Daria Terescenco
email: daria.terescenco@univ-lehavre.fr

Nicolas Hucher
email: nicolas.hucher@univ-lehavre.fr

Geraldine Savary
email: geraldine.savary@univ-lehavre.fr

Celine Picard
email: celine.picard@univ-lehavre.fr

From interface towards organised network: questioning the role of the droplets arrangements in macroscopically stable O/W emulsions composed of a

Keywords:

    From interface towards organised network: Questioning the role of the droplets arrangements in macroscopically stable O/W emulsions composed of a conventional non-ionic surfactant, TiO2 particles, or their mixture Daria Terescenco, Nicolas Hucher, Géraldine Savary, Celine Picard

Introduction

An emulsion is a system in which one liquid is dispersed in a second liquid, the two liquids being partially or totally immiscible. The liquid droplets of the disperse phase are dispersed in a liquid medium, the continuous phase [START_REF] Tadros | Emulsion Formation, Stability, and Rheology[END_REF]. To disperse two immiscible liquids, a third component is needed to assure the stability of the interface. For this purpose surfactants and colloidal particles are often used.

The surfactant is an amphiphilic molecule able to reduce the oil/water interfacial tension and it forms an electrostatic and steric barrier at the interface between two liquids [START_REF] Rosen | Emulsification by Surfactants[END_REF]. The mechanism of the particle stabilization is different: partial wetting of the surface of the solid particles by water and oil is the origin of the strong anchoring of solid particles at the oil-water interface [START_REF] Aveyard | Emulsions stabilised solely by colloidal particles[END_REF][START_REF] Chevalier | Emulsions stabilized with solid nanoparticles: Pickering emulsions[END_REF] acting as a barrier against coalescence [START_REF] Schmitt | Colloidal particles as liquid dispersion stabilizer: Pickering emulsions and materials thereof[END_REF][START_REF] Menon | Characterization of oil-water interfaces containing finely divided solids with applications to the coalescence of water-in-oil Emulsions: A review[END_REF][START_REF] Tambe | Factors Controlling the Stability of Colloid-Stabilized Emulsions: I. An Experimental Investigation[END_REF][START_REF] Monégier Du Sorbier | Influence of the electrostatic interactions in a Pickering emulsion polymerization for the synthesis of silica-polystyrene hybrid nanoparticles[END_REF]. The hydrophilic-lipophilic balance number (HLB) of surfactant molecules is the parameter which determines both the type (oil in water or water in oil) and stability of emulsions using them [START_REF] Binks | Chapter 1:Emulsions -Recent Advances in Understanding[END_REF], while it is the wettability of particles at the oil-water interface that is crucial in optimizing the stability of solid-stabilized emulsions [START_REF] Binks | Particles as surfactants-similarities and differences[END_REF][START_REF] Binks | Synergistic Stabilization of Emulsions by a Mixture of Surface-Active Nanoparticles and Surfactant[END_REF]. Numbers of solid particles are good candidates for the interface stabilization: clay, chitosan, cyclodextrin [START_REF] Yang | An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications[END_REF], starch [START_REF] Marto | Safety assessment of starch-based personal care products: Nanocapsules and pickering emulsions[END_REF], iron oxides [START_REF] Udoetok | Stabilization of Pickering Emulsions by Iron oxide Nano-particles[END_REF], titanium dioxide [START_REF] Stiller | Investigation of the stability in emulsions stabilized with different surface modified titanium dioxides[END_REF], silica [START_REF] Torres | Preparation of o/w emulsions stabilized by solid particles and their characterization by oscillatory rheology[END_REF][START_REF] Binks | Pickering emulsions stabilized by hydrophilic nanoparticles: in situ surface modification by oil[END_REF][START_REF] Frelichowska | Topical delivery of lipophilic drugs from o/w Pickering emulsions[END_REF][START_REF] Arditty | Some general features of limited coalescence in solidstabilized emulsions[END_REF], etc. Moreover, particles can be employed in association with surfactants, as stabilizing additives of such disperse systems in different fields of practical interest [START_REF] Binks | Particles as surfactants-similarities and differences[END_REF]. Some studies show that surfactants can be added to colloid-stabilized emulsions and, as a consequence, to considerably increase the emulsion stability [START_REF] Tambe | Factors Controlling the Stability of Colloid-Stabilized Emulsions: I. An Experimental Investigation[END_REF][START_REF] Zhai | Chemical and Physical Aspects of Macroemulsions Stabilized by Interfacial Colloids[END_REF]. Midmore et al., for example, discussed the stability of the silica/Brij and silica/Tween systems and compared them to the classical emulsions stabilized only by the surfactant [START_REF] Midmore | Synergy between silica and polyoxyethylene surfactants in the formation of O/W emulsions[END_REF]. Two most important observations were then made: at very low surfactant concentrations, where the surfactant cannot form a stable emulsion, the silica/surfactant system is quite stable. The authors also explained that one should take into account the chemical structure of the emulsifier to govern its interactions with both particles and the oil phase.

The surfactant can adsorb on particle surfaces and modify their wettability in a manner to improve or reduce its efficacy. Or, if surfactant does not adsorb on particles, it is possible that both species will compete for the oil-water interface but one may dominate [START_REF] Binks | Synergistic Stabilization of Emulsions by a Mixture of Surface-Active Nanoparticles and Surfactant[END_REF].

Emulsions stabilization by a mixture of particles and surfactant have been already investigated in the literature; the majority of the studies focus on the silica particle [START_REF] Ravera | Effect of Nanoparticles on the Interfacial Properties of Liquid/Liquid and Liquid/Air Surface Layers[END_REF][START_REF] Pichot | O/W emulsions stabilised by both low molecular weight surfactants and colloidal particles: The effect of surfactant type and concentration[END_REF][START_REF] Vashisth | Interfacial displacement of nanoparticles by surfactant molecules in emulsions[END_REF][START_REF] Zhu | Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant[END_REF][START_REF] Katepalli | Response of Surfactant Stabilized Oil-in-Water Emulsions to the Addition of Particles in an Aqueous Suspension[END_REF], but other types of particles are also discussed, like kaolinite [START_REF] Nallamilli | Synergistic stabilization of Pickering emulsions by in situ modification of kaolinite with non ionic surfactant[END_REF], magnetite nanoparticles [START_REF] Vílchez | Antagonistic Effects between Magnetite Nanoparticles and a Hydrophobic Surfactant in Highly Concentrated Pickering Emulsions[END_REF], carbon black particles [START_REF] Katepalli | The Response of Carbon Black Stabilized Oil-in-Water Emulsions to the Addition of Surfactant Solutions[END_REF] combined to anionic, cationic or non-ionic surfactants of different chain length (sodium dodecyl sulphate, dodecyltrimethylammonium bromide, cetrimonium bromide, polysorbates, etc.). However, in most cases, with the noticeable exception of the work authored by Pichot et al. [START_REF] Pichot | O/W emulsions stabilised by both low molecular weight surfactants and colloidal particles: The effect of surfactant type and concentration[END_REF], the particles were unable to stabilize the interface in absence of surfactant, and it was difficult to understand the role of each of them on the competition for the interface.

The research is moving forward towards the application interest for this type of systems. Particle-stabilized emulsions are of great interest in heavy oil processing, food science, controlled release systems, functional materials, microencapsulation [START_REF] Ngai | Particle-Stabilized Emulsions and Colloids: Formation and Applications[END_REF], pharma-cosmetic applications.

For this reason, in our study, in addition to the interest for the interface stabilization, we also focused on the development of stable and totally emulsified systems, taking into account not only the droplet dispersion but also the droplet interaction mechanism. This is a new approach, never discussed in the literature before. Once the interface is stabilised, the droplets organisation should be understood. The created network will depend on the droplets size, their polydispersity, their interactions, and will govern the emulsions rheological, textural and, consequently, applicative properties.

The emulsifying systems were thoughtfully chosen. Classical emulsifier mixture of non-ionic surfactants Steareth-2/Steareth-21, often used for topical applications, is well known for its stabilizing properties [START_REF] Pasquali | Birefringent emulsions stabilized with steareth-2 and steareth-21[END_REF] and was already used for the previous studies of the NP-based emulsions [START_REF] Rossano | Effects of aging on structure and stability of TiO2 nanoparticle-containing oil-in-water emulsions[END_REF][START_REF] Rowenczyk | Development of preservative-free nanoparticles-based emulsions: Effects of NP surface properties and sterilization process[END_REF]. This emulsifier was used in combination with, or it was totally replaced by TiO 2 particles coated with silica and cetyl phosphate.

The aim of this study is to bring new explanations about the role of the particle alone and in combination with a conventional surfactant (a mixture of non-ionic polyethoxylated fatty ethers) not only at the oil/water interface but also on the macroscopic and applicative properties of the emulsions containing them. The novelty of this work lays in the investigation of the particle direct impact of the emulsion microscopic, macroscopic and thermal properties, and through the comparison with the reference system (surfactant/particles or surfactant exclusive stabilisation), to point out its unique contribution in emulsion structuration.

Materials and methods

Materials

The raw materials used in this study were kindly provided by the suppliers listed in Table 1. The selected raw materials were of cosmetic grade considering the further interest of emulsions topical application.

The surfactant chosen for this study is a combination of two ethoxylated ethers: Steareth-2 is a polyoxyethylene (2) stearyl ether and the Steareth-21 a polyoxyethylene [START_REF] Midmore | Synergy between silica and polyoxyethylene surfactants in the formation of O/W emulsions[END_REF] stearyl ether [START_REF] Ash | Handbook of Green Chemicals[END_REF].

Each emulsion contained 10% of emulsifying system: 10% TiO 2 ; 10% Steareth-2/21 (6/4); 5% TiO 2 and 5% Steareth-2/21 (6/4). The oil/water for each system ratio was fixed at 40/60 and a preservative was introduced to avoid bacterial proliferation.

Methods

Emulsions formulation

TiO 2 10 emulsionthe particles were first pre-dispersed in the water phase with an Ultrasonic Processor (Fisher Scientific FB-705) during 300 sec at 80% amplitude. Next, the oil phase is added to the water phase during 30 seconds under vigorous stirring (11000 rpm) using a rotor-stator T25 digital ultra-turrax (IKA, Freiburg, Germany) equipped with the dispersing head S25N-25F. Then, the homogenization is continued for 1.30 minutes to form an emulsion. Finally, after adding the preservative, the mixture was put under stirring (Turbotest, radial flow turbine of 55 mm diameter, VMI Raynerie) at 500 rpm for 15 minutes.

St10 emulsion-the oil phase and 10% of emulsifier were heated up to 80°C under mechanical stirring (400 rpm), as well as the water phase. Then, as for the previous system, the oil phase was added to the water for 30 seconds and the homogenisation was continued for 1.30 minutes at 11000 rpm. The formulation was left cooling down to 30°C under stirring (Turbotest, radial flow turbine of 55 mm diameter, VMI Raynerie) at 400 rpm. Next, the preservative was added and the emulsion was left under the same stirring for more 15 minutes. TiO 2 5St5 emulsion was prepared using both protocolsparticles were pre-dispersed in the water phase and then heated up to 80°C, the surfactant is mixed with the oil phase and also heated up to 80°C. Next steps are identic to the previous emulsions.

Once prepared, the mixtures were split into three samples, stocked at 4°C, 40°C and ambient temperature, respectively, for stability monitoring.

Contact angle measurement

20 mg of TiO 2 particles coated with silica and cetyl phosphate were compressed under 5 tonnes to form compact pellets. Water and caprylic capric triglycerides contact angle measurements on the obtained pellets were performed using a portative goniometer PGX+ (ScanGaule, Gravigny, France) connected to the PGPlus software. It is equipped with a high-resolution camera to acquire images, with a specific lighting system associated with a mirror to visualize the liquid droplet deposited onto the surface. A syringe (Ø=0.77mm) is used to depose the first droplet on the surface and its volume is increased by the addition of five successive drops to approximately 7µL. Photography is taken after each liquid addition and the advanced contact angle is determined on the identified triple point (intersection of the liquid, solid and vapour phases using the software.). Advancing contact angle was measured at least in triplicate for both liquids.

Microscopy

The emulsions microstructure was observed by means of an optical microscope (Leica DMLP microscope) equipped with a digital camera at a magnification of x100 under the bright field. Leica IM 1000 software was used to analyse the micrographs.

Droplet size distribution

The emulsion droplets size were measured by static light scattering (SLS) using a laser diffraction particle size analyser SALD-7500 nano (Shimadzu Co., Ltd, Japan) equipped with a violet semiconductor laser (405 nm). The emulsions were diluted with deionized water to achieve the absorption parameter equal to 0.2. Considering the important chemical inhomogeneity of the medium (presence of the particles and their agglomerations besides to the oil phase droplets), and to get rid of the optical properties of all of them, the Fraunhofer theory was used. The droplets median values (D50) were acquired, representing the size in microns that splits the distribution with half above and half below this diameter.

Rheological measurements

Rheological tests were performed using a controlled stress rheometer (HR2, TA instruments). All the measurements were carried out in duplicate at 25°C. The solvent trap was used to prevent sample drying. Once loaded, the samples were left at rest for two minutes prior to any measurement. Samples were analysed once and then changed prior to further analysis.

Flow curves were obtained by recording shear stress and viscosity values at increasing shear rates ranging from 0,001 to final 5000 1/s (continuous ramp, logarithmic mode) for 300s. 60 mm aluminium parallel plate device was used for the test and the gap was fixed at 200µm.

The linear viscoelastic region was obtained through oscillatory measurements conducted at 1Hz with an increasing strain from 0.1% to 100% (logarithmic mode).

For each system were collected:

-G' (storage modulus) in the linear region of the deformation; -Tan δ (=G"/G') in the linear region of the deformation;

- 90%G' : the end point of the linear viscoelastic region was determined as a strain (%) when the storage modulus (G') value was dropped 10% from the linear level [START_REF] Korhonen | Rheological properties of three component creams containing sorbitan monoesters as surfactants[END_REF].

- G'=G" : the strain value at G'=G" (tanδ=1) which corresponds to a loss of viscoelastic properties and to the transition to the product flow process.

The frequency sweep ramp was performed from 0.01 to 10 Hz at a constant strain remaining within the previously marked linear viscoelastic region of each sample. Then, storage (G'), loss moduli (G") were reported for the rheological characterization of investigated samples.

Differential scanning calorimetry

Emulsions thermal properties were described by the differential scanning calorimeter Pyris 1 DSC (PerkinElmer). A small amount of the sample (up to 20 mg) was weighted in 50 µL aluminium pan and hermetically closed. An empty aluminium pan was used as a reference. The cooling/heating program was applied in the temperature range from 10°C to -60°C with a 5°C/min rate. Two consecutive cooling/heating cycles were applied to each sample to confirm the repeatability of the observations.

Both, the temperature corresponding to the peaks maxima (T°C) and the enthalpy values (ΔH J/g) of each peak were collected.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was performed on a Setsys apparatus (Setaram, Caluire-et-Cuire, France) with the Setsoft software. Around 40 mg of emulsion were weighed and heated under the air atmosphere by the following procedure (Program 1):

-Isotherm: 10 min at 25°C; -Ramp: 25°C to 350°C at 5°C/min; -Isotherm: 2 min at 350°C.

The weight loss of the sample is registered as a function of temperature. The first derivative of the TGA curve (the DTG curve) was also plotted to visualise the dynamics of the weight loss (mg/min).

To refine the obtained results, a second procedure was developed and focused on the slow water evaporation (Program 2):

-Isotherm: 120 min at 70°C; -Ramp: 70°C to 500°C at 2°C/min; -Isotherm: 10 min at 500°C.

Texture analysis

Analysis of emulsions texture properties was performed using a texture analyzer TAXT Plus (Stable Micro Systems, Cardiff, UK).

A compression test was carried out with the cylindrical probe P/35. 1 mL of the product was compressed by the probe at a constant speed of 1 mm/s up to a 0.5 mm gap between the probe and the base.

An extrusion test was carried out with a compression disc of 45 mm diameter in a 180 mL container (50 mm diameter) filled with cream on 45 mm height. The probe compressed and extruded the product up and around the edge of the disc at a constant speed test of 1 mm/s to a distance of 30 mm, and then returned to its start position.

Spreading properties were recorded in compression traction mode, equipped with friction module A/FR (ASTM-D 1894-90). A polypropylene plastic sheet was fixed to the base and the Helioplate™ HD 6 (Helioscreen, Marseille, France) plate, consisting of 50×50 mm 2 PMMA substrate and with homogeneous roughness on the entire surface, equal to 6 µm, was fixed to the sled (63.5x63.5 mm 2 ). 200 µL of the cream was applied onto a 20x20 mm 2 surface of the PMMA plate in four lines, parallel to the movement of the sled. The sled was then pulled across the polypropylene support at a constant speed (3 mm/sec) on 120 mm and the force of the emulsion spreading was measured. The test conditions were adapted from similar studies of the spreading properties developed in our laboratory [START_REF] Gilbert | Predicting sensory texture properties of cosmetic emulsions by physical measurements[END_REF][START_REF] Terescenco | Influence of the emollient structure on the properties of cosmetic emulsion containing lamellar liquid crystals[END_REF].

For each test, the positive area under the curve was calculated (A + g.sec) to express the work to spread, to compress or to extrude the product. Each test was performed at least three times.

During the stretching test, 100 µL of the product is compressed and then stretched between the cylindrical probe P/0.5R and the base at 40 mm/s, respecting a 0.8 mm gap. The filament stretching was monitored with a camera to identify the breakpoint of the filament in order to measure its breaking length, as developed by Gilbert et al. [START_REF] Gilbert | Stretching properties of xanthan, carob, modified guar and celluloses in cosmetic emulsions[END_REF].

Data analysis

Results were presented as the mean value ± standard deviation (SD).

XLSTAT software (Addinsoft, Paris, France) was used to perform the statistical analyses of the collected data. The ANOVA test (two-way analysis of variance) was applied to the results to spot the significant differences between the emulsions (P<0.05). Mean intensities were compared using the Tukey multiple comparison tests to distinguish different groups of products.

Results

3.1.

Building up an emulsionformulating stable, totally emulsified systems using an emulsifier, TiO 2 particles or surfactant/particles mixture All the emulsions were formulated at 10% of emulsifier and with oil phase (CCT)/water phase ratio equal to 40/60. The Steareth-2 possesses the HLB value (hydrophilic/lipophilic balance) equal to 4.9 and the Steareth-21 value equals to 15.5, thus the 60/40 mixture oriented the emulsion towards O/W type with a final HLB=9 [START_REF] Courtney | Emulsifier Selection/HLB[END_REF].

Meanwhile, the particles behave differently. Their ability to form an emulsion is guided by their physical and chemical properties: size, specific area, and the most important, the coating. According to Bernauer et al. [START_REF] Bernauer | OPINION ON Titanium Dioxide (nano form) coated with Cetyl Phosphate, Manganese Dioxide or Triethoxycaprylylsilane as UVfilter in dermally applied cosmetic[END_REF], the silica (16%) and cetyl phosphate (6%) coated TiO 2 particles are of 70 nm size (number weighted median). They have a rutile crystalline form (>98%) and a volume specific surface area of 150 m²/cm 3 . All these parameters will favour the adsorption of the particle on the liquid/liquid interface, and, consequently, it's capacity to emulsify the water and the oil phase. The discussed TiO 2 particle is considered amphiphilic, as shown by contact angle measurements (Figure 1) that indicated good compatibility with both phases; it can be dispersed in either the oil or the water phase, even if it has more affinity with the oil phase. It is important to note that in our study, the incorporation method had a major impact on emulsion formation. The incorporation of the particles to the more suitable oil phase will not permit the emulsion formation (during the emulsification step, the two phases remain non-homogeneous). On the other hand, the initial dispersion of the particles in the water phase makes possible the formation of the emulsion and the total incorporation of the oil phase.

The obtained systems were first analysed in terms of the microstructure. As can be seen from Figure 2, the gradual changing of the emulsions microscopic properties is totally dependent on the emulsifying system.

The TiO 2 10 emulsion is heterogeneous; with a polydisperse droplet population and the agglomerations of particles present in the continuous phase (Figure2b). The St10 surfactant forms fine droplets as presented in Figure2d. The addition of the particles to the surfactant (Figure2c) has a consequence on the droplet size and the size distribution.

There is an appearance of a small population of large drops due probably to coalescence. The same phenomenon was also observed by Binks et al. for the systems stabilized by C 12 E 7 emulsifier coupled with hydrophilic fumed silica particles [START_REF] Binks | Synergistic Stabilization of Emulsions by a Mixture of Surface-Active Nanoparticles and Surfactant[END_REF]. Several studies observed the destabilization of the emulsion stabilized by the surfactant coupled to the particles due to the loss of the surfactant from water interfaces and its adsorption on particle surfaces [START_REF] Binks | Synergistic Stabilization of Emulsions by a Mixture of Surface-Active Nanoparticles and Surfactant[END_REF]. In this study, the structure of the emulsion was also modified by the particles (increasing twice the D50 in volume from St10 to TiO 2 5St5), but the systems remain macroscopically stable at 4°C, 40°C and room temperature for more than 3 months. The full emulsification, without any water or oil phase separation during the time, made possible the further macroscopic analysis of the systems.

How the matrix is affected by the emulsifying system? Rheological and thermal investigation

The first step in the macroscopic approach combined three rheological tests. Figure 3 presents the evolution of the elastic (G') and viscous (G") moduli as a function of the oscillation strain and the calculated parameters are reported in Table 2. Three studied emulsions clearly showed a different response to the applied oscillation strain. The storage modulus, G', of TiO 2 10 was at least one order of magnitude higher than the emulsions containing Steareth-2/21 emulsifier.

Viscoelastic and flow behaviour

Meanwhile, it shows a quick response to the deformation, as revealed by a short linear plateau length  90%G' =0.28%.

Since the TiO 2 10 emulsion does not contain texturing agents, the only network that is formed within the matrix is the arrangement of the droplets. As a consequence, this nonhomogeneous system is more elastic at rest but is quickly deteriorated once solicited.

It is noteworthy that the differences between these two systems intervene at higher deformations: the loss of the viscoelastic properties is strongly delayed for the mixture TiO 2 5St5 ( G'=G" = 49.87%).

It is interesting to notice that the viscoelastic properties of the studied emulsions are not directly governed by the droplet sizes. While the droplet sizes increase with the particle quantity in the emulsion, for the oscillatory strain measurements, it is more likely that the mixture of both surfactant and particle stabilizers favours the formation of a more flexible and resistant system.

The flow behaviour of the studied systems can be split into three domains, as presented in Figure 4. Region I corresponds to low shear rates (from 0.01 to 1 s -1 ): the TiO 2 10 is the most viscous at low shear rates, followed by St10 and, finally, the mixture. According to Barnes [41], the first zone corresponds to the draining of the emulsion under gravity. At low shear rates, the products behave similarly to the oscillation test according to the elastic modulus G': the combination of two emulsifying systems decreases the viscosity of the product. When the shear rate increases (region II), the TiO 2 10 and St10 reverse their order and, finally, in the region III, the emulsion stabilized by TiO 2 particles shows the lowest viscosity and the most important shear thinning behaviour.

According to the literature, high shear values can create new interfaces in the system, leading to imperfect coverage and when two imperfectly-covered droplets interact, bridges can be formed in the places where particles are shared between both interfaces [START_REF] French | Making and breaking bridges in a Pickering emulsion[END_REF]. Consequently, the further coalescence of the bridge-connected droplets will favour the important shear thinning behaviour.

On the contrary, two surfactant containing emulsions present a similar mechanic resistance of the network build by Steareth-2/21, resulting in a comparable appearance of the viscosity evolution function of the shear rate. Consequently, decreasing the quantity of the surfactant from 10% to 5% decreases the viscosity of the emulsion but does not affect the general shear thinning behaviour.

The results point out the fact that all the studied systems (particles, non-ionic emulsifier or their mixture) are able to form droplets and stabilise their interface over time. Meanwhile, they also influenced the droplets interactions within the matrix, which results in individual response to the oscillatory and continuous solicitations.

Frequency response

The frequency test results can give complementary information about the investigated systems. According to Tadros et al. [START_REF] Tadros | Application of rheology for assessment and prediction of the long-term physical stability of emulsions[END_REF], one usually obtains part of the frequency dependence of G′ and G″ when analysing viscoelastic emulsions. This frequency dependence can be observed for St10 and TiO 2 5St5 (Figure 5), two emulsions containing the conventional emulsifier. This suggests the idea that for the TiO 2 5St5 mixture emulsifier, it is more likely that the Steareth emulsifier controls the network properties and prevails over the TiO 2 particles. Probably, this Steareth network is developed in 3 dimensions and, at higher frequencies, there is less time for droplet "sliding" one on each other and the droplet arrangements are responding more like a solid. For this reason, the moduli values are increasing with higher frequencies. Frequently, when the storage modulus is independent of the frequency this is due to the "strong gel-like" behaviour of the analysed product [START_REF] Lorenzo | Viscoelastic characterization of fluid and gel like food emulsions stabilized with hydrocolloids[END_REF]. But, this explanation cannot be applied in case of the TiO 2 10 emulsion. However, one can observe that the G' value is almost independent of the frequency and that of G" gradually decreases. This specific behaviour can be mainly attributed to the formation of a 3D network of the flocculated TiO 2 particles on the oil droplets [START_REF] Kawazoe | Characterization of silicone oil emulsions stabilized by TiO2 suspensions pre-adsorbed SDS[END_REF].

The rheology approach demonstrates that there are at least two types of networks formed in the analysed samples. The one formed by Steareth surfactant is more flexible during the longer times of solicitation, being able to rearrange and less flexible with increasing frequency and the system has less time to "relax". In the meantime, the frequency test confirms that TiO 2 10 emulsion also forms a network from flocculated solid particles as shown by the high consistency of the product at rest. But, this network, developed in the continuous phase, is quickly destroyed as showed by the oscillatory strain and flow tests.

Emulsions thermal response

Till today, the emulsions thermal properties were studied in some specific cases, like in crude oil case (water in oil emulsion) [START_REF] Clausse | Freezing Within Emulsions: Theoretical Aspects and Engineering Applications[END_REF] or in the food industry (oil in water emulsion) [START_REF] Bazmi | Thermaltransitions and fat droplet stability in ice-cream mixmodel systems[END_REF].

Considering the O/W emulsions with potential applicative interest, the thermal properties of such systems were never or very little discussed to our knowledge. That's why thermal studies were performed on the investigated samples. Each emulsion gave a specific response to the thermal solicitation depending on the emulsifier system. Two techniques were adapted: DSC for low temperatures investigation and the TGA for high ones.

For both techniques, it was clearly observed that the use of the surfactant of Steareth-2/21 type changes the behaviour of raw materials. In Figure 6, it can be seen that the use of the Steareth delays the solidification of the water and oil phases. The continuous phase (water) freezing was very intense for the studied systems as it represents the continuous phase of the emulsions. In order to explore more precisely the droplet organisation, the interest was focused on the oil phase solidification.

Table 3 gives information about the temperatures of the oil phase solidification of pure raw materials: pure CCT, the mixture St/CCT (1/7.12) and TiO 2 /CCT near to the studied emulsions. Firstly, it can be observed that the addition of solid raw materials (Steareth and TiO 2 ) to the CCT switched the solidification point of the oil to higher temperatures due to the physical state of these substances at such temperatures. Then, when subjected to the dispersion state and the presence of an aqueous continuous phase (emulsions), the solidification of these emulsifying mixtures was also influenced and tends to decrease. Finally, the addition of the Steareth in the emulsion brought closer the solidification of the oil phase to the pure CCT freezing point.

It is obvious that the presence of the Steareth-2/21 molecules or of TiO 2 particles mixed to the pure CCT changes its solidification mechanism. It is susceptible to be present not only at the water/oil interface but also in both internal and external phases [START_REF] Drelich | Mixed O/W emulsions stabilized by solid particles: A model system for controlled mass transfer triggered by surfactant addition[END_REF]. Indeed, the TiO 2 particles, present in the water phase and at the interface are susceptible to migrate towards the oil phase, because of their affinity with CCT. However, the freezing behaviour of the emulsions is not exclusively linked to the behaviour of pure raw materials, as shown by statistically different temperatures between the raw materials and the emulsions (Table 3), the emulsions droplet dispersion should be also considered.

In the case of St10 emulsion, for example, the droplet sizes are small, resulting in their important number and strong organisation, making the internal phase less accessible to the solidification. When increasing the TiO 2 quantity in the emulsion, the formation of bigger and less numerous droplets favours a quicker solidification of the internal phase.

These results show that the solidification point of the oil phase depends on the size and number of oil droplets, their dispersion and the positioning of the raw materials inside or on the interface of the droplets of the emulsion.

To complete the observations, the TGA analyses were executed. Firstly, classical heating from 25°C to 350°C was performed on each sample to evaluate the weight loss (Table 4). Evidently, the total weight loss was statistically different between the three studied emulsions because of the TiO 2 solid particles residue at a 5% level in TiO 2 5St5 emulsion and at a 10% level for TiO 2 10 emulsion. Meanwhile, the weight loss corresponding to the water evaporation (Step 1) and oil phase evaporation (Step 2) is difficult to interpret. The evaporation dynamics are different, but not well represented only by the weight loss value. To better observe the described phenomena, the complementary test was performed. The heating of each sample was realized in two steps: an isotherm at 70°C for 2 hours, followed by the heating step, from 70°C to 500°C. During the first step (Figure 7a), the temperature was maintained at 70°C to favour slow water evaporation. At that temperature, the surfactant Steareth-2/21 is already in a liquid state but still assuring the emulsion stability. Pure water evaporation dynamics is given as a reference and it shows a similar DTG curve with TiO 2 10 emulsion continuous phase evaporation. For TiO 2 10 emulsion the process is quicker, finished in one step with almost no retention of the bulk water. Only the end of the evaporation reveals the presence of the droplet network, with 10%-15% water retention over 15 more minutes. In contrast, both emulsions containing Steareth-2/21 molecules exhibit slower water evaporation, pointing out the retention of water over a larger time scale, due to water/surfactant interaction. The following suggestion can be proposed: at these high concentrations (5% and 10%) the surfactant is placed not only on the interface but also in oil and aqueous phases. Unlike solid particles, the amphiphilic molecules are able to build different structures (micellar organisation, liquid crystalline phases, etc.) and to retain an amount of the bulk water inside these structures. During the second step, the weight loss corresponds to the loss of the caprylic/capric triglycerides for TiO 2 10 and of CCT/Steareth for the emulsions TiO 2 5St5 and St10 (Figure 7b). During a slow heating process, one can observe that the oil phase evaporation peaks are not identical for the three emulsions.

St10 emulsion, which holds the water during the first step, is gradually losing its CCT/Steareth mixture, showing that the surfactant and the oil phase are degraded in the same range of temperatures. On the contrary, for the internal phase evaporation of TiO 2 10 emulsion, TiO 2 particles create a physical barrier at the oil droplet surface and are retaining the CCT evaporation, which occurs abruptly at higher temperatures with the boiling of the oil. For the TiO 2 5St5 mixture, the behaviour is more similar to the St10 emulsion in terms of degradation temperatures, as if the Steareth-2/21 molecules were the principal stabilizing mechanism. Interestingly, the mixture and TiO 2 10 share the same shoulder at the end of the degradation process, pointing out the possible adsorption of the small amount of oil phase on the particle surface.

The combination of the microstructure observation with the rheology response and the thermal properties of the studied systems is an innovative way to understand the distribution of the raw materials inside the matrix and to interpret the organisation of the dispersed phase. Once the arrangement of the model systems was described, the interest was focused on the textural properties, in connection with a potential applicative interest (food industry, pharmaceutical application, etc.).

How the emulsifying system impacts the applicative properties? Textural approach

Textural analyses were performed to describe the emulsions applicative properties. This approach is often used for the characterisation of formulated systems and gives information about the network organisation and its response to different types of solicitation. It is an interesting tool to complete the results of the rotational rheology [START_REF] Gilbert | Rheological and textural characterization of cosmetic emulsions containing natural and synthetic polymers: relationships between both data[END_REF] and to evaluate the firmness, consistency or other textural parameters [START_REF] Lukic | Effect of Small Change in Oil Phase Composition on Rheological and Textural Properties of w/o Emulsion[END_REF], dermal and pharmaceutical efficiency [START_REF] Calixto | Interactions between UV filters and active substances in emulsion: Effect on microstructure, physicochemical and in-vivo properties[END_REF]. To our knowledge, very few studies in the literature deal with the texture properties of such emulsions and the compared effect of different type of interfacial agents.

Moreover, some of the textural tests were developed to predict the sensory properties of the texture of emulsions. For example, parameters from the extrusion test are a part of the "integrity of shape" attribute model that is related to the properties at rest of a product. Parameters from the compression test are correlated to the "compression force" evaluated by a sensory panel that is directly related to the consistency of an emulsion. And the stringiness of the emulsion can be quantified through a compression/stretching test [START_REF] Gilbert | Predicting sensory texture properties of cosmetic emulsions by physical measurements[END_REF] that is related to the extensional properties of products. Connecting data from texture analysis to microstructure and droplets arrangements of emulsions is a key to understand the differentiated impact of raw materials on emulsions formulation but also interaction with skin and surfaces.

Based on the literature, four tests were selected: compression, extrusion, spreading and stringiness, to collect the characteristics of the emulsion with an applicative interest. The obtained results are presented in Table 5. [START_REF] Gilbert | Rheological and textural characterization of cosmetic emulsions containing natural and synthetic polymers: relationships between both data[END_REF]: the squeezing of the small quantity of the sample provokes the beginning of network breakdown, identic with the II zone of the flow test (1-100 1/s). Similarly, the force necessary to extrude the products is most important for St10, followed by the TiO 2 10 and finally, by TiO 2 5St5 emulsion, which was proved (by oscillatory strain test) to possess the most flexible, but less consistent organisation.

The compression and extrusion tests are dealing with partial 3D network destruction, but still keeping the individual droplet interactions. These interactions can be accessed by the spreading and stringiness tests.

The product is spread between two nonpolar surfaces: PMMA and polypropylene. The examples of the spreading traces are given in Figure 8. The traces show the fragility and quick breakdown of the TiO 2 10 emulsion with a nonhomogenous trace and less affinity with nonpolar surfaces. The Steareth-2/21 containing emulsions present homogenous spreading, pointing at the flexibility of the system, but also the strong droplet interactions as shown by higher A+ values. This behaviour was already evoked during the frequency test: the slow spreading of the sled is similar to low frequencies effect when the network of the viscoelastic system has time to rearrange and to adapt its structure. TiO 2 10, behaving more like an elastic system, is immediately broken down. Interestingly, TiO 2 5St5 exhibit the highest value of spreading, in a relationship with their ability to deformation resistance ( G'=G") ) . The attempt of the Steareth-2/21 containing systems to keep its organisation and stretching properties is more pronounced than in case of TiO 2 stabilised system. This test gives a complementary proof of the Steareth-2/21 surfactant main role in the emulsion stabilisation and network formation, even if it is introduced in the system containing the TiO 2 particles, which are also able to stabilise the interface. However, the stringiness values show poor stretching properties of all the studied emulsions. Consequently, the type of system stabiliser does not significantly impact the emulsions capacity to stretch. Considering the emulsions applicative interest, the poor stretching properties is a good characteristic for our products.

Conclusion

This study focused on development (composition, formulation process) and the analysis of three emulsions stabilised by TiO 2 particles, by a conventional polyethoxylated fatty alcohol surfactant/co-surfactant namely Steareth-2/21 surfactant and by the mixture of both. The aim was to raise the scale from the interface stabilisation towards building up a macroscopically stable and totally emulsified system and to access the mechanism of droplets interactions and organisation which is less investigated today.

Depending on the interfacial agent used, the size, droplet size distribution and droplets network organisation are different. Probably due to a different decrease of the interfacial tension, emulsions containing TiO 2 particles exhibit bigger droplet size and higher polydispersity compared to conventional ones. Moreover, we showed that conventional surfactant has a major impact on the microstructure of the emulsions as one of the emulsions containing the mixture of TiO 2 particles and surfactants is very close to the conventional ones. Meanwhile, the stability over time is not affected by the type of the emulsifying system.

Combined to the microstructure analyses, the study of the rheological and thermal analyses revealed the impact of the different interfacial agent on the droplets network organisation and its heterogeneity but also how the TiO 2 particles, surfactants and the mixture of both organized between both phases and interphase. Once again, it puts in evidence the major impact of the conventional surfactants compared to the TiO 2 particles on the viscoelastic properties and the thermal properties. Thanks to the mixture of TiO 2 particles and surfactants, it is possible to obtain a system with higher deformability.

The thermal analysis was successfully used here, on oil-in-water emulsions, and brought complementary and original data to explain the role of the ingredients on the organisation of the microstructure.

This study brings new interesting results, valuable from a theoretical and practical point of view: by modulating the composition of the emulsifying system, one can develop unique texture properties of the emulsions. This approach is very promising as these textural data are related to topical applications of products and should be considered when developing products of practical interest. Contrary to the systems developed in the literature, the emulsions developed during this study present great applicative potential.

Figure 1 .

 1 Figure 1. Images of the droplets and calculated values of the contact angle formed between water and caprylic capric triglycerides liquids on a TiO2/silica/cetyl phosphate pellet.

Figure 2 .

 2 Figure 2. (a) Photography of three homogenous and totally emulsified emulsions as well as their bright field micrographs (*100), medianvol (D50) values and particle size representation in volume for (b) TiO2 10, (c) TiO25 St5 and (d) St10 emulsion respectively.

Figure 3 .

 3 Figure 3. Evolution of the elastic (G') and viscous (G") moduli as a function of the oscillation strain of three studied emulsions stabilised exclusively by TiO2 particles at 10%, by a surfactant/particle mixture TiO25% and Steareth2/21 at 5%, or by the Steareth2/21 10% alone. The destruction of the system is represented by the G' and G" intersection.

Table 2 .

 2 Calculated G' (Pa) and tanδ values at the plateau; the plateau length expressed by oscillation strain point at 10% of G' plateau decrease; and the network destruction point represented by the oscillation strain value at G' and G" intersection for TiO210 emulsion, TiO25St5 and St10 one.

Figure 4 .

 4 Figure 4. Viscosity evolution of three emulsions as a function of the shear rate. The shear-thinning behaviour of the samples is divided into three regions: I 0.01-1 1/s (low shear rates); II 1-100 1/s (medium shear rates); III 10-10000 1/s (high shear rates).

Figure 5 .

 5 Figure 5. Double logarithmic plots of G' and G" as a function of frequency. Frequency independent TiO210 emulsion, with no moduli evolution and frequency dependant TiO25St5 and St10 with moduli, increasing at higher frequencies.

Figure 6 .

 6 Figure 6. DSC freezing profiles of TiO210, TiO25St5 and St10 emulsions. The sharp peak corresponds to the water freezing, while the second represents the oil phase solidification. The addition of the Steareth-2/21 emulsifier delays the solidification of both: continuous and internal phases.

Figure 7 .

 7 Figure 7. TG derivative (mg/min) is plotted as a function of (a) time for isothermal step at 70°C and (b) temperature during the heating step from 70°C to 500°C. Emulsions are represented by black lines, while the pure water (a) and CCT (b) reference are plotted in grey.

Figure 8 .

 8 Figure 8. Example of spreading traces (spreading over 120 mm distance at 3 mm/sec between a polypropylene and PMMA surface) for (a) TiO210, (b) TiO25St5 and (c) St10 emulsions.

Table 1 .

 1 INCI, CAS, trade name and the supplier of each raw material, as well as the composition of the three formulated emulsions TiO210,

	St10, TiO25 St5
	INCI	CAS	Trade name	Supplier	TiO 2 10	St10	TiO 2 5 St5
	TiO 2 , silica, cetyl phosphate	13463-67-7; 7631-86-9; 3539-43-3	Eusolex ® T-Easy	Merck	10%	-	5%
	Steareth-2	9005-00-9	Massocare ® S2	Massó	-	6%	3%
	Steareth-21	9005-00-9	Massocare ® S21	Massó	-	4%	2%
	Caprylic capric triglycerides (CCT)	73398-61-5	Triglycerides C8C10 55/45	Stéarinerie Dubois	35.6% 35.6% 35.6%
	Deionized Water				53.4% 53.4% 53.4%
	Phenoxyethanol and Methylparaben	122-99-6; 99-76-
	and Ethylparaben and Propylparaben	3; 120-47-8; 94-	Sepicide HB	Seppic	1%	1%	1%
	and Butylparaben	13-3; 94-26-8;

Table 3 .

 3 Temperatures of solidification of oil phases: of three studied emulsions EM_TiO210, EM_TiO25St5, EM_St10 and of the raw materials pure CCT and the TiO2/CCT, St/CCT mixtures. a-c Values with a different letter in one column indicate that corresponding products are significantly different from the considered measurements.

	Sample	Oil phase solidification T°C
	TiO 2 /CCT	-19.18±0.07	a
	EM_TiO 2 10	-21.26±0.29	a
	St/CCT	-21.59±0.27

a EM_TiO

2 5 St5 -24.72±0.24

		b
	CCT	-26.89±2.69

b EM_St

10 -30.94±0.05 c

Table 4 .

 4 Percentage of the total weight loss, water loss during Step 1 (25°C-170°C) and oil phase loss during Step 2 (170°C-350°C). a-c Values with a different letter in one column indicate that corresponding products are significantly different from the considered measurements.

		Total weight loss(%) Step 1(%) Step 2(%)
	TiO 2 10	85.61±0.7 c	46.9±3.4 b 38.7±2.8 b
	TiO 2 5St5	91.7±0.2 b	53.3±0.2 a 38.4±0.1 b
	St 10	97.1±0.4 a	51.9±0.7 ab 45.2±0.3 a

Table 5 .

 5 Positive areas A+(g.sec) under the force curves, corresponding to the work of compression, extrusion and spreading of the TiO2 10%, TiO25% Steareth 2/21 5%, and Steareth 2/21 10% containing emulsions. The stringiness of the same emulsions is represented in mm. a-c Values with a different letter in one column indicate that corresponding products are significantly different from the considered measurements. St10 exhibit the highest value compared to TiO 2 10 and finally the TiO 2 5St5. It is mainly due to shearing during compression

		Compression A+(g.sec) Extrusion A+(g.sec) Spreading A+ (g.sec) Stringiness (mm)
	TiO 2 10	118.1±3.5 b	4608±95 b	628.9±14.5 c	5.9±0.4 b
	TiO 2 5St5 87.5±3.3 c	4122±72 c	1065.3±12.2 a	10.1±0.2 a
	St10	207.9±1.1 a	5833±192 a	1007.6±22.8 b	10.8±0.4 a
	For the compression test,


El Sawaf

Ossen

Nicolas Grimault
email: nicolas.grimault@cnrs.fr

Patrick Chevret
email: patrick.chevret@inrs.fr

Audibility of warning signals: methods to evaluate the combined effects of hearing protectors and hearing impairment

Keywords: Noise, Environment, Annoyance I-INCE Classification of Subject Number: 36, 61, 76, 79

Workers exposed to high noise levels are generally required to wear hearing protection devices (HPDs). For security reasons, it is important to evaluate whether the audibility of warning signals in noise is impeded when using HPDs, especially for hearing-impaired workers. The typical method to assess the effect of HPDs is to perform subjective experiments which require large populations with various degrees of hearing impairment to get statistically significant results. To ease the evaluation of the combined effects of HPDs and hearing impairment, two alternative methods are proposed. First, sound simulations have been used to reproduce the combined effects of impairment and HPDs to perform listening tests on normal hearing listeners. Qualitatively, the effects are successfully simulated. Second, a detection model has been developed to predict masked thresholds of warning signals in presence of background noise. Even though it overestimates the detrimental effects of HPDs, the model is qualitatively in agreement with the experimental data.

INTRODUCTION

In many workplaces, acoustic warning signals are often used to alert workers in case of dangerous situations. However, several factors may decrease the efficiency of these signals [START_REF] Wilkins | A field study to assess the effects of wearing hearing protectors on the perception of warning sounds in an industrial environment[END_REF][START_REF] Arz | Effect of wearing hearing protectors on the audibility of railway warning signals -an experimental study[END_REF], notably the presence of background noise, the hearing status of the workers or the use of hearing protection devices (HPDs). As such, it is important to be able to assess the effect of HPDs on the workers' detection ability. The effect of HPDs is usually assessed by performing subjective experiments to measure the masked threshold of a signal, (i.e. the level at which the signal becomes audible in presence of a background noise) both with and without wearing a HPD [START_REF] Wilkins | Hearing protection and warning sounds in industry-a review[END_REF]. The HPD is then considered as improving or deteriorating detection depending on whether the masked threshold while wearing the HPD is higher (deterioration) or lower (improvement) than the masked threshold without HPD. However, this method is time consuming and the recruitment of listeners is a major difficulty. Indeed, to yield statistically significant results, it is necessary to constitute a large population with various degrees of hearing impairment, with enough listeners for each degree of impairment. Because of this difficulty, two alternative methods to ease the evaluation of the effect of HPDs on audibility are presented hereafter.

These methods should take into account two hearing capabilities that govern the audibility of warning signals: the absolute thresholds and the width of the auditory filters. The elevated absolute thresholds of hearing-impaired listeners can deteriorate detection because the attenuations of HPDs add up to the absolute thresholds to the point where the signal becomes inaudible [START_REF] Arz | Effect of wearing hearing protectors on the audibility of railway warning signals -an experimental study[END_REF]. Another source of deterioration is the increase in the upward spread of masking [START_REF] Arz | Effect of wearing hearing protectors on the audibility of railway warning signals -an experimental study[END_REF]. Because the HPDs usually have higher attenuation in high frequencies than in low frequencies, the masking effect of a low frequency noise will almost not be changed while the high frequency components of a signal to be detected will be attenuated. Hearing-impaired listeners may be more affected as they already have a poorer frequency selectivity (their auditory filters are wider) when their absolute thresholds exceeds around 30 dB HL [START_REF] Florentine | Frequency selectivity in normally-hearing and hearing-impaired observers[END_REF][START_REF] Brian | Cochlear hearing loss: physiological, psychological and technical issues[END_REF]. On the other hand, the non linear growth of masking with noise levels can also improve detection: when a noise level is attenuated by the HPDs, it leads to weaker masking effects which in turn allows for lower masked thresholds [START_REF] Wilkins | Hearing protection and warning sounds in industry-a review[END_REF].

The first method to evaluate the effects of the HPDs combine to impairment consists in using sound simulations to reproduce the effects of hearing impairment and HPDs. By altering signals so they are perceived by normal hearing listeners as if they were hearingimpaired (and/or wearing HPD), listening tests can be performed on a limited number of normal hearing listeners (which are a lot easier to recruit than hearing-impaired listeners) and thus speed up the experiment.

The second method is a predictive model based on Glasberg and Moore's excitation pattern model [START_REF] Brian | Derivation of auditory filter shapes from notched-noise data[END_REF]. The model is able to compute masked thresholds for pure and complex sounds while taking into account the sound attenuations of a HPD and the hearing status of the listeners (absolute thresholds and auditory filters).

In this paper, the results of these two methods are compared to the result of a previous subjective experiment held on hearing-impaired listener.

EXPERIMENT ON HEARING-IMPAIRED LISTENERS

The experiment on hearing-impaired listeners is only briefly presented here since it is presented in details in a companion paper proposed at Inter-noise 2019 [START_REF] Arz | Effect of wearing hearing protectors on the audibility of railway warning signals for hearing-impaired listeners -an experimental study[END_REF].

Participants

Seventy-three listeners aged from 18 to 81 (mean age = 53.3 years; SD = 14.5 years) with various hearing status from normal to highly impaired participated in a detection experiment. Their hearing status were assessed by measuring the absolute thresholds at the 11 standard audiometric frequencies from 125 to 8000 Hz as well as the equivalent rectangular bandwidths (ERBs) of the auditory filters centered at 500, 1000, 2000 and 3000 Hz via a notched-noise experiment [START_REF] Patterson | Auditory filter shapes derived with noise stimuli[END_REF][START_REF] Patterson | Auditory filters and excitation patterns as representations of frequency resolution[END_REF]. Four hearing groups were then considered using an average loss indicator named BIAP corresponding to the mean of the absolute thresholds at 500, 1000, 2000 and 4000 Hz on the best ear:

-NH group: normal hearing with BIAP≤20dB HL; 28 listeners -group HI1: weak to medium impairment with 20<BIAP≤30; 16 listeners -group HI2: medium to severe impairment with 30<BIAP≤40; 17 listeners -group HI3: severe impairment with BIAP>40; 12 listeners

The average audiograms for each group are shown figure 1.

Detection task

Warning signals from the French National Railway Company (SNCF) were used in the detection task: seven warning signals and two background noises. The masked thresholds were estimated using a two interval forced choice (2IFC) procedure with a two-down one-up adaptive rule leading to a 70.7 % detection [START_REF] Levitt | Transformed up-down methods in psychoacoustics[END_REF]. The levels of the background noises were fixed at 86 dB(A). The masked thresholds were estimated in three conditions: without HPD, with an earmuff and with custom-moulded earplugs. The experiment results for the four groups will be used as a basis for comparison with the two methods proposed.

SOUND SIMULATION

The principle of this method is to modify audio signals so they are perceived by normal hearing listeners as if they were hearing-impaired listeners and/or wearing HPDs.

Hearing impairment and HPD simulation

To simulate hearing impairment, the hearing loss simulator (HLS) developed by Grimault et al [START_REF] Grimault | A real time hearing loss simulator[END_REF] was used. This simulator compensates the cochlear compression of the healthy ear to reproduce the in-out function of an impaired ear to simulates the impaired frequency selectivity as well as the elevated absolute thresholds. As input, it only requires the audiogram to be simulated.

The HPDs simulated were those tested in the experiment with hearing-impaired listeners (see section 2). The attenuations of the earmuff have been measured in narrowband on the listeners during the experiment each time they had to put it on, an average attenuation was then computed from the several attenuation measured. The attenuations of the earplugs have been measured in narrowband on an artificial head as it was too difficult to place a microphone in the ear of the subjects to measure the attenuation without altering the attenuations of the earplugs too much. The HPDs simulation is achieved by applying narrowband attenuation on the magnitude of the signals' spectrum, while preserving the phase.

Finally, the signals were first modified to simulate the HPDs, then they were modified by the HLS to simulate impairment.

Participants & simulated impairments

Eight young normal hearing listeners (absolute thresholds < 20 dB HL at all tested frequencies for both ears) performed the same experiment as the one described in section 2 except they were listening to the simulation signals of impairment and HPDs. The simulated hearing impairments correspond to the averaged audiograms of the three hearing-impaired groups (HI1, HI2 and HI3 as described in section 2.1) shown figure 1. Thus, each listener performed the detection task in four different conditions: with three different impairments simulated and with their own normal hearing status (no simulation of impairment). Results are shown in section 5.

PREDICTIVE MODEL

The proposed model is based on the excitation pattern model of Glasberg and Moore [START_REF] Brian | Derivation of auditory filter shapes from notched-noise data[END_REF] upon which is applied the signal detection theory (SDT) [START_REF] Green | Signal detection theory and psychophysics[END_REF].

In a detection task where the listener has to choose between two stimuli ("noise alone" and "noise + signal") which one contains the signal it is possible to compute a detectability index d :

d = ∆µ σ . (1)
This index d informs on the difficulty of the detection task and yields the information of the targeted detection percentage; ∆µ is linked to the levels of the stimuli and, at threshold, yields the level of the target signal. Finally, σ is the internal noise of the listener doing the task. The values of internal noise according to hearing status were evaluated from the thresholds without HPDs measured in the experiment presented in section 2.2. This evaluation is presented in another paper that shows that the values of σ increase with increasing impairment. To take into account the ear's ability to integrate information over a large frequency spectrum, the detectability index d is expressed in terms of the detection index (d i ) in each independent frequency band [START_REF] David | Detection of Multiple Component Signals in Noise[END_REF] (σ is considered equal in all bands):

d = Nb i=1 d 2 i = Nb i=1 ∆µ 2 i σ 2 i = Nb i=1 ∆µ 2 i σ (2)
with Nb the number of independent bands. From Eq. 2, once the internal noise values are known, it is then possible to choose a detection percentage, expressed through d , and to compute the level of the target signal (expressed through ∆µ i ).

To take into account hearing impairment, the excitation pattern model [START_REF] Brian | Derivation of auditory filter shapes from notched-noise data[END_REF] is modified in two ways. First, since hearing-impaired listeners may have wider auditory filters [START_REF] Florentine | Frequency selectivity in normally-hearing and hearing-impaired observers[END_REF][START_REF] Brian | Cochlear hearing loss: physiological, psychological and technical issues[END_REF], the average ERBs of the different impaired group are used to compute the equivalent widths of the filters used by Glasberg & Moore's model to compute the excitation pattern. The measured ERBs are also used to define the frequency bands used in Eq.2; which are fewer but larger for hearing-impaireds. Second, the absolute hearing thresholds are accounted for by limiting the levels of the excitation pattern: if the level in a band is below the absolute threshold then this band does not contribute to detection and ∆µ i is equal to 0 in this band. In short, the average ERBs are first used to modify the way the excitation pattern is computed, then the average absolute hearing thresholds are used to correct the values of ∆µ i in each band.

The average audiograms and ERBs of the 4 groups (NH, HI1, HI2 and HI3) have been used as input to predict the thresholds for the signals used in the experiment presented section 2, targeting the same detection percentage (70.7%), which implies d = 0.78 [START_REF] Swets | Signal detection and recognition by human observers: contemporary readings[END_REF]. Results for this method are shown in section 5.

RESULTS & DISCUSSION

Figures 2 and3 show the boxplot distribution of the HPDs effects (averaged on the 7 alarms) on the different groups computed as the difference between the masked thresholds with hearing protectors and the masked thresholds without hearing protectors. The results are presented for the three methods considered:

-Experiment on hearing-impaired listeners (that serves as a reference, see section 2) -Sound simulation (see section 3) -Predictive model (see section 4).

The results from the experiment on hearing-impaired listeners show that the perception is improved for NH and HI1 groups, whereas it is deteriorated for HI2 and HI3 groups. The sound simulation method yields fairly good results as it is able to properly reproduce the effect of a HPD observed from the experiment on hearing-impaired listeners: the higher the hearing loss, the higher the deterioration due to the HPD. The sound simulation method was compared to the experiment on HI listeners through Wilcoxon rank sum tests. The tests show no significant difference between the two methods (p>0.05 for all groups and for both HPDs). The remaining discrepancies between these two methods are relatively small. These discrepancies could be explained by the fact that each HPD was simulated using a unique set of attenuation values (see section 3.1) whereas in the experiment with hearing-impaired listeners, the attenuations experienced by each listener may vary. Because the sound simulation results tend to overestimate the detrimental effect of HPD for hearing-impaired listeners (the median value of the effect of the HPD are higher), it can be hypothesised that the attenuations experienced by several listeners were lower than those simulated. Considering the predictive model method, it does predict higher deterioration for higher impairments but the detrimental effect of the HPD is clearly overestimated (as compared to the experiment on hearing-impaired listeners). Indeed, the amelioration observed for groups NH and HI1 (from the experiment on hearing-impaired listeners) is not reproduced by the model while the deterioration effect for groups HI2 and HI3 is greatly overestimated. When comparing the effects of the earmuff evaluated by the model and the experiment on HI listeners, Wilcoxon tests show no significant difference (p>0.05) for groups HI2 and HI3 while there is significant differences between methods for groups NH and HI1 (p<0.05). For the earplugs, the Wilcoxon tests show significant differences between the model and the experiment on HI listeners (p< 0.05) for all groups. As already mentioned for the sound simulation method, this overestimation could be explained by the fact that the unique set of attenuations values used in the predictive model are larger than those experienced by several listeners. Another possible explanation could be that the variation of the width of the auditory filters with sound level is not properly taken into account. Indeed, the beneficial effect of a HPD on normal hearing listeners comes from an improved frequency selectivity (i.e. sharper auditory filters) thanks to the reduced sound levels under the HPD which lead to less masking effects. The lack of beneficial effect observed for the NH group with the predictive model method (see figures 2 and 3) suggests that the sharpening of the filters due to the reduction of the sound level is underestimated in the model. This aspect deserves further work.

CONCLUSIONS

As an alternative to subjective tests on hearing-impaired listeners which require the recruitment of large populations with various degrees of hearing impairment, two methods have been proposed to evaluate the effects of HPDs on the audibility of warning signals.

The sound simulation method yields satisfactory results as the effects of HPD observed from the experiment on hearing-impaired listeners are well reproduced. The next step would be to test different sets of attenuation values to evaluate how sensitive detection is to these attenuations. More generally, this method is also an interesting prevention tool as it allows workers to experiment what an impaired audition sounds like, thus encouraging them to properly protect their hearing.

The predictive model method also yields qualitatively good results but it strongly overestimates the detrimental effect of the HPD observed from the experiment on hearingimpaired listeners. This overestimation can be explained by the fact that the unique set of attenuations used in the model are larger than those experienced by several of the hearingimpaired listeners. Another explanation could be that the sharpening of the filters due to the reduction of the sound levels for the listeners who performed the tests is larger than the sharpening used in the model. Overall, the proposed predictive model could be used to easily predict masked thresholds of warning signals for different detection percentages of the target signal.

Figure 1 :

 1 Figure 1: Averaged audiograms for the four hearing groups defined in section 2.1.

Figure 2 :

 2 Figure 2: Effects of the earmuff, assessed by the three different methods as the difference between the masked thresholds with and without HPD.

Figure 3 :

 3 Figure 3: Effects of the earplugs, assessed by the three different methods as the difference between the masked thresholds with and without HPD.


Jean-Pierre Arz
email: jean-pierre.arz@inrs.fr

Effect of wearing hearing protectors on the audibility of railway warning signals for hearing-impaired listenersan experimental study

Keywords: Hearing Protection, Warning Signals, Hearing Impaired I-INCE Classification of Subject Number: 36, 79

In order to evaluate the effect of wearing hearing protector devices (HPDs) on the audibility of railway warning signals, masked thresholds measurements were performed in the laboratory with and without wearing the HPDs. Seven railway warning signals and two HPDs (custom molded earplugs and a passive earmuff) were tested on normal-hearing (NH, N=11) and hearing-impaired (HI, N=60) listeners with various hearing loss profiles. The results show that for NH subjects, the audibility is generally improved when wearing the HPDs (i.e. the protected thresholds are lower than the unprotected thresholds). On the contrary, for HI subjects, the protected thresholds are higher than the unprotected thresholds and this detrimental effect of HPDs tends to increase with increasing hearing loss. To guarantee the security of HI workers, it was considered that their protected thresholds should not be higher than the unprotected thresholds of the NH subjects. Based on this statement, the results show that the security of HI workers is guaranteed up to around 25 dB of hearing loss (dB HL), in average at 500, 1000 and 2000 Hz in the best ear.

INTRODUCTION

Acoustic warning signals are often used in workplaces to alert workers of a potentially dangerous situation. In practice, the audibility of warning signals may be compromised by several factors, notably the hearing status of the workers and the wearing of hearing protection devices (HPDs) [START_REF] Wilkins | A field study to assess the effects of wearing hearing protectors on the perception of warning sounds in an industrial environment[END_REF][START_REF] Wilkins | Hearing protection and warning sounds in industry -a review[END_REF].

In Europe, HPDs must (respectively should) be worn when daily noise exposure levels exceed 85 dB(A) (respectively 80 dB(A)) [3]. However, article 11 states that derogations may be granted in exceptional conditions if the "use of individual hearing protectors would be likely to cause a greater risk to health or safety than not using such protectors".

Until 2009, the French National Railway Company (SNCF) was granted from such derogation (Article L4111-4 of the French Labour Code) because the risk of not hearing warning signals when wearing HPDs could result in fatal accidents. Since May 2009, the derogation was not granted anymore and SNCF therefore initiated listening tests to evaluate the influence of wearing HPDs on the perception of warning signals. In a previous experimental study only considering normal hearing (NH) listeners [START_REF] Arz | Effect of wearing hearing protectors on the audibility of railway warning signals -an experimental study[END_REF], it was found that wearing earplugs hardly deteriorates the perception of railroad warning signals (as compared to no HPD). For hearing-impaired (HI) listeners however, it is known that wearing HPDs can have a more detrimental effect [START_REF] Lazarus | Signal recognition and hearing protectors with normal and impaired hearing[END_REF]. This more detrimental effect for HI listeners may be due to two distinctive phenomena : first, elevated absolute thresholds (referred as "Case 1 elevation" in [START_REF] Giguère | Modeling the interaction between the hearing protector attenuation function and the hearing loss profile on sound detection in noise[END_REF]) and second, broadened auditory filters (referred as "Case 2 elevation" in [START_REF] Giguère | Modeling the interaction between the hearing protector attenuation function and the hearing loss profile on sound detection in noise[END_REF]).

Therefore, the present experimental study was conducted to evaluate the effect of wearing HPDs on the detection of railroad warning signals for HI listeners.

METHOD

Participants

Seventy-five listeners aged from 18 to 81 years (mean age = 51.5 years; SD = 15.8 years) participated to the experiment. Fifteen listeners had normal hearing (no absolute thresholds greater than 20 dB HL at any of the audiometric frequencies from 125 to 8000 Hz and on both ears). The other listeners were grouped into four hearing classes according to their mean absolute threshold at 500, 1000 and 2000 Hz on their best ear (denoted PAM in this paper):

-20 listeners are in the class 10<PAM≤20 dB HL (class HI1) -18 listeners are in the class 20<PAM≤30 dB HL (class HI2) -14 listeners are in the class 30<PAM≤40 dB HL (class HI3) -8 listeners are in the class PAM>40 dB HL (class HI4).

Figure 1 shows the mean audiograms for the five hearing classes considered.

Warning signals, noises and HPDs

Seven warning signals used by the French National Railway Company (SNCF) have been tested. Four of them are dedicated to warn track workers and were tested in a ballast plough background noise. The three other signals are used to warn drivers and were tested in the background noise of a railway vehicle at maximal speed. The two masking noises dominate in the low frequency range (f<500 Hz) and have most of their energy below 3 kHz. Six out of the seven warning signals are harmonic sounds that differ greatly by their frequency content (i.e. they have different fundamental frequencies and different repartitions of their dominant harmonic components). The non-harmonic warning signal is made of the sum of two pure tones at 3430 and 4084 Hz.

Two HPDs were tested: silicon custom molded earplugs and passive earmuffs.

Masked thresholds measurements

Masked thresholds were estimated using an adaptive, two-interval, forced choice (2IFC) procedure with a two-down one-up adaptive rule. This procedure leads to a 70.7 % of detection [START_REF] Levitt | Transformed up-down methods in psychoacoustics[END_REF]. The levels of the noises were fixed at 86 dB(A) while the warning signals started at 86 dB(A) and varied according to the listener's answers. The initial step size of 5 dB was first reduced to 3 dB after the first three reversals and finally to 1 dB after two more reversals. Thresholds were computed as the average level of the last four reversals.

For each situation (i.e. for a given warning signal and a given protection condition), the masked threshold measurement was repeated three times and the retained masked threshold is the mean of the three thresholds. When the standard deviation of the three thresholds exceeded 3 dB, a fourth measurement was performed and the retained threshold was computed as the mean of the three nearest thresholds.

RESULTS

Figure 2 shows the box-and-whisker plots of the measured masked thresholds with and without wearing the HPDs. For the sake of conciseness, the thresholds have been averaged across the seven warning signals, leading to a rather large spread of the values. First, regarding the thresholds without HPDs, it can be observed that the median values of the five hearing classes considered increase with increasing hearing impairment. However, the differences on the medians are rather small: for instance, the median of the most hearing-impaired class (HI4) is just around 6 dB higher than the median of the NH class.

Secondly, regarding the thresholds obtained when wearing the HPDs, the differences between the hearing-impaired classes and the NH class tend to be greater (than without HPD): for instance, the HI4 class median is around 12 dB higher than the NH class median. This is true for the two HPDs tested. However, when looking at the NH class results, it appears that the thresholds with HPDs are lower than the thresholds without HPDs.

Figure 3: Differences ∆ between masked thresholds with HPDs and masked thresholds without HPDs for the five hearing classes and the two HPDs tested. The results have been averaged across the seven warning signals tested.

To gain better insights into the influence of wearing HPDs as a function of hearing impairment, Figure 3 shows the differences ∆ between the thresholds with HPDs and the thresholds without HPDs. For the NH class, these differences almost always have negative values (around -2 dB on the medians) so wearing the HPDs seems to improve the detection. The result that the perception is generally improved for NH listeners when wearing HPDs (as compared to no HPD) is consistent with the results of other studies [START_REF] Arz | Effect of wearing hearing protectors on the audibility of railway warning signals -an experimental study[END_REF][START_REF] Lazarus | Signal recognition and hearing protectors with normal and impaired hearing[END_REF]. By contrast, for HI listeners, the ∆ values tend to increase with increasing hearing impairment and become mostly positive from the HI3 class, indicating a deterioration of the perception.

From further analyses made individually for each warning signal, it appears that the effect of the HPDs greatly depends on the frequency content of the warning signals. The signals that are less affected by hearing impairment are those who have a fundamental frequency that dominates in low frequency (f<500 Hz). This is certainly due to the fact that the absolute thresholds are generally moderate at these low frequencies (see Figure 1) and that the HPDs tested also have rather low attenuation values below 500 Hz; hence these signals are not or little affected by the elevation of absolute thresholds (see "Case 1 elevation" in [START_REF] Giguère | Modeling the interaction between the hearing protector attenuation function and the hearing loss profile on sound detection in noise[END_REF]). By opposition, the non-harmonic warning signal composed by two pure tones in high frequencies (3430 and 4084 Hz) is greatly affected by the hearing impairment: the median value of the deterioration reaches up to 10 dB for the earplugs and 13 dB for the earmuffs for the HI4 class. This is certainly due to the fact that both the absolute thresholds values and the attenuation values of the HPDs are large at these frequencies (see "Case 1 elevation" in [START_REF] Giguère | Modeling the interaction between the hearing protector attenuation function and the hearing loss profile on sound detection in noise[END_REF]). This signal, which has no energy below sufficient energy below 1500 Hz when HPDs are worn.

DISCUSSION

To guarantee the security of HI workers, it was considered that their protected thresholds should not be higher than the unprotected thresholds of the NH subjects. Based on this statement, statistical analyses (Wilcoxon rank sum tests) performed individually for each warning signal showed that the security is not guaranteed for the two most impaired classes considered (i.e. from PAM > 30 dB HL) and for the two HPDs tested (p<0.05). For the HI2 class (20 < PAM ≤ 30 dB HL), the security is guaranteed only for four warning signals (out of the seven signals tested). However, additional analysis showed that the security is guaranteed for the whole seven signals as soon as the unprotected thresholds of the NH subjects are majored by 2 dB (i.e. the protected thresholds of the HI2 are not higher than the unprotected thresholds of the NH subjects majored by 2 dB). This discrepancy of 2 dB seems rather small compared to the variations of the signal levels observed in the field and due to the variations of the position of the workers respective to the positions(s) of the alarm device(s). Thus, if this 2 dB difference can be tolerated, the audibility of the HI listeners when wearing the earplugs or the earmuffs is not statistically different from the audibility of the NH class without HPD up to the hearing class HI2 (20 < PAM ≤ 30 dB HL). Moreover, by considering another hearing class such as 15 < PAM ≤ 25 dB HL (N=20), results show that the protected thresholds for this class are not statistically different from the unprotected thresholds of the NH group. Overall, these results suggest that the security of HI workers is guaranteed up to around 25 dB of hearing loss (dB HL), in average at 500, 1000 and 2000 Hz in the best ear.

Furthermore, for security purposes, this laboratory study should be complemented by other listening tests in the field. Indeed, this study only evaluated the influence of HPDs by comparisons between the thresholds with and without wearing the HPDs. It did not evaluate the influence of HPDs on other aspects such as the localization of the warning signal or the perceived urgency of the warning signals [START_REF] Vaillancourt | Comparison of sound propagation and perception of three types of backup alarms with regards to worker safety[END_REF].

Another limitation of this experimental study is that it is not possible to evaluate, for each warning signal, which phenomenon among absolute thresholds or broadened auditory filters (see [START_REF] Giguère | Modeling the interaction between the hearing protector attenuation function and the hearing loss profile on sound detection in noise[END_REF]) is mainly responsible for the degradation of the audibility. To do so, a predictive model is currently being developed and is presented in a companion paper at Inter-Noise 2019 [START_REF] El Sawaf | Audibility of warning signals: methods to evaluate the combined effects of hearing protectors and hearing impairment[END_REF].

CONCLUSION

This experimental study on the influence of wearing HPDs on the audibility of warning signals show that the audibility is generally improved for NH listeners whereas it tends to be deteriorated for HI listeners and the deterioration increases with increasing hearing impairment.

To guarantee the security of HI workers, it was considered that their protected thresholds should not be higher than the unprotected thresholds of the NH subjects. Based on this statement, the results show that the security of HI workers is guaranteed up to around 25 dB of hearing loss (dB HL), in average at 500, 1000 and 2000 Hz in the best ear.

Besides, to gain better insights into which phenomenon (among elevated absolute thresholds or broadened auditory filters) is responsible for the degradation of the audibility, a predictive model of masked thresholds is currently under development. Such a model should ease the evaluation of the influence of HPDs by multiplying the scenarios of background noises, warning signals, attenuations of HPDs and hearing impairments to guarantee the security of the workers.

Figure 1 :

 1 Figure 1: Mean audiograms for the five hearing classes. PAM is the mean absolute threshold at 500, 1000 and 2000 Hz on the best ear.

Figure 2 :

 2 Figure 2: Masked thresholds measured for the five hearing classes considered and in three conditions: without HPDs, with the earplugs and with the earmuff. The results have been averaged across the seven warning signals tested.