side walls, obstacles, or unwanted suspended dust, to cite a few. These direct reflections

can significantly alter the images and thus reduce the quality of the measurement. To mitigate the impact of reflected lights and eliminate interference from non-desirable suspended particles, one common solution is to use fluorescent particles. Fluorescent dyes, such as Rhodamine B or Nile Red, with different absorption and emission wavelengths, allow for effective blocking of undesired incident light sources using appropriate bandpass filters.

Commercial fluorescent particles can be quite expensive, especially considering the amount of particles needed for specific applications. To make the use of fluorescent particles more affordable, various manufacturing methods for elaborating fluorescent objects have been developed and published. A first one is to stain a fluorescent dye on the surface of commercial polymer based objects. This was applied on spheres, fragments and fibers made of various polymers with Nile red [3,4], and Rhodamine B [5]. The advantage of staining protocols is their versatility. However, stained particles seem to have a weak light emission intensity even with high power lasers, as can be seen on the emission response in Fig. 1 of Nel et al. [4]. As a result, staining methods are not suitable for large-scale experiments, where large fields

<footnote><sup>a</sup> INSA Lyon, CNRS, Ecole Centrale de Lyon, Université Claude Bernard Lyon 1, LMFA, UMR5509, 69621, Villeurbanne, France</footnote>

<footnote><sup>b</sup> Université Claude Bernard Lyon 1, INSA Lyon, Université Jean Monnet, CNRS UMR 5223, Ingénierie des Matériaux Polymères, F-69621, Villeurbanne cedex, France</footnote>

<footnote>* Corresponding author at: Université Claude Bernard Lyon 1, INSA Lyon, Université Jean Monnet, CNRS UMR 5223, Ingénierie des Matériaux Polymères, F-69621, Villeurbanne cedex, France.</footnote>

<footnote>E-mail address: okba.mostefaoui@insa-lyon.fr (O. Mostefaoui).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Schematic representation of the particle production protocol where the blue solution (S1) is water with dissolved surfactants and the pink solution (S2) the polymer dissolved in chloroform with Rhodamine B.</figcaption></figure>

<table><caption>**Table 1**
Experimental parameters and elaborated particles characteristics associated.</caption><thead><tr><th colspan="4">Experimental parameters</th><th colspan="3">Particles characteristics</th></tr><tr><th>Stirrer</th><th>Polymer</th><th>ω (rpm)</th><th>C<sub>i</sub> (g/L)</th><th>d̅ (μm)</th><th>d<sub>0.5</sub> (μm)</th><th>σ<sub>d</sub> (μm)</th></tr></thead><tbody><tr><td>H</td><td>PS/ABS</td><td>150</td><td>25</td><td>196.1</td><td>146.3</td><td>137.7</td></tr><tr><td>H</td><td>PS/ABS</td><td>300</td><td>25</td><td>149.7</td><td>121.6</td><td>86.0</td></tr><tr><td>H</td><td>PS/ABS</td><td>300</td><td>12.5</td><td>208.1</td><td>195.6</td><td>134.3</td></tr><tr><td>H</td><td>PMMA</td><td>300</td><td>12.5</td><td>170.7</td><td>154.2</td><td>79.0</td></tr><tr><td>H</td><td>PS/ABS</td><td>450</td><td>25</td><td>118.2</td><td>86.3</td><td>77.4</td></tr><tr><td>H</td><td>PS/ABS</td><td>600</td><td>25</td><td>75.2</td><td>60.1</td><td>43.15</td></tr><tr><td>H</td><td>PS/ABS</td><td>750</td><td>25</td><td>62.4</td><td>46.7</td><td>35.8</td></tr><tr><td>R</td><td>PMMA</td><td>3000</td><td>25</td><td>9.9</td><td>7.66</td><td>6.13</td></tr></tbody></table>

H : Heidolph stirrer & R : Rayneri stirrer.

of view are required, with intense light emission by the particles for velocimetry measurements. Another method is to create particles with a fluorescent dye embedded, increasing therefore the emission response in fluorescence. This method was used by Pedocchi et al. [6], who added Rhodamine WT in a liquid polyester resin, which was ground and sieved after solidification. However the particles obtained had non-regular shapes, which in the case of large-scale experiments can induce undesired effects. To the best of our knowledge, no simple protocol permits to produce spherical polymer based particles of a controlled diameter with fluorescent dye embedded into the bulk.

The present paper aims to introduce a novel and reproducible protocol for manufacturing custom fluorescent spherical particles, embedding a fluorophore (Rhodamine B) within each particle. This method differs from the existing surface staining techniques, and permits to increase the fluorescent light emission by the particles. Moreover, we aim to provide an affordable method for adjusting particle diameter by appropriately selecting four parameters: the type of stirrer and its stirring speed, the type of polymer, and the surfactant concentration. The protocol we propose is an iterative process and consists of three steps : an elaboration step, followed by a characterization step and concluded by an adjustment of the first step. These steps are to be repeated until the required tracer particles are obtained.

## 2. Elaboration protocol

The particle production protocol is inspired by an emulsion protocol to elaborate nanoparticles of polyethylene [7], and by a methodology to create phosphorescent paraffin particles of various sizes involving the dispersion of melted paraffin wax in hot water [8]. The protocol involves the dispersion of two non-miscible liquid solutions: a water-based solution (named S1) and a dissolved polymer-based solution where the fluorescent dye is added (named S2). Mixing both solutions

ensures the production of droplets of S2, which are then solidified with an appropriate procedure. After solidification, particles are recovered by filtration. An overview of this protocol is sketched in Fig. 1.

In this work, the aqueous solution (S1) is composed of distilled water and surfactants, that are added to reduce the droplets sizes and stabilize them during solidification. The selected surfactants are ranging from 1 to 2 g of sodium dodecyl sulfate (SDS, 436143-Sigma-Aldrich) and ranging from 1.5 to 3 g of polyvinyl alcohol (PVA, Kuraray Poval 40–80), which are dissolved in 200 mL of distilled water, resulting in a total initial surfactant concentration of 12.5 to 25 g/L. A fixed proportion of 60% of SDS and 40% of PVA is maintained. The mixture is placed in a glass beaker on a heating mantle with a magnetic stirrer for three hours at 50 °C until all surfactants are completely dissolved. The polymer-based solution (S2) comprises either polymethyl methacrylate (PMMA, ρ = 1.15 g/cm³) or polystyrene/acrylonitrile-butadiene-styrene (PS/ABS, ρ = 1.05 g/cm³) copolymer, both being conveniently soluble in chloroform at room temperature. Solution S2 is composed of 1 g of polymer, and 10 mg of Rhodamine B dissolved in 10 g of chloroform mixed with a magnetic stirrer for three hours until fully homogenized.

To generate two distinct ranges of particle diameters, two different stirrers are used: a high-speed overhead stirrer (Rayneri Turbotest) which generates small microscopic particles, and a low-speed stirrer (Heidolph Hei-TORQUE Expert 400) which generates larger microscopic particles. Photos of both stirrers with zooms of the corresponding mixing blades are shown in Fig. 2.

For both setups, the protocol is the same. To create droplets of S2, 100 mL of S1 are placed into a semi-spherical glass container where 5 mL of S2 are added drop by drop (Fig. 1(a)) and mixed at 3000 rpm for the high-speed stirrer or at 150 to 750 rpm for the low-speed one. During the mixing process, the chloroform from S2 gradually dissolves in the aqueous solution and evaporates through the free surface, so that the mixing and solidification step takes place simultaneously. The mixing process lasts for 2 to 3 h until the chloroform is completely evaporated and the particles are solidified (Fig. 1(b)). The mixture is then filtered using a paper filter and washed with deionized water in a Büchner funnel connected to a vacuum pump (Fig. 1(c)). The filtered particles are finally left to dry overnight under a hood. Once dried, the particles are easily collected with an average recovery around 90% of the initial mass of the polymer.

## 3. Particle characterization

This section focuses on the characterization of two sets of particles. The first set comprises PMMA particles synthesized with the Rayneri stirrer at 3000 rpm and a surfactant concentration of 25 g/L. The second set consists of PS/ABS particles synthesized with the Heidolph
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Laboratory stirrer and blades used to produce respectively (a) small microscopic fluorescent particles (Rayneri Turbotest) and (b) large microscopic fluorescent particles (Heidolph Hei-TORQUE Expert 400).</figcaption></figure>

stirrer at 300 rpm and the same surfactant concentration (Table 1). Mi- croscopic photographs of both sets of particles are depicted in Fig. 3(a) and (b), respectively.

Several analyses are then conducted on both sets of particles to assess their suitability as large fluorescent buoyant objects — for large-scale experiments for particles produced with the low-speed stirrer and small tracers to be used in small-scale experiments for the particle produced with the high-speed stirrer. First, their diameter distribution and average value are measured using a Microscope Olympus BX51, with image analysis estimation performed on over a thousand randomly sampled particles from both sets of particles. Each measurement is replicated three times to assess its reproducibility. The diameter dis- tribution probability density function is illustrated in Fig. 4. Using the Rayneri stirrer, a monodispersed distribution of fluorescent particles is obtained, with a peak diameter of 6–7 µm and a mean diameter of 9.9 µm, with most particles ranging between 0 and 12 µm in diame- ter. For the Heidolph stirrer, a bidispersed distribution of fluorescent particles is obtained, with small diameters around 70 µm and larger ones around 200 µm, resulting in an average diameter of 149.7 µm. The reproducibility for both sets is considered acceptable, as the statistical data—mean and standard deviation—show a variation of less than 20% between replicates.

Second, particle density is determined using a helium pycnometer (Micromeritics AccuPyc II 1345). The average density of the fluorescent

PMMA and PS/ABS particles remains to that of the raw material (respectively 1.15 g/cm³ and 1.05 g/cm³).

Finally, the emission spectrum of the fluorescent particles in re- sponse to the required excitation wavelength is measured. It is worth noting that the change of polymer can shift the maximum emission wavelength by several tens of nanometers due to their intrinsic sur- face hydrophilicity [9]. A Nd:YAG laser (Litron Bernoulli) emitting at 532 nm is used as excitation light source and the emission spectra of the PMMA or PS/ABS particles is measured using a spectrofluorime- ter (Jasco FP-8350) (Fig. 4). The maximum emission wavelength is $\lambda_{PMMA}$ = 600 nm for the PMMA made particles and $\lambda_{PS/ABS}$ = 630 nm for the PS/ABS made particles (see Fig. 5).

4. Particle diameter adjustments

To modify the particles’ average diameter, several adjustments can be made based on the theory proposed by Calabrese et al. [10] re- garding the size of drops in a stirred dispersion. The key parameters influencing particle diameter are the shear stress in the tank, the interfacial tension between the two fluids (the aqueous and polymer solution), and the viscosity ratio between both phases. These param- eters are controlled by the stirring speed of the stirrer, the surfactant concentration, and the solution viscosity, respectively.

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Microscopic photographs of (a) fluorescent small PMMA particles elaborated with the Rayneri stirrer at 3000 rpm and (b) fluorescent large PS/ABS particles elaborated with the Heidolph stirrer at 300 rpm. Both at a surfactant concentration of 25 g/L.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Probability Density function (PDF) of the diameter distribution of triplicate batches: (a) for smaller PMMA particles generated using the Rayneri at 3000 rpm with about 3000 randomly sampled particles per batch, and (b) for larger PS/ABS particles generated using the Heidolph stirrer at 300 rpm with about 500 randomly sampled particles per batch. Both sets are elaborated at a surfactant concentration of 25 g/L.</figcaption></figure>

The influence of the low-speed stirrer rotational speed on the particle diameters is first evaluated, as depicted in Fig. 6. This figure illustrates the average diameter distribution of PS/ABS particles at different stirring speeds, ranging from 150 to 750 rpm, with a fixed surfactant concentration of 25 g/L. Increasing the stirrer rotational speed tends to decrease the particle average diameter. At a speed of 150 rpm, particles diameter reach up to 500 µm, at 750 rpm, particle diameters drop to 50 µm. A monotonous decreasing trend is observed between the average/median diameter and the stirring speed, in agreement with Calabrese et al. [10] drop size theory linking the increasing stirring speed to a decrease in the drop size diameter. Nevertheless, for each stirring speed, the standard deviation of the particle diameter remains high (as already observed in Fig. 4). These diameter spreads may be caused by the non-homogeneous vertical mixing, especially for low speed tests or by foaming observed for the highest speeds. The use of an additional sieving step would then mitigate this wide diameter distribution.

Besides, the influence of the surfactant concentration and the use of a different polymer is assessed by using two different concentrations and two polymers while maintaining a stirring speed of 300 rpm. Fig. 7(a) and (b) indicates that, respectively, increasing the surfactant

concentration or using PMMA instead of PS/ABS, narrows the dispersion of the particle diameters, reduces the number of large particles and thus reduces the particles median diameter ($d_{0.5}$).

All the parameters sets used for this study — stirrer, polymer used, the stirring speed $\omega$ and the surfactants concentration $C_s$ — and their resulting particle characteristic — the average diameter $\bar{d}$, the median diameter $d_{0.5}$, the diameter standard deviation $\sigma_d$ — are reported in Table 1.

## 5. Particles test in a velocimetry measurement

The tree tested sets of PMMA particles are (i) unstained 100 µm PMMA not-fluorescent particles provided from LaVision (1108946, LaVision GmbH), (ii) the same 100 µm PMMA particles stained on their surface with Nile Red following Nel et al. [4] protocol, (iii) and our custom-made PMMA particles embedded with Rhodamine B, elaborated at 300 rpm and 25 g/L of surfactant concentration in the Heidolph stirrer. Our PMMA particles were sieved in the [80–100] µm size range to remove the largest particles. These three sets are introduced into the retention tank of a closed-circuit open channel flow to evaluate their quality and effectiveness for 3D Particle Tracking Velocimetry (PTV) measurements. Fig. 8 displays raw images captured using a 2.5 MP MiniShaker TR® camera equipped with an 8 mm lens and a bandpass

<figure><img src="image_8.png" /><figcaption>Fig. 5. Emission spectra of the PMMA particle displayed in red, and PS/ABS particles displayed in blue, for an excitation wavelength of 532 nm.</figcaption></figure>

<caption>Fig. 6. Diameter distribution boxplot of the particles diameters as a function on the stirring speed for PS/ABS particles elaborated with the Heidolph stirrer and a fixed surfactant concentration of 25 g/L. The dividing line represents the median diameter and the square the average value.</caption>
<figure><img src="image_9.png" /><figcaption>Fig. 7. PDF of diameter particles elaborated at a fixed stirring speed of 300 rpm - (a) for two surfactants concentrations (25.0 and 12.5 g/L) with PS/ABS and (b) for two polymers (PMMA and PS/ABS) at a surfactant's concentration of 12.5 g/L.</figcaption></figure>

filter of 605 ± 30 nm. The camera offers a field of view of 100 cm × 60 cm, with a resolution of 0.5 mm/pixel, it is positioned underneath the channel bed at a distance of 80 cm allowing to observe the entire 30 cm width of the channel. To optimize light capture, the camera aperture is set to f/1.4. Illumination of the particles within the water flow is achieved using a Nd:YAG laser (Litron Bernouilli) emitting at 532 nm with a total power of 2 × 200 mJ, scattered over the entire volume of interest using two cylindrical lenses (with focal lengths of 15 mm and 100 mm) arranged in series. The laser output is positioned at a distance of 1 m perpendicular to the channel's VOI. Under identical flow conditions and channel settings, for the two sets of fluorescent particles, the illumination power is set to 100% (400 mJ) and bandpass filters mounted on the objectives, while for the set of non-fluorescent particles, the light emission power is reduced to 20% (20 mJ) and no bandpass filters mounted on the objectives. Acquisition images of the three different configurations is shown, respectively in Fig. 8(a), (b) and (c).

Based on these observations, the Signal-to-Noise Ratio (SNR) values is computed for the three images as: SNR = 20*log(Signal Power/MSE), where N is the number of pixels in the image, the “Signal Power” represents the power of the actual signal of interest = $\sqrt{(1/N) \times \sum_{n=1}^{N} I_n^2}$, with $I_n$ being the pixel intensities and MSE denotes the mean squared root of noise in the signal, computed as MSE = $\sqrt{(1/N) \times \sum_{n=1}^{N} M_n^2}$, with $M_n$ the pixel intensity of the minimum image for each acquisition image [11]. The SNR calculated and averaged for our custom-made particles is 14.41, compared to 4.02 for the stained particles and 3.32 for the non-fluorescent ones. Our fluorescent embedded particles display a significantly better signal than the other two.

Ultimately, it was determined that fluorescent embedded PMMA particles proved to be more effective than fluorescent embedded PS/ABS particles in experiments with limited light excitation intensity. Under the same illumination power, PMMA particles resulted in an

average emission intensity five times higher than that of PS/ABS particles within a similar size range.

## 6. Discussions

To determine whether the particles can serve as flow tracers in water, their Stokes number St is computed as:

$$St = \frac{\rho_p d_p^2 u_0}{18 \mu_g l_0} \qquad (1)$$

where $\rho_p$ is the particle density, $d_p$ is the particle diameter, $\mu_g$ is the fluid dynamic viscosity, $l_0$ represents a characteristic dimension, and $u_0$ is a characteristic flow velocity. For our [80–100] µm PMMA particles embedded with Rhodamine B, considering the characteristics a typical flow velocity of $u_0$ = 0.2 m/s and dimension $l_0$ = 0.3 m, $St = 4 * 10^{-4}$, which is significantly smaller than 1. Therefore, these particles can be considered as tracers in the present flow configuration [12]. In other words, their time response to the fluid motion is negligibly small. In the case of this specific flow, the range of particles Stokes number can go from $1 * 10^{-2}$ to $4 * 10^{-6}$.

Furthermore, as observed in Fig. 4 for both stirrers, wide distributions of diameter particles are generated. Additionally, the Heidolph stirrer particle elaboration results in a bidisperse diameter distribution for all parameter sets studied. This bidisperse distribution is likely due to the use of a perforated blade, as shown in Fig. 2. The perforated blade generates two distinct levels of shear stress in the tank: a stronger shear stress between the tank and the edge of the blade, creating the smaller diameter particles, and a limited shear stress within the holes of the blade, producing the biggest diameter particles. A possible solution to mitigate the wide and/or bidisperse particle diameter distribution may be through a more adequate mixing blade and tank geometry. The improved design of the tank and mixing blade should permit to reach a more homogeneous shear stress throughout the tank.

<figure><img src="image_10.png" /><figcaption>Fig. 8. Raw image of - (a) unstained industrial PMMA particles, (b) Nile Red surface stained industrial PMMA particles and (c) our custom made spherical fluorescent PMMA particles sieved in the range [80–100] µm- illuminated with a Nd:Yag 532 nm laser captured with a MiniShaker TR® camera equipped with an 8 mm lens and a 605 ± 30 nm bandpass filter for the two sets of fluorescent particles.</figcaption></figure>
To produce fluorescent particles of larger diameters than exposed herein, in the range of millimetric size, several strategies come to light. First, a reduction in stirring speed emerges as a plausible route, potentially mitigating shear stresses and facilitating the formation of large droplets in the tank. However, it is imperative to maintain a high enough speed e.g., at 150 rpm in our case to elevate the dispersed phase in the tank, particularly as the chloroform is heavy with a density equal to 1.5. A second approach may involve the utilization of a low-density solvent to counteract potential limitations posed by the use of low stirring speeds. A third approach could be to increase the viscosity of the water-based solution while maintaining a high enough stirring speed, thus reducing the shear stresses. Finally, for researchers seeking to use larger particles, we recommend using the staining protocol described by Nel et al. [4]. Indeed, even though such protocol presents a lower emission intensity than the protocol described in this study, as the emitted light is intricately linked to surface area, it should not pose significant problem if the particle is large enough.

Moreover, it is expected that the materials available to researchers (such as the container, reactants, polymer, stirrer, and its blades) will differ from those used in this study, leading to slight variations in particle characteristics. After characterizing the particles, the protocol may require several adjustment iterations based on the sensibility study insights given in this study to obtain the desired particle diameter and density.

Finally, the electrical properties, mechanical properties and the chemical ageing state of the particles, are expected to strongly affect interactions with other particles (homo-aggregation) and other objects such as pollutants or microorganisms (hetero-aggregation). These factors, in turn, influence the particles transport characteristics [13, 14]. Future studies should better characterize these properties on the particles produced with the present protocol.

## 7. Conclusion

In this work, we propose an easy-to-implement and affordable technique for producing spherical fluorescent particles embedded with Rhodamine B, with diameters ranging from 5 to 500 µm and made of PMMA and/or PS/ABS. These fluorescent embedded particles exhibit a signal-to-noise ratio that is four to five times higher than fluorescent surface-stained or non-fluorescent particles, resulting in a better detection. Moreover, the resulting production cost is approximately 2-3 euros per gram, once the equipment is available, thus much cheaper than commercial fluorescent particles which typically cost between 50 and 300 euros per gram.Finally, our method offers flexibility in generating spherical fluorescent particles matching the particle sizes and materials commonly used for laser velocimetry purposes.

This protocol is shown to be fairly replicable. Additionally, a sensitivity study was conducted on various factors including stirring speed, type of polymer used, surfactant concentration, and the use of two different stirrers with different blade configurations. The main results are that increasing the stirring speed, the surfactants concentration and using PMMA rather than PS/ABS reduces the median, average and standard deviation diameters. After particle elaboration within a given diameter range, we recommend including a final sieving step to achieve a narrower diameter distribution. Additionally, particles that do not fall within the desired range can be dissolved back into the chloroform solution. This step prevents losses, reduces microplastic pollution, and increases the overall efficiency of the protocol.

Overall, this protocol successfully produces isotropic fluorescent spherical particles with diameters ranging from 5 to 500 µm best suited for most multiphase water flows. The use of our fluorescent embedded particles strongly increases the fluorescent emission intensity compared to fluorescent surface staining and is recommended for experiments with limited light excitation intensity experiments. From all configurations tested, the smallest elaborated particles [5–25] µm exhibit a very low Stokes number, making them ideal tracers for small-scale water

flow experiments with limited velocity magnitudes. Particles presenting diameter range of [25–75] µm and [75–150] µm, are to be used as tracers, respectively, for water flows presenting intermediate scales and for large scale experiments such as open channel flow. Oppositely the largest particles diameter [150–500] µm can be used as debris in the context of transport study.

## CRediT authorship contribution statement

**Okba Mostefaoui:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Data curation, Conceptualization.

**Diego Lopez:** Writing – review & editing, Supervision, Conceptualization. **Emmanuel Mignot:** Writing – review & editing, Supervision, Conceptualization. **Valérie Massardier-Nageotte:** Writing – review & editing, Supervision, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Acknowledgments

Authors would like to thank Catherine Salami for insights on the elaboration protocol made during her master internship. Authors would also like to thank Dr. Stéphane Chambert of the ICMBS (Institut de Chimie et Biochimie Moléculaires et Supramoléculaires) for granting us access to the spectrofluorometer.

## Funding

This work was supported by the LABEX iMUST of the University of Lyon (ANR-10LABX-0064), created within the program "Investissements d'Avenir" set up by the French government and managed by the French National Reasearch Agency (ANR). This work has been supported by the EUR H2O'Lyon (ANR-17-EURE-0018) of "Université de Lyon" (UdL), within the program "Investissements d'Avenir" operated by the French National Reasearch Agency (ANR) through the funding of a PhD extension and a intership grant. This work was also carried out as well with the support from INSA Lyon as part of Okba Mostefaoui's "INSA Lyon Environmental Issues" MESR thesis grant.

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