WO2015071225A1

WO2015071225A1 - Device for characterizing the electroacoustic potential of a solution

Info

Publication number: WO2015071225A1
Application number: PCT/EP2014/074194
Authority: WO
Inventors: Raphael PUSSET; David Jacob; Guillaume MERIGUET; Emmanuelle Dubois; Jean Chevalet
Original assignee: Cordouan Technologies
Priority date: 2013-11-13
Filing date: 2014-11-10
Publication date: 2015-05-21
Also published as: FR3013121A1; FR3013121B1

Abstract

According to one aspect, the invention relates to a device for characterising the electroacoustic potential of a solution, suitable for characterising a solution with a given acoustic impedance. The device comprises a module (100) for generating and formatting an acoustic wave train along a given propagation axis (Δ), with at least one given acoustic frequency and one analysis cell (200). The analysis cell comprises a chamber (20) for receiving the solution to be analysed (S) and at least two electrodes (21, 22) intended to be immersed in the solution to be analysed. The chamber comprises at least one orifice (204, 205) for filling the chamber with the solution and draining said solution therefrom; said chamber being coupled at one end to a module for generating the acoustic wave train and closed at the opposite end by a wall (208) which is absorbent for the acoustic wave, impedance-matched with the solution to be analysed. Electronic processing means (300) allow determination of the amplitude and/or the phase of the electroacoustic potential of the solution from an electrical signal measured between the electrodes.

Description

Device for characterizing the electroacoustic potential of a solution

STATE OF THE ART

TECHNICAL FIELD OF THE INVENTION The present invention relates to a device for characterizing the electroacoustic potential of a solution, in particular for measuring the electrophoretic mobility of charged species in solution, as well as a method of characterization, and applies more particularly to the characterization of electrolytes and suspensions, for example of the colloid type.

State of the art

Colloids are used in many industrial applications, such as biomedicine, the food industry, the cosmetics industry. The colloids, or more generally the dispersed media, are suspensions of one or more solid or liquid substances dispersed homogeneously in a solution. The particle sizes in the case of colloids typically range from 1 to 1000 nanometers. A colloid can thus be a substance in liquid or semi-solid form (gel) which contains in suspension species or particles small enough for the mixture to be homogeneous. However, most species suspended in polar environments carry an electrical charge that controls their interactions with the surrounding environment such as other charged species, ions, molecules, etc. The electrical charge is responsible for the repulsive forces and / or electrostatic attraction between these species, which determine their tendency to agglomerate, sediment or remain stable over time. Characterizing the electrical charge of suspended species is therefore very important for understanding many phenomena such as, for example, recycling by extraction processes, coagulation and sedimentation phenomena, for example for applications in the field of treatment of drinking water and wastewater.

The electric charge of a species in solution can be characterized in known manner by a quantity called electrokinetic potential or "zeta potential"("potentialζ") determined from a measurable quantity called the electrophoretic mobility μ _ε . if the potential ζ of a suspended species for example is not high enough, the repulsive force between species may become too weak to compensate for the attraction forces and to stabilize the suspensions. To access the electrophoretic mobility μ _ε of a charged species, it is known to measure the movement speed of the charged species when the system is subjected to an external force field, for example an electric force field. Thus the most common method is based on electrophoresis, which consists in applying an electric field in a solution in order to set in motion the charged species. The analysis of the displacements of the particles thus set in motion can then be made for example by optical techniques, for example Doppler techniques. However, these techniques are limited since the solutions are very concentrated or opaque, due to light absorption phenomena and multiple scattering. Moreover, for solutions of high conductivity, the application of an electric field can induce a heating of the sample as well as electrochemical phenomena to the electrodes which degrade the solution and disturb the measurement.

One solution to these limitations is a very rare technique of electroacoustic measurement. The principle of this technique is based on the analysis of the electrical response (the electroacoustic potential) of a solution containing charged species, subjected to an acoustic wave.

The founding article of this technique is that of Debye, in 1933, which predicts the electroacoustic phenomenon for ions (P. Debye, "A method for the determination of the mass electrolutic ions", J. Chem Phys., Vol. 1, pp 13-16, 1933). A solvent containing charged species is subjected to an ultrasonic wave which causes an oscillatory movement of the solvent. The charged species, present in the solvent, follow this movement differently according to their masses and their coefficients of friction. Thus, an infinitely massive species remains in place while a species of the same density as the solvent perfectly follows the movement. For a binary electrolyte and if the ions do not have the same density, the anions and the cations are a priori separated, creating an electric field, and at the scale of the acoustic wavelength, an electric potential, measurable using electrodes immersed in the solution.

This phenomenon has been studied experimentally by other scientific teams.

The published application GB 2192282 discloses an integrated device for determining the zeta potential of a colloidal solution, into which are sent to a chamber for analyzing continuous acoustic waves so as to obtain a standing wave regime. Measurements made at the level of electrodes placed in the analysis chamber make it possible to determine the phase of the electrostatic potential.

R. Zana and E. Yaeger highlighted the "false effects" that could occur in the implementation of the technique described by Debye, highlighting the difficulty of applying this technique to obtain reliable quantitative measurements ( see, for example, R. Zana and E. Yaegger, Modem Aspects of Electrochemistry No. 14, Ultrasonic vibration potentials, pp. 1-61, J. O'M Bockris, BE Conway, RE White (Eds), 1982). In these same works, R. Zana and E. Yaeger described an improved montage allowing notably to limit these "false effects". In this arrangement, the use of pulsed acoustic waves makes it possible to overcome electromagnetic problems by temporally decoupling acoustic wave emission and detection. The assembly described is thus formed of two tanks filled with water separated by a tube forming a delay line, the solution to be analyzed being contained in a cell immersed in the second tank. The tube also makes it possible to spatially filter the acoustic wave emitted. Moreover, the electroacoustic potential is measured between two electrodes mounted on a rotating device in order to vary the projected distance on the propagation axis of the acoustic wavefront, in order to optimize the detected signal. The described technique has thus made it possible to quantitatively determine the partial ionic volumes in an electrolyte.

In parallel with this work, the technology has also been developed by other teams (see for example Dukhin et al US 6,449,563). Variants of this technology have also been described, such as for example a reverse process to that described by Debye, and based on the principle that an alternating electric field applied to a charged solution will induce an acoustic wave (see for example O'Brien et al US 4,497,208).

All of these theoretical and experimental works led to the pairing of industrial devices from 1985. Today, two commercial systems for the measurement of potential ζ are mainly known. A first system is Acoustosizer ™ from Colloidal Dynamics ™ whose operating principle is based on the work of O'Brien (see above). A second system is the DT1200 ™ Dispersion Technology ™ whose operating principle is based on the work of Dukhin (see above). Both systems operate in pulsed mode. Specifically, in the device developed by Dispersion Technology ™, a probe integrating both the emission source of the acoustic wave and the electrodes is immersed in a container containing the solution that is seeks to analyze. This technology, attractive from the point of view of simplicity of implementation, however, has a number of disadvantages. In particular, the necessary volume of solution for the analysis is important (typically a few tens of milliliters); Moreover, the reflections of the acoustic wave in the container are not controlled and the results obtained are not always consistent with the results obtained by the other technologies.

An object of the invention is to provide a device for characterizing the electroacoustic potential of a solution that is compact and modular for industrialization purposes, and that allows reliable and reproducible characterizations, even on small volume samples.

SUMMARY OF THE INVENTION

According to a first aspect, the present application relates to a device for characterizing the electroacoustic potential of a solution adapted to the characterization of a given acoustic impedance solution, comprising a module for generating and shaping a train of acoustic waves according to a given propagation axis, at least one given acoustic frequency, an analysis cell and electronic processing means.

The analysis cell comprises a chamber intended to receive the solution to be analyzed, with at least one orifice for filling and emptying the chamber with the solution; it is mechanically coupled at one of its ends to the module for generating and shaping the acoustic wave train, and it is closed at the other end by a wall of absorbent material for the acoustic wave, adapted in impedance with the solution to be analyzed. The analysis cell further comprises at least two electrodes intended to be immersed in the solution to be analyzed. The electronic processing means make it possible to determine from an electrical signal measured between the electrodes, the amplitude and / or the phase of the electroacoustic potential of the solution.

The module for generating and shaping an acoustic wave train comprises a transducer for transmitting the acoustic wave train, and a tube for propagating the acoustic wave forming a delay line, mechanically coupled to the transducer. The acoustic wave train has a finite temporal envelope, and the delay line is sized to temporally decouple acoustic wave emission and detection, allowing "pulsed" operation at a given rate. From the amplitude of the electroacoustic potential for example, it is possible to determine in particular the electrophoretic mobility of charged species in solution. The phase can indicate for example on the sign of the charged species.

The applicants have shown that this original architecture of a characterization device according to the first aspect makes it possible to produce an extremely compact device, with a necessary volume for the analysis solution of less than one milliliter, while presenting a very good measurement reliability, thanks in particular to the reproducibility of the assembly and the limitation of false effects inherent for example to acoustic reflections.

The "analysis solution" is understood in the present description as a fluid in the broad sense and may comprise a liquid solution of greater or lesser viscosity, for example an electrolyte, a semi-solid (gel), a dispersion or a suspension solid objects (for example a colloid) or liquid objects (emulsions).

The term "absorbent material", without further specification in the present description, an acoustic wave absorbing material, that is to say a material having an attenuation coefficient for an acoustic wave of a given wavelength greater than about 250 m ^-1 , which is an attenuation length, defined by the reciprocal of the attenuation coefficient, of less than 0.004 m Such a material typically absorbs 90% of the acoustic energy incident on a thickness of 1 cm.

Media defining an interface are said to be "impedance-adapted", with no further precision in the present description, if the acoustic impedances of the two media are in a ratio of 1.5 at most (ie if an incident acoustic wave of a medium to the other is transmitted with a transmission better than 95%).

The wall of absorbent material may be a wall integrated in the chamber or may be an attached wall, for example a sealed or removable sealing plug.

According to one variant, the generation and shaping module comprises a transducer for transmitting the acoustic wave train, and a tube for propagation of the acoustic wave forming a delay line, mechanically coupled to the transducer. Advantageously, the tube is filled with a suitable material impedance with the solution to be analyzed. The transducer is for example a piezoelectric transducer, for example a single-frequency transducer or working over a wider frequency range, typically a few MHz, thanks to a frequency scanning mechanism.

In practice, the transducer may be chosen to be best suited to the type of solutions to be analyzed and to the material of which the tube forming the delay line is filled, so that to optimize the coupling of the power of the acoustic wave in the tube forming the delay line. In the case of an aqueous solution for example, the transducer may be advantageously adapted to water and the tube filled with water. The tube may also be filled with a solid material that is the most impedance matched to the solution to be analyzed and the transducer chosen accordingly.

According to a variant, when the tube forming the delay line is filled with a liquid or semisolid material (gel), the device comprises a window forming a sealed interface between the tube and the chamber, transparent to the wavelength acoustic.

In the present description, a window is said to be "transparent to the acoustic wavelength" if this window passes at least 90% of an incident acoustic wave of a given wavelength.

Advantageously, the window forming the interface is formed of a film of chemically inert material with the greatest number of solvents that can be found in the solutions to be analyzed, for example a film of polyethylene or PTFE (Polytetrafluoroethylene), arranged at the input of the analysis cell or at the outlet of the tube forming the delay line. By choosing a sufficiently thin film, typically of thickness less than λ / 5 with λ acoustic wavelength, the interface formed is transparent to the acoustic waves.

According to one variant, the interface between the tube forming the delay line and the analysis cell, for example the end of the tube if it is solid, or the acoustically transparent window, may be covered with a sufficiently thin layer to be transparent to the waves and ensuring chemical immunity with the solution to be analyzed. This layer can be obtained by deposition, for example, of a PTFE film.

In the same way, the wall of absorbent material of the chamber can be covered on its surface in contact with the solution to be analyzed with a layer providing chemical immunity with the acoustically transparent solution to be analyzed.

Advantageously, the tube forming the delay line has a wall of absorbent material at the acoustic wavelength and adapted in impedance with the material of which the tube is filled. For example, the same absorbent material can be used for the wall of the tube and the closure cap of the chamber.

The wall of the tube, if it is made of acoustically absorbing material and is impedance-matched with the material of which the tube is filled, makes it possible to very effectively absorb the non-axial emissions of the near field and the far field. It is thus possible to to minimize the length of the tube forming the delay line to have a plane wavefront in the region where the electrodes are located for the measurement.

Advantageously, the chamber of the analysis cell is modular and comprises a set of mechanically coupled and dismountable parts including a first support part of at least one electrode and a second part comprising the orifices (s) for filling and emptying the chamber with the solution to be analyzed.

An analysis cell thus formed makes it possible in particular to easily change the electrodes depending on the solvent or wear without having to replace the entire device. Disassembly of parts also allows easier maintenance and cleaning of each part separately, which minimizes the risk of cross-pollution.

According to one variant, the chamber of the analysis cell further comprises a third part providing the mechanical interface with the generation and shaping module. This piece can carry the window forming the sealing interface with the tube forming the delay line, when it is necessary.

According to one variant, the internal volume of the chamber has variable transverse dimensions, the volume widening in the direction of propagation of the acoustic wave. This configuration makes it possible to adapt to the angle of divergence of the acoustic wave and to limit the disturbances that would result from reflections on the internal walls of the chamber. The internal volume may have a generally frustoconical shape.

According to one variant, the parts forming the module for generating and shaping the acoustic wave train and the parts forming the chamber of the analysis cell are substantially symmetrical in their revolution, centered relative to one another. This architecture makes it possible to gain compactness and to have a precise and reproducible positioning of the parts relative to each other, to guarantee a good reproducibility of the measurements.

Advantageously, the acoustic frequency of the wave emitted by the acoustic wave train generation module is between 300 KHz and 5 MHz, advantageously of the order of Megahertz. This frequency range is optimal for benefiting from a short acoustic wavelength and limiting the axial dimensions of the device, for example the distance separating the electrodes, or the length of the tube forming the delay line, without having the limitations. too high frequencies, in particular the acoustic attenuation due to the nano-objects in the suspensions for example and an increased mechanical tolerance on the inter-electrode distance. The duration of the wave train is advantageously chosen short enough to limit the length of the delay line but must have a sufficient number of alternations to allow a measurement with a good signal-to-noise ratio. Typically, the applicants have shown that for an acoustic frequency around the Megahertz, a wave train comprising between 10 and 30 alternations, for example around 20 alternations, gave reproducible results.

According to one variant, the device further comprises a system for measuring the speed of the solvent, also called particle velocity (or solvent particle velocity). The system for measuring the speed of the solvent comprises, for example, an axial hydrophone adapted to measure the pressure of the acoustic wave, from which the particle velocity can be deduced, for example integrated in the wall of absorbent material of the chamber. to point along the axis of propagation of the acoustic wave.

According to one variant, two of said electrodes are arranged in planes perpendicular to the propagation axis of the wavefront, spaced a predetermined distance apart. This distance is equal to (2n + 1) λ / 2, where n is a positive integer or zero and λ is the wavelength of the acoustic wave in the solution to be analyzed. Alternatively, one of the electrodes is a reference electrode disposed along the length of the cell and which allows the measurement of a mean potential.

In the case where two electrodes are arranged in planes perpendicular to the propagation axis of the wavefront, distant from (2n + 1) λ / 2, it is advantageous to provide a variable distance between two electrodes, in order, for example, to adapt to the acoustic emission wavelength in the solution to be analyzed and optimize the sensitivity of the measurement.

According to a variant, at least one of the electrodes may be partially covered with an electrical insulating material so that only the zones in the center of the field of the acoustic wavefront of analysis are useful for measuring the electroacoustic potential. Applicants have shown that this configuration does not reduce the sensitivity but rather increases it by removing the signal components from the detection on the edge areas of the acoustic wave that can be out of phase with the signal components from the detection in the center of the field.

According to a variant, an electrode may be arranged on the wall of absorbent material of the chamber, said wall being deformable axially, while at least one other is immersed in the analysis solution, carried for example by a support member. electrode; the analysis cell can then comprise pressure means on the movable wall to move the electrode along the axis of propagation of the acoustic wave. This configuration has the advantage of having an adjustable distance between the electrodes, the electrodes remaining on the same axis parallel to the axis of propagation.

According to a variant, the analysis cell may comprise three electrodes intended to be immersed in the solution to be analyzed, arranged in planes perpendicular to the propagation axis of the wavefront. A 3-electrode configuration increases sensitivity by making simultaneous measurements between the electrodes two by two. Advantageously, the distance between the electrodes is variable.

According to a second aspect, the present application relates to a method for characterizing the electroacoustic potential of a given acoustic impedance solution comprising:

the generation and formatting of an acoustic wave train according to a given propagation axis, at least one given acoustic frequency, by means of a module for generating and shaping a train of acoustic wave comprising a transducer for transmitting the acoustic wave train and a tube for propagation of the acoustic wave, mechanically coupled to the transducer and forming a delay line;

filling a chamber of an analysis cell by means of an orifice arranged in the chamber with the solution to be analyzed, the chamber being mechanically coupled at one of its ends to the module for generating and putting into operation forming the acoustic wave train, and being closed at the other end by a wall of absorbent material for the acoustic wave, impedance-matched with the solution to be analyzed;

measuring an electrical signal between two electrodes immersed in the solution to be analyzed;

the processing of the electrical signal to determine the amplitude and / or the phase of the electroacoustic potential of the solution to be analyzed.

According to a variant, the method according to the second aspect further comprises a measurement of the power of the acoustic wave and / or the speed of the solvent at the level of the measurement electrodes.

From the amplitude of the electroacoustic potential and the speed of the solvent, it is possible to determine the electroacoustic potential per unit of velocity and to deduce, for example, the electrophoretic mobility. For example, the measurement of the power of the acoustic wave and / or the speed of the solvent is carried out by means of an axial hydrophone arranged in the wall of absorbent material of the chamber; advantageously, the hydrophone points along the axis of propagation of the acoustic wave.

Alternatively, the measurement of the power of the acoustic wave and / or the speed of the solvent can be performed by means of a suitable calibration solution.

According to one variant, a measurement of the acoustic wavelength can be made using two electrodes, one being mobile along the axis of propagation of the acoustic wave.

According to a variant, the method comprises measuring a first electrical signal between a first and a second electrode and measuring a second electrical signal between the first electrode and a third electrode, in order to gain sensitivity on the measurement of the potential. Electroacoustic.

Advantageously, the method comprises adjusting the position of at least one of the electrodes along the axis of propagation of the acoustic wave to adapt to the acoustic emission wavelength in the solution to be analyzed. and optimize the sensitivity of the measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures: FIG. 1, an example of a device for characterizing the electroacoustic potential of a solution according to an exemplary embodiment;

Figure 2, a diagram illustrating the principle of acoustophoresis;

Figures 3 and 4, figures illustrating variants of a device for characterizing the electroacoustic potential of a solution according to the present description; Figures 5 and 6, curves showing the detected electrical signal as a function of time by means of a device according to the present description, respectively for a solution containing K4S1W12O40 ions and BaCl ₂ ions;

FIGS. 7 and 8, curves showing the detected electrical signal as a function of time by means of a device according to the present description, respectively for a solution colloidal silica particles, and for a colloidal solution of the same nature but with incorporation of a salt increasing the electrical conductivity.

DETAILED DESCRIPTION In the figures, the identical elements are identified by the same references. For questions of readability of the figures, the size scales between elements represented are not respected.

FIG. 1 represents an example of a device according to the present description adapted to characterize the electroacoustic potential of a solution S.

The device generally comprises a module for generating and shaping an acoustic wave train 100 along a propagation axis Δ, an analysis cell 200 and electronic processing means 300.

The analysis cell comprises a chamber 20 intended to receive in an internal volume 209 of the chamber 20 the solution S to be analyzed. In the example of Figure 1, the chamber comprises an orifice 204 for filling the chamber and an orifice 205 for emptying the solution; the chamber 20 is mechanically coupled at one of its ends to the module 10 for generating and shaping the acoustic wave train, and it is closed at the other end by a wall 208 of absorbent material for the wave acoustic, adapted in impedance with the solution to be analyzed. In the example of FIG. 1, the analysis cell comprises two electrodes 21, 22 intended to be immersed in the solution to be analyzed, and arranged in planes perpendicular to the propagation axis of the wavefront, distant from each other. a predetermined distance.

In operation, the acoustic wave created at a given acoustic frequency and shaped by the generation and shaping module 100 propagates to the chamber 20 of the analysis cell containing the solution S with charged species . As illustrated in FIG. 2, the charged species + and -, present in the solvent, follow differently the oscillatory movement of the solvent (represented by the wave P in FIG. 2) subjected to the ultrasonic wave as a function of their masses and of their friction coefficients. For a binary electrolyte and if the ions do not have the same density, the anions (-) and the cations (+) are separated, creating an electric field E, and at the scale of the acoustic wavelength λ , an electric potential ΔΥ, called electroacoustic potential. The peak-to-peak voltage is measurable by means of electrodes immersed in the solution and separated by a distance along the propagation axis of the acoustic wave equal to (2n + 1) λ / 2, where n is a positive integer or zero.

According to one variant, the electrodes may comprise a measurement electrode and a reference electrode, arranged for example along the length of the analysis cell in order to measure a mean potential. The electrical signal is then half of the electrical signal measured with two electrodes, as described above.

In the example of FIG. 1, the electrodes 21, 22 are each connected to electronic processing means 300 by means of electrical connection wires 31, 32. The electronic processing means 300 comprise, in particular, a circuit for detecting the response. electrical system containing a filtering and amplification system. The detection comprises a differential measurement of the signals detected on each of the electrodes, allowing a subtraction of the noise. It differs in this sense from the detection made with a reference electrode and a measurement electrode. The recorded electrical response gives aces to the amplitude A of the electroacoustic potential AV as well as to its phase Φο. The characteristic quantity related to this measurement is the electroacoustic potential per unit of velocity, also called CVP according to the abbreviation of the Anglo-Saxon expression "Colloid Vibration Potential", which is obtained by dividing the amplitude of the electroacoustic potential by the velocity v _s of the solvent, ie CVP = A / v _s . From the electroacoustic potential per unit of velocity (CVP) and the characteristics of the solution, including, for example, the conductivity, it is then possible to deduce, for example, using known models, electrophoretic mobility or zeta potential values of the charged species in the solution (see for example AV Delgado et al., "Measurement and interpretation of electrokinetic phenomena", Pure Appl. Chem., Vol.77, No. 10, pp. 1753-1805, 2005).

In the example of FIG. 1, the module 100 for generating and shaping an acoustic wave train at the acoustic frequency chosen for implementing the method comprises a transducer 10, for example a piezo transducer. -electric and a tube 11 forming a delay line.

The delay line makes it possible to temporally separate the emission and the detection; indeed, there appears a very strong disturbance during the emission period which would greatly complicate the extraction of the signal. The temporal dissociation of measurement and excitation involves working with a sinusoidal wave train of N periods. The length of the line delay is chosen based on the acoustic wavelength and the number of periods required to acquire.

The working frequency is therefore a first parameter to be defined. The range of ultrasound is very large, however the electroacoustic measurements of the literature have been made only between 80 kHz to 18 MHz. By way of example, the acoustic wavelength in water for a frequency of 100 KHz is 15 mm; it is 1.5 mm for a frequency of 1 MHz and 0.15 mm for a frequency of 10 MHz. Increasing the frequency makes it possible to reduce the length of the delay line but also the dimensions of the cell, since the inter-electrode distance must be a multiple of half the wavelength in the solution to be analyzed. High frequencies also minimize chemical reactions at the electrodes that can disturb the electrical measurement and degrade the electrodes. However the accuracy on the separation of the electrodes becomes critical at very high frequency. In addition, increasing the frequency increases the acoustic attenuation due to nano-objects in solution, which must then be taken into account. Applicants have shown that a very good compromise is to choose a frequency in a frequency range between 300 kHz and 5 MHz. With a frequency of the order of megahertz, for example, the inter-electrode distance will be 0.75 mm minimum. In practice, it will be possible to choose an interelectrode distance of 2.25 mm corresponding to approximately 3λ / 2. This choice makes it possible to favor a volume of the reduced chamber, less than a few milliliters. Moreover, at this frequency, the chemical reactions to the electrodes and the acoustic absorption remain negligible in the majority of cases.

To produce an acoustic wave at one megahertz, transducers of different types and sizes can be used. This is for example a piezoelectric material which transforms an electric excitation into a pressure wave.

Commercially available transducers have various acoustic impedances suitable for different materials, metals (for example for non-destructive testing) or liquids (generally suitable for water). Indeed, at an interface, it is the difference in acoustic impedance of each of the materials constituting the interface that controls the transmission and reflection of the acoustic wave. According to a variant, the transducer is adapted to the solid, and the delay line formed for example of a tube made of solid material, which has obvious industrial advantages. In this case, we will seek for the solid material which is formed the tube, an impedance material as close as possible to that of the solution to be analyzed, to limit the acoustic power losses. According to another variant, in the case of measurements on liquid solutions, with a transducer adapted to water, one can minimize the losses at the interfaces if the materials are correctly chosen; this implies a delay line consisting of water and therefore of a suitable container.

Another characteristic of the transducer is its transverse dimension, for example the diameter in the case of a cylindrical transducer. The transverse dimension indeed controls the far-field near-field limit and the divergence of the transmitted acoustic wave. Thus, in the case of a transducer adapted to water, the values of distances far-field transducer are typically of the order of 4 mm, 40 mm and 100 mm for transducer diameters respectively 5 mm, 15 mm , 25 mm. The divergence of the acoustic wave for each of these diameter values is respectively 42 °, 14 °, 8 °. Ideally, a plane acoustic wave is sought at the measurement zone. It can be approached with a measurement zone located in the far field and a divergence angle that favors the flatness of the wave at the measurement zone. The length of the delay line is therefore advantageously greater than this distance while remaining close to avoid losing too much acoustic power. As a result, the smaller transducer seems more favorable because it also reduces the size of the cell. However, the delay line is advantageously long enough to be able to temporally separate transmission and detection knowing that the minimum length of a pulse train is governed by the rise time of the electronic card. In practice, the applicants have shown that a delay line of at least four centimeters is a good compromise for a pulse train comprising between 10 and 30 alternations at an acoustic frequency around the megahertz. The piezoelectric transducer has, according to an advantageous variant, an outer diameter of approximately 16 mm with an active part, ie the piezoelectric pellet, with a diameter of approximately 14 mm. This compromise makes it possible to have a far-field transducer-distance of about 35 mm and a divergence angle of 15 degrees that meet the needs of a substantially flat acoustic wave in the measurement zone.

The delay line is advantageously formed of a tube 11 mechanically coupled to the transducer, and filled with a material 111 impedance-adapted with the solution to be analyzed. Advantageously, the wall 112 of the tube forming the delay line is made of absorbent material at the acoustic wavelength and adapted in impedance with the material of which the tube is filled. Thus, along such a delay line, all the acoustic waves which diverge with respect to the axis of propagation in the tube are absorbed, and therefore do not cause no interference with the main propagating wave train. The material used for the wall 112 of the tube is advantageously a material having a high attenuation coefficient _att (a short attenuation length). Applicants have shown that polyurethane is particularly well suited, for example the product known as APTFLEX®, because of its very high attenuation coefficient (a _att = 42000 m- ¹ , attenuation length 24 microns ) and its very good impedance matching Z with water (Z = 1.5 10 ⁶ Pa sm ^-1 ). For example, we can choose an APTFLEX® wall a few millimeters thick.

In the example of Figure 1, the tube is filled with a liquid, for example water; the device then advantageously comprises an interface 40 ensuring the seal between the tube 11 and the chamber 20, transparent to the acoustic wavelength. The interface 40 thus closes the tube at its end. Applicants have shown that a polyethylene film, which has good chemical immunity with most solvents, is a good compromise for providing the sealing interface. The attenuation coefficient of the polyethylene is 50 m ^-1 and its acoustic impedance of 1.76 10 ⁶ Pa sm ^-1 . Its impedance is therefore well adapted to that of water, which makes it possible to avoid parasitic reflections at the interface, and by choosing a sufficiently thin film, typically less than a few tenths of a millimeter thick, the absorption is negligible.

The assembly thus described makes it possible to have an acoustic wave train whose wavefront is appropriate for the measurement. Advantageously, the acoustic power transmitted at the measurement zone can be determined and the speed of the solvent deduced from this measurement in order to obtain the CVP, as defined above. These measurements can be made by means of a hydrophone of which an implementation variant will be described later.

In the example of Figure 1, the chamber 20 of the analysis cell is modular and comprises a set of interchangeable assemblable parts that fit on the delay line. The chamber thus comprises a first support piece 202 of at least one electrode (two in the example of FIG. 1) and a second part 203 comprising the orifices 204 and 205 for filling and emptying the chamber by the solution . The second piece 203 is advantageously provided with an inlet pipe at the bottom and an outlet pipe at the top to prevent the cell from the presence of bubbles which disturb the acoustic wave. In the example of FIG. chamber further comprises a third part 201 providing the mechanical interface with the tube 112 forming the delay line. In the example shown in Figure 1, the part 201 supports the polyethylene wall 40 for forming a sealed interface between the tube 112 and the chamber.

This advantageous configuration makes it possible in particular to replace the electrodes easily, to modify the materials if necessary and also to easily clean the internal volume of the chamber while maintaining a rapid disassembly mounting of the cell. According to one variant, the various parts are symmetrical with revolution, centered on the delay line for example by means of positioning screws (not shown) which ensure the rigidity of the system and the maintenance of the assembly in a perfectly sealed manner. This arrangement makes it possible to carry out low and perfectly reproducible measurements by using a minimum volume of analysis solution, typically of the order of one milliliter. Advantageously, the parts have increasing internal diameters to respect the angle of divergence of the acoustic wave and avoid acoustic disturbances related to reflections on the walls. It is not necessary that these parts are impedance-matched with the solution to be analyzed because they do not disturb the propagation of the acoustic wave because of their shapes. Thus, parts made of inexpensive and easily machinable material, such as Plexiglas, PEEK (polyetheretherketone) or PTFE, are perfectly adapted.

By against the wall 208 of the chamber located at the end opposite to that which is coupled with the tube 112 forming the delay line is absorbent material, acoustic impedance adapted to that of the solution to be analyzed. As previously for the walls of the tube 112, a material of the APTFLEX® (polyurethane) type advantageously makes it possible to eliminate the reflections of the acoustic wave which would necessarily disturb the measurement in a small-volume cell by using long wave trains.

According to one variant, all the walls of the chamber may comprise an absorbent material with an acoustic impedance matched to that of the solution to be analyzed.

As will be shown hereinafter, very good results have been obtained with a device of the type of that described in FIG. 1, a modular, compact device (the total length of the device is less than 10 cm) and which requires a volume of sample for analysis below one milliliter.

As explained above, the measurement of the electroacoustic potential is made in a variant between two electrodes (21, 22, FIG. 1) spaced apart from (2n + 1) λ / 2, the relative positioning of the two electrodes being precisely adjusted. The electrodes are advantageously located in the middle of the chamber in the zone where the acoustic wave is the most homogeneous. Their size (diameter) is advantageously low in front of the wavelength. Electrodes are advantageously formed of tensioned metal son, advantageously electrically isolated from the central measurement area as will be illustrated later. The material is, for example, stainless steel, a metal that is very chemically and mechanically resistant and inexpensive compared to noble metals. In the form of 50 μιη diameter wire, it has no effect on the sound wave and makes it possible to maintain good accuracy in the measurement. Thus, two stainless steel wires are advantageously fixed on either side of the electrode holder piece 202 whose thickness fixes the inter-electrode gap, for example at 3λ / 2. This distance is thus 2.25 mm in the case of an aqueous solution, with an acoustic frequency of 1 MHz. In theory the electrodes could be placed at λ / 2 which would reduce the volume of the sample, but the electrode holder could then be too fragile.

As precisely described, the electrodes are connected to the electronic processing means 300 by means of connection wires 31, 32, for example very low noise coaxial type connection wires. The electronic means 300 comprise in particular a selective differential amplifier which has a fairly narrow bandwidth centered on the frequency of the acoustic wave, for example a megahertz, a high gain, for example an adjustable gain between 1000 and 4000, a high impedance of entry, all low noise.

FIGS. 3 and 4 show variants of a device for characterizing the electroacoustic potential according to the present description, comprising respectively 2 and 3 electrodes, one of these electrodes being arranged on the wall 208 of absorbing material.

In these examples, the chamber is formed of a single electrode holder part 202 which also includes the orifices for filling and emptying the chamber. Alternatively, an arrangement of the chamber with several interchangeable parts as described above is also possible. Furthermore, in this example, the tube 1 1 forming the delay line is formed of a solid material, for example quartz or a polymeric material, coated with a wall 112 of absorbent material, adapted in impedance with the solid material of which the tube is formed. Thus, it is not necessary in this example to have a sealing interface, the chamber being advantageously closed by the tube itself mechanically coupled to the chamber, possibly by means of centering and fixing parts (not shown ). It is possible to provide at the end of the tube in contact with the inner volume of the chamber a layer 115 of a material having good chemical immunity with the solvents which can be made of the solution to be analyzed. This layer is for example a deposit of PTFE sufficiently fine not to introduce disturbances of the acoustic wave propagating. Alternatively, in these examples, if the tube is filled with a liquid or semisolid, a sealing wall may be provided, as previously described.

The device further comprises in these examples actuating means 25, for example a stepping motor, for applying a pressure on the wall 208 supporting one of the electrodes, referenced 23 in the figures.

Thus, as shown in FIG. 3, this advantageous configuration makes it possible to very precisely adjust the inter-electrode distance between a fixed electrode 21 and the moving electrode 23. With this arrangement, the moving electrode moves along the propagation axis of the acoustic wave Δ; there is therefore no risk of disturbances of the measurement which would be due to a shift of one of the electrodes in edge of the cham of the acoustic wave. The geometry of the axially deformable wall 208 advantageously allows precise positioning and deflection of the order of at least half a wavelength, while maintaining the tightness of the chamber. For example, as illustrated in FIGS. 3 and 4, the wall 208 may have a solid "top-hat" shape with a greater thickness in the middle and thinner on the edges.

In the example of FIG. 3, the fixed electrode 21 which extends through the chamber is partially covered with a coating 211 of electrical insulating material, for example a polymerized insulating varnish or a silicone film, the electrode being left uncoated in the measurement zone corresponding to the center of the field of the acoustic wave. This particular arrangement allows for limited detection at the center of the field, where the acoustic wave is the most uniform / flat; this eliminates the effect of any dimensional or geometric defects of the electrodes which tend to spatially average the value of the potential and thus reduce the signal-to-noise ratio.

In the example of Figure 3 is also shown a hydrophone 50 integrated in the wall of absorbent material 208 of the chamber to point along the axis of propagation Δ of the acoustic wave. As previously explained, the hydrophone allows a control during the measurement of the power of the acoustic wave. It thus makes it possible to measure the speed of the solvent. In the case of the presence of an electrode 23 on the wall 208 of absorbent material, it is possible to shift the hydrophone slightly from the axis of propagation of the acoustic wave.

The device represented in FIG. 4 shows an example with 3 electrodes 21, 22, 23, one being as in the preceding example, mobile along the axis of propagation of the acoustic wave (electrode 23). With 3 electrodes, it is possible to make two measures of the electroacoustic potential, and to gain in sensitivity. For this, the electrodes are arranged so as to have two inter-electrode distances equal to (2n + 1) λ / 2.

The electrodes are as previously connected to the electronic processing means 300 by connection son 31 - 33. Advantageously, the immersed electrodes are partially covered as previously, with an electrical insulating coating, respectively 211, 221.

FIGS. 5 and 6 represent electroacoustic potential measurements obtained with a device of the type of FIG. 1. The curves are obtained respectively with a solution of K4S1W12O40 (potassium silicotungstate) of pH 4.2 and a solution of BaCl ₂ (chloride barium). The volume of the solution to be analyzed is less than 4 milliliters. The frequency of the acoustic wave is one megahertz and the pulse train comprises 18 oscillations forming a pulse train of 18 μβεα

It is remarkable to note that even with low mass ions (BaCl ₂ ), a perfectly exploitable signal is obtained for the measurement of electrophoretic mobility.

It has been inferred as well from the amplitude of the electroacoustic potential shown in the curve of Figure 5 an electrophoretic mobility of 5.2 +/- 0.1 mV xlO ^ ^'V ¹ for K4SiWi ₂ 04O solution.

The curve obtained also makes it possible to determine the phase of the electroacoustic potential.

For this, one can for example consider the sign of the potential after a given time from the beginning of the signal, corresponding to a given number of periods of the train of the electroacoustic wave.

FIGS. 7 and 8 represent measurements of electroacoustic potential also obtained with a device of the type of FIG. 1. This time, the curves are obtained with a colloidal solution of silica particles (ludox®) of concentration 0.5% wt in mass. The curve of FIG. 7 is obtained with a conductivity solution 0.0072 s / m; The volume of the solution to be analyzed is less than 4 milliliters. The frequency of the acoustic wave is one megahertz and the pulse train comprises 18 oscillations forming a pulse train of 18 μβεΰ. FIG. 8 represents a curve obtained with the same colloidal solution but in which a salt (KO) has been added to modify the conductivity of the solution (conductivity 0.15 s / m).

As predicted by the theoretical models, the amplitude of the electroacoustic potential in the solution of high conductivity is much lower (factor 30 on the amplitude); however, a perfectly exploitable signal is again obtained for the measurement of electrophoretic mobility.

It has been inferred as well from the amplitude of the electro-potential curves shown in Figures 7 and 8 an electrophoretic mobility of 2.5 +/- 0.05 xlO ^ mV ^'V ¹ for Ludox® solution without KO and 2.4+ / - 0.35 xlO ^ mV ^'V ¹ for the solution of Ludox® with KCl.

Although described through a number of detailed exemplary embodiments, the device and the method for characterizing the electroacoustic potential of a solution according to the invention comprise various variants, modifications and improvements which will become obvious to the skilled person. the art, it being understood that these various variants, modifications and improvements are within the scope of the invention, as defined by the following claims.

Claims

A device for characterizing the electroacoustic potential of a solution, adapted to the characterization of a given acoustic impedance solution, comprising:

A module (100) for generating and shaping an acoustic wave train according to a given propagation axis (Δ), at least one given acoustic frequency comprising a transducer (10) for transmitting the train; an acoustic wave and a tube (11) for propagation of the acoustic wave, mechanically coupled to the transducer and forming a delay line;

An analysis cell (200) comprising: a chamber (20) for receiving the solution to be analyzed (S), comprising at least one orifice (204, 205) for filling and emptying the chamber with the solution, the chamber being mechanically coupled at one of its ends to the acoustic wave train generation module, and being closed at the opposite end by a wall of absorbent material (208) for the acoustic wave, impedance-adapted with the solution to be analyzed; o At least two electrodes (21, 22, 23) intended to be immersed in the solution to be analyzed, electronic processing means (300) for determining from an electrical signal measured between the electrodes, the amplitude and / or the phase of the electroacoustic potential of the solution.

Device according to Claim 1, in which the tube forming the delay line has a wall (112) of absorbent material at the acoustic wavelength and is filled with a given material (111), the absorbent material forming the wall ( 112) of the tube being impedance-adapted with the material (111) of which the tube is filled.

Device according to any one of the preceding claims, wherein the tube being filled with a liquid or semi-solid material, the device comprises a window (40) forming a sealed interface between the tube (11) and the chamber (20) , transparent to the acoustic wavelength.

4. Device according to any one of the preceding claims, wherein the chamber (20) of the analysis cell is modular and comprises a set of mechanically coupled and dismountable parts including a first support part (202) of at least an electrode and a second piece (203) including said orifice for filling the chamber with the solution.

5. Device according to any one of the preceding claims, wherein the internal volume (209) of the chamber has variable transverse dimensions, the volume widening in the direction of propagation of the acoustic wave.

6. Device according to any one of the preceding claims, wherein the acoustic frequency is of the order of Megahertz.

7. Device according to any one of the preceding claims, further comprising a hydrophone (50) integrated in the wall of absorbent material (210) of the chamber.

8. Device according to any one of the preceding claims, wherein at least one of the electrodes is partially covered with an electrically insulating material (211), so that only the zones in the center of the field of the acoustic wave fon analysis are useful for measuring the electroacoustic potential.

9. Device according to any one of the preceding claims, wherein at least two electrodes are arranged in planes perpendicular to the propagation axis of the wavefront, spaced a predetermined distance.

10. Device according to claim 9, wherein the distance between said electrodes is variable.

11. Device according to claim 10, wherein the wall of absorbent material (208) of the chamber is axially deformable, an electrode (23) is arranged on the wall of absorbent material (208) and the analysis cell comprises means pressure device (25) on the wall of absorbent material (208) for moving the electrode along the axis of propagation of the acoustic wave (Δ). A method of characterizing the electroacoustic potential of a given acoustic impedance solution comprising:

the generation and formatting of an acoustic wave train according to a given propagation axis, at least one given acoustic frequency, by means of a module for generating and shaping a train of acoustic wave comprising a transducer (10) for transmitting the acoustic wave train and a tube (1 1) for propagation of the acoustic wave, mechanically coupled to the transducer and forming a delay line;

the processing of the electrical signal to determine the amplitude of the electroacoustic potential of the solution to be analyzed.

The method of claim 12, further comprising adjusting the position of at least one of the electrodes along the axis of propagation of the acoustic wave.

The method of any of claims 12 or 13, further comprising measuring the power and / or velocity of the solvent at the measurement electrodes.