WO1999030842A1 - Method and device for monitoring phase transitions - Google Patents

Abstract

The invention concerns a sensor (110, 120) comprising a mass element tapering from a fixing support towards a free end immersed in a medium (I), for generating and/or detecting a vibration. The mass element is formed of a triangular plate. On one of said element two main surfaces, is isolated, by etching, in the zone located in the proximity of the fixing support, an electrode of small dimension. A ground electrode is provided on the mass element other main surface. The invention also concerns a method for monitoring phase transition in a medium (I), consisting in measuring the variation of a vibration travel velocity in the medium (I) based on the progress rate of the transition, using at least one sensor (110, 120) immersed in the medium (I), while a temperature gradient is imposed and controlled between at least two points of the medium (I).

Description

 "Method and device for monitoring phase transitions"

The present invention relates to the field of the study of phase transitions in a medium. For example, the invention relates to the study of sol-gel or gel-sol transitions. More specifically, the invention relates to a device and a method for carrying out measurements on the propagation of acoustic waves in the medium to be studied.

Numerous means have been proposed for probing physical media, gases, liquids, gels or solids, by ultrasound. See for example the following articles: "Resonant Vibration of a Cone" (The Journal of the Acoustical Society of America, vol.36, p.309-312, 1964), "Design of Sonic Amplitude Transformers for High Magnification" ( The Journal of the Acoustical Society of America, vol. 35, p. 1367-1379, 1963) and "Torsionally Résonant Amplitude Transformers for High Magnification" (The Journal of the Acoustical Society of America, p.1 - 9, 1964), by E. EISNER, "Solid Cône in Longitudinal Half-Wave Résonance" by D. ENSMINGER (The Journal of the Acoustical Society of America, Vol. 32, p. 194-196, 1960) and the patent FR 2 693 271.

All of these means are not entirely satisfactory. The object of the present invention is to provide an improvement to existing methods and devices.

This object is achieved by the present invention thanks to a method for monitoring phase transitions in a medium, comprising a step consisting in measuring the variation of the speed of propagation of a vibration in the medium as a function of the rate of advancement of the transition, thanks to at least one probe immersed in the medium, characterized in that a temperature gradient is imposed and controlled between at least two points of the medium.

According to another aspect of the invention, it comprises an acoustic probe comprising a piezoelectric mass element comprising an excitation source and a resonator, each consisting of a zone of the mass element in continuity of material with each other

Preferably, this mass element is formed by a flat tapered plate from the zone comprising the excitation source towards the free end of the mass element, the zone situated between the excitation source and the free end, constituting the resonator.

Advantageously, the mass element is formed of a triangular flat plate, the two main faces of which have an area greater than the areas of the faces situated along the thickness of the plate. Even more advantageously, an excitation electrode of small dimension is formed by a zone. of material deposited on one of the main faces, while a ground electrode is placed on the other main face

The method and the probe according to the invention can also be used independently of one another.

An electrical excitation applied between the small electrode and the mass electrode makes it possible to produce a mechanical deformation of the mass element by piezoelectric effect at the level of the electrode. This mechanical deformation then propagates throughout the the mass element, without there being any discontinuity between the part corresponding to the excitation source and the rest of the mass element which acts as a resonator The amplitude of the vibration thus produced is therefore optimized

Other aspects, objects and advantages of the invention will appear on reading the detailed description which follows

The invention will also be better understood using the references to the accompanying drawings,

- Figure 1 shows a synopsis of the entire measuring device for implementing the method according to the invention, - Figure 2 schematically shows an example of a tank of the measuring device for implementing the method according to the invention,

FIG. 3 is a schematic representation of an acoustic probe according to the invention,

FIG. 4 is a median section along the largest dimension of an acoustic probe according to the invention,

FIG. 5 is a block diagram of the measurement of the time of flight of an acoustic vibration according to the method according to the invention,

- Figure 6 is a curve obtained by the implementation of the method and the acoustic probe according to the invention, showing the evolution of the flight time of an acoustic vibration as a function of the progress of the ge fication transition milk,

FIG. 7 schematically represents a variant of the tank for implementing the method according to the invention,

FIG. 8 schematically represents another variant of the tank for implementing the method according to the invention,

- Figure 9 schematically shows a device for implementing the method according to the invention comprising a single probe and a reflecting wall

In the following, the qualifier "acoustic" will be used without obviously intending to reduce the measurement spectrum to only audible signals

In a preferred embodiment of the method according to the invention, it is used to follow the evolution of the gefhication of the milk

As shown in FIG. 1, a transmission signal is produced from a clock 10, a deviator 20 and a generator of pulses 30

This signal is then transmitted to an emitter probe 1 10, which emits a vibration in medium I After propagation in medium I, the vibration is detected at the level of a receiving probe 120, to give an amplified signal in an amplifier 40 then filter by a filter 50 The signals transmitted and received are displayed on a digital oscilloscope 60 then recorded on a microcomputer 70 via a GPIB interface card 80. The acquisition and processing of the recorded data is then carried out by appropriate software on the microcomputer 70. According to a preferred embodiment shown in FIG. 2, the device for implementing the method according to the invention comprises a tank 100 and the two probes 110, 120.

The tank 100 is thermostatically controlled by a medium II which serves as a water bath 130. The assembly consisting of the tank 100 and the water bath 130 is in a medium III constituted by the environment.

The medium II is at a temperature T2, the medium I is at a temperature T1 close to T2 and the medium III is at a temperature T3, lower than T1 and T2. The temperature of the tank 100 is controlled to the nearest 0.1 ° by virtue of a circulation of water in the water bath 130. The water bath 130 is supplied by a water inlet 132. A water outlet 134 is placed on the water bath 130 so as to allow a homogeneous circulation and cooling of the medium II. This control bath allows a study over a wide temperature range.

The tank 100 includes walls thermostatically controlled at a first temperature T2 and is open to the environment III thermostate at a second temperature T3 different from the first.

There are thus at least two zones thermostatically controlled at different temperatures to impose and control a temperature gradient in the medium I. As illustrated by FIG. 3, the emitting probe 110 comprises a mass element 112, a fixing support 114 and two electrodes 116, 118.

The emitting probe 110 is cut from a flat plate of piezoelectric material on the two main faces of which a conductive material 111 has been deposited (FIG. 4). This cut is made so as to isolate, in this flat plate, the mass element 112 in a triangular shape. The mass element 112 then comprises two main faces 122, 124 parallel to each other and covered with a conductive material 111. As illustrated in FIG. 4, a zone of conductive material constituting the excitation electrode 116 is isolated from the rest of the conductive material covering the whole of one 122 of the main faces 122, 124 by etching a strip 123 of conductive material at the periphery of the zone constituting the excitation electrode 116. This etching is carried out by the technological means of known microelectronics of the skilled person. The excitation electrode 116 thus isolated is preferably small and located near a part of the mass element 112 able to cooperate with the fixing support 114.

A ground electrode 118 consists of a deposit of conductive material covering the other 124 of the main faces 122, 124.

The area of the mass element 112, located between the excitation electrode 116 and the mass electrode 118, constitutes the source of excitation 126.

The area of the mass element 112, located between the excitation source 126 and the free end 128 of the mass element 112 situated opposite the fixing support 114, constitutes a resonator 113.

With this type of embodiment, there is no discontinuity between the excitation source 126 and the resonator 113.

A comparative study was made on two types of probe. These last two are the same size and shape, but different in the way in which we excite the structure:

- a simple etching in the structure reducing the latter to a set of continuous physical properties, as described above; and

- embedding and bonding of a piezoelectric excitation source 126 in a mass element 112. This latter possibility generates discontinuities between the excitation source 126 and the mass element 112, especially when it is a question of materials with distant physical properties.

The maximum values of the amplitude of the vibrations at the free end 128, associated with the same electrical excitation, are presented in the following table

It can therefore be seen that the emitting probe 110 according to the present invention makes it possible to approximately multiply by 100 the amplitude of the vibrations generated in the medium I

Advantageously, the largest dimension of the mass element 112 is between 0.5 and 5 cm

The mass element 112 preferably has the shape of an isosceles triangle whose height is approximately twice as large as the base. The base is provided with the fixing support 114.

By way of nonlimiting example, the mass element 112 can have a base 16 mm long and a height of 32 mm. It can have a thickness of the order of 1 or 2 mm. The excitation electrode 116 can have the In the form of a 10 mm diameter disc According to a variant of the invention, the excitation electrode 116 can be produced by depositing a conductive material, on one of the main faces 122, 124, locally through a mask for example, on a plate or a mass element 1 12 of piezoelectric material, bare before depositing It is also possible to envisage carrying out the excitation electrode 116 by locally doping the piezoelectric material so that it has the conductivity sufficient electrical to establish contact or by applying a conductive element to the surface of the mass element 112, or by any other technique known to those skilled in the art

The receiving probe 120 can be produced in the same way as the sending probe 110

Typically, the mass element 112 of emitting probes 110 and receiving probes 120 is a piezoelectric ceramic material. Advantageously, this ceramic material is PZT.

The type of probe described above can be used, as mentioned above, to follow a ghefication transition in a dairy product, but more generally, will be used to follow phase transitions in a medium, involving changes of viscoelastic properties of the medium

The method according to the invention consists in studying the evolution of the viscoelastic properties of the medium I, by measuring the speed of propagation of a vibration in this medium I, that is to say by measuring the time of flight of this vibration, while superimposing a thermal gradient and a regulation of the temperature of the medium I The viscoelastic properties of the medium I, depend on the temperature as we will explain below

Preferably, the emitting probe 110 and the receiving probe 120 are immersed on the surface of the medium I contained in the tank 100

Advantageously, the free end 128 of the emitting probes 110 and receptors 120 is immersed over a height of 5 mm. However, it may in other cases be considered to completely immerse the emitting probes 110 and receiving 120, or else again to apply them on the surface of medium I, in the case of solid media for example

The distance between the emitting probe 110 and the receiving probe 120 is fixed by the operator according to the echoes received Typically, a distance of the order of a centimeter constitutes a good compromise Only the first echo, corresponding to "longitudinal mode", is considered (here called "longitudinal mode", any movement which takes place in the direction of the largest dimension of the emitting probe 110). Such a distance makes it possible to overcome the influence of the multiple echoes reflected on the walls of the tank 100 and to be limited to a one-way trip in the medium I studied.

Preferably, the distance between the emitting probe 110 and the receiving probe 120 is such that the measurement of the propagation time of the vibration is carried out under near field conditions. A measurement in the near field is a measurement in which there is a direct coupling between the emitting probe 110 and the receiving probe 120 via the medium to be characterized. Roughly, it will be estimated that the near field condition is satisfied if the distance between the emitting probes 110 and receiving 120 is less than three times the wavelength λ of the vibration in the medium I, but more preferably less than once the wavelength λ of the vibration in the medium I.

However, for certain applications, it may be advantageous for the distance between the emitting probe 110 and the receiving probe 120 to be of the order of ten times the wavelength λ in medium I. Therefore, more generally , the distance between the emitting probe 110 and the receiving probe 220 is less than or equal to a few wavelengths in medium I.

Advantageously, the frequency of the vibration is of low frequency. This frequency can be between 10 and 300 kHz but preferably it is between 50 and 250 kHz, or more precisely still between 70 and 200 kHz.

Each vibration mode is associated with a specific frequency and displacement. For example, if for an excitation close to 90 kHz, only the "longitudinal" mode appears, that is to say leading to a displacement along the axis of the emitting probe 110 taken in its largest dimension, this "longitudinal" mode will preferably be excited

The variation in the time of flight of the vibration received at the level of the receiving probe 120, correlated with the evolution of the medium I, is identified by the first zero crossing (FIG. 5)

The excitation by an electric shock of small excitation sources 126, as in the case of the present invention, makes it possible to generate a diverging acoustic wave

FIG. 6 represents a curve of evolution of the time of flight of the wave with the progress of the reaction for a kinetics of gaffification of the milk, produced by superimposing a thermal gradient on the surface of the medium

I, regulating the temperature of this medium I

We can identify five portions on this curve which we interpret by associating with these five portions, five phases of the geffication transition - phase 1 would correspond to the release of a macropeptide (a hydrophilic fragment of kappa casein) generated by hydrolysis of kappa casein by chymosin,

- phase 2 would correspond to the aggregation of casein micelles to form clusters of finite size, in fact like kappa caseins are associated with the other types of casein (alpha, beta) in a particular structure called micelles, hydrolysis of kappa casein by chymosin leads to the aggregation of micelles, during these two phases, the temperature of medium I remains constant and homogeneous, which results from the propagation of heat in medium I by free convection, - the phase 3 would correspond to the competition between two effects which are the increase in the flight time of the vibration, i.e. the decrease in speed with the decrease in temperature which results from the appearance of an elastic component linked to a change in free convection / conduction regime and the reduction in this flight time, resulting from the elastic appearance which appears in medium I during evolution, - phase 4 would correspond to the connectivity threshold, that is to say the appearance of a giant macromolecular chain, and

- phase 5 would correspond to a firming of the gel

As this curve shows, the present invention makes it possible to clearly distinguish the different phases of the geffication transition and therefore to precisely control, at the industrial level, the transformation of dairy products.

Indeed, the fact of imposing a gradient makes it possible to obtain the competitive effect at the origin of the inflection zone of the curve corresponding to phase 3 which was not the case with the methods and devices of prior art

We can make the following hypothesis to interpret this result when the elastic component appears in medium I, the heat is no longer propagated by free convection but by conduction, the surface at the interface of medium I and medium III being at temperature T3, lower than the temperature T1 within the medium I, the heat of the medium I is gradually released towards the surface by conduction and the propagation of the vibration between the emitting probes 110 and receiving 120 takes place in a zone of the medium I subjected at a temperature gradient between temperature T3 and temperature T1

This configuration for achieving the gradient is particularly advantageous since it is easy to implement in an industrial environment. However, it is possible to conceive of numerous variants of the process, device and probe described above without departing from the scope of the present invention. The tank 100 can comprise at least one wall or zone of medium II, thermostatically controlled at a first temperature T2 different from a second temperature T3 at which is thermostated at least one other wall or another zone of medium II For example as shown in FIG. 7, the tank is closed and comprises two zones regulated at different temperatures T2 and T3 In an analogous manner, FIG. 8 represents another variant of the device according to the invention comprising a tank 100 with a cooling means 129 by which a zone of the medium I is thermostatically controlled at a first temperature T3 different from a second temperature T2 at which the wall of the tank is thermostatically controlled 100.

The present invention can also be designed with a single probe 115 and a reflecting surface 125 for acoustic waves. The probe 1 15 then makes it possible alternately to transmit a signal and to receive it after reflection on the reflecting surface 125. A measuring device according to the invention can comprise several, but at least one, probe (115) allowing both the emission and reception of the acoustic signal.

The application to monitoring a phase transition in a dairy product (medium I) is mentioned above very generally. However, a person skilled in the art will understand that the device according to the invention allows in particular an evaluation of a time characteristic of the setting or of the visco-elastic characteristics of the medium undergoing evolution after setting, etc. as much information which is invaluable for the industrialist having to follow the manufacture and the qualification of its production.

A method, a device and a probe for monitoring phase transitions have been described above. In this detailed description, the case of the processing of dairy products (yoghurts, creams, cheeses, etc.) was taken as an example, and more particularly the ghefication of milk. However, it should be understood that the invention is compatible with other types of applications for which it can be very useful. For example, it can be useful in other areas of the food industry, for cosmetics, monitoring resin polymerization, painting or even other areas.

Claims

1. Acoustic probe comprising a piezoelectric mass element (112) comprising an excitation source (126) and a resonator (113), this probe being characterized in that the excitation source (126) and the resonator (113) are constituted each of a zone of the mass element (112) in continuity of material with one another. 2. Acoustic probe according to claim 1, characterized in that the mass element (112) is formed by a flat tapered plate from the zone comprising the excitation source (126) towards a free end (128) of the element mass (112), the area between the excitation source (126) and the free end (128), constituting the resonator (113). 3. Acoustic probe according to one of the preceding claims, characterized in that the mass element (112) is formed by a triangular planar plate whose two main faces (122, 124) have an area greater than the areas of the faces located according to the thickness of the plate. 4. Acoustic probe according to claim 3, characterized in that an excitation electrode (116) is formed by an area of conductive material produced on one (122) of the main faces (122, 124). 5. Acoustic probe according to claim 4, characterized in that the excitation electrode (116) is produced by isolating an area of conductive material in a conductive material covering all of one (122) of the main faces (122, 124). 6. Acoustic probe according to claim 5, characterized in that the zone of conductive material constituting the excitation electrode (116) is isolated from the rest of the conductive material covering all of one (122) of the main faces (122, 124) by etching a strip (123) of conductive material at the periphery of the area constituting the excitation electrode (116). 7. Acoustic probe according to one of claims 3 to 6, characterized in that the main faces (122, 124) are parallel between them. 8. Acoustic probe according to one of claims 4 to 7, characterized in that a ground electrode (118) consists of a conductive material covering all of that (124) of the two main faces (122, 124) which is different from that (122) carrying the excitation electrode (116). 9. Acoustic probe according to one of the preceding claims, characterized in that it is used to follow phase transitions in a medium, implying changes in viscoelastic properties of the medium (I). 10. Acoustic probe according to one of the preceding claims, characterized in that it is used to follow a ghefication transition in a dairy product. 11. Acoustic probe according to one of the preceding claims, characterized in that the largest dimension of the mass element (112) is between 0.5 and 5 cm. 12. Acoustic probe according to one of claims 3 to 11, characterized in that the mass element (112) is an isosceles triangle whose height is approximately twice as large as the base. 13. Method for monitoring phase transitions in a medium (I), comprising a step consisting in measuring the variation in the speed of propagation of a vibration in the medium (I) as a function of the rate of progress of the transition, thanks to at least one probe (110, 120, 115) according to one of claims 1 to 12, immersed in the medium (I), characterized in that a temperature gradient is imposed and controlled between at least two points middle (I). 14. Method according to claim 13, characterized in that the vibration is of low frequency. 15. Method according to claim 14, characterized in that the frequency of the vibration is between 10 and 300 kHz, advantageously between 50 and 250 kHz and more preferably between 70 and 200 kHz 16 Device for measuring the variation in the speed of propagation of a vibration as a function of the rate of advancement of a phase transition of a medium, comprising a tank (100) to contain the medium (I), at least a probe (110, 120, 115) according to one of claims 1 to 12, for emitting and detecting a vibration and characterized in that it further comprises at least two zones thermostatically controlled at different temperatures to impose and control a temperature gradient in the medium (I) 17 Device according to claim 16, characterized in that the tank (100) comprises walls thermostatically controlled at a first temperature (T2), and is open to an environment (III) thermostate at a second temperature (T3) different from the first 18 Device according to claim 16, characterized in that the tank (100) comprises at least one wall, or a zone of the medium thermostatically controlled at a first temperature (T2) different from a second temperature (T3) at which is at least thermostatically controlled another wall or another zone of the medium (I) 19 Device according to one of claims 16 to 18, characterized in that it comprises at least one transmitting probe (110) and at least one receiving probe (120) from each other by a fixed distance, less than three times the wavelength (λ) of the vibration and more preferably less than once the wavelength (λ) of the vibration in the medium ( I) 20 Device according to one of claims 16 to 19 characterized in that it comprises at least one transmitting probe (110) and at least one receiving probe (120) distant from each other by a distance of l order of ten times the wavelength (λ) of the vibration in the medium (I) 21 Device according to one of claims 16 to 20, characterized by the fact that at least one probe (110, 115) comprises a small excitation source (126) for emitting an acoustic signal in the medium in the form of a diverging wave. 22. Device according to one of claims 16 to 21, characterized in that the operating frequency of the probe (110, 115, 120) is between 10 and 300 kHz, advantageously between 50 and 250 kHz and more preferential between 70 and 200 kHz. 23. Device according to one of claims 16 to 22, characterized in that it comprises at least one probe (115) allowing both the emission and the reception of the acoustic signal.