Nicolas Leboucher

Christophe Dumouchel

Denis Lisiecki

Yves Wanner

Injector Internal Geometry and Sub-Atmospheric Back Pressure Influence on Low Weber Number Liquid Flow

The experimental work presented in this paper investigates the influence of the injector internal geometry on the primary atomization process in sub-atmospheric back pressure condition. A series of four injectors is studied. High-rate shadowgraph films of the liquid flow issuing from the nozzle are performed (66 667 fr/s). In order to have exploitable images, the injection pressure is maintained low (less than 1 MPa). On each image, the interface length per unit liquid surface area, e 2 (0), is measured. The examination of this parameter and of the images reveals that under atmospheric pressure, the primary atomization process is rather independent of the injectors. However, as the ambient pressure decreases, the injectors show different behaviour and the atomization process becomes intermittent. This behaviour is attributed to the apparition of cavitation in the injector. The characteristics of this intermittency are evaluated. Among other results, we observed that atomization process produced by cavitating flows are more sensitive to the injector internal geometry, the intermittency is due to the production of vortex cavitation, and, contrary to what is usually reported in the literature, cavitation does not enhance atomization in the present case. These results, as well as others, are presented and discussed in this paper.

Introduction

It is now well recognized that, whatever the injection pressure, the nature of the flow inside the injector influences primary atomization processes and the subsequent spray characteristics [1; 2]. It is therefore required to conduct specific investigations on the link between injector internal geometry and the liquid behaviour just at the nozzle exit. Such investigations must address also the important question concerning the characterization of atomizing liquid system.

The internal flow characteristics that are known to influence the issuing liquid flow are the distribution of velocity, the level of turbulence and the liquid cavitation. The velocity distribution, including axial and non-axial components, shapes the liquid flow. This is a very important step in atomization. Swirl injector is a well-known device based on this concept. It produces a thin conical sheet from the development of a high rotating motion inside the nozzle [START_REF] Lefebvre | Atomization and Sprays[END_REF]. The turbulence of the issuing liquid flow imposes initial perturbations of different scales that structure the whole atomization process. In low-Weber atomization processes, the final spray characteristics are directly dependent on the initial turbulence level [START_REF] Dumouchel | [END_REF].

Liquid cavitation is a rupture in liquid continuum due to excessive stress and that appears as soon as the pressure decreases below the liquid vapour pressure. It is characterized by a change of phase of the liquid. Cavitation triggers in high injection pressure condition such as diesel or gasoline direct-injection and its effect on primary atomization processes has been widely investigated in these conditions [2]. Generally speaking, these investigations agree to say that cavitation enhances atomization but the reasons evoked to explain this are not fully established. Geometrically induced cavitation takes place at the nozzle orifice entrance and declines in three regimes, i.e., developing cavitation, super-cavitation, hydraulic flip [5]. Another type of cavitation structures due to the presence of hydrodynamic vortex in the liquid flow may also appear [START_REF] Soteriou | ILASS-Europe[END_REF][START_REF] Arcoumanis | [END_REF]. The origin of these vortex or string cavitation structures has been experimentally investigated in injector up-scale model [8][9][10]. These investigations point out that string cavitation cannot exist in the absence of geometrically induced cavitation. It is more than probable that the influence of cavitation on the primary atomization depends on origin of cavitation.

According to the literature [12,13] the appearance of geometrically induced cavitation imposes a stabilization of the mass flow rate as the back pressure decreases for a constant value of the injection pressure. This behaviour has been widely used to determine the condition of cavitation and the corresponding critical cavitation number. This number involves the injection pressure, the ambient pressure and the liquid vapour pressure. Numerous experimental results of the literature point out the dependence between the critical cavitation number and the injection pressure. Furthermore, the critical cavitation number shows that liquid cavitation can be triggered Corresponding author: Christophe.Dumouchel@coria.fr 2 by decreasing the ambient pressure [13]. In previous investigations addressing the influence of cavitation on liquid atomization processes, cavitation was always triggered by increasing the injection pressure.

In this experimental investigation, injections at low injection pressure are performed under sub-atmospheric pressure. Low injection pressure range is chosen in order to ease the visualization and analysis of the liquid flow at the nozzle exit, i.e., the initial step of the primary atomization process. Working at sub-atmospheric pressure allows getting rid of the aerodynamic effect and concentrating on the effect of the internal injector geometry. We expect that this will allow triggering liquid cavitation also. In this case, cavitation would be triggered at low injection pressure allowing its effect on the atomization process to be observable.

Experimental Setup and Diagnostics

The experimental setup is schematized in Fig. 1. The liquid is kept in a reservoir (fuel tank in Fig. 1) at the exit of which it is filtered. A combination of two pumps (low pressure LP and high pressure HP) provides an absolute injection pressure P i ranging from 0 to 16 MPa and that is regulated and measured by a high pressure sensor just before the injector. The injector is fixed on top of a closed chamber in which the ambient pressure is controlled in the range [0.004 MPa; 0.2 MPa]. The interval chamber volume is 8.5 litres (23.4 x 19 x 19 cm 3 ). The chamber is equipped with four windows (one on each side, height: 12 cm; width: 8 cm). In order to limit liquid stagnation on the windows, an airdrift system sweeps each window by a controlled air-flow. The sub-atmospheric pressure in the chamber is controlled by a Sterling Pump (SIHI-Dry) that allows reaching 0.004 MPa. This pump can work even with contaminated air: the air does not have to be dry and the presence of liquid droplets is not penalizing.

A series of four injectors of the type of gasoline direct injection device is available. These injectors are conceived to work in transient conditions and are equipped with a needle controlling the closing and the opening of the injection device. In the closing position, the needle rests on the wall of the nozzle sac volume. The nozzle discharge orifices are located at the bottom of the sac volume. All injectors have three identical cylindrical orifices with the same diameter d or = 190 µm and the same length, i.e., L/d or 1. These orifices are regularly distributed at the injector nozzle and make an angle of 23 degrees with the axis of the injector body. One of the injector is referred as standard (Inj. 1) and the three others present some differences concerning the orifice inlet profile (Inj. 2) and the roughness of the needle and of the sac (Inj. 3 and 4, respectively). The references of the injector are given in Table 1. When positioned on top of the chamber, the injector is orientated so that one of the jets injects vertically and can be fully observed through the window.

The liquid used is Shellsol D40 whose physical properties are close to those of gasoline. The liquid physical properties are: density L = 766 kg/m 3 , surface tension = 0.025 N/m, dynamic viscosity µ L = 0.9 10 -3 kg/(ms), vapour pressure P v = 300 Pa.

The present study focuses on the behaviour of the injector during the fully-open stage only and the transient opening and closing phases of the injector are ignored. During the fully-open stage, the needle has reached its upper position and the injector works in a steady state condition. The mass flow rate Q m of the fully-open stage is measured for each injector as a function of the couple (P i ; P a ) by measuring the liquid mass M(t i ) injected per injection as a function of the injection time t i . (The injection time is defined as the duration of the electronic signal send to the injector.) For a fixed pressure drop P i = P i -P a , this mass can be written as:

) ( ) ( i FO m i t M t Q t M ( 1
)
where t FO designates the time duration of the fully-open stage and M(t i ) is the mass of liquid injected during the transient stages, i.e., during the opening and the closing of the injector. By assuming that t FO = t it(t i ) where the time t might be a function of the injection time, Eq. (1) becomes: Great sac roughness Table 1 Description and nomenclature of the injectors

) ( ) ( ) ( i m i i m i t t Q t M t Q t M (2)
For great injection times, M and t are independent of the injection time and therefore, according to Eq. (2), M(t i ) increases linearly with t i and the mass flow rate is the slope of this evolution. Thus, Q m is obtained by measuring M(t i ) for a range of sufficiently large injection times, i.e., greater than 2 ms. The discharge coefficient C D of the injector fully-open stage is then calculated by ) 2 3 ( 4

2 i L or m D P d / Q C .
High-speed shadowgraph images of the jet issuing from the injectors are performed with a Phantom V12.1 Camera (256x256 pixels). The acquisition rate is fixed at 66 667 frames/s corresponding to a time delay of 15 µs between consecutive images and the exposure time is 0.3 µs. The physical field covered by the optical arrangement is 2.6 mm x 2.6 mm which makes a spatial resolution equal to 10 µm/pixel. The light source is a continuous 300 W Xenon arc source.

In complement to the high-speed films, shadowgraph snapshots are performed with a camera MatrixVision (1600 x 1200 pixels) which offers a high spatial resolution and a nanolite light source. In this second configuration, the light source has a time duration equal to 20 ns and plays the role of the shutter. The physical field covered with this second optical configuration is 6 mm x 8 mm corresponding to a spatial resolution equal to 5 µm/pixel.

Experimental Results

Figure 2 presents the mass flow of the fullyopen stage as a function of the injection and ambient pressures. The point series shown in the graphs of this figure correspond to specific injection pressure P i and the increase of pressure drop is obtained by reducing the ambient pressure. For each injector, the discharge coefficient measured under atmospheric pressure is indicated in each figure as well as the relationship it imposes between the pressure drop and the mass flow rate. Inj. 2 shows the greatest discharge coefficient. Experimental investigations of the literature reported greater discharge coefficients for cylindrical discharge orifices with rounded inlet geometry [14]. The present observation agrees with this behaviour. We note however that the increase of the discharge coefficient for Inj. 2 is moderate compared to what is reported in the literature which is likely due to the fact that its orifice-inlets have been smoothed but not fully rounded.

For each injector, Fig. 2 highlights a saturation of the mass flow rate as the relative injection pressure increases. According to the literature [1,11,12] the stabilization of the mass flow rate as the ambient pressure decreases has been reported is due to the apparition of cavitation in the injector. In these works, the injec- Figure 3 Estimation of K crit for each injector tion pressure is usually great (above 10 MPa) and the ambient pressure is reduced from this value to the atmospheric pressure. We see here that working at low injection pressures with ambient pressures below the atmospheric reports similar behaviour. Except for Inj. 2, note that for P i = 1.0 MPa, the stabilization of the mass flow rate occurs at atmospheric pressure. Therefore, the presence of cavitation at this injection pressure seems probable. Following Payri's methodology [1,11,12], we estimate the critical cavitation number K crit under which cavitation occurs as the number K = (P i -P v )/(P i -P a ) where P a is given the value at which mass flow rate stabilization occurs. The results are presented in Fig. 3. As reported elsewhere [1,11,12], we find that K crit increases with the injection pressure. Figure 3 shows that K crit depends on the injector internal geometry also: cavitation takes place less easily in the smoothed orifice inlet injector and more easily in the devices with greater roughness.

Figure 2 indicates also that, when the mass flow rate stabilization is reached, further decrease of the ambient pressure induces a decrease of the mass flow rate. This behaviour, which has not been observed by others, clearly evidences a strong modification of the internal liquid flow that should have repercussion on the atomization process. Examination of images of the liquid flow at the nozzle exit confirms this point.

Examples of images extracted from high-speed films are presented in Fig. 4. These images show the influence of P a on the atomization process when P i = 0.3 MPa for Inj. 1 and Inj. 4. These images show two different atomization processes. AP1: The jet issuing from the injector shows a highly perturbed interface with the emergence of lateral ligaments that ensure an early disintegration. AP2: The liquid system issuing from the injector is much wider than in AP1 and the lateral ligaments are far less developed and totally absent sometimes. Breakup and drop formation seem to be delayed compared to AP1. The presence of each atomization process is dependent on the ambient pressure and on the injector. At P a = 0.1 MPa, only AP1 is observed whatever the injection pressure and the injector. For ambient pressure of 0.01 MPa and less, the liquid atomization process is intermittent between AP1 and AP2. The percentage rate of each atomization process will be estimated later but note in Fig. 4, that for the lowest ambient pressure, Inj. 4 reported the AP2 only.

The emergence of AP2 in Fig. 4 is not due to an increase of the pressure drop when the ambient pressure decreases. To illustrate this Fig. 5 presents two images for a constant pressure drop ( P i = 0.3 MPa) at two different ambient pressures (0.1 and 0.004 MPa). These images are snapshots and concern Inj. 1. At P a = 0.1 MPa only AP1 has been observed (left image in Fig. 5). At P a = 0.004 MPa, the second atomization process has been observed intermittently (right image). Therefore, the triggering of AP2 is well related to the decrease of the ambient pressure.

The intermittency between AP1 and AP2 during the injection motivated performing high-speed films in order to estimate the appearance rate of each process. To achieve this, the film images were treated and transformed into two grey levels images on which liquid appears in black on a white background. Using classical image treatment tools, the boundary of the continuous liquid system, i.e., the one attached to the nozzle, is detected. The length L of this bound- This ratio is noted e 2 (0) since it corresponds to the value of the surface-based scale distribution e 2 (D) for the scale D = 0 [15]. This distribution has been used to characterize atomization processes and to develop new atomization models [15,16]. e 2 (0) corresponds to the amount of interface per unit area of surface which is an interesting information in liquid atomization: efficient atomization processes show great value of e 2 (0). If we go back to the identification of AP1 and AP2 (Fig. 4) we see that the very deformed jets of AP1 should be characterized by greater values of e 2 (0) than those of AP2 that are much wider (A s is greater) and less tortuous (L is smaller). For the operating conditions shown in Fig. 4, Fig. 6 presents the temporal evolution of the ratio e 2 (0). The signals obtained at P a = 0.1 MPa characterize the atomization process AP1: e 2 (0) oscillates between 0.004 and 0.008 and does not report significant differences between Inj. 1 and Inj. 4. (The mean of e 2 (0) are equal to 0.0056 and 0.0059 for Inj. 1 and 4 respectively.) At P a = 0.01 MPa, some differences arise: the Inj. 4 signal is not very different than the one obtained for the greater ambient pressure but the Inj. 1 signal shows two major differences. First, the e 2 (0) values are not as great as for P a = 0.1 MPa and, second, we note that at around 2.5, 4.5 and 5.5 ms, e 2 (0) collapses during a small time interval. This behaviour is the mark of the triggering of AP2. It is not reported for Inj. 4. (We must add here that as far as the appearance of each atomization process is concerned, differences from one injection to another could be observed.) Finally, for the lowest ambient pressure P a = 0.004 MPa, the difference between Inj. 1 and Inj. 4 is much more pronounced. For Inj. 1, we see that AP2 lasts longer, whereas for Inj. 4, AP2 is stable during the all injection. We can add here, that, as illustrated in Fig. 7, no specific frequency was reported whatever the operating conditions including the injection, the injection pressure and the ambient pressure: the intermittency between AP1 and AP2 is not related to a specific frequency. Note that the absence of frequency is a characteristic of super-cavitation regime [13].

Generally speaking, a similar behaviour was observed for the other injectors. Figure 8 compares the number fraction distribution of e 2 (0) for each injector at the two extreme ambient pressures and a constant injection pressure (P i = 0.3 MPa). These distributions are produced from the temporal evolution of e 2 (0) as those shown in For the smallest ambient pressure, differences between the four injectors are numerous. The distribution reported by Inj. 1 clearly shows two peaks corresponding to the two atomization processes. Note that the right peak (corresponding to AP1) has been left shifted with the decrease of the ambient pressure. The distributions reported by Inj. 2 and Inj. 3 are still mono-modal but they are narrower and left shifted compared to those obtained at atmospheric pressure. For these two injectors, the atomization process intermittency between AP1 and AP2 has been observed but the values of e 2 (0) of each process seem to be too close to each other (as discussed later) to generate a bi-modal distribution in Fig. 8. Finally, the distribution obtained for Inj. 4 is very narrow and located at a small value of e 2 (0). This result is representative of the atomization process AP2 which is stable for this operating condition (see Fig. 6). In complement to the results presented in Fig. 8, the estimations of percentage of each atomization process as well as the corresponding average values of the ratio e 2 (0) have been determined. The results are gathered in Table 2. It is important to add here that the percentage of presence of each atomization process might report significant variations from one injection to another. This is the reason why the accuracy of the percentage shown in Table 2 is low (5%) and that the results for three ambient pressures (the lowest, the greatest and a medium value) are reported only. We believe however that this percentage gives interesting indications. On the other hand, we noticed that the average values of e 2 (0) obtained for each atomization process is rather well reproducible from one injection to another.

As already mentioned, Table 2 reminds that, whatever the injector, the atomization process AP1 is the only one observed when P a = 0.1 MPa. It is also interesting to note that for this case, the average value of e 2 (0) during the fully-open stage is not very much dependent on the injector. As the ambient pressure decreases, several observations can be made. First, the AP1-percentage decreases and the AP2-percentage increases. Note that these variations (which are estimations as pointed out above) depend on the injector as well as on the ambient pressure. At P a = 0.004 MPa, the smallest AP2-percentage is obtained for Inj. 2. This injector has the lower critical cavitation number (see Fig. 3) indicating a lower propensity to trigger cavitation. Similarly, the injectors with the greatest critical cavitation number (Inj. 3 and 4) report the greatest percentage of AP2 at P a = 0.004 MPa. As observed in Fig. 8, the mean value of e 2 (0) for AP1 and AP2 are close to each other for Inj. 2 and 3. This is the reason why, the distribution for these injectors in Fig. 8 are mono-modal. The evolution of the average of e 2 (0) with P a is also instructive. We see that, for the two atomization processes, the average of e 2 (0) decreases with the ambient pressure. This means that, whereas the pressure drop increases (since P i is constant), the primary atomization process seems to be less and less effective when P a decreases, i.e., when cavitation is triggered. This observation is unusual since it is generally admitted that cavitation globally enhances atomisation. This behaviour is not observed in the present working conditions. Finally, it is interesting to note that the characteristics of the atomization processes at the lowest ambient pressure including the percentage of each process and the corresponding average of e 2 (0) depend on the injector. As said above, this is not the case under atmospheric pressure.

This indicates that, as cavitation is triggered, the atomization process becomes sensible to injector characteristics that have a negligible influence otherwise. Of course this last result should be confirmed by proper spray drop-size distribution measurements. This point is currently under consideration. Finally, high-speed films allow estimating the velocity V s of liquid ligaments and structures just downstream the exit section. These estimations have been performed for Inj. 4 at different ambient pressures by taking five couples of images and by measuring on each couple 10 velocities V s by recognizing liquid structures on each image. The structure velocities are shown in Fig. 9 where mass-flow rate velocities V q (evaluated from the mass flow rate and the section of the orifices) are also represented. Whereas these two velocities are of the same order of magnitude under atmospheric condition, we see that V s is always far greater than V q when the ambient pressure decreases. Since no acceleration of the liquid from the nozzle exit to the position where images were taken is possible, this difference indicates that the issuing flow occupies a portion of the nozzle exit section only. This behaviour is a known characteristic of cavitating flows. We note also that the structure velocity shows a linear relationship with the square root of the pressure drop. For cavitating flow, such behaviour is observed for the effective velocity [1]. The correspondence between the effective and the liquid structure velocity would suggest flow detachment from the orifice wall down to the nozzle exit section and therefore an independency between the atomization process and the structure velocity. As far as this point is concerned, it is important to remind here the very short length of the nozzle discharge orifice and to point out that, as illustrated in Fig. 9, V s doesn't vary much with the atomization process.

Finally, considering the value of the structure velocity for the atomization process AP2 as well as the width of the liquid flow in this condition (see Fig. 4 for instance), it can be shown that the structure of the flow for this atomization process is likely an annular liquid sheet. Visualization of the liquid jet at the end of the injection (Fig. 10) confirms the axisymmetric and hollow structure of the liquid flow.

Summary and Conclusions

The present experimental investigation shows that even in the absence of aerodynamic forces, i.e., at low injection pressure, the ambient pressure may have an important influence on the liquid atomization process. This influence is due to a deep modification of the injector internal liquid flow. Several points indicate that this modification is due to the apparition of liquid cavitation: as the ambient pressure decreases, the mass flow rate stabilizes, the atomization process becomes intermittent and the velocity of the issuing liquid flow is much greater than the average mass flow-rate velocity. Indeed, according to Payri's works, mass flow rate stabilization as the pressure drop increases is a characteristic of cavitation. Several observations underlined the highly unstable nature of cavitating flows [5,13,17]. Finally, cavitating flows may not cover the whole orifice section which results in velocity greater than the flow-rate mean velocity [1]. Thus, this demonstrates that the influence of cavitation on liquid atomization process can be investigated at low injection pressure, i.e., in a condition for which the qualification and quantification can be approached by visualizations and image analyses as done here. In the present investigation, the atomization processes are identified by their interface length per unit of liquid surface area. This information is important in atomization and succeeds in identifying and quantifying different processes. It is observed that cavitation triggers an intermittent liquid atomization process and that the characteristics of this intermittency are functions of the injector internal geometry. This intermittency is never related to a given frequency. A similar result was reported in super cavitation regime [13]. This regime is characterized by a propagation of the cavitation structures down to the nozzle exit section. One of the important geometrical characteristics of the injector investigated here is the small discharge orifice length that is likely in favour of the apparition of a super cavitation regime.

In this work, the intermittency is characterized by an alternation between two atomization processes. One of them corresponds to the production and atomization of an annular liquid sheet. This specific geometry is believed to result from the development of string or vortex cavitation [8][9][10]. These structures, which are known to be very unstable, find their origin in the vortex development in the nozzle sac volume of multi-hole injectors. Our injectors are equipped with such a region.

As far as the efficiency of the atomization processes is concerned we find that it becomes more sensitive to injector internal geometrical characteristics in case of cavitation and that, contrary to what is often reported, it seems to be not enhanced by cavitation. Although it has to be corroborated by proper spray characterisation, this last observation is believed to be related to the absence of reattachment of the liquid flow in the discharge orifices because of their too short length. Thus, turbulence is considerably reduced and the subsequent initial perturbation level of the liquid flow at the nozzle exit is low.

Figure 1

 1 Figure 1 Experimental Setup

Figure 2

 2 Figure 2 Fully-open stage mass flow rate as a function of the injection and ambient pressures P i and P a (the line illustrates the dependence with the discharge coefficient measured under atmospheric pressure P a = 0.1 MPa)

Figure 4 Figure 5

 45 Figure 4 Images from the high-speed films. Influence of the ambient pressure on the liquid flow issuing from the injector (P i = 0.3 MPa, AP1: Atomization process 1; AP2: Atomization process 2)

Figure 6 Figure 7

 67 Figure 6 Temporal evolution of e 2 (0) as a function of the ambient pressure (P i = 0.3 MPa, left: Inj. 1, right: Inj. 4)

Figure 8

 8 Figure 8 Number fraction distribution of e 2 (0). Influence of the injector and of the ambient pressure (P i = 0.3 MPa)

Figure 9 Figure 10

 910 Figure 9Comparison between average velocity V q and the structure velocity V s (Inj. 4, P i = 0.3 MPa)

Table 2

 2

	3 MPa)

Estimation of the percentage of each Atomization Process and corresponding average value of e 2 (0) (µm -1 ) (P i = 0.3 MPa, Figures in italic indicate average performed on five injections)

Acknowledgements

The authors thank MAGIE FUI project for the financial support.


S Tanchatchawan

P Vallikul

P Yongyingsakthavorn

C Dumouchel

An adaptive tomographic technique to reconstruct local drop size distribution of liquid spray at multi-resolution

This paper proposes an adaptive deconvolution algorithm to reconstruct local drop-size distributions within liquid sprays at multiple resolutions by using the laser diffraction measurement data. The algorithm employs the strip integration by which the scattered light intensities from the local regions along the measured line-of-sight being integrated and scanned over the entire region at multiple resolutions. In the strip integration model, the contribution of the intensity scattered from each local region to the measured line-of-sight data is controlled by the area of intersection between the local region and the laser beam past the region. The algorithm allows the number of input line-of-sight measurement data to be larger than the number of local regions at some selective regions where the drop-size distributions being determined. This means that the local drop-size-distributions at the selected regions can be reconstructed at higher spatial resolutions than those at other regions. The algorithm has been tested by measuring drop-size-distributions of two water sprays whose drop-size distributions are spatially nonhomogenous. The first spray is a two-dimensional like flat spray generated from an in-house designed nozzle "the single-hole triple disk nozzle" where the spray is constraint by the internal flow of the liquid before the injection. The second spray is a solid cone spray generated from a small industrial pressure swirl atomizer. It has been shown from the measurement results that the proposed algorithm is capable of reconstructing drop-size distribution of the liquid spray at multiple spatial resolutions.

Introduction

Line-of-sight data obtained from the measured forward scattered intensity of laser beams past through a spray has widely been used to reconstruct local intensity scattered from the points along the line-ofsight via the deconvolution processes [1][START_REF] Yule | Eighteenth symposium (international) on combustion[END_REF][START_REF] Dodge | [END_REF][4][5]. In the deconvolution schemes, the measured line-of-sight intensity data, ̅

, was modeled as a line integration of the local forward scattered intensity, from local points along the laser beam; the integration begins at the laser source (

)and ends at the detector ( ), see Figure 1. In this sense, the local intensity, , is the intensity per unit path length, . Since the laser beam has a finite width, , then the model local intensity has spatial resolution equal to one beam width, . Boyaval and Dumouchel (2001a) proposed to increase spatial resolutions of the reconstruction results by redefining the mathematical representation of the line-of-sight measurement data [START_REF] Boyaval | ILASS-Europe[END_REF]. They modeled the single line-of-sight data as a combination of two line integrations of forward scattered light past regions and In this model, two local intensities, and , of half width of the laser beam are spatially filled within one measurement. Then the local intensity in this two line integration model has higher spatial resolution than that of the one line integration model. With this concept in mind, multiple line integrations (more than two lines per one laser beam width) can also be included into the line-of-sight data. Local intensities for all line, however, have a single spatial resolution, equal to the number of local intensity by one beam width. If multiple resolutions of the local intensity were to be modeled then some weighting schemes on the width of the local region to which it represent are needed.

W W W I I I a I b I I ) , ( y x I w 1 w 2 w 3 w 4 a b W/2 L   L dx x I I 0 ) (      dx x I x I I L b a    0      L W dydx y x I I 0 0 , (1a) (1b) (1c)
Figure 1. Line-of-sight models: single line integration (1a), double line integration(1b), and strip integration (1c)

In this paper we propose to model the line-of-sight data by the strip integration. The line-of-sight data is modeled by the integration of the local intensity along the line-of-sight, , and over the beam-width, ,

̅ ∫ ∫ ( ) (1)
where ( ) is the intensity per unit path-length per beam-width (or local intensity per unit area), see Fig. 1. The inner integrand can further be subdivided into strips of arbitrary width, [START_REF] Yule | Eighteenth symposium (international) on combustion[END_REF] Within each strip, the local intensity is ( ) where . Therefore, the single line-of-sight data is modeled as the combination of strip integrations,

∫ (∫ ( ) ∫ ( ) ∫ ( ) ) (3)
The width, , and the position, , of the region of the local intensity can be modeled at arbitrary sizes. In other words, the local intensities can be modeled at multiple resolutions.

Measurement of drop-size distributions of a flat spray

The strip integration concept has been applied to measure drop-size distributions along a horizontal plane of a water spray, injected from a three-disc nozzle [START_REF] Dumouchel | [END_REF]. The nozzle operates at the pressure of 4 bar with corresponding mass flow rate of 7.4 kg/hr. The spray issued from the nozzle is a kind of flat-spray type. This is due to the asymmetrical flow inside the nozzle. The thickness of the spray is assumed constant along the span-wise direction while the drop-size distribution varies. Drop-size-distributions on a 10-cm-plane below the nozzle exit were determined in this study, see Figure 2.

Figure 2 shows that the span of the spray is 10 cm and that there are 16 laser beams scan across. The beam width is 10-mm. From the left edge, each laser beam is scanned 2.5 mm apart for 3 cm span such that the local intensities can be resolved at high resolution within this span. Beyond the 3-cm-span, each laser beam is scanned at 10 mm apart. the resolution of the local intensity within this region is coarser. The region of interest in this study is within the first span.

Based on (3), the relation between the pathintegrated and the local scattered intensities can be written as In (4), the path integrated data, ̅ ( ), where , are known line-of-sight measurement data. The local scattered intensities, ( ), where , are unknowns. Since the number of measured line-ofsight data is greater than the number of unknowns, , then the problem defined by ( 4) is an over determined problem. Maximum entropy technique [8] has been used to solve for the local intensities.

̅ ( ) ∑ ( ) ( ) (4)
Once the local intensities had been solved, the local drop-size-distribution at the local region could be determined. Procedures to determine drop-sizedistribution from the known local scattered intensities can be found in our earlier work [9]. The drop-sizedistribution in this work is the volume frequency, ( ) ∑ (5) The mean diameter, , is the sauter mean diameter,

∑ ∑ (6)
where denotes the size class consider, is the number of drops in size class , is the middle diameter of size class , is the number of class, is the width of class and is the liquid volume contained in class Figure 4 shows the measurement results of the mean diameters of the spray for three local regions near the left edge of the spray. The width of the local regions is equal to the width of the laser beam in other words the resolution of the local mean diameter is one to one (1:1). Figure 5 shows the measurement results of dropsize-distribution of the first two local regions of the above case (shown in Figure 4), the local regions are between 0-10 mm and 10-20 mm. It can be seen that the drop-size-distribution has two modes at the edge region. Further away from the edge, the drop-size-distribution becomes single mode distribution.

Multi-resolutions reconstructions to determining mean diameters and drop-size-distributions have been performed within the regions between 0 to20 mm from the edge of the spray. The region was divided into four local regions. The first three are determined at resolution equal to 2.25 to 1 and the fourth region is determined at the resolution 1.5 to 1. Figure 6 shows the results of mean diameters as multi-resolutions reconstructions.

Figure 7 shows the resulting drop-size-distributions of the four regions. By measuring at higher resolutions one can resolve drop-size-distributions within a finer width. The drop-size-distributions obtained are consistent with those obtained at the coarser local regions.

Figure 6. Mean diameters, measuring with multiresolutions

Measurement of drop-size-distributions of a solid cone spray

The pressure-swirl atomizer was used to generate an axi-symmetrical solid-cone water spray [9]. The measurement has been conducted on a cross-sectional plane of the spray at 7 cm below the nozzle exit. The injection pressure used is 12 bar giving the mass flow rate 4 kg/hr. Figure 8 shows schematic diagram of the spray and the measurement plane. The axi-symmetrical plane has the radius of 60 mm and the line-of-sight measurements have been scanned across the half plane. Two scanned modes have been performed. Between 0 and 30 mm, each measured lineof-sight is 2.5 mm apart and beyond that each measurement is 10 mm apart.

In the axi-symmetrical spray, the local regions are modeled as concentric rings and a centre core where the local intensity within each region, ( ) is assumed constant. Mathematical model of the line-ofsight, ̅ ( ) can be written based on (3) but in discrete form as

̅ ( ) ∑ ( ) (7)
is the intersection area between the local concentric ring at and the plan form area of the beam past . Figure 9 shows the pertinent parameters appearing in [START_REF] Dumouchel | [END_REF]. Note that only half-length line-ofsights are shown in the Figure 9.

Figure 10 shows the reconstructed mean diameters for each local region whose width is equal to the width of the laser beam. The resolution of the reconstruction is one to one. It can be seen that the mean diameter increases from the center core toward the outer edge. Figure 11 shows the corresponding drop-sizedistributions within the local regions. It can be seen that there are important changes both in the mean diameters and in the widths when comparing the drop-sizedistributions within concentric rings 5-15 mm and 15-25 mm. This means that there are steep variations between the two local regions. Reconstruction at higher resolutions within those local regions are needed.

yM yi y1

... ... Reconstruction results of the mean diameters at multi resolutions are shown in Figure 12. It can be seen that the mean diameters increase with the radius; the values are consistent with the results in Figure 10. The reconstruction results show that within concentric rings 14-20 and 20-26 mm, the mean diameters are almost equal. Figure13 shows, however, that drop-sizedistributions within the two regions have significantly different widths. Beyond the radius of 20 mm the width of the drop-size-distributions are broadened. Furthermore the incomplete local spray information in the previous case has been fulfilled.

A M,1 /2 A M,j-1 /2 A M,j /2 A M,N-1 /2 A M,N /2 A i,1 /2 A i,j-1 /2 A i,j /2 A 1,1 /2 r N r N-1 r j r j-1 r 1 W A M,j-2 /2 A i,j-2 /2 r j-2

Conclusions

The proposed adaptive tomographic reconstruction technique has been applied to reconstruct drop-sizedistributions within different types of sprays. The developed algorithm is able to resolve the distributions at multiple resolutions. This is the main advantage of the proposed technique over the conventional techniques where the solutions obtained are at single (and often coarse) resolution and the adaptation of the width of the local reconstruction region is not possible. It has also been shown that by using the strip-area as the reconstruction basis, the algorithm can handle the different shapes of the local reconstructed regions, e.g. rectangular or circular ring areas.

Figure 2 .Figure 3 .

 23 Figure 2. Description of flat water spray

Figure 4 . 1 Figure 5 .

 415 Figure 4. Means diameter of the three local regions from the left edge.

Figure 7 .

 7 Figure 7. Drop-size-distributions, measuring with multi-resolutions

Figure 8 .

 8 Figure 8. Schematic diagram of a solid-cone spray and the measurement plane

Figure 9 .

 9 Figure 9. Drawing defines pertinent parameters relating line-of-sight and local intensity.

Figure 10 .Figure 11 .Figure 12 .Figure 13 .

 10111213 Figure 10. Mean diameters of solid cone spray, measuring with single resolution

Acknowledgements

The first author is being supported under the PhD scholarship program by the Ministry of Sciences and Technology (MOST), Thailand and Thailand Institute of Scientific and Technological Research (TISTR). Research facilities are partially provided by the Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi and the Graduate School, King Mongkut's University of Technology North Bangkok.


Pingping Dong

Qing Jiao
email: jiaoqing@nbu.edu.cn

Zengcheng Zhang

Miao Jiang

Changgui Lin

Xianghua Zhang

Hongli Ma

Baochen Ma

Shixun Dai

Tiefeng Xu

Controllable Li3PS4-Li4SnS4 solid electrolytes with affordable conductor and high conductivity for solid-state battery

Keywords: Sulfide solid electrolyte, Glass ceramics, Low grain impedance, All-solid-state batteries

diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Electrochemical energy storage device such as Li second battery was believed to be the key technology for increasing requirement of pure electric vehicle, power backup device, and grid energy storage [START_REF] Liu | Anomalous High Ionic Conductivity of Nanoporous β-Li 3 PS 4[END_REF][START_REF] Rajagopala | Evaluation of Li 6 P 2 S 8 I Solid Electrolyte for All Solid-State Lithium Battery Applications[END_REF][START_REF] Yi | High Capacity and Superior Cyclic Performances of All-Solid-State Lithium-Sulfur Batteries Enabled by a High-Conductivity Li 10 SnP 2 S 12 Solid Electrolyte[END_REF][START_REF] Shan | Mixed Alkaline Earth Effects on Crystallization Behavior of Basalt Glasses and Liquids[END_REF][START_REF] Kraft | Inducing High Ionic Conductivity in the Lithium Superionic Argyrodites Li 6+x P 1-x Ge x S 5 I for All-Solid-State Batteries[END_REF][START_REF] Seoa | Broad Temperature Dependence, High Conductivity, and Structure-Property Relations of Cold Sintering of LLZO-based Composite Electrolytes[END_REF]. All-solid-state battery system was proposed to miniaturize the battery pack and improve the safety of battery [START_REF] Rajagopala | Evaluation of Li 6 P 2 S 8 I Solid Electrolyte for All Solid-State Lithium Battery Applications[END_REF]. Compared with the commercial Li + batteries, solid electrolytes exhibit comparable safe stability and energy density [START_REF] Rajagopala | Evaluation of Li 6 P 2 S 8 I Solid Electrolyte for All Solid-State Lithium Battery Applications[END_REF][START_REF] Yi | High Capacity and Superior Cyclic Performances of All-Solid-State Lithium-Sulfur Batteries Enabled by a High-Conductivity Li 10 SnP 2 S 12 Solid Electrolyte[END_REF].

Innovative sulfide electrolyte materials characterized by excellent ionic conductivity and wide stable electrochemical window have become the focus of ASSLSB. The sulfur solid electrolyte provided low elastic modulus and good formability due to its excellent ductility (Young's modulus=20 GPa), expecting that the bulk impedance is markedly decreased by cold pressing [START_REF] Shan | Mixed Alkaline Earth Effects on Crystallization Behavior of Basalt Glasses and Liquids[END_REF][START_REF] Kraft | Inducing High Ionic Conductivity in the Lithium Superionic Argyrodites Li 6+x P 1-x Ge x S 5 I for All-Solid-State Batteries[END_REF]. Moreover, most solid sulfur electrolytes (SSE) can be proceeded by simply mechanical milling to avoid unnecessary reactions between active materials and SSE during hot sintering [START_REF] Seoa | Broad Temperature Dependence, High Conductivity, and Structure-Property Relations of Cold Sintering of LLZO-based Composite Electrolytes[END_REF].

Recently the typical sulfide electrolyte material as Li 10 GeP 2 S 12 (LGPS) superionic conductor was explored by Kamaya et al. [START_REF] Kamaya | A Lithium Superionic Conductor[END_REF]. It provided a traditional three-dimensional framework structure and ensured effective lithium-ion transport channels. Moreover, the ionic conductivity is comparable with the commercial lithium batteries up to 12 mS cm -1 .

However, Ge element as a raw material is relatively expensive and rare, limiting the commercialization of LGPS battery. Hence, high-conductivity, good electrochemical characteristics and low-cost SSE should be further explored [START_REF] Wang | Fast Ion Conduction and Its Origin in Li 6-x PS5 -x Br 1+x[END_REF][START_REF] Zhu | First Principles Study on Electrochemical and Chemical Stability of Solid Electrolyte-Electrode Interfaces in All-Solid-State Li-Ion Batteries[END_REF][START_REF] Feng | Enhanced Ion Conduction by Enforcing Structural Disorder in Li-Deficient Argyrodites Li 6-x PS 5-x Cl 1+x[END_REF][START_REF] Ohno | Further Evidence for Energy Landscape Flattening in the Superionic Argyrodites Li 6+x P 1-x M x S 5 I (M = Si, Ge, Sn)[END_REF][START_REF] Arnold | Halide Doping effect on Solvent-Synthesized Lithium Argyrodites Li 6 PS 5 X (X= Cl, Br, I) Superionic Conductors[END_REF][START_REF] Zhou | New Family of Argyrodite Thioantimonate Lithium Superionic Conductors[END_REF][START_REF] Wang | Reaction Mechanism of Li 2 S-P 2 S 5 System in Acetonitrile Based on Wet Chemical Synthesis of Li 7 P 3 S 11 Solid Electrolyte[END_REF][START_REF] Liang | Li 10 Ge(P 1-x Sb x ) 2 S 12 Lithium-Ion Conductors with Enhanced Atmospheric Stability[END_REF][START_REF] Ahmad | Enhanced Air-Stability and High Li-Ion Conductivity of Li 6.988 P 2.994 Nb 0.2 S 10.934 O 0.6 Glass-Ceramic Electrolyte for All-Solid-State Lithium-Sulfur Batteries[END_REF]. Many researchers attempted to employ Sn or Si instead of Ge atom to construct similar structure for lithium-ion diffusion [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF][START_REF] Vinado | Electrochemical and Interfacial Behavior of All Solid-State Batteries using Li 10 SnP 2 S 12 Solid Electrolyte[END_REF][START_REF] Nam | First-Principles Design of Highly Functional Sulfide Electrolyte of Li 10-x SnP 2 S 12-x Cl x for All Solid-State Li-Ion Battery Applications[END_REF][START_REF] Sun | Superionic Conductors: Li 10+δ [SnySi 1y ] 1+δ P 2 δS 12 with a Li 10 GeP 2 S 12 -type Structure in the Li 3 PS 4 -Li 4 SnS 4 -Li 4 SiS 4 Quasi-ternary System[END_REF][START_REF] Kato | High-Power All-Solid-State Batteries using Sulfide Superionic Conductors[END_REF][START_REF] Inagaki | Conduction Mechanism of Li 10 GeP 2 S 12 -Type Lithium Superionic Conductors in a Li-Sn-Si-P-S System[END_REF][START_REF] Hori | Structure-Property Relationships in Lithium Superionic Conductors Having Li 10 GeP 2 S 12 -Type Structure[END_REF][START_REF] Bron | Li 10 Si 0.3 Sn 0.7 P 2 S 12 -A Low-Cost and Low-Grain-Boundary Resistance Lithium Superionic Conductor[END_REF][START_REF] Kuhn | A New Ultrafast Superionic Li-Conductor: Ion Dynamics in Li 11 Si 2 PS 12 and Comparison with Other Tetragonal LGPS-Type Electrolyte[END_REF], which one of examples is the designed component of Li 10+δ [Sn y Si 1-y ] 1+δ P 2-δ S 12 SSE [START_REF] Sun | Superionic Conductors: Li 10+δ [SnySi 1y ] 1+δ P 2 δS 12 with a Li 10 GeP 2 S 12 -type Structure in the Li 3 PS 4 -Li 4 SnS 4 -Li 4 SiS 4 Quasi-ternary System[END_REF]. Successfully controlled of element replacement, the increased lattice volume promoted the high ionic conductivity of σ=1.1×10 -2 S cm -1 at 25 °C.

When atoms with small radius are replaced by large ones, the ion transport framework expands to increase the volume of diffused lithium ions, thus facilitating lithium-ion conduction in channels. In the system of Li 10 Ge 1-x Sn x P 2 S 12 SSE, the variation of conductivity decreases contrary to the increase in lattice volume due to the change of the combination bond on the potential energy of moving ions [START_REF] Krauskopf | The Bottleneck of Diffusion and Inductive Effects in Li 10 Ge 1-x Sn x P 2 S 12[END_REF][START_REF] Culver | Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li 10 Ge 1-x Sn x P 2 S 12[END_REF]. Therefore, the nature of substitution effect is a serious problem in the ionic transition for lithium-ion battery and must be further clarified.

In this work, the relatively cheap Sn element was used to completely replace the expensive Ge to explore the designed (1+x) Li 3 PS 4 -(2-x) Li 4 SnS 4 SSE. The P/Sn ratio was adjusted to control of the crystal bones for evaluating the rule of lithium-ion transition. The conversion of PS 4 and SnS 4 structural tetrahedron allows the control of charge density surrounding Li + ions in the host framework. Ion's transport was discussed in terms of the potential energy of the coordination environment. The results obtained the glass-ceramics with high ionic conductivity 2.1 mS/cm and good electrochemical property after heat technology at room temperature (25°C). In addition, the battery LCO/2.5Li 3 PS 4 -0.5Li 4 SnS 4 /Li was assembled to evaluate the cycle performance. It enabled the exploitation of ASSLSB with desirable electrochemical properties, which revealed the expected superiorities of these devices.

Experiment

Material Synthesis

The sulfur solid electrolyte system of (1+x) Li 3 PS 4 -(2-x) Li 4 SnS 4 (x =0, 0.5, 1, 1.5, 1.7) was synthesized through high energy mechanical ball milling and subsequent heat treatments.

The initial materials including P 2 S 5 (99 %, Aladdin), Li 2 S (99.9 %, Aladdin) and SnS 2 (>99%, Aladdin) were weighed according to the ratio. The mixed initial feedstock (3g) was placed in a 45 mL ZrO 2 jar containing 12 ZrO 2 balls and ground at 510 rpm for 30h.

Mechanical ball milling of x = 1.5 sample was selected for the heat treatment in a quartz ampule, with 500 °C, 550 °C, 600 °C, and 650 °C for 72h. The heating rate was 2.2 °C/min, and the cooling rate was 0.6 °C/min. All the preparations and sample treatments were proceeded in an Argon gas environment (O 2 <0.1ppm, H 2 O<0.1ppm), including the synthesis and electrochemical tests.

Structural Characterization

X-ray diffraction (XRD) test was performed using Bruker D2 Phase analyzer (10 mA, 30 kV) with Cu Kα radiation to determine the amorphous characteristic and crystalline state.

Diffraction data was collected within the range of 10°-60 ° with 0.015° step size. A polymer film was used to prevent contact with air. Structural evolution was measured by Raman spectra equipped with an Ar ion laser (Renishaw In Via, Gloucestershine, UK) under the 488 nm excitation. Thermogravimetric/differential thermal (TG/DSC) (Germany, NETZSCH STA 449F3) was performed using a thermal analyzer at a heating rate of 10°C min -1 to determine the crystallization temperature ( T c ). And the morphology of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 samples before and after heat treatment was observed by SEM (SEM, JEOL, JSM-7001F).

Electrochemical Characterization

The impedance was measured using the AC impedance method in the Ar atmosphere by a Solartron electrochemical workstation (type: PGSTAT302N, Metrohm). The SE particles were cold-pressed at 180 MPa (diameter=10 mm, thickness= 0.8 mm). Stainless-steel (SS) sheets were then placed on either side of the electrolyte sheet as current collectors. Conductivity was calculated based on the equation σ = D / (R×A), D expresses the thickness, A denotes contact area, and R stands for the resistance of pellets. Electrochemical window was evaluated for solid electrolyte by cyclic voltammetry. SS and lithium metal were used as the cathode and anode of the cell, and measurements were conducted using the Solartron 1287 electrochemical interface with 1 mV s -1 scanning rate from -0.1V to 5V on an asymmetric Li/2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE /SS cell.

All-solid-state battery LiCoO 2 /2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE/Li was prepared as follows. The cathode is comprised of LiCoO 2 (99.9% Sigma-Aldrich), the 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE and conductive carbon. First, LiCoO 2 , carbon black and 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE with a ratio of 14:1:5 were weighted and ground in an agate mortar for 5 min. Then, the mixture (2mg) was pressed into the particle bulk at 180 MPa. After the 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE (100 mg) was pressed into the particle bulk, the positive electrode was together pressed into double-layer particles under the same pressure. Finally, lithium foil was attached to one side of the double-layer particles with 90 MPa. The mixture served as the positive electrode, and lithium metal plate was regarded as the negative electrode. The LiCoO 2 /2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE/Li cell battery was thus assembled and tested for charge-discharge process in glovebox. [START_REF] Seoa | Broad Temperature Dependence, High Conductivity, and Structure-Property Relations of Cold Sintering of LLZO-based Composite Electrolytes[END_REF][START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF]. Also, the part diffraction peaks are consistent with Li 3 PS 4 phase which peaked at 17°, 25.7°, 28.7°, 29°, 30.7°, 38.9°, 46.7°, 50.8°, respectively. It was indicated that the prepared electrolyte samples precipitated the high conductive phase similar to conductors Li 3 PS 4 which contributed to the ions transport inside the network channels. Therefore, the sample structure was well crystalized, and a similar tetragonal structure was formed within the precipitated crystals.

Result and Discussion

Structural Characterization

Moreover, the overall diffraction peaks exhibited a trend of first increase and then decrease with heat treatment. When the temperature was raised to 550 °C, the XRD peaks became sharp and strong, revealing a high crystallinity was formed. However, the XRD peaks near 36° almost disappeared, and other peaks were weakened at 650 °C. It revealed that the optimal annealing temperature is 550 °C, which is hosted to the construction of network for lithium-ion transport. The crystallization temperature (T c ) of amorphized sample was measured by DSC to identify the optimum heat treatment temperature for glass-ceramics phases. As shown in Fig. S1 in the supplement file, the initial crystallization range of the 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE sample is about 400-600 °C, which is basically selected as the heat treatment temperature range.

Raman test was conducted to clarity the framework of electrolyte after mechanical ball milling and heat treatments. In Fig. 2(a), the spectra of samples after milling show obvious peaks at approximately 380, 418 and 430 cm -1 . The peaks at 418 and 430 cm -1 were mainly caused by the stretching vibration of ν(PS 4 ) tetrahedrons [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF]. And the vibration mode of P 2 S 6 4-units was contributed to the peak nearby 380 cm -1 . With increase of P substitution, these above peaks presented enhanced vibration intensity. However, the intensity of the peaks at 418 cm -1 and 430 cm -1 significantly declined when x=1.7, probably owing to the transformation of different structural sites of PS 4 3-tetrahedron [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF][START_REF] Stoffler | Amorphous versus Crystalline Li 3 PS 4 : Local Structural Changes During Synthesis and Li Ion Mobility[END_REF][START_REF] Preefer | Rapid and Tunable Assisted-Microwave Preparation of Glass and Glass-Ceramic Thiophosphate "Li 7 P 3 S 11 " Li-Ion Conductors[END_REF][START_REF] Hassouna | A Structural, Spectroscopic and Electrochemical Study of a Lithium Ion Conducting Li 10 GeP 2 S 12 Solid Electrolyte[END_REF][START_REF] Yang | Dual Doping: An Effective Method to Enhance the Electrochemical Properties of Li 10 GeP 2 S 12 -Based Solid Electrolytes[END_REF][START_REF] Minami | Preparation and Characterization of Lithium Ion Conducting Li 2 S-P 2 S 5 -GeS 2 Glasses and Glass-Ceramics[END_REF]. It indicated that PS 4 3-units were easier formed at x=1.5 and linked to restructure after further addition. As exhibited in Fig. 2(b), the Raman spectrum of LiSn(Ge)PS were almost consistent with previous reports [START_REF] Kamaya | A Lithium Superionic Conductor[END_REF][START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF][START_REF] Hassouna | A Structural, Spectroscopic and Electrochemical Study of a Lithium Ion Conducting Li 10 GeP 2 S 12 Solid Electrolyte[END_REF][START_REF] Yang | Dual Doping: An Effective Method to Enhance the Electrochemical Properties of Li 10 GeP 2 S 12 -Based Solid Electrolytes[END_REF][START_REF] Minami | Preparation and Characterization of Lithium Ion Conducting Li 2 S-P 2 S 5 -GeS 2 Glasses and Glass-Ceramics[END_REF]. After continuous heat treatment of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 , the peaks intensity increased first and then descended as a whole. New peaks obviously appeared at 255, 285, 345, 405, 551 and 570 cm -1 , which of the peak at 345 cm -1 was ascribed to the vibration of SnS 4 4-tetrahedron [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF][START_REF] Kanazawa | Mechanochemical Synthesis and Characterization of Metastable Hexagonal Li 4 SnS 4 Solid Electrolyte[END_REF][START_REF] Zhang | Li 4-x Sb x Sn 1-x S 4 Solid Solutions for Air-Stable Solid Electrolytes[END_REF]. The peaks near 255, 418, 551, 570 cm -1 and the broad asymmetric peaks in the 270-310 cm -1 range were mainly caused by PS 4 3-tetrahedron vibrations [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF][START_REF] Hassouna | A Structural, Spectroscopic and Electrochemical Study of a Lithium Ion Conducting Li 10 GeP 2 S 12 Solid Electrolyte[END_REF][START_REF] Yang | Dual Doping: An Effective Method to Enhance the Electrochemical Properties of Li 10 GeP 2 S 12 -Based Solid Electrolytes[END_REF][START_REF] Minami | Preparation and Characterization of Lithium Ion Conducting Li 2 S-P 2 S 5 -GeS 2 Glasses and Glass-Ceramics[END_REF], that exhibited a most connection of SnS 4 and PS 4 for the crystalline structure [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF]. While the characteristic stretching vibrations of P-S bonds at 384 and 406 cm -1 were associated with the P 2 S 6 4-and PS 4 3-anions, respectively [START_REF] Hassouna | A Structural, Spectroscopic and Electrochemical Study of a Lithium Ion Conducting Li 10 GeP 2 S 12 Solid Electrolyte[END_REF][START_REF] Yang | Dual Doping: An Effective Method to Enhance the Electrochemical Properties of Li 10 GeP 2 S 12 -Based Solid Electrolytes[END_REF][START_REF] Minami | Preparation and Characterization of Lithium Ion Conducting Li 2 S-P 2 S 5 -GeS 2 Glasses and Glass-Ceramics[END_REF]. The enhanced vibration mode of PS 4 3-units at 418 cm -1 suggested that large Li + coordinated with sulfur involved in the ions transport process. In this case, the Li + were most transported inside the non-crystal structure, leading to the increased impedance. Fig. 3(b) further provides the typical EIS profiles of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE with different heat treatments. Impedance diagrams are usually composed of semicircular arcs and oblique lines (electrode grain and grain boundary) [START_REF] Bron | Li 10 SnP 2 S 12: An Affordable Lithium Superionic Conductor[END_REF][START_REF] Bron | Li 10 Si 0.3 Sn 0.7 P 2 S 12 -A Low-Cost and Low-Grain-Boundary Resistance Lithium Superionic Conductor[END_REF]. Herein, the lacked semicircle with only one slope at 550 °C was observed, indicating that the contribution of the grain resistance from semi-circular arc is almost negligible. With increase of heat temperature, the value of the impedance decreases first and then increases obtaining the minimum impedance of 50Ω, when the temperature increased to 650 °C, the impedance value then increased which of the result was in accordance with Raman analysis that the declined PS 4 unit as conducive to lithium ions. It has been reported by Tatsumisago M. that a heat-treated Li 2 S-P 2 S 5 glass-ceramic conductor realized an extremely high ionic conductivity of 1.7 x 10 -2 S cm -1 . The optimum conditions of the heat treatment reduced the grain boundary resistance and the influence of voids, increasing the Li + ionic conductivity of the solid electrolyte [START_REF] Seino | A Sulphide Lithium Super Ion Conductor is Superior to Liquid Ion Conductors for Use in Rechargeable Batteries[END_REF]. Also, SEM pictures before and after heat treatment of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 was supplemented in Fig. S2. For 2.5Li 3 PS 4 -0.5Li 4 SnS 4 , a small number of crystals were precipitated after ball milling, and the whole electrolyte was still in the glass state with tendency of phase separation. After 550 °C heat treatment, a large number of crystals were precipitated, the contact between grains was closer and the density was intensified, which reduced the grain boundary impedance and improved the ionic conductivity. In addition, the EDS spectra in supplement display the elements are uniformly distributed inside the sample before and after heat treatment, even better after heat treatment. Therefore, it was believed that the conductivity was optimized by modification of the electrolyte component and further heat influence. Fig. 3(c) displays the ionic conductivity at different x values after mechanical ball mill and different heat treatment temperatures, where the maximum ionic conductivity of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE at 550℃ was 2.1×10 -3 S cm -1 .

Ionic Conductivities

Electronic conductivity can be calculated by the formula σ e = DI/SE, where D is the sample thickness, S represents the contact area, E expresses the polarization voltage, and I is the electronic current. A difference of seven orders of magnitude was found between electron conductivity and ionic conductivity exhibit in Fig. 3(d). Therefore, the electronic conductivity σ e =1.02 × 10 -9 S cm -1 was almost negligible, implying that the grain conductivity contributes to the fast Li + transport and is not caused by electronic conduction. Fig. 3(e) displays the impedance value of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE along with the temperatures, which was high up to 29.5 mS cm -1 at 100 °C. Accordingly, the ionic conductivity showed good fitting on the basis of the Arrhenius Law. As shown in Fig. 3(f), the activation energy (Ea) values were calculated as 0.335 eV, 0.325 eV, 0.332 eV, 0.336 eV, respectively at 500-650 °C. The results showed that the activation energy was low to about 0.325 eV when heat treated at 550 ℃, which is a relatively low value compared with the result of the result of Li 10 SnP 2 S 12 (0.31eV) reported by Bron P. [START_REF] Bron | Li 10 Si 0.3 Sn 0.7 P 2 S 12 -A Low-Cost and Low-Grain-Boundary Resistance Lithium Superionic Conductor[END_REF]. When the heat treatment temperature was properly controlled, crystal impedance was reduced, and ionic conductivity was enhanced.

Consequently, the binding ability of sulfur atoms (P-S bond) could be adjusted to the nearby Li + by optimization of Sn/P ratio in response to the migration of lithium ions [START_REF] Krauskopf | The Bottleneck of Diffusion and Inductive Effects in Li 10 Ge 1-x Sn x P 2 S 12[END_REF][START_REF] Culver | Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li 10 Ge 1-x Sn x P 2 S 12[END_REF].

Cyclic Voltammetry

According reports, there is usually a narrow electrochemical window (about 1.8~2.3 V vs Li + /Li) for chalcogenide solid electrolytes such as Li 10 GeP 2 S 12 [START_REF] Arnold | Halide Doping effect on Solvent-Synthesized Lithium Argyrodites Li 6 PS 5 X (X= Cl, Br, I) Superionic Conductors[END_REF]. Fig. 4 evaluated the electrochemical stability of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE by Cyclic Voltammetry (CV) in Ar atmosphere. The assembled battery adopted the stainless steel and lithium applied for the poles of working electrode. No significant redox reaction occurred at -0.1 to 5 V in CV curve, except for oxidation and reduction peaks at approximately 0 V (vs Li/Li + ), which were attributed to the lithium ions dissolution (Li→Li + + e -) and deposition (Li + + e -→Li) [START_REF] Liang | Li 10 Ge(P 1-x Sb x ) 2 S 12 Lithium-Ion Conductors with Enhanced Atmospheric Stability[END_REF]. It was believed that good electrochemical stability was obtained and the electrochemical window high up to 5V was realized.

Structural Model of Li + Transportation

It is suggested that Li + diffusion depends on the potential energy surface which of the distribution is decided by the interaction between the movable Li + and the matrix including the electrostatic interaction between Li + ions [START_REF] Krauskopf | The Bottleneck of Diffusion and Inductive Effects in Li 10 Ge 1-x Sn x P 2 S 12[END_REF][START_REF] Culver | Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li 10 Ge 1-x Sn x P 2 S 12[END_REF]. As depicted in Fig. 5(a), the quadrilateral frame structure Li 10 SnP 2 S 12 was composed of SnS 4 4-and PS 4 3-tetrahedral anions containing interstitial-state Li + ions. The main frame structure provides channels for ion migration along the c direction, and Li + alternately moves through Li(1) and Li(3) sites (Fig.

5(b))

, leading to the rapid ions diffusion [START_REF] Seoa | Broad Temperature Dependence, High Conductivity, and Structure-Property Relations of Cold Sintering of LLZO-based Composite Electrolytes[END_REF][START_REF] Nam | First-Principles Design of Highly Functional Sulfide Electrolyte of Li 10-x SnP 2 S 12-x Cl x for All Solid-State Li-Ion Battery Applications[END_REF][START_REF] Hori | Structure-Property Relationships in Lithium Superionic Conductors Having Li 10 GeP 2 S 12 -Type Structure[END_REF]. In addition to the effect of structure control on electrical conductivity, the nature of the chemical environment around Li ions also influences its migration. Krauskopf proposed an idea for solid-electrolyte inductive impact, in which the bonding changes of the host frame in solid electrolyte modified the potential energy pattern of the mobile ions [START_REF] Krauskopf | The Bottleneck of Diffusion and Inductive Effects in Li 10 Ge 1-x Sn x P 2 S 12[END_REF][START_REF] Culver | Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li 10 Ge 1-x Sn x P 2 S 12[END_REF]. In the crystal ceramic structure, the S atoms in the M-S (M=P, Sn) bonds are arranged on both sides of the c access channel. The polarity of Sn-S bond is greater than that of the equivalent P-S bond as schematized in Fig. 5(c).

Therefore, the negative charge density of P-S atom was decreased indicating that the electrostatic of S•••Li interaction is lower than that of Sn analogue in the P-substituted system. The efficiently reduced energy barrier of lithium-ion diffusion along channels improved the conductivity of Li + and optimized electrochemical performance [START_REF] Krauskopf | The Bottleneck of Diffusion and Inductive Effects in Li 10 Ge 1-x Sn x P 2 S 12[END_REF][START_REF] Culver | Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li 10 Ge 1-x Sn x P 2 S 12[END_REF].

All solid -state battery

A prototype cell of 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE in ASSLSBs was assembled in Fig. 6(a). As previous reported [START_REF] Hayashi | Development of Sulfide Solid Electrolytes and Interface Formation Processes for Bulk-Type All-Solid-State Li and Na Batteries[END_REF][START_REF] Zhang | High Capacity, Superior Cyclic Performances in All-Solid-State Lithium-Ion Batteries Based on 78Li 2 S-22P 2 S 5 Glass-Ceramic Electrolytes Prepared Via Simple Heat Treatment[END_REF], the change of interface resistance between cathode and electrolyte was much larger than that between electrolyte and lithium [START_REF] Zhang | High Capacity and Superior Cyclic Performances of All-Solid-State Lithium-ion Batteries Enabled by a Glass-Ceramic Solo[END_REF][START_REF] Zhang | FeS Nanosheets as Positive Electrodes for All-Solid-State Lithium Batteries[END_REF]. Thus, the variation of the interfacial resistance was mainly attributed to the continuous shuttle effect of Li + ions at the electrolyte and cathode interface during cycling, which lead to the contact failure between cathode and solid electrolyte.

Hence, stable interface contact is crucial for ASSLSB with good cycling performance.

Conclusions

Glass-ceramic (1+x) Li 3 PS 4 -(2-x) Li 4 SnS 4 SSEs were successfully synthesized through mechanical ball milling and continuous heat treatments. The resistance of the grain boundary was effectively reduced by controlling P/Sn ratio and heat treatments. Owing to the less electronegativity of P than Sn, the weak coulomb attraction between S and nearby Li + cations was formed. Thus, Li + was transported faster in the designed channel, and the ionic conductivity was significantly improved. When the heat temperature reached 550 °C, the obtained 2.5Li 3 PS 4 -0.5Li 4 SnS 4 SSE showed the higher ionic conductivity (2.1×10 -3 S cm -1 )

and lower E a with 0.325 eV. Furthermore, the constructed ASSLSB delivered a wide and stable electrochemical window from -0.1 V to 5 V. A good cyclic performance was attained with capacity retention rate kept at 78.7% and the coulomb efficiency always outperformed 99.5%. The work improved the understanding of effect on the chemical bond energy of structure-property relationships in ionic conductors. It paved the way for the enhancement of ionic conductivity in SSE and the acceleration of all-solid-state battery applications.

Fig. 1 (

 1 Fig. 1(a) displays the XRD patterns of samples after mechanical ball milling. Consistency

Fig. 3 (

 3 Fig.3(a) presents the impedance plots of all samples after milling. The impedance of the

Fig. 6 (

 6 Fig.6(b) shows the cyclic performance including the columbic efficiency from 3V to 4.2V

Fig. 2 (

 2 Fig. 2(a) Raman spectrum of all samples (1+x) Li 3 PS 4 -(2-x) Li 4 SnS 4 after mechanical

Fig. 3 (

 3 Fig. 3(a) Impedance of (1+x) Li 3 PS 4 -(2-x) Li 4 SnS 4 SSE by milling. The inset corresponds to

Fig. 4 Fig. 5 (Fig. 6 (

 456 Fig. 4 Cyclic voltammetry (CV) curve of Li/2.5Li 3 PS 4 -0.5Li 4 SnS 4 /SS cell with the voltage

Acknowledgement

This work was financially supported by National Natural Science Foundation of China (Grant no. 51972176)and sponsored by K. C. Wong Magna Fund in Ningbo University.

List of


Gerardo Salinas

Filippo Malacarne

Giorgia Bonetti

Roberto Cirilli

Tiziana Benincori

Serena Arnaboldi

Alexander Kuhn

Wireless electromechanical enantio-responsive valves

Keywords: Responsive materials, bipolar electrochemistry, conducting polymers, chiral recognition

la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Chemically responsive valves are essential devices in channel-based microfluidics.1-3 Such systems selectively manipulate/control a small flow of liquid inside channels or compartments triggered by an external input. Commonly, microfluidic valves are designed by using stimuli-responsive polymers as active material.1,2 Different polymer-based valves, controlled by electric4,5 or magnetic fields,6 infrared light,7,8 temperature,9 and pH10 have been developed. Nonetheless, the design of alternative chemically responsive devices, sensitive to different and far more complex physicochemical parameters, such as chirality, is an interesting challenge. Chirality is a fundamental symmetry property of elementary particles, molecules or even macroscopic objects.11 Commonly a system is defined as chiral if it exists as a pair of 'left-handed' and 'right-handed' mirror images that cannot be superimposed (enantiomers). Due to their numerous applications in medicine, chemistry or biochemistry, chiral molecules have gained considerable attention.11 For example, for biological systems, a specific pharmaceutical compound can be designed for a defined biological receptor, where chirality is used to tune the nature of the interaction.12 As such, enantiomeric interactions ultimately control and perturb biological functions, thus, enantiorecognition is of tremendous importance in biological systems. Although different spectroscopic methods have been developed for the efficient enantiorecognition of chiral probes,13-15 their coupling to delivery systems is still a challenge. An interesting approach is to combine the outstanding enantiorecognition capability of inherently chiral conducting polymers with the highly efficient electromechanical actuation of polypyrrole,16-19 triggered by bipolar electrochemistry (BE). BE has gained considerable attention due to its broad range of applications from electroanalysis20,21 and electrosynthesis22,23 to motion generation. [START_REF] Moo | Nano/Microrobots Meet Electrochemistry[END_REF] The approach is based on the asymmetric polarization of a conducting object due to the presence of an external electric field (ε).25-28 This produces a polarization potential difference (ΔV) between the extremities of the object. When electroactive species are present in solution and ΔV exceeds the thermodynamic threshold potential (ΔVmin), redox reactions take place asymmetrically at both sides of the device, the so-called bipolar electrode (BPE). With such a concept, Bouffier et al. designed a wireless electrochemical valve based on a bubble accumulation/release mechanism.29 Nonetheless, the closing process of the valve is irregular, due to the random detachment of bubbles. Thus, externally driven electromechanical actuation is presented as a promising alternative for the design of wireless enantioselective valves. Such electrochemically driven actuation is based on an asymmetric uptake/release of ions inside the polymer matrix, which causes a swelling/shrinking process.30,31 Recently such wireless actuation has been successfully used as a powerful tool for the transduction of chiral information based on an electromechanical readout.32-34 The approach is based on the coupling of the reduction of polypyrrole (Ppy) (Scheme 1A) with the enantioselective oxidation of a chiral probe on the surface of an inherently chiral oligomer. Due to the energetically different interactions between the chiral oligomer surface and the probe in solution, enantiodiscrimination in terms of difference in oxidation potentials can be achieved. Thus, when an external electric field is applied, the chiral BPE present in solution is polarized, leading to a preferential oxidation of one of the two antipodes. Consequently, only when the right enantiomer of the deposited inherently chiral oligomer reacts specifically with the corresponding molecular probe in solution, an electromechanical deformation of the Ppy can be observed. Herein, we present a wireless enantioselective valve, obtained by combining the electromechanical properties of a Ppy strip with the enantioselectivity of an inherently chiral oligomer, oligo-2,2'-bis[2-(5,2'-bithienyl)]-3,3'-bithianaphthene (BT2T4) (Scheme 1B). The enantioselective valve is then placed in a bipolar setup containing a solution with the antipodes of a chiral analyte, i.e. L-or D-3,4-dihydroxyphenylalanine (L-or D-DOPA) (Scheme 1C, D). The setup is designed in analogy to what has been reported for enantioselective actuators.33-34 The functionalized Ppy strip is properly tailored to cover a capillary tube loaded with a dye. In the presence of the enantiomers of a chiral analyte dissolved in solution, and by applying a specific voltage, the mechanical deformation of the Ppy occurs only in the case when the right analyte antipode is oxidized on the oligomer with the matching configuration. The induced upward bending allows the opening of the tube and the release of the dye. The Ppy lid can be easily closed again by switching the polarity of the applied electric field. The process is reversible meaning that the opening/closing loop can be repeated several times until no dye is left in the reservoir.

Materials and Methods

SYNTHESIS OF THE OLIGO-PPY FILM

The polymeric valve was designed in a two-step process. First, electropolymerization of the Ppy film was carried out using an aqueous 0.25 M dodecylbenzene sulfonate (DBS, Sigma Aldrich) and 0.2 M pyrrole (Sigma-Aldrich) solution, by chronopotentiometry applying 4 mA for 1.5 h. A classic threeelectrode set-up, coupling two gold-coated glass slides, used as working and counter electrode, respectively, and a Ag/AgCl as reference electrode, was used. After polymerization, the Ppy film was washed with water, dried, and peeled off from the gold substrate. In the second step, the oligomerization of the chiral monomer, 2,2'-bis[2-(5,2'-bithienyl)]-3,3'-bithianaphthene (BT2T4), was carried out in a 0.1 M LiClO4/ACN solution containing 1 mM of monomer, applying 2 mA for 40 min (Scheme S1A). Only a fraction of the Ppy film and the smooth side was used as working electrode, together with a Pt mesh and Ag/AgCl as counter and reference electrodes, respectively. After deposition, the free-standing oligo-(BT2T4)-polypyrrole films were cut into strips of 15 x 5 mm. The bipolar devices were fixed on a support with the smooth side upwards. All solutions were prepared with deionized water (MilliQ Direct-Q®).

BIPOLAR ENANTIOSELECTIVE VALVE

For the bipolar chiral valve, the enantiopure (R)-or (S)-oligo-BT2T4-Ppy device was fixed in the center of a bipolar cell, comprising a capillary glass tube loaded with a blue dye solution (Crystal violet, Sigma-Aldrich). Two graphite feeder electrodes were positioned at the extremities of the cell at a distance of 5 cm. Enantiomeric recognition was carried out in 8 mL of an aqueous 0.2 M LiClO4 solution, in the presence of 5 mM L-or D-DOPA at a constant applied electric field (1.6 V cm-1 or 1.7 V cm-1, respectively). For the double bipolar valve, the enantiopure (R)-and (S)-oligo-Ppy bipolar devices were fixed face-to-face at the center of a bipolar cell, containing two glass tubes filled with the blue and a pink dye (Rose Bengal, Sigma-Aldrich), respectively. Double bipolar recognition was carried out in an aqueous 0.2 M LiClO4 solution, in the presence of an equimolar solution (5 mM) of L-DOPA and L-Tryptophan (L-Tryp), at a constant applied electric field (1.6 V cm-1). Experiments were monitored by using a charge-coupled device (CCD) camera (CANON EOS 70D, Objective Canon Macro Lens 100 mm 1:2.8). Images were processed with ImageJ software.

Results and Discussion

In order to evaluate the opening/closing mechanism of the valve, triggered by bipolar electrochemistry, a non-enantioselective Ppy valve was tested. The bipolar electrode was designed as follows. From a 15 mm Ppy strip, a 5 x 2 mm (length and width) section, corresponding to the dimensions of the cathodic part of the actuator, and a broader part (10 x 5 mm), corresponding to the anodic extremity, were cut properly. This asymmetric geometry was used in order to increase the area where the anodic reaction takes place. This allows to maximize the current density at the cathodic side of the BPE, producing a more pronounced bending of the cantilever. The device was then immobilized at its larger extremity on a support with the smooth face upwards. The obtained non-modified valve was placed in a bipolar cell containing an aqueous 0.2 M LiClO4, 5 mM L-DOPA solution. Only the extremity of the actuator was used to cover a tube loaded with dye. The tip of the cantilever was painted with a non-conductive commercial nail varnish in order to improve its adhesion to the opening of the tube and to facilitate the bending of the actuator, due to a localized swelling/shrinking process (Scheme S1B). When an electric field of 2 V cm-1 is applied, the asymmetric polarization of the Ppy device takes place. This leads to the oxidation of L-DOPA and the reduction of Ppy at the anodic and cathodic extremities of the polymeric film, respectively. Since during the reduction of Ppy, the uptake of cations preferentially takes place at the rough face of the film, oriented downwards in the present set-up, the asymmetric swelling causes an upward bending of the cantilever. This leads to the opening of the tube and consequently to the release of the dye (Figure S1a,b,c). Due to the efficient and reversible uptake and release of ions by Ppy, it is possible to induce a downward bending by inverting the polarity of the feeder electrodes, which allows the closing of the dye reservoir (Figure S1d). It is important to highlight that under these conditions the oxidation of Ppy at the δ+ extremity is accompanied by the reduction of either the Ppy substrate at its δ-end, or the quinoidal species formed during the oxidation of DOPA on the other side. Nonetheless, this umpolung can be repeated for several cycles without damaging the Ppy strip, making the process reversible (Movie S1).

After having demonstrated that in such a set-up the Ppy strip can act as an electrochemically responsive valve, an analog, but enantioselective, system has been built. Theoretically, by functionalizing the Ppy strip with inherently chiral oligomers, the opening and the closure of the valve should be controlled essentially by the diastereomeric interactions between the inherently chiral oligomers and the chiral analytes dissolved in solution. The outstanding enantiorecognition capability of these oligomer substrates in terms of redox potentials has been demonstrated previously.35 This is a crucial point in the development of the enantioselective valve since it is possible to oxidize exclusively one enantiomer on the surface of the functionalized Ppy, if the proper electric field is applied. The overall experimental setup is illustrated in Scheme 1D. The hybrid enantioselective valve, composed of enantiopure (R)-or (S)-oligomer BT2T4 (orange) electrodeposited onto Ppy (blue), was used as a bipolar electrode. The experimental procedure is the same as the one described above for the non-modified Ppy valve. When applying the optimized electric field, the involved redox reactions that take place at the respective extremities of the BPE are the oxidation of DOPA and the reduction of Ppy. For example, the (R)-oligo-Ppy valve shows actuation when applying an electric field of 1.7 V cm-1 in the presence of D-DOPA (Figure 1A, Movie S2 upper video), whereas, in the presence of L-DOPA no mechanical deformation was observed under the same conditions (Figure 1B, Movie S2 bottom video). As stated above, the selective oxidation of D-DOPA, coupled with the reduction of Ppy, triggers the upward bending, allowing the opening of the valve accompanied by a massive release of blue dye. In addition, by periodically inverting the polarity of the electric field, it is possible to observe a reversible opening/closing of the reservoir. In order to confirm the enantioselectivity of the valve as a function of the configuration of the inherently chiral oligomer, a mirror experiment was carried out by depositing the opposite oligomer configuration ((S)-oligo-BT2T4) on the Ppy strip, and testing the device in an aqueous 0.2 M LiClO4 solution of 5 mM L-or D-DOPA. Under these conditions, the (S)oligo-Ppy device exhibits actuation only when it is immersed in a solution of L-DOPA (Figure S2). From these results, one can conclude that such electromechanical systems can act as enantioselective valves since the opening/closing mechanism is governed by the diastereomeric interactions between the chiral probe in solution and the inherently chiral oligomer. This also opens up the possibility to design systems that selectively open or close in the presence of more than one relevant probe dissolved in the same solution.

Thus, to extend this concept to more sophisticated systems, a double enantioselective valve was designed. In this device, the opening/closing mechanism is controlled by the orientation of the applied electric field and the respective diastereomeric interactions. First, we evaluated the electric field-induced control by placing two non-enantioselective Ppy actuators face-to-face, at the centre of a bipolar cell comprising two glass tubes loaded with a blue and a red colorant, in an aqueous 0.2 M LiClO4 solution of 5 mM L-DOPA (Scheme S2). When a high enough electric field is applied, the Ppy valve that is facing the positive feeder electrode opens, due to the uptake of ions, whereas the opposite valve closes, due to the release of ions from the polymer. The inverse behaviour is observed when changing the orientation of the electric field. As it can be seen in Figure S3, by applying an electric field of 2 V cm-1, it is possible to trigger a continuous opening/closing process of the two non-enantioselective Ppy valves. For the same electric field value, but opposite polarities, the process is switching from a state where the red valve is open and the blue valve is closed, to the inverse configuration. In addition, the umpolung is reversible and can last for several cycles.

After demonstrating the efficient electric field control of the opening/closing mechanism of the double valve system, its diastereomeric selectivity has been evaluated. In this case a (S)-and (R)oligo-Ppy device were placed face-to-face, at the center of a bipolar cell comprising two glass tubes loaded with a blue and a pink dye, in an aqueous 0.2 M LiClO4 solution. Although it is relatively straightforward to assume that in the presence of L-and D-DOPA, at a given applied electric field, the umpolung process will operate, we were interested in testing more complex systems, including mixtures of molecules with the same chiral configuration. In previous electrochemical experiments it has been demonstrated that L-DOPA reacts preferentially with (S)-oligo-BT2T4, whereas L-Tryp has a higher affinity for (R)-oligo-BT2T4.36,37 Thus, by applying an optimized electric field, each enantioselective valve will open only in the presence of the analyte with the right stereochemical configuration.

In Scheme 2 one opening and closure cycle of the double bipolar valve is illustrated in the simultaneous presence of L-DOPA and L-Tryp. Under the influence of the proper electric field, the (R)-oligo-Ppy actuator, which is facing the positive feeder electrode, favors the selective oxidation of L-Tryp, with a concomitant deformation of its cantilever and a subsequent release of blue dye. At the same time, the (S)-oligo-Ppy actuator is not deformed, keeping the tube closed due to the ion release (Scheme 2A). When switching the polarity, reactions are inverted, thus on the (S)-oligo-Ppy the oxidation of L-DOPA occurs, coupled to the opening of the tube loaded with red colorant, whereas the other strip is reoxidized and shrinks, thus preventing the release of blue dye (Scheme 2B). To test this mechanism, we first studied the opening/closing of the double enantioselective system in the presence of a 5 mM L-Tryp solution (Movie S3). At a constant electric field (1.6 V cm-1) only the Ppy strip functionalized with the (R)-oligomer undergoes mechanical deformation, causing the release of dye. Even more importantly, when the orientation of the electric field is changed, the opposite strip does not react with L-Tryp either, since it is modified with the oligomer with the wrong stereochemistry ((S)-oligomer) (Figure S4). Finally, the double enantioselective opening/closing mechanism was tested in an aqueous 0.2 M LiClO4 solution in the presence of an equimolar mixture (5 mM) of L-DOPA and L-Tryp. Figure 2 presents three cycles of closing/opening of the enantioselective valves by switching the polarity. Under an electric field of 1.6 V cm-1, the (R)-oligo-Ppy valve, which is facing the negative feeder electrode, remains closed, whereas the (S)-oligo-Ppy valve, which is facing the positive electrode, opens (Figure 2A). By switching the polarity, the reactions are inverted causing the opening of the (R)-oligo-Ppy valve and the closing of the (S)-oligo-Ppy device (Figure 2B). Once again, due to the outstanding electromechanical properties of Ppy, the double umpolung mechanism is reversible and lasts several cycles (Movie S4). FIGURE 2 Sequential closing and opening cycles of two bipolar chiral valves (1.5 cm long) obtained by switching the polarity between feeder electrodes and by functionalizing the Ppy actuators with (R)oligo-BT2T4 and (S)-oligo-BT2T4, dipping in an aqueous 0.2 M LiClO4 solution of 5 mM L-DOPA and 5 mM L-Tryp. Feeder electrodes are placed at a distance of 5 cm. A constant electric field of 1.6 V/cm is applied. The two tubes are loaded with two dyes, a blue and a pink one, respectively.

Conclusion

In the present contribution, the first enantioselective valve driven by bipolar electrochemistry is reported. This approach is based on the diastereomeric interactions between inherently chiral oligomers, used to functionalize a polypyrrole strip, and the chiral analytes dissolved in solution. The system is triggered by the electric field and can be considered as a wireless chiral valve. The design of the device allows the selective release of dye by applying a constant electric field and by switching the polarity between the feeder electrodes. These systems can detect different chiral analytes in a single experiment by reacting just with one of the two enantiomers of a given chiral probe. The described set-up has characteristic dimensions in the millimeter scale, but can be in principle scaled down to smaller dimensions. The major advantage of these devices is their wireless nature, combined with a direct transduction of molecular chirality into a release of specific compounds. Thus, these valves might be interesting, among others, for the field of microfluidics in the frame of wireless drug release.

SCHEME 1

 1 SCHEME 1 Chemical structures of (A) the oxidized polypyrrole, (B) the oligo-(S) and oligo-(R)-2,2'bis[2-(5,2'-bithienyl)]-3,3'-bithianaphthene (oligo(S)-and oligo-(R)-BT2T4), and (C) D-and L-DOPA molecules. (D) Schematic illustration of the bipolar set-up used for the wireless enantioselective valve with a representation of the chemical reactions, the electromechanical actuation and the release of the dye. The orange part stands for the BT2T4 oligomer, whereas light and dark blue symbolizes the Ppy film and the dye, respectively. The brown colour represents the varnish used to cover the Ppy tip.

FIGURE 1

 1 FIGURE 1 Bipolar enantioselective valve (1.5 cm long) operating with a Ppy actuator functionalized with (R)-oligo-BT2T4 dipping in an aqueous 0.2 M LiClO4 solution of (A) 5mM D-DOPA and (B) 5mM L-DOPA. The picture sequence from top to bottom represents the opening and closing process of the valve by switching the polarity between feeder electrodes at a constant applied electric field of 1.7 V cm-1. The red arrows show the upward and downward movement of the cantilever.Two enantioselective devices, together with a small capillary-type reservoir loaded with blue dye, were fixed separately in a bipolar cell containing an aqueous 0.2 M LiClO4 solution of 5 mM L-or D-DOPA.

SCHEME 2

 2 SCHEME 2 Schematic illustration of the bipolar set-up used for the double recognition with wireless enantioselective valves with a representation of the related chemical reactions, the resulting electromechanical actuation and the final release of the dyes. The orange part stands for the BT2T4 oligomers, whereas the light blue color symbolizes the Ppy film. The dark brown color stands for the varnish used to cover the Ppy tip. The tubes are loaded with two dyes represented here as dark blue

Acknowledgments

This work has been funded by the H2020 European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 741251, ERC Advanced grant ELECTRA) and under the HORIZON-ERC-2021 work program (grant agreement no 101040798, ERC Starting grant CHEIR)

Supporting information

Additional supporting information may be found in the online version of this article at the publisher's website.