Djilani Ahmed Abdourahman

Arthur Geniere

Mélodie Auriol

Florent Dalas

Anne- Laure Biance

Marie Le Merrer
email: marie.le-merrer@univ-lyon1.fr

Generation and stability of cement soap films †

Foaming a cementious suspension is a complex process that involves many multiscale chemical, physical and dynamical mechanisms. As a first step, we investigate here experimentally the possibility of withdrawing a single liquid soap film from a suspension of cement. We then determine the film lifetime and if particles are entrained or not. We vary the cement concentration, grain size, rheological properties and withdrawing velocity. We observed that the rheology of the cement paste, characterized through its yield stress, plays a key role in the film formation. We show that an optimum exists, as a low yield stress promotes film creation but is detrimental to the film stability. Another key result is that the rheology alone is not enough to describe film formation: the particle size in the suspension is also crucial, with large particles promoting film creation. Finally, we found that the withdrawing velocity also affects the ability to create films and the possibility to drag particles in them. Experiments performed with a silica suspension for comparison confirm these findings.

Introduction

Aerated porous solids are interesting materials combining the properties of their solid matrix, such as mechanical properties or fire resistance 1 , to the advantages of air: they are light, have good thermal and phonic insulating properties and reduce the quantity of raw material content, helping industry to address crucial environmental challenges. Besides, they have a porous structure with a large surface to volume ratio. The solid material can be polymeric, metallic 2 or ceramic. Examples of ceramic foams include materials made from sintered alumina or silica colloids 3,4 , hydroxyapatite or calcium phosphate foams used as scaffolds for bone replacement applications 5,6 , and of course cement or concrete foams 7 . Concrete foams allow the building industry to improve the thermal efficiency of building materials, as well as to reduce the environmental footprint due to the production of Portland cement 7 , the most commonly used one, whose composition is presented in the following.

Elaborating a solid cement foam first requires to foam, i.e. incorporate air in, a cement paste. This first step is not as simple as one could expect, because of the properties of the paste. First, the paste is not only a reactive fluid, but it also has a complex rheology, solid-like at low stresses, but able to flow, at stresses larger than a yield stress σ Y 8 , with a large apparent viscosity.

Unfortunately, highly viscous suspensions show lower foamabilities 9 and the complex rheology (in particular the yield stress) also modifies the stability of the foamed structure 9,10 . Secondly, the paste is also a suspension of particles, typically 1 to 100 microns in size, hence comparable to the typical thickness of fresh soap films [11][12][13] . This then raises new questions on the ability of particles to be trapped in the films. Finally, in order to stabilize the liquid-gas interfaces, surfactants are incorporated in the paste. If they are ionic and have a charge opposite to that of the particles (anionic surfactants for positively charged Portland cement grains), surfactants may adsorb on the particles. A first consequence is that the paste rheology is modified 14 . But this also renders the particles hydrophobic and surface-active, enabling the formation of so-called particle-stabilised foams 9,[15][16][17] . This favors the formation of particle-laden films which may ultimately solidify as cement films 18 and form closed cell foams with a low thermal conductivity [START_REF] Gibson | Cellular Solids: Structure and Properties[END_REF] . To summarize, the role of physico-chemical conditions on the fresh foam structure is now understood.

On the contrary, little is known on the dynamical aspects of the foaming of cement pastes, and the foaming of suspensions of particles in general. In the foaming process, the paste and foamed paste undergo strong shear, which might play a key role, in particular by breaking aggregates of particles which might further re-assemble in the films [START_REF] Deleurence | [END_REF] . Finally, how dynamical processes and the presence of particles modify film stability remains an open question. Whereas the formation of soap films is known to be governed by surface mechanical properties 21 , which are dramatically modified by the presence of particles 22 , less is known about film stability, which depends on the mechanical deformation or stress 23 and on the presence of particles that can either inhibit 24 or trigger rupture 25 .

To get some insight on the role of dynamical parameters-shear rates and stresses-on the foamability of fresh cement pastes, we focus here on the particular case of the generation of a single soap film. We investigate the influence of the solid mass fraction, the average particle size and the velocity of film making and determine the ability to generate foam films, whether they contain particles and their stability.

Materials and methods

Samples

We perform experiments on two different systems: suspensions of non reactive silica beads and reactive fresh cement pastes, composed of cement (Lafarge-Holcim) and of an aqueous surfactant solution. The surfactant nature and concentration are specified below. In some suspensions, a polymeric rheology modifier is also incorporated, as explained below.

The mass fraction of solid material in the cement (resp. silica) suspension, denoted as w, is varied in the range 0.5 -0.75 (resp. 0.49 -0.83). This corresponds to a solid volume fraction in the range 0.24 -0.56 (resp. 0.27 -0.66).

On the one hand, the silica particles (Silibeads) are used as received. Their average size varies between 3 and 30 µm, so they are not colloidal. To probe the effect of particle size, two granulometric distributions have been used, whose characteristics are reported in Table 1. At these mass fractions, the obtained silica suspensions look like pastes.

On the other hand, the cement powder consists in a mix of calcium silicates and aluminates, with a grain size ranging from a few microns to a few tens of microns. In this study, two different cement types are used, whose granulometric properties are summarized in Table 1. Most of the experiments were performed with the product LHY-05381 (CEM I 52.5R from Le Havre Lafarge plant), which we refer to as regular. For a few experiments, a cement with smaller particles (LHY-04190 CEM I 52.5N from Saint-Pierre-la-Cour Lafarge plant), which we refer to as fine, was used. In both cases, the grains have a polyhedral shape. Average grain sizes and standard deviations are then varied by a factor 2 to 4. These cements are composed of 95% of clinker (calcium and aluminium silicates) and 5% of calcium sulfate.

When dispersed in water, the cement particles undergo attractive interactions, mostly due to van der Waals forces and also to ion-ion correlations due to the presence of divalent calcium ions coming from the cement dissolution 26 . Consequently, the fresh dispersion is a paste, i.e. a yield stress fluid, whose yield stress and apparent viscosity increase with the cement content 8 . The fresh paste is always used between 10 and 45 minutes after elaboration, before the acceleration of the setting reaction in the ma-terial (3 hours 27 ) and to avoid significant changes in the paste rheology. This is supported by the fact that we did not observe any time dependency of our measurements.

In order to stabilize the liquid-gas interfaces, as well as to ensure a good capture of the cement particles in the film, an ionic surfactant with an electrical charge opposite to that of the particles is used. For suspensions of negatively charged silica, we use the cationic surfactant TTAB (Tetradecyl Trimethyl Ammonium Bromide). Cement suspensions are composed of various mineral phases whose surface potentials strongly depend on the physicochemistry of the solution which evolves upon time 28,29 . However, it is well known that anionic additives like superplasticizers or anionic surfactants efficiently adsorb on cement particles to modify their rheology or their contact angle 14,18,[START_REF] Flatt | Understanding the Rheology of Concrete[END_REF] . We therefore incorporate Sodium Dodecyl Benzene Sulfonate (SDBS), an anionic surfactant, in the cement suspensions. Previous works have shown that the surfactant-induced hydrophobization of particles, which ensures their capture in the soap films, is favored when the surfactant is concentrated enough to adsorb on the solid surfaces, but not too large to prevent surfactant aggregates or bilayers 14,16,18 . The optimal concentrations were found close to 10% of the critical micellar concentration 18 so we choose this value in our study. The initial concentration of SDBS in the solution for cement is therefore fixed to 0.167 mM, and the concentration of TTAB in the solution for silica is 0.45 mM. Finally, in some experiments (cement samples B and C), we also add to the solution polycarboxylate grafted polyethylene oxide (PCE, Chryso Premia 162) which acts as a polymeric superplasticizer [START_REF] Flatt | Understanding the Rheology of Concrete[END_REF] . The polymers are known to graft onto the cement surface and to promote steric repulsion between the otherwise attractive grains. The addition of PCE inhibits the aggregation of the cement particles, and thus results in a viscosity and yield stress decrease, at a given cement content. To ensure that the cement surfaces are well covered by the polymer, the concentration of superplasticizer is chosen to be proportional to the cement concentration and to its specific surface area determined from BET test and reported in table 1 [START_REF] Flatt | Understanding the Rheology of Concrete[END_REF] . The concentration of PCE used is therefore fixed to 0.12 % of the cement mass (resp. 0.44%) for the regular cement LHY-05381 (resp. fine cement LHY-04190). When a superplasticizer is used, the cement is first mixed with one half of the water with PCE, before the second half that contains the SDBS surfactant is added.

Yield stress measurements

The rheology of the fresh pastes is that of a yield stress fluid, i.e. they can only flow for shear stresses larger than a yield value σ Y . We therefore measure the yield stress values for our samples.

In order to assess a large number of formulations with a simple procedure, we used a mini-cone test, a labscale adaptation of the so-called slump test [START_REF] Roussel | [END_REF]32 as depicted in figure 1: a normalized cone (38 mm lower and 20 mm upper diameter, 55 mm height) deposited on a glass plate is filled with the paste and slowly lifted. Under the action of gravity forces, the paste spreads on the surface and we measure the final diameter D f of the slump. At the end of the process, gravitational pressure is balanced by the yield 1 Characteristics of the solid materials (silica beads, cement) used in our experiments. The cement particle size has been obtained by laser granulometry and the surface specific area (SSA) by BET method. The ratio between superplasticizer (PCE) and cement weights is kept constant for a given sample, whatever the cement mass fraction w. The silica average particle size has been deduced from sieve analysis provided by producer.

stress σ Y that then reads [START_REF] Roussel | [END_REF]32 :

σ Y 225ρgΩ 2 4π 2 D 5 f (1)
Here Ω = 38 mL denotes the cone volume, ρ is the fluid density and g is the acceleration due to gravity. Note that Eq.1 is accurate only when surface tension is negligible, i.e. for D f smaller than ≈ 20 cm or σ Y larger than 0.3 Pa 33 , which is not always verified here. However, as our study is mostly comparative, we will still use Eq.1 even though this assumption is not always strictly valid. This method is also limited at high yield stress, when the fluid will not spread on a distance significantly larger than the bottom diameter of the cone. This corresponds to yield stresses larger than 500 Pa in our experimental conditions. Figure 2 shows our yield stress measurements as a function of the particle mass fraction w for the different systems considered. As expected, we observe that the paste yield stress σ Y increases with the solid fraction w, as predicted by the Yodel model 34 and that it decreases upon addition of superplasticizer (going from samples A to B in figure 2a). Yield stress values without superplasticizer have the same orders of magnitude as those reported in the literature for cement pastes with anionic surfactants at concentrations much lower than the critical micellar concentration, around 1 Pa for w = 0.5. 14 We also observe that both cement samples with superplasticizer (B and C) show comparable yield stresses at a fixed cement concentration.

Experimental set-up

The experimental setup is depicted in figure 3. A 25 mL volume of fresh paste is poured in a beaker placed on a translation stage. The stage can be moved vertically at a prescribed velocity. Above the paste, we fix a square frame (inner side length L = 16 mm), with cylindrical sides (diameter 4 mm) while the lower side is kept open. The frame is made of acrylic-base resin and is obtained by 3D printing. A camera (IDS UI) is placed in front of the frame for recording images (typical frequency 30-50 Hz), which are subsequently analysed with the software ImageJ. The experiment is made as follows: the frame is initially dipped in the paste by rising the beaker and we lower the translation stage in order to withdraw the frame from the fresh cement paste at a velocity V . In this study, two withdrawing velocities, V = 2.8 mm/s and V = 28 mm/s have been used. The withdrawing is stopped when the frame is about to leave the paste, so that the withdrawn foam film, if it exists, touches the bath on its lower side (figure 4). The film is recorded from the side until its bursting, which takes typically a few tens of second. For each sample, this operation is repeated approximately 10 times. Between each withdrawing event, the sample has been re-homogenized with a spatula for around 15 seconds in order to limit sedimentation of the cement particles in the beaker. In the following, we vary the solid cement fraction in the paste, the particle size and the pulling velocity to define and understand what are the best conditions to obtain a stable soap film. For more details, we report in Table 2 the various experimental conditions tested, and the number of experiments performed, one experiment corresponding to typically 10 withdrawing events.

Results

First observations

Depending on the experimental conditions, two different behaviors are observed. On the one hand, some withdrawing events lead to the formation of particle laden films (case (i)), which can last more than 10 s (and up to more than 200 s) after the end of the withdrawing stage (figure 4a and supplementary movie 1 †). On the other hand, in case (ii), a film cannot be created, i.e. breaks before the end of the opening or immediately after (less than 5 s, and most often less than 1 s). A typical image sequence is shown in figure 4b (see also supplementary movie 2 †). In the following, we therefore try to characterize which experimental conditions lead to the first or second situation.

Probability to make a cement foam film 3.2.1 Influence of cement mass fraction

In order to characterize the foam film formation from the cement suspension, we therefore calculate for each experiment the probability P to make a foam film, defined as the number of films corresponding to case (i), divided by the number of withdrawing events (typically 10 in each experiment). We consider that a film is stable if it lasts more than 3 seconds for the cement suspensions, and 1 second for the silica suspensions. Indeed, even if they are almost always predictive, our experiments show in some conditions fluctuations of the outcome, which can be attributed to external factors such as mechanical vibrations of our set-up or internal ones, such as particle polydispersity or suspension heterogeneities. Error bars reflect the number of withdrawing events and experiments we performed in each case. We first investigate the influence of the cement mass fraction w, at fixed velocity V = 28 mm/s. Figure 5 shows that, for all sample types, P decreases with w. As the particle concentration is increased and the fresh cement becomes more and more pasty and viscous, it becomes more difficult to create a foam film. For instance, for sample A (regular cement LHY-05381, no superplasticizer), the probability to make a soap film drops abruptly from P ≈ 1 to P ≈ 0 for w in the range 0.55 -0.65.

In cement-based applications, it is however crucial to ensure a minimum cement mass fraction in the fresh paste, as it is directly related to the mechanical properties of the final setted cementitious material. A natural direction to improve the possibility to form cement films at high cement fraction is then to incorporate a superplasticizer, which is expected to improve the paste workability, that is, to decrease the paste yield stress and apparent viscosity [START_REF] Flatt | Understanding the Rheology of Concrete[END_REF] .

Figure 5 therefore shows that the same qualitative behavior is observed when superplasticizer is incorporated -from samples A to B-but the drop in foamability P is observed for a higher cement mass fraction, typically w = 0.7 instead of w = 0.6. This suggests that making the cement paste more fluid is a necessary step to improve foamability, here the possibility to make a cement film. In the following, we therefore quantify how this probability P is influenced by the rheology of the fresh cement and silica pastes, via their yield stress σ Y .

Influence of yield stress and particle size

Based on slump test measurements of the yield stress σ Y (section 2.2, we plot in figure 6 the probability to make a foam film P as a function of σ Y , for various sample types, for both cement and silica pastes. For each sample type, the yield stress is varied with the solid mass fraction.

As expected, we find that in all cases the probability P drops when increasing σ Y . Comparing cement samples A and B (respectively without or with superplasticizer), we also observe that this change of behaviour occurs at a larger yield stresses without superplasticizer (figure 6a): the transition occurs for σ Y ≈ 0.5 -1 Pa without superplasticizer (sample A), compared to σ Y ≈ 2 -10 Pa with superplasticizer (sample B). This suggests that the paste rheology is not the only parameter that determines the ability of the withdrawn cement to form a stable foam film.

Beyond their macroscopic effect on rheology, superplasticizers like PCE are known to drastically reduce the aggregation of the cement particles, by decreasing the particle-particle attraction through steric effects [START_REF] Flatt | Understanding the Rheology of Concrete[END_REF] . The effective grain (i.e. aggregate) size is therefore expected to be larger without the fluidizing additive. We therefore wonder what is the influence of the cement grain size on the cement apparent foamability.

To assess this point, we therefore conducted additional experiments with a finer cement LHY-04190 with superplasticizer (sample C, see table 1). While the cement grain size has little influence on the paste yield stress (compare samples B and C in figure 2a), the transition from P ≈ 1 (cement films can be created) to P ≈ 0 (no cement films) occurs for a much lower yield stress σ Y ≈ 0.2 -0.3 Pa for the finer cement (sample C). This result confirms the expectations from the effect of superplasticizer: for a constant suspension yield stress the smaller the grain/aggregate size, the more difficult it is to create a cement foam film (figure 2a). At this point, it should be mentioned that there are also variations in the chemistry of samples A, B and C that could explain the observed trend. However, the effect of the particle size is also observed for our simpler model system, the silica pastes, for which the solution chemistry is the same for all samples. For σ Y ∼ 200 Pa, we find that P = 1 for particle size in the range 0 -50 µm (sample a), while it drops to P ≈ 0.3 for finer particles (sample b, range 0 -20 µm).

Influence of withdrawing velocity: particle capture

As we are interested in the role of dynamic conditions, we now investigate how the withdrawing velocity modifies the probability to make pasty soap films. We performed experiments with similar conditions (same setup and samples), but reducing the withdrawing velocity by a factor of 10. At the smaller velocity V = 2.8 mm/s, a new regime is observed: particle-free film can be created, as reported in the inset of figure 7a. We observed this behavior for both cement sample A (regular cement LHY-05381, no superplasticizer) and silica sample b (0-20 µm particles), under some specific experimental conditions.

More precisely, for both samples, figure 7 shows the probability to make a cement or particle laden film P C , as well as the probability P to make a foam film, which may indifferently contain particles or not, as a function of the solid mass fraction w. P C = P means that all created films contain particles, while P C = 0 and P = 1 means that we always draw particle-free films. For the larger velocity (V = 28 mm/s), particles are present in almost all cases where a film is created. For the smaller velocity, we find that particle-laden films can be created for larger solid fractions w, and that particle-free films are still stable at even larger w.

Lifetime of pasty soap films

In order to elaborate a mineral foam with disconnected cells, cement paste foams have to be stable enough and not to collapse in order to enable setting to take place inside the cement laden films. This requires that the cement foam film lasts long enough. To quantify the cement film stability, we therefore measure the lifetime of the withdrawn cement laden films.In figure 8, we plot the average lifetime of pasty foam films T (obtained at V = 28 mm/s) as a function of the solid mass fraction w for two cement samples and one silica suspension.

The lifetime T is defined as the time elapsed between the end of the drawing phase and the film bursting.

For the cement sample B (figure 8a), we observe that T significantly increases (it is multiplied by a factor close to 6) as w increases from 0.5 to 0.75. For the silica suspension (figure 8b), we observe that the soap film lifetime goes through a maximum for w ≈ 0.6. We discuss these seemingly antagonist behaviors in section 4. Finally, the film lifetime is also increased by a factor close to 3, at fixed solid fraction, when the particle size is decreased (from cement samples B to C, figure 8a).

It should also be noted that the longest lifetimes are measured at concentrations for which very few drawing events actually result into a stable film, as shown by the probability results reproduced in figure 8. This suggests that for a given V -which could be fixed by the processing shear rates in a macroscopic foamthere exists an optimal cement concentration, which makes a compromise between the possibility to form cement laden films and their subsequent stability.

Discussion

Capture of particles in the film

We first discuss the ability for the created soap film to capture particles. This point is crucial for the applications in cementitious materials processing, as the presence of particles is a prerequisite to make solids with a closed porosity.

In particular, for a fixed cement concentration w, we find that particles can be captured only for the larger velocity (figure 7). This observation is reminiscent of experiments of dip-coating (supported films) of granular suspensions 35,36 which evidenced that only liquid and no particles were dragged below a threshold velocity.

A naive interpretation of this phenomenon is that for very viscous (or high yield stress) pastes, it is more favorable to suck water out of the paste than to actually deform it as a whole. The typical stress associated with the deformation of the paste is the yield stress σ Y . Based on Darcy law, and assuming the paste permeability reads αd 2 , a typical stress to suck liquid out of the cement suspension scales as η 0 V /(αd), with η 0 the dynamic viscosity of the interstitial solution. We will thus expect to get foam films containing cement particles for σ Y (η 0 V )/(αd), that is, for low enough concentrations, high velocity or small particles.

In this simple model, the effect of particle size is reminiscent of what is observed for the entrainment of suspension in dip-coating experiments in various geometries 35,37 .

In addition, we can estimate the velocity at which a transition is expected for the samples shown in figure 7. For cement sample A (cement without superplasticizer, figure 7a), we have σ Y ∼ 10 Pa for w = 0.65. We have no characterization of the aggregate size in our pastes without superplasticizer, but optical measurements in the literature 38 suggest it can be as large as five times the initial particle size, that is d ∼ 100 µm. From these and taking 39 α ∼ 10 -2 , we expect a crossover from particle laden to particle free films for V = V c = σ Y αd/η 0 ∼ 1 cm/s, which is indeed compatible with the different behaviours evidenced for V = 28 and 2.8 mm/s.

For the silica sample b at the lower velocity V = 2.8 mm/s (figure 7b), we also observe that the particles are no longer captured when w ≈ 0.77 -0.79. The estimated critical stress σ c ∼ η 0 V /(αd) is 100 Pa (taking d ≈ 3 µm), of the same order of magnitude as the yield stress values (figure 2b).

Estimation of the thickness h of the film

We have found in section 3.2.2 that the cement grain size d modifies the ability to withdraw a cement film from the paste. More precisely, we find that reducing the particle size lowers the probability to form a film for a fixed suspension yield stress σ Y . It would therefore be useful to compare d to a natural lengthscale in this film drawing problem, which is the thickness h of the created film.

Prediction by continuum mechanics versus particle size

Starting from the pioneering work of Mysels 40 and up to recently 11,12 , a lot of efforts have been devoted to the case of (particle-free) soap film withdrawing. From a modeling point of view, it was first proposed [START_REF] Mysels | Soap films: studies of their thinning and a bibliography[END_REF] that the thickness of the film is selected by a balance between viscous forces and surface tension: h ∼ aCa 2/3 , where a = γ/ρg is the capillary length, which compares gravity to surface tension effects and Ca = ηV /γ with γ the fluid surface tension and η its viscosity. In more details, the selected thickness thus depends on the surfactant dynamics and its diffusion and availability in the thin film, but under appropriate conditions, these dependencies with withdrawing velocity and fluid viscosity are reasonable. For yield stress fluids like cement paste, Derjaguin and Levi 42 have shown that this expression can be modified in order to account for the actual viscous stress in the film, which is, in the limit of vanishing velocities, σ Y instead of ηV /h. The modified prediction for the film thickness is therefore h ∼ a 3 (σ Y /γ) 2 : the film thickness is expected to increase with the yield stress, i.e. in our experiments, with the solid mass fraction w. This law was put in default by experiments [START_REF] Maillard | [END_REF]44 in similar yet different case of a plate pulling a film of a polymer microgel, due to elastic interactions between the plate and the fluid in the bath. However, in our free film configuration, these elastic interactions do not exist, and there is, to the best of our knowledge, no experimental verifica- tion or contradiction of the prediction of Derjaguin and Levi. We therefore use it to estimate a typical film thickness.

In cement pastes containing Sodium Dodecyl Sulfate surfactant, very similar to the SDBS used in the present study, the surface tension at 0.1 times the critical micellar concentration was found close to 50 mN/m 18 . For sample B (regular cement LHY-05381 with superplasticizer), the transition to no film creation occurs at w ≈ 0.7, corresponding to a cement paste density of 1900 kg/m 3 , which yields a = 1.6 mm. For σ Y = 1 Pa, we therefore estimate h ∼ 2 µm, 10 times smaller than the average particle size (24 µm). For sample C (fine cement), the transition occurs at w ≈ 0.63 and σ Y ≈ 0.25 Pa, corresponding to h ∼ 0.12 µm, typically 1/60 of the average particle size 7.1 µm. For the cement pastes, we therefore expect the prediction of Derjaguin and Levi, based on continuum equations, to fail in this case due to the granular character and the particle size of the fresh cement suspension.

In the slightly different geometry of dip-coating (film withdrawn from a bath by a solid plate), recent works 35,36 have 1 cm Fig. 7 Probabilities to make a cement or particle-laden film P C (solid symbols) and a foam film P (empty symbols) as a function of w, for two different withdrawing velocities V = 2.8 mm/s (blue diamonds) and V = 28 mm/s (red squares). A foam film may indifferently contain particles or not. (a) cement sample A (regular cement, no PCE), (b) silica sample b (particle size 0-20 µm). Inset: image of a particle-free soap film obtained for sample A at w = 0.7 and V = 2.8 mm/s.

shown that, as the velocity and the viscous stress are decreased, the entrained film saturates to a thickness comparable to the particle diameter d. Similarly, we can expect the thickness of the entrained soapy cement film to scale as the typical particle size.

On the contrary, for the silica suspension of case b, the transition occurs at w ≈ 0.72 and σ Y ∼ 150 Pa, yielding h ∼ 4 cm (with γ ≈ 60 mN/m the surface tension for the pure surfactant solution 14 ). This is orders of magnitude larger than what is observed by naked eye on the sample (which actually form a film). This discrepancy could be explained by the effect of gravity, neglected in the thin film model of Derjaguin and Levi but which should play a role at large thicknesses.

Light absorption by the cement films

To compare the quantity of particles entrained, we measured for one sample the light intensity transmitted through the film for various solid fractions. The cement grains both absorb and scatter 2 for the number of films considered. The error bars correspond to the standard deviation.

light, so that lower intensities indicate that more particles are entrained. We report in figure 9 the transmitted light intensity, normalized by the incident light, I/I 0 as a function of w, for cement sample B. We observe that the transmitted intensity decreases as the paste solid fraction and the velocity increase, indicating that more particles are captured. This suggests that the film thickness increases with the withdrawing velocity and the solid fraction, as expected since the viscous stresses get larger for both cases. However, extracting the thickness from the transmission coefficient would require a thorough optical calibration of the cement pastes, beyond the scope of the present study. Besides, it could also be modified by the local concentration of particles in the film, which may be different from the one in the bath. Discriminating between the two effects is unfortunately not possible with such images and would require independent measurements of the liquid and solid content, possibly through fluorescence measurements.

Possible criteria for film generation

To summarize, we find in our experiments that the probability to make a soap film P decreases at large concentrations w, corresponding to large yield stresses of the suspension σ Y . For a given yield stress, an effect of the particle size d is observed, large particles promoting film formation. We discuss here possible mechanisms which could set the stability of the freshly generated particle film.

For soap films of pure liquid, we expect the film to remain stable if its weight (per unit width) ρghL does not exceed the surface tension variation in the film ∆γ at the origin of Marangoni stresses 23 . If we consider that continuum equations are still valid for our granular films, we can use the prediction of Derjaguin and Levi h ∼ a 3 σ 2 Y /γ 2 to predict that the film will break for σ 2 Y > γ∆γ/(La). Increasing the particle concentration increases the yield stress σ Y and destabilises the film. Taking γ = 50 mN/m and ∆γ = 1 mN/m (typical value from the literature 23 ), we find that the destabilisation should occur for σ Y ∼ 2 Pa. This result, which seems at first order in good agreement with our measurements on cement paste, does not explain many of our observations: the effect of the particle size and the larger yield stress for the silica suspensions.

Taking into account the granular nature of the suspension, and assuming that the entrained film thickness scales as h ∼ d as in dip-coating experiments 35,36 , we expect unstable films for larger particle size, which is incompatible with our observations. Several ingredients are also likely to play a non-trivial role on the film stability. First, the role of the strong film-induced confinement on the paste rheology should be considered. Second, we expect the solid particles to play a surface-active role in our experiments, given the charge and concentrations of chosen surfactants. In this case, particle-stabilized monolayers are expected to be stable for capillary pressures up to γ/d 45 . Smaller particles might therefore induce more resistant films. On the other hand, since we work at fixed SDBS surfactant concentration, finer particles could be less covered by the surfactant, hence less surfaceactive, since they have a larger specific area. Finally, the role of film inhomogeneities should also be investigated. We indeed observed zones of the film (without cement visible for instance in figure 4b, are more prone to bursting, as reported in the literature in other configurations for model particulate systems 24,25 .

Film stability

We now discuss the stability of the fresh cement film, which can be characterized by its lifetime T , of the order of tens of seconds. For cement films, we found that T increases with the solid fraction w. This could be due to the higher viscosity of the paste as w increases, resulting in a slower drainage of the film.

On the other hand, for the silica films, we observe a maximum lifetime for an intermediate solid mass fraction w ≈ 0.6. The decrease in lifetimes at high solid fractions could then be due to a lower solid fraction in the film, resulting in a less viscous film. We indeed observed that particle-free films could be generated at high solid fraction but the intermediate situation of a film poorly charged in particles may also be possible.

Finally, we also observe a slight increase of the film lifetime when the particle size is smaller, reminiscent of observations performed on expanding liquid sheets 25 . This point is attributed to the fact that large particles create large heterogeneities in film thicknesses, leading to particle bare zones of very unstable thin films 24,25 . The influence of the particle size on the film lifetime could also be linked to the surface active role of the particles, as mentioned in the previous section.

Conclusion

In this work, we investigated the creation and stability of pasty foam films, created by withdrawing a rectangular frame from a bath of paste, which is either a fresh cement or a suspension of silica particles. This setup is a model experiment to study the dynamics of soap film creation, the elementary process of foaming. We have investigated the effect of the solid mass fraction, rheology modifier (superplasticizer), particle size and withdrawing velocity.

The first result is that cement films can only be created at low enough concentrations, i.e. for less viscous pastes. On the other hand, we found that the film stability is improved at high solid fraction, so that there exists an optimal concentration to ensure both foamability and foam stability, as also encountered in macroscopic 3D foams 9,10 .

Second, we evidenced that, beyond the paste rheology, the cement grain size plays a non-trivial role on the foamability: for a fixed suspension yield stress, large particles promote film creation.

Finally, we found that the withdrawing velocity also affects the ability to create a soap film, and its content (presence of particles). This suggests that the dynamics of the process of generating the foam might also modify the structure of the fresh liquid foam, and of the final solid foam (obtained after setting of the cement or silica sintering).

It would be interesting in the future to test to which extent these results could be applied to the elaboration of macroscopic 3D foams. In particular, in various foaming processes -direct foaming, or mixing of paste and foam-the foamy paste undergo deformations whose rate could affect the foam structure.

At the film scale, this study also calls for an extensive characterisation of the entrained film (thickness and local concentration of particles), which could be envisaged through the combination of various optical methods like optical transmission and laser-induced fluorescence measurements.

Fig. 1

 1 Fig. 1 Slump test used to measure the yield stress of the cement paste.

Fig. 2

 2 Fig. 2 Yield stress σ Y as a function of w. (a) Cement pastes: blue diamonds: sample A (regular cement, no PCE), black circles: sample B (regular cement with PCE), red squares: sample C (fine cement with PCE). (b) Silica suspensions: blue diamonds: sample a (particle size 0-50 µm), red squares: sample b (particle size 0-20 µm). The solid lines are guides to the eye following the Yodel model 34 .

Fig. 3

 3 Fig. 3 Sketch of the experimental setup.

Fig. 4

 4 Fig. 4 Image sequences of two withdrawing events for w = 0.6 (sample A, regular cement without PCE, V = 28 mm/s). (a) Case (i): Stable cement film (lifetime 18.7 s). The timelapse between successive frames is 3.75 s. (b) Case (ii): The withdrawn film breaks before the end of the frame pulling. The timelapse between successive frames is 0.05 s, much shorter that in the first image sequence. See also corresponding movies as ESI †.

Fig. 5

 5 Fig. 5 Probability to create a foam film P as a function of the solid mass fraction w for V = 28 mm/s. (a) Cement pastes: blue diamonds: sample A (regular cement, no PCE), black circles: sample B (regular cement with PCE), red squares: sample C (fine cement with PCE). Each data point corresponds to averages over 1-5 different experiments, see Table 2 for details. (b) Silica suspensions: blue diamonds: sample a (particle size 0-50 µm), red squares: sample b (particle size 0-20 µm).

Fig. 6

 6 Fig. 6 Probability to create a foam film P as a function of the yield stress σ Y . (a) Cement pastes: blue diamonds: sample A (regular cement, no PCE), black circles: sample B (regular cement with PCE), red squares: sample C (fine cement with PCE). (b) Silica suspensions: blue diamonds: sample a (particle size 0-50 µm), red squares: sample b (particle size 0-20 µm). The withdrawing velocity is fixed to V = 28 mm/s.

PFig. 8

 8 Fig.8 Pasty film lifetime T (black) and probability to make a film P (red) as a function of the solid mass fraction w: (a) cement samples B (solid symbols, regular cement with PCE) and C (empty symbols, fine cement with PCE), (b) silica sample b (particle size 0-20 µm). The withdrawing velocity is V = 28 mm/s. Each point corresponds to an average over 3-49 stable films, see Table2for the number of films considered. The error bars correspond to the standard deviation.

Fig. 9

 9 Fig.9 Normalized transmitted intensity I/I 0 as a function of the solid mass fraction w for sample B (regular cement with PCE) at two withdrawing velocities V = 2.8 mm/s (empty symbols) and V = 28 mm/s (solid symbols).

  Table

		Sample		Density (kg/m 3 )	Particle size (µm)
		Cement type	SSA (m 2 /g) PCE ratio			average std
	A LHY-05381 (regular)	0.8	-	3130		24	21
	B LHY-05381 (regular)	0.8	0.12%	3130		24	21
	C	LHY-04190 (fine)	4.5	0.44%	3130		7.1	5.1
		Silica type			range average std
	a	SiLiBeads 5210			2500	0-50	28	12
	b	SiLiBeads 5209			2500	0-20	3.4	2.8

Table 2

 2 For each experimental condition (one sample, one solid fraction, one withdrawing velocity), we report the number of experiments performed. One experiment corresponds to approximately 10 withdrawing events. When filled, the last column indicates the number of films used to calculate the average film lifetimes reported in figure8.

	Sample	V	w	number of	number
		(mm/s)		experiments of films
			0.50	3
			0.55	2
		28	0.60	2
			0.65	3
	A		0.70	2
			0.55	1
		2.8	0.60	1
			0.65	1
			0.70	2
			0.50	2	10
			0.60	5	49
	B	28	0.65	3	29
			0.70	5	19
			0.75	4	4
			0.55	1	7
	C	28	0.60	1	4
			0.65	1
			0.713	1
			0.767	2
			0.775	1
	a	28	0.780	2
			0.803	2
			0.810	1
			0.817 *	2
			0.49	1	8
			0.59	1	11
			0.69	1	11
		28	0.71	1	9
			0.721	2
	b		0.725	3
			0.73	1	3
			0.77	1	-
			0.49	1
			0.77	1
		2.8	0.79	1
			0.81	1
			0.83	1

* At this high concentration, note that the suspension is not fluid anymore.

[1][2][3][4][5][6][7][8][9][10][11]

Acknowledgements

This work was funded by the National Association of Research and Technology (ANRT, France) and the LafargeHolcim Innovation Center. We thank Véronique Schmitt, Rénal Backov, Martin Mosquet, Isabelle Javierre, Pierre-Henri Jézéquel and Pauline Petit for stimulating discussions. We thank Gilles Simon for his help for the experimental setup.


Orianne Dumas
email: orianne.dumas@inserm.fr

Cleaners and airway diseases

Keywords: disinfectants, cleaning products, occupational airway diseases, asthma, COPD

Purpose of review: Evidence for adverse respiratory effects of occupational exposure to disinfectants and cleaning products (DCPs) has grown in the last two decades. The relationship between DCPs and asthma is well-documented, but questions remain regarding specific causal agents. Beyond asthma, associations between DCPs and COPD or chronic rhinitis are plausible and have been examined recently. The purpose of this review is to summarize recent advances on the effect of occupational exposure to DCP and chronic airway diseases.

Recent findings:

Recent epidemiological studies have often focused on healthcare workers and are characterized by efforts to improve assessment of exposure to specific DCPs. Despite increasing knowledge on the effect of DCPs on asthma, the burden of work-related asthma caused by DCPs has not decreased in the past decade, emphasizing the need to strengthen prevention efforts. Novel data suggest an association between occupational exposure to DCPs and other chronic airway diseases, such as rhinitis, COPD and poor lung function.

Summary: Epidemiological and experimental data showed that many chemicals contained in DCPs are likely to cause airway damages, indicating that prevention strategies should target multiple products. Further research is needed to evaluate the impact of DCPs exposure on occupational airway diseases beyond asthma.

Introduction

Evidence for adverse respiratory health effects of exposure to disinfectants and cleaning products (DCPs) has grown in the last two decades [START_REF] Clausen | Chemicals inhaled from spray cleaning and disinfection products and their respiratory effects. A comprehensive review[END_REF][START_REF] Dumas | Damaging effects of household cleaning products on the lungs[END_REF]. Due to their widespread use both in the general population [START_REF] Lemire | Domestic exposure to irritant cleaning agents and asthma in women[END_REF][START_REF] Weinmann | Association of household cleaning agents and disinfectants with asthma in young German adults[END_REF] and at the workplace [START_REF] Folletti | Update on asthma and cleaning agents[END_REF], DCPs raise important public health concerns. In the context of the COVID-19 pandemic, the use of DCPs has further increased in healthcare settings, as well as in other public-and work-places and in individual homes [START_REF] Zheng | Increased Indoor Exposure to Commonly Used Disinfectants during the COVID-19 Pandemic[END_REF]. Some workers experience particularly high level of exposure to DCPs, which is a major cause of work-related asthma (WRA) [START_REF] Folletti | Update on asthma and cleaning agents[END_REF][START_REF] Rosenman | Cleaning Products and Work-Related Asthma, 10 Year Update[END_REF]. Because cleaning and disinfection are crucial in some environments, such as healthcare, to prevent the spread of infections, it remains a challenging issue for prevention of occupational airway diseases [START_REF] Tarlo | Opportunities and obstacles in translating evidence to policy in occupational asthma[END_REF].

Although the relationship between occupational exposure to DCPs and asthma is now welldocumented, as reviewed in 2017 by Folletti et al. [START_REF] Folletti | Update on asthma and cleaning agents[END_REF], research in this field is still very active, as questions remain in particular regarding specific causal agents. Beyond asthma, it is plausible that inhalation of irritants and sensitizers from DCPs may be associated with other chronic respiratory diseases, such as chronic obstructive pulmonary disease (COPD) or chronic rhinitis [START_REF] Dumas | Association of Occupational Exposure to Disinfectants With Incidence of Chronic Obstructive Pulmonary Disease Among US Female Nurses[END_REF][START_REF] Folletti | Asthma and rhinitis in cleaning workers: A systematic review of epidemiological studies[END_REF], and research work on these novel hypotheses has emerged in the past few years. The purpose of this review is to summarize and discuss recent advances on the effect of occupational exposure to DCPs and chronic airway diseases, including asthma, rhinitis and COPD.

Asthma

Evidence for a relationship between occupational exposure to DCPs and asthma emerged in the early 2000's, with consistent findings from work-related asthma (WRA) surveillance schemes, reporting elevated proportions of asthma cases in cleaning and healthcare occupations, and population-or workforce-based epidemiological studies examining associations between exposure to DCPs and asthma [START_REF] Zock | Update on asthma and cleaners[END_REF]. Today, both types of studies remain useful as they provide complementary data to better characterize the relationship between DCPs and asthma, and up-to-date information to evaluate the related public health burden [START_REF] Jaakkola | Assessment of public health impact of work-related asthma[END_REF]. This section summarizes studies published since the last review by Folletti et al. [START_REF] Folletti | Update on asthma and cleaning agents[END_REF].

Work-related asthma surveillance networks and case series

Recent data from WRA surveillance networks or case series in the UK and North America showed that 8-12% of all WRA cases were attributed to cleaning products [START_REF] Rosenman | Cleaning Products and Work-Related Asthma, 10 Year Update[END_REF][START_REF] Carder | Occupational and work-related respiratory disease attributed to cleaning products[END_REF][START_REF] Walters | Cleaning agent occupational asthma in the West Midlands, UK: 2000-16[END_REF][START_REF] Li | Work-related asthma from cleaning agents versus other agents[END_REF].

Consistent with previous reports [START_REF] Zock | Update on asthma and cleaners[END_REF], most cases were from the healthcare and cleaning industries [START_REF] Rosenman | Cleaning Products and Work-Related Asthma, 10 Year Update[END_REF][START_REF] Carder | Occupational and work-related respiratory disease attributed to cleaning products[END_REF][START_REF] Walters | Cleaning agent occupational asthma in the West Midlands, UK: 2000-16[END_REF]. However, a variety of industries were reported, such as education and leisure (including swimming-pool employees), reflecting the common use of DCPs in many workplaces. Regarding causal agents, the most frequent in The Health and Occupation Research (THOR) network in the UK were aldehydes, with a tendency to decrease over time, and chlorine, which maintained its contribution over the reporting period [START_REF] Carder | Occupational and work-related respiratory disease attributed to cleaning products[END_REF]. In the US, the most frequently reported causes were cleaning materials or unspecified general purpose cleaners, floor strippers or waxes, bleach, and disinfectant cleaners [START_REF] Rosenman | Cleaning Products and Work-Related Asthma, 10 Year Update[END_REF]. In a Canadian WRA case series, although the most frequent exposures included surfactants, alcohols, disinfectants and acids, workers were commonly exposed to multiple products including both potential sensitizers and irritants, and a specific causative agent was rarely identified [START_REF] Li | Work-related asthma from cleaning agents versus other agents[END_REF].

In the US, cases identified by surveillance systems from 1998 to 2012 were compared to data collected from 1993 to 1997 [START_REF] Rosenman | Cleaning Products and Work-Related Asthma, 10 Year Update[END_REF]. No decrease in the proportion of cases attributed to cleaning products was observed between the two periods. Similarly, data from the THOR network showed that in contrast with a general declining trend in occupational asthma incidence between 1989 and 2017, no reduction in cases attributed to cleaning agents was observed [START_REF] Carder | Occupational and work-related respiratory disease attributed to cleaning products[END_REF]. These data suggest that despite the increased amount of data in the scientific literature on the relationship between DCPs and asthma in the last decades, no decrease is observed in the occurrence of cleaning-related WRA.

Epidemiological studies

Epidemiological studies on the association between occupational exposure to DCPs and asthma are summarized in Table 1. An increasing number of studies have been conducted among healthcare workers, who are particularly frequently exposed to DCPs, including high level disinfectants. Recent studies are also characterized by efforts to improve assessment of exposure to specific DCPs.

Several results have been published in the context of the Nurses' Health Study II, a prospective cohort study of 116 429 US female nurses. A job-task-exposure matrix was designed in this population to evaluate exposures to specific DCPs [START_REF] Quinot | Development of a job-task-exposure matrix to assess occupational exposure to disinfectants among US nurses[END_REF]. A cross-sectional analysis first investigated the association between exposure to specific DCPs and asthma control among 4,102 nurses with asthma [START_REF] Dumas | Occupational exposure to disinfectants and asthma control in US nurses[END_REF]. Using the job-task-exposure matrix, high levels of exposure to formaldehyde, glutaraldehyde, hypochlorite bleach, hydrogen peroxide and enzymatic cleaners were associated with poor asthma control; exposure to quaternary ammonium compounds and alcohol were not. A separate analysis examined the association between hand/arm hygiene tasks and asthma control, and suggested an adverse effect of products used for surgical hand/arm antisepsis [START_REF] Dumas | Association of hand and arm disinfection with asthma control in US nurses[END_REF]. A prospective analysis in the Nurses' Health Study II further examined the association of occupational exposure to DCPs with asthma incidence [START_REF] Dumas | Occupational exposure to disinfectants and asthma incidence in U.S. nurses: A prospective cohort study[END_REF]. Exposure to DCPs was not associated with incident asthma. However, this analysis, conducted in a population of late career nurses (mean age: 55 years), was likely affected by a healthy worker effect. The Nurses' Health Study 3, a prospective cohort of early-to mid-career nurses in the US and Canada, offered an opportunity to limit this bias when investigating the association between occupational exposures and asthma. In a prospective analysis including 17,280 female nurses (mean age: 34 years), nurses who reported >5 years of use of high level disinfectants had increased risk of incident asthma (adjusted hazard ratio [95%CI], 1.38 [1.03-1.85]) [START_REF] Dumas | Occupational use of high-level disinfectants and asthma incidence in early to mid-career nurses: a prospective cohort study[END_REF]. No information was available on exposure to lower level disinfectants and common cleaning products.

In a cross-sectional study including 2030 healthcare workers in New York City, Caridi et al. [START_REF] Caridi | Occupation and task as risk factors for asthmarelated outcomes among healthcare workers in New York City[END_REF] found that the task of cleaning surfaces was associated with increased risk of current asthma. Occupations with the highest risk of current asthma were environmental service workers and registered nurses. In the same population, Su et al. [START_REF] Su | Clustering asthma symptoms and cleaning and disinfecting activities and evaluating their associations among healthcare workers[END_REF] identified health clusters based on 27 asthma symptom/care variables, and exposure clusters based on 14 products application during cleaning and disinfecting activities, separately by hierarchical clustering.

This strategy aimed at addressing both asthma heterogeneity and the multiplicity of DCPs exposures when examining their association. Several exposure clusters, reflecting various combination of DCPs use, were associated with different asthma clusters. In particular, the exposure cluster "housekeeping/chlorine" was associated with "undiagnosed/untreated asthma" and "asthma attacks/exacerbations". "General cleaning/laboratory" was associated with "asthma attacks/exacerbations". "Disinfection products" was associated with "mild asthma symptoms" and "undiagnosed/untreated asthma". Smaller cross-sectional studies conducted in various countries and settings reported overall similar results. In a survey of 347 workers employed at a veterans administration medical center in the US, several cleaning and disinfection tasks were associated with increased risk of current asthma, although results should be interpreted with caution as only unadjusted associations were presented [START_REF] Kurth | Current asthma and asthma-like symptoms among workers at a Veterans Administration Medical Center[END_REF]. In a study of 239 certified nurse aides in Texas [START_REF] Patel | Work-Related Asthma Among Certified Nurse Aides in Texas[END_REF], no significant association was observed between cleaning and disinfection tasks or products and bronchial hyperresponsivenness-related symptoms. However, this study of relatively small size may have lacked power to detect significant asssociations, despite elevated odds ratios for some exposures. In addition, nurse aides are more often employed outside hospital than other healthcare professions (e.g., registered nurses) and may thus have lower level of exposure to DCPs [START_REF] Caridi | Occupation and task as risk factors for asthmarelated outcomes among healthcare workers in New York City[END_REF][START_REF] Dumas | Determinants of disinfectant use among nurses in U.S. healthcare facilities[END_REF]. In a health hazard evaluation conducted by the National Institute for Occupational Safety and Health (NIOSH) in a US hospital including 163 workers, air samples were collected and analyzed for hydrogen peroxide, peracetic acid, and acetic acid [START_REF] Hawley | Respiratory symptoms in hospital cleaning staff exposed to a product containing hydrogen peroxide, peracetic acid, and acetic acid[END_REF][START_REF] Casey | Health problems and disinfectant product exposure among staff at a large multispecialty hospital[END_REF].

Workers in the department with the highest air measurements had higher risk of current asthma compared with the US population. Lower and upper airway symptoms were examined in a subsample of workers (n=50). No significant association between exposure and lower airway symptoms was reported [START_REF] Hawley | Respiratory symptoms in hospital cleaning staff exposed to a product containing hydrogen peroxide, peracetic acid, and acetic acid[END_REF]. In a cross-sectional study in New Zealand, cleaning workers (n=425) had higher risk of current asthma than reference workers [START_REF] Brooks | Respiratory health in professional cleaners: symptoms, lung function, and risk factors[END_REF]. Finally, Weinmann et al. [START_REF] Weinmann | Association between Occupational Exposure to Disinfectants and Asthma in Young Adults Working in Cleaning or Health Services: Results from a Cross-Sectional Analysis in Germany[END_REF] examined the relationship between exposure to disinfectants and asthma among 356 German young adults working in health or cleaning services. They reported a three-fold increased risk of asthma after only twelve months of occupational exposure to disinfectants.

Rhinitis and nasal symptoms

In 2013, Siracusa et al. [START_REF] Siracusa | Non-IgE-mediated and irritant-induced work-related rhinitis[END_REF] reviewed agents and activities newly identified as potential causes of work-related rhinitis, with a focus on irritant-induced rhinitis. Healthcare and cleaning work, or DCPs, were among the most frequently reported activities or agents. Literature reviews on asthma and rhinitis in cleaners [START_REF] Folletti | Asthma and rhinitis in cleaning workers: A systematic review of epidemiological studies[END_REF] and healthcare workers [START_REF] Mazurek | Occupational Respiratory Allergic Diseases in Healthcare Workers[END_REF] also suggested that exposure to cleaning products was associated with rhinitis and nasal symptoms, although data remain limited. In a recent cross-sectional study of 16,099 adults in Norway, Clarhed et al. [START_REF] Clarhed | Chronic Rhinosinusitis Related to Occupational Exposure: The Telemark Population Study[END_REF] examined the association between occupational exposures to several airbone irritants or sensitizers evaluated by questionnaire, and chronic rhinosinusitis defined based on selfreported symptoms of nasal blocakge or nasal discharge. Participants ever exposed to cleaning agents had signigicantly increased risk of chronic rhinosinusitis (adjusted odds ratio:

1.2, 95% confidence intervall: 1.0-1.3). In the health hazard examination conducted by the NIOSH in a US hospital (described in the previous section), significantly increased risk of both acute and chronic nasal symptoms was found in workers with high exposure to a product containing hydrogen peroxide, peracetic acid, and acetic acid [START_REF] Hawley | Respiratory symptoms in hospital cleaning staff exposed to a product containing hydrogen peroxide, peracetic acid, and acetic acid[END_REF]. In a cross-sectional study of 543 healthcare workers in France, exposure to quaternary ammonium compounds was associated with work-related nasal symptoms [START_REF] Gonzalez | Asthma among workers in healthcare settings: role of disinfection with quaternary ammonium compounds[END_REF].

COPD and lung function

Due to the irritant properties of many chemicals contained in DCPs, hypotheses regarding a potential impact of DCPs exposure on COPD and lung function have emerged in the past few years [START_REF] Mirabelli | Occupation and three-year incidence of respiratory symptoms and lung function decline: the ARIC Study[END_REF][START_REF] Svanes | Respiratory Health in Cleaners in Northern Europe : Is Susceptibility Established in Early Life ?[END_REF]. Studies conducted on this question since 2016 are summarized in Table 2.

In a large mortality study based on linkage of Belgian census, register and death certificate data, higher risk of COPD mortality was found in cleaners compared to non-manual workers [START_REF] Van Den Borre | Health risks in the cleaning industry: a Belgian census-linked mortality study (1991-2011)[END_REF]. In a population of 13,259 Hispanic adults in the US, occupational exposure to cleaning and disinfecting solutions was associated with increased risk of chronic bronchitis [START_REF] Kim | Association of birthplace and occupational exposures with chronic bronchitis in US Hispanics/Latinos, 2008-2011[END_REF], consistently with earlier results from a population-based study in Northern Europe [START_REF] Svanes | Respiratory Health in Cleaners in Northern Europe : Is Susceptibility Established in Early Life ?[END_REF]. In the the UK Biobank, a first analysis including data from 228,614 adults showed that current employment as a cleaner was associated with increased risk of spirometry-defined COPD [START_REF] De Matteis | Occupations associated with COPD risk in the large population-based UK Biobank cohort study[END_REF] A more recent analysis of the UK Biobank examining complete occupational history in a subsample of 94,551 participants found no association between ever working as a cleaner, or duration of cleaning employment, and COPD [START_REF] De Matteis | The occupations at increased risk of COPD: analysis of lifetime job-histories in the population-based UK Biobank Cohort[END_REF]. These seemingly contradictory results emphasize the need for a better assessment of occupational exposure to DCPs (e.g., through the use of job-exposure matrices) over the lifetime in large cohorts, to clarify exposure trajectories associated with COPD risk.

In a cross-sectional study of US working adults, the highest prevalence of self-reported COPD was found among healthcare support occupations [START_REF] Doney | Prevalence of Chronic Obstructive Pulmonary Disease Among US Working Adults Aged 40 to 70 Years[END_REF]. In a prospective analysis of 73,262 US female nurses from the Nurses' Health Study II, occupational exposure to DCPs, evaluated by questionnaire and a job-task-exposure matrix, was significantly associated with an increased risk of developing COPD [START_REF] Dumas | Association of Occupational Exposure to Disinfectants With Incidence of Chronic Obstructive Pulmonary Disease Among US Female Nurses[END_REF]. The association was observed among participants both with and without asthma. High-level exposure to several specific disinfectants (i.e. glutaraldehyde, bleach, hydrogen peroxide, alcohol, and quaternary ammonium compounds) was significantly associated with COPD incidence.

In a cross-sectional study in New-Zealand, cleaners had lower lung function (FEV1, FVC) than reference workers [START_REF] Brooks | Respiratory health in professional cleaners: symptoms, lung function, and risk factors[END_REF]. In a longitudinal analysis of 6,235 adults from the European Community Respiratory Health Survey, cleaning activities (both cleaning at home and working as a cleaner) were associated with accelerated FEV1 and FVC decline [START_REF] Svanes | Cleaning at Home and at Work in Relation to Lung Function Decline and Airway Obstruction[END_REF]. This result was observed in women only and independent of asthma. No association was observed between cleaning activities and FEV1/FVC decline or airway obstruction. However, there were only few cases of incident airway obstruction and this analysis had limited power [START_REF] Svanes | Cleaning at Home and at Work in Relation to Lung Function Decline and Airway Obstruction[END_REF][START_REF] Cummings | The Long-Term Effects of Cleaning on the Lungs[END_REF]. In an older study in a large French cohort, an association between occupational exposure to detergents and lung function was also observed independently of asthma [START_REF] Moual | Occupational exposures estimated by means of job exposure matrices in relation to lung function in the PAARC survey[END_REF].

Mechanisms

Occupational asthma can be induced by allergic (sensitizer-induced) and nonallergic (irritantinduced) mechanisms, and both types of mechanisms have been described for DCPs, depending on the specific causal agent [START_REF] Folletti | Update on asthma and cleaning agents[END_REF][START_REF] Dumas | Do chronic workplace irritant exposures cause asthma?[END_REF]. Similarly, occupational rhinitis can be classified as allergic or non-allergic [START_REF] Siracusa | Non-IgE-mediated and irritant-induced work-related rhinitis[END_REF]. Irritant-induced asthma or rhinitis can be caused by a single high level (accidental) exposure to irritants or by chronic, lower level ('daily-life') exposure to irritants [START_REF] Siracusa | Non-IgE-mediated and irritant-induced work-related rhinitis[END_REF][START_REF] Dumas | Occupational irritants and asthma: An Estonian cross-sectional study of 34 000 adults[END_REF][START_REF] Casas | Irritants and asthma[END_REF]. Chronic exposure to inhaled irritants from DCPs may cause injury of the airway epithelium, oxidative stress and neutrophilic airway inflammation [START_REF] Casimirri | Biomarkers of oxidative-stress and inflammation in exhaled breath condensate from hospital cleaners[END_REF][START_REF] Van Den Broucke | Irritant-induced asthma to hypochlorite in mice due to impairment of the airway barrier[END_REF][START_REF] Mcgovern | Neutrophils mediate airway hyperresponsiveness after chlorine-induced airway injury in the mouse[END_REF], which are also relevant to COPD pathogenesis. Among all cleaning agents reported as likely causative agents of occupational asthma in THOR surveillance network, 15 low molecular weight organic compounds were identified [START_REF] Carder | Occupational and work-related respiratory disease attributed to cleaning products[END_REF].

According to a Quantitative Structure Activity Relationship (QSAR) model, 7 of these 15 agents had a respiratory sensitization potential. In a smaller subset of 80 patients with occupational asthma attributed to cleaning agents, sensitization mechanism was suggested by the clinician's evaluation in 84% of cases [START_REF] Walters | Cleaning agent occupational asthma in the West Midlands, UK: 2000-16[END_REF], but could be confirmed by specific inhalation challenges only in a small proportion. Among patients with occupational asthma from cleaning agents in Canada [START_REF] Li | Work-related asthma from cleaning agents versus other agents[END_REF], 42% had a diagnosis of irritant-induced asthma. However, irritantinduced asthma may be largely underestimated among occupational asthma cases reported by physicians [START_REF] Reilly | The Burden of Work-related Asthma in Michigan, 1988-2018[END_REF], as for this type of asthma the causal agent cannot be identified with certainty in a specific worker [START_REF] Vandenplas | EAACI position paper: Irritant-induced asthma[END_REF]. Epidemiological studies examining specific asthma phenotypes such as allergic vs. non allergic asthma are generally consistent with a predominant role of irritant exposures and the hypothesis of nonallergic mechanisms for workplace exposure to most DCPs [START_REF] Wang | Risks of exposure to occupational asthmogens in atopic and nonatopic asthma: a case-control study in Taiwan[END_REF][START_REF] Lillienberg | Occupational Exposure and New-onset Asthma in a Population-based Study in Northern Europe (RHINE)[END_REF][START_REF] Dumas | Cleaning and asthma characteristics in women[END_REF].

Mechanistic studies on respiratory effects of inhaled chemicals from DCPs have been reviewed by Clausen et al. [START_REF] Clausen | Chemicals inhaled from spray cleaning and disinfection products and their respiratory effects. A comprehensive review[END_REF], with a focus on ingredients from cleaning sprays. Although there is a limited number of inhalation studies in human, pulmonary effects have been reported after inhalation of several agents including chlorine, chloramine, acids, aldehydes and quaternary ammonium compounds [START_REF] Clausen | Chemicals inhaled from spray cleaning and disinfection products and their respiratory effects. A comprehensive review[END_REF]. Rodent models examining several quaternary ammonium compounds showed that inhalation of these chemicals caused pulmonary damage and neutrophilic inflammation, and suggested amplified effects when they were mixed with other chemicals such as ethylene glycol [START_REF] Clausen | Chemicals inhaled from spray cleaning and disinfection products and their respiratory effects. A comprehensive review[END_REF]. Chlorine exposure is also commonly used in experimental models examining effects of inhaled irritants on the airway [START_REF] Dumas | Do chronic workplace irritant exposures cause asthma?[END_REF][START_REF] Van Den Broucke | Irritant-induced asthma to hypochlorite in mice due to impairment of the airway barrier[END_REF][START_REF] Van Den Broucke | Contribution of mast cells in irritant-induced airway epithelial barrier impairment in vitro[END_REF]. Recent studies have focused on the airway epithelial barrier, which plays an important role in asthma, rhinitis and COPD [START_REF] Van Den Broucke | Irritant-induced asthma to hypochlorite in mice due to impairment of the airway barrier[END_REF][START_REF] Van Den Broucke | Contribution of mast cells in irritant-induced airway epithelial barrier impairment in vitro[END_REF]. Using a mice model of airway epithelial damage aiming at mimicing human accidental exposure to high level of an irritant agent followed by lower dose irritant exposure (as sometimes observed in irritant-induced asthma), Van Den Broucke et al. showed that airway exposure to hypochlorite resulted in airway hyperreactivity and pulmonary inflammation in mice with previously damaged epithelial barrier [START_REF] Van Den Broucke | Irritant-induced asthma to hypochlorite in mice due to impairment of the airway barrier[END_REF]. A study examining the effects of laundry detergents (which may contain both irritants and sensitizers) on human bronchial epithelial cells also supported a disruptive effect on the epithelial barrier function [START_REF] Wang | Laundry detergents and detergent residue after rinsing directly disrupt tight junction barrier integrity in human bronchial epithelial cells[END_REF].

Injury of the epithelium caused by irritant exposure may increase epithelial permeability, potentially facilitating penetration of allergens [START_REF] Wang | Laundry detergents and detergent residue after rinsing directly disrupt tight junction barrier integrity in human bronchial epithelial cells[END_REF][START_REF] De Genaro | Low dose of chlorine exposure exacerbates nasal and pulmonary allergic inflammation in mice[END_REF]. In mice with pre-existing allergic lung inflammation, chlorine exposure has also been shown to contribute to potentiation of Th2 response [START_REF] De Genaro | Low dose of chlorine exposure exacerbates nasal and pulmonary allergic inflammation in mice[END_REF], and to aggravate airway hyperresponsiveness and inflammation by activation of innate immune cells [START_REF] Shim | Aggravation of asthmatic inflammation by chlorine exposure via innate lymphoid cells and CD11cintermediate macrophages[END_REF].

Oxidative stress has been suggested as one of the mechanisms for the impact of inhaled irritant cleaning products on the airway epithelium [START_REF] Dumas | Do chronic workplace irritant exposures cause asthma?[END_REF][START_REF] Tarlo | Occupational asthma[END_REF]. In accordance with this hypothesis, an Italian study of hospital cleaners reported that potential biomarkers of oxidative stress in exhaled breath condensate were higher in cleaners compared to control workers, while Fractional exhaled Nitric Oxide levels were similar in both groups [START_REF] Casimirri | Biomarkers of oxidative-stress and inflammation in exhaled breath condensate from hospital cleaners[END_REF].

Finally, a new interesting hypothesis regarding mechanisms of health effects of DCPs involves

a role of the microbiome. DCPs, which aim at removing or inactivating microorganisms, are likely to influence indoor environmental microbiota, and possibly gut or airway microbiota, which may in turn impact respiratory health [START_REF] Dumas | Damaging effects of household cleaning products on the lungs[END_REF][START_REF] Tun | Postnatal exposure to household disinfectants, infant gut microbiota and subsequent risk of overweight in children[END_REF][START_REF] Velazquez | From one species to another: A review on the interaction between chemistry and microbiology in relation to cleaning in the built environment[END_REF]. In a Canadian birth cohort, the use of disinfectants or eco-friendly products for home cleaning was associated with significant differences in infants' gut microbiota composition [START_REF] Tun | Postnatal exposure to household disinfectants, infant gut microbiota and subsequent risk of overweight in children[END_REF]. In adults, although DCPs have not been specifically examined, a few recent studies suggested that occupational exposures to chemicals [START_REF] Lai | Impact of occupational exposure on human microbiota[END_REF], as well as ozone exposure [START_REF] Niu | Ozone exposure leads to changes in airway permeability, microbiota and metabolome: a randomised, double-blind, crossover trial[END_REF], may alter the gut or airway microbiota.

Conclusion

The adverse effect of occupational exposure to DCPs in asthma on now well-established. However, recent data showed that the burden of WRA caused by DCPs exposure has not decreased in the past decade, emphasizing the need to strengthen prevention efforts [START_REF] Rosenman | Cleaning Products and Work-Related Asthma, 10 Year Update[END_REF][START_REF] Cummings | The Long-Term Effects of Cleaning on the Lungs[END_REF].

Novel data suggest an association between occupational exposure to DCPs and other chronic airway diseases, such as rhinitis, COPD and poor lung function, and support the conduct of further studies. Epidemiological and experimental data show that many chemicals contained in DCPs are likely to cause airway damage, indicating that prevention strategies should target multiple products and may require the development and evaluation of "green chemistry" or non-chemical disinfection methods [START_REF] Tarlo | Opportunities and obstacles in translating evidence to policy in occupational asthma[END_REF][START_REF] Goodyear | Cleaning and disinfection in home care: A comparison of 2 commercial products with potentially different[END_REF]. Improving local ventilation and use of adequate personal protective equiment has also been suggested [START_REF] Tarlo | Opportunities and obstacles in translating evidence to policy in occupational asthma[END_REF]. Recent studies aiming at improving characterization of DCPs exposures [START_REF] Clausen | Chemicals inhaled from spray cleaning and disinfection products and their respiratory effects. A comprehensive review[END_REF][65][66][67][68][69][70]] represent a first step toward the development of exposure reduction strategies.

Key points:

• The relationship between occupational exposure to disinfectants and cleaning products and asthma is well-documented, but questions remain regarding specific causal agents.

• Recent epidemiological studies have often focused on healthcare workers and are characterized by efforts to improve assessment of exposure to specific DCPs.

• The burden of work-related asthma caused by disinfectants and cleaning products has not decreased in the past decade, emphasizing the need to strengthen prevention efforts.

• Novel data suggest an association between occupational exposure to disinfectants and cleaning products and other chronic airway diseases, such as rhinitis, COPD and poor lung function, and support the conduct of further studies.

• Epidemiological and experimental data show that many chemicals contained in disinfectants and cleaning products are likely to cause airway damage, indicating that prevention strategies should target multiple products. This study provides updated data on the occurrence of work-related asthma associated with cleaning products from surveillance programs in five US states. Of all work-related asthma cases, 12% (1199) were related to cleaning products. This proportion was unchanged since the mid 1990's, emphasizing the need to strengthen prevention efforts.

[Note: this reference is currently reported as a conference abstract; the corresponding paper is currently in revision (minor revision). Reference will be changed upon acceptance]

This prospective study of 17,280 early to mid-career female nurses (mean age: 34 years), in the US and Canada, showed that occupational use of high level disinfectants among nurses was associated with significantly increased risk of developing asthma, adding longitudinal evidence to an association between occupational exposure to disinfectants and asthma. either at work or at home was associated with accelerated FEV1 and FVC decline. This result was observed in women only and independent of asthma. consequences for respiratory health. Am. J. Infect Workers in the department with the highest air measurements had higher risk of current asthma (SMR 3.47; 95% CI [1.48-8.13]) compared with the U.S. population. In a smaller sub-sample (n=50) who completed a post-shift survey, increased exposure was associated with acute nasal and eye irritation but not significantly with lower airway symptoms. Tasks: cleaning fixed surface associated with increased risk of current asthma; Chemical exposures: alcohols associated with "mild asthma symptoms" and "asthma attacks/ exacerbations". Bleach associated with "mild asthma symptoms", "undiagnosed/ untreated asthma", and "asthma attacks/exacerbations". Enzymes associated with "undiagnosed/untreated asthma" and "asthma attacks/ exacerbations". Exposure clusters: "housekeeping/chlorine" associated with "undiagnosed/untreated asthma" (OR 3.11; 95% CI 1.46-6.63) and "asthma attacks/exacerbations" (2.71; 1. 25-5.86). "General cleaning/laboratory" associated with "asthma attacks/exacerbations" (2.35; 1.07-5.13). "Disinfection products" associated with "mild asthma symptoms" (1.81; 1.09-2.99) and "undiagnosed/untreated asthma" (3.42; 1.24-9.39). Weinmann et al., 2019 [START_REF] Weinmann | Association between Occupational Exposure to Disinfectants and Asthma in Young Adults Working in Cleaning or Health Services: Results from a Cross-Sectional Analysis in Germany[END_REF] Germany, cross-sectional study, workers in cleaning or health service (aged 20-24 years), n=265 Doctordiagnosed asthma Exposure to disinfectants evaluated by questionnaire Exposure to disinfectants for more than twelve months associated with doctordiagnosed asthma (OR: 2.99, 95% CI: 1.36-6.55). COPD-chronic obstructive pulmonary disease; FEV1 -forced expiratory volume in 1 second; FVC -forced vital capacity; HR hazard ratio; JTEM -job-task-exposure matrix; LLN -lower limit of normal; PR -prevalence ratio; SMR -standardized mortality ratio. All associations presented are based on models adjusted for potential confounders.

  PA, Frederiksen M, Sejbaek CS, et al. Chemicals inhaled from spray cleaning and disinfection products and their respiratory effects. A comprehensive review. Int. J. Hyg. Environ. Health. 2020;229:113592. This review focused on the respiratory effects associated with the use of cleaning spray (either at home or at work) provides a recent and comprehensive synthesis of results from epidemiological and clinical studies, toxicological studies of cleaning chemicals, as well as human and field exposure studies. * Rosenman K, Reilly MJ, Pechter E, et al. Cleaning Products and Work-Related Asthma, 10 Year Update. J. Occup. Environ. Med. 2020;62:130-137.

*

  * Dumas O, Varraso R, Boggs KM, et al. Association of Occupational Exposure to Disinfectants With Incidence of Chronic Obstructive Pulmonary Disease Among US Female Nurses. JAMA Netw. Open. 2019;2:e1913563. This prospective cohort study of 73,262 US female nurses from the Nurses' Health Study II showed that occupational exposure to cleaning products and disinfectants was significantly associated with a 25% to 38% increased risk of developing chronic obstructive pulmonary disease, independent of asthma and smoking. * Dumas O, Gaskins AJ, Boggs KM, et al. Occupational use of high-level disinfectants and asthma incidence in early to mid-career nurses: a prospective cohort study. Eur. Respir. J. 2019;54:OA3282.

Table 1 . Epidemiological studies on the association between occupational exposure to disinfectants and cleaning products and asthma outcomes published in 2017-2020

 1 . Control. 2018;46:410-416. [65] Quinot C, Amsellem-Dubourget S, Temam S, et al. Development of a bar code-based exposure assessment method to evaluate occupational exposure to disinfectants and cleaning products: A pilot study. Occup. Environ. Med. 2018;75:668-674. [66] Marbac M, Sedki M, Boutron-Ruault MC, et al. Patterns of cleaning product exposures using a novel clustering approach for data with correlated variables. Ann. Epidemiol. Su FC, Friesen MC, Stefaniak AB, et al. Exposures to volatile organic compounds among healthcare workers: Modeling the effects of cleaning tasks and product use. Ann. Work Expo. Heal. 2018;62:852-870. [69] Benjamin ML, Arnold S, Rao M, et al. Ventilation and posture effects on inhalation exposures to volatile cleaning ingredients in a simulated domestic worker cleaning environment. Indoor Air. 2020;(in press). [70] Lin N, Rosemberg M-A, Li W, et al. Occupational exposure and health risks of volatile organic compounds of hotel housekeepers: Field measurements of exposure and health risks. Indoor Air. 2020;(in press).

	2018;28:563-569.e6.
	[67] Lovén K, Isaxon C, Wierzbicka A, et al. Characterization of airborne particles from
	cleaning sprays and their corresponding respiratory deposition fractions. J. Occup.
	Environ. Hyg. 2019;16:656-667.
	[68]

Table 2 . Epidemiological studies on the association between occupational exposure to disinfectants and cleaning products and lung function or COPD published in 2016-2020

 2 95% CI 0.45-6.51), instrument cleaning including endoscopy (1.33, 0.66-2.68]), surface cleaning including sprays (1.39, 0.35-5.60), glutaraldehyde (1.33, 0.66-2.68), ortho-phthalaldehyde (1.33, 0.66-2.68) BHR -bronchial hyperresponsiveness; JTEM -job-task-exposure matrix; HR -hazard ratio; OR -odds ratio; PR -prevalence ratio; SMR -standardized mortality ratio. All associations presented are based on models adjusted for potential confounders, unless otherwise specified.Cleaning activities (both cleaning at home and working as cleaner) associated with accelerated FEV1 and FVC decline in women only. Results were similar in participants without asthma. No association between cleaning activities and FEV1/FVC decline or airway obstruction. Current job / job history Currently working as domestic cleaner (PR 1.43, 95%CI: 1.28-1.59) or industrial cleaner (1.96, 1.16-3.31) was associated with increased risk of COPD. Results were similar in never-smokers and participants without asthma. Ever working as a cleaner or duration of cleaning employment was not associated with COPD. Weekly use of disinfectants to clean surfaces only and to clean medical instruments was associated with incident COPD (surfaces: HR 1.38, 95%CI 1.13-1.68; instruments: 1.31, 1.07-1.61). Using the JTEM, high-level exposure to glutaraldehyde, bleach, hydrogen peroxide, alcohol, and quaternary ammonium compounds was significantly associated with COPD incidence Brooks et al., 2020[START_REF] Brooks | Respiratory health in professional cleaners: symptoms, lung function, and risk factors[END_REF] Cleaners had lower lung function (FEV1, FVC) than reference workers; use of bleach and specific sprays among cleaners associated with lower FEV1 %predicted, compared to reference workers.

	Dumas et al.,	USA, 2009, prospective cohort study	Asthma	Current exposure to	Weekly use of disinfectants to clean surfaces only or to clean medical
	2020 [19]	(~6-year follow-up), female registered	incidence	cleaning products /	instruments was not associated with incident asthma (surfaces: HR 1.12, 95%CI
		nurses (mean age: 55 years),		disinfectants evaluated by	0.87-1.43; instruments: 1.13, 0.87-1.48). No association was observed between
		n=61,538		questionnaire and a JTEM	exposure to specific products evaluated by the JTEM and asthma incidence
	Dumas et al.,	USA & Canada, 2010-2018,	Asthma	Duration of exposure to high	Nurses who reported >5 years of use of high level disinfectants (11%) had
	2020 [20]	prospective cohort study (~4-year	incidence	level disinfectants evaluated	increased risk of incident asthma (HR 1.38, 95% CI 1.03-1.85)
		follow-up), female nurses (mean age:		by questionnaire
		34 years), n=17,280
	Brooks et al.,	New Zealand, 2008-2010, cross-	Current	Exposure to cleaning tasks	Cleaners had increased risk of current asthma. Bleach (OR 1.87; 95% CI 1.12-
	2020 [28]	sectional study, 425 cleaners (mean	asthma	and products evaluated by	3.14) and decalcifiers (2.77; 1.11-6.92) associated with current asthma,
		age: 45 years) and 281 reference		questionnaire	compared to reference workers
		workers (mean age: 40 years)
	Patel et al., 2020	USA, 2016-2017, cross-sectional	New-onset	Exposure to cleaning	New-onset asthma not analyzed because of low number of cases (n=11). No
	[24]	study, certified nurse aides (mean age	asthma,	products / disinfectants	significant association between cleaning/disinfections tasks and products and
		38 years), n=239		during the longest held job,	BHR-related symptoms. Highest OR observed for patient care cleaning and

Acknowledgements

None

Funding

Anses PNREST 2020/1/047 -CONETRE

Conflict of interest

None

Word count: 2854