FR2744528A1 - Measuring device for visco-elastic characteristics of a fluid - Google Patents

Abstract

The measuring device has a measuring cell comprising two cavities(1,2) which communicate with each other through a rectilinear capillary tube(4). The cell(1,2,4) is hermetically sealed and the cavities(1,2) each contain a piezo-electric pressure sensor(5,6). The cavities are electrically connected to a central unit(11) designed to process the detected signals from the sensors and to deduce the visco-electric characteristics of the fluid. One of the cavities(1) is associated with a vibrating element(12) which gives a low amplitude oscillating movement to the fluid contained in the measuring cell. The cell operates over a frequency interval whose upper bound exceed 1000 Hertz. The two cavities(1,2) and the tube(4) are integrally filled by the fluid to be analysed.

Description

DEVICE FOR DETERMINING THE VISCO ELASTIC CHARACTERISTIOUS OF A FLUID
The invention relates to a new device for determining the viscoelastic characteristics of a fluid. Such devices are generally designated by the term rheometer.

 The rheometer object of the present invention is of the capillary type.

 Many fields require knowledge of the viscoelastic characteristics of the fluids they use, in particular in the food industry, but also in that of cosmetics, petrochemicals, paints and varnishes, plastics, etc. .., where we want to be able to study and characterize oils, gels, suspensions or solutions.

 To this end, the determination of rheological properties, and more specifically, viscoelastic coefficients of a fluid were to date accessible by rheometers, in particular capillary rheometers and rotary rheometers.

 Rotary rheometers consist of measuring the resistance torque to the flow of the fluid to be analyzed. If, of course, with this type of rotary rheometer, it is possible to operate in an oscillating regime, however, the frequency of oscillation of the fluid is limited to a few tens of hertz, taking into account in particular the use of mobile mechanical parts generating a parasitic inertial effect.

 The other main type of known rheometers is constituted by capillary rheometers, which operate on the principle of the continuous flow of a fluid in a calibrated capillary. The fluid is subjected to a known and adjustable pressure difference and various measurements can be carried out, in particular flow speed, pressure at different locations along the tube, flow rate of the fluid at the outlet of the tube. It is possible, by means of this capillary rheometer, to measure the viscosity.

 Among these capillary rheometers, there has been described in document GOA-2 233 461 a measurement cell comprising a capillary tube connecting two reservoirs, constituting a module in equilibrium. The first tank is mounted in equilibrium on the knife of a scale, and the second tank is suspended by means of a weighing system. The fluid whose rheological characteristics are to be determined passes under controlled and measured pressure from the first to the second reservoir, and the flow rate is determined by measuring the charge in the second reservoir by means of the measurement system at regular time intervals. .

 If such a device makes it possible to determine the viscosity of a fluid at zero frequency, on the other hand, it is not possible to determine the visco-elastic characteristics at finite frequency, a fortiori at high frequency.

 The object of the invention is to provide a capillary rheometer with oscillating flow, allowing the determination of the visco-elastic characteristics of fluids at high frequencies, in particular liable to exceed 1000 Hertz.

This device for determining the visco-elastic characteristics of a fluid, comprises a measuring cell made up of two cavities communicating with each other by means of a rectilinear capillary tube. This device is characterized:
- in that the measuring cell is hermetically dose, after introduction into it of the fluid to be characterized;
- in that said cavities are each provided with a pressure sensor, said sensors being electrically connected to a central unit intended to process the signals picked up and restore the viscoelastic characteristics of the fluid;
- And, in that one of the cavities is associated with a vibrating member, capable of setting in motion the fluid contained in the measurement cell thus formed.

In other words, the invention consists in measuring the pressures at the ends of a capillary, in which an oscillating flow of the fluid to be characterized is induced, by means of an excitation member. It is from the dynamic characteristic connecting the flow of a type flow
Alternative pooiseuille in the capillary tube, away from pressures between the ends of this tube, which one reaches the visco-elastic coefficients of the fluid. These coefficients are obtained by digital processing, carried out by the central unit, consisting of a microcomputer provided with software provided for this purpose.

 Advantageously, the installation operates on a frequency interval the upper limit of which is greater than 1000 Hertz.

 For certain measurements, the two cavities and the tube are completely filled with the fluid to be analyzed. Furthermore, and as already specified, the measuring cell constituted by the two cavities and the capillary tube is hermetically dose. In fact and in the case where the fluid to be studied consists of a solution, this cell prohibits any concentration drift.

 The pressure sensors advantageously consist of piezoelectric elements.

 The oscillating fluid setting member consists of a vibrating pot associated with an elastic membrane closing an orifice mounted on one of the two cavities, or even by a piezoelectric exciter immersed in one of the cavities.

 Advantageously, the entire measurement cell is placed in a thermostatically controlled enclosure, so as to maintain the temperature constant for the duration of the experiment, according to a determined value.

 An illustrative and non-limiting example is presented, in support of the appended figures.

 Figure 1 is a schematic representation of the operating principle of the installation according to the invention.

 FIG. 2 is a diagrammatic representation in section of the measurement cell according to the invention.

 FIG. 3 is a diagrammatic representation in section of the calibration cavity of the pressure sensors, implemented by the invention.

FIG. 4 is a curve representing the evolution, as a function of the frequency, of the real and imaginary parts of the complex viscosity of a solution of the following characteristics:
- CTAB concentration: 0.35 M
- KBr concentration: 0.40 M
- temperature: 31 OC
FIG. 5 is a curve representing the evolution, as a function of the frequency, of the real and imaginary parts of the complex elastic modulus of the same solution.

 Figure 6 is a Cole-Cole diagram corresponding to the results of Figure 5.

 The following description implements a measurement cell more particularly suitable for measuring pressures using piezoelectric sensors. It is understood that the invention cannot be limited to this single embodiment, and that any pressure sensor could be used.

 Schematically shown in Figure 1 the device according to the invention. Basically, it comprises a measuring cell consisting of two cavities (1, 2) connected to each other by a straight capillary tube (4), this capillary having for this application a length of 3 cm, for an internal diameter of 0.3 mm .

Each of these two cavities (1, 2) comprises a piezoelectric pressure sensor (5, 6), whose time pressure signals, respectively vi (t) and v2 (t) are collected by a signal analyzer (11), restoring the frequency response T (jo) = vl (jo) of the cell thus
v2 (joe) incorporated.

 The signals are acquired for example by means of an analyzer or an acquisition card. A digital processing carried out by the central unit (11), typically consisting of a microcomputer, then makes it possible to extract from this response the viscoelastic coefficients sought.

 Furthermore, the device also comprises a vibrating pot (12) acting on an elastic member (13) closing off the cavity (1), as can be seen in FIG. 1. In this way, this vibrating pot makes it possible to generating an oscillating flow of the fluid in the capillary tube. This oscillating flow can be of small amplitude, when it is required for certain applications, to remain in the linear approximation.

 According to a preferred characteristic of the invention, the two reservoirs or cavities (1, 2) as well as the capillary (4) are completely filled with the fluid to be analyzed. The cell thus formed being completely closed, the flow is made possible by the compressibility of the fluid in the cavity (2), and to a lesser extent, by the deformation of the sensitive part of the pressure sensor (6).

 Advantageously, the device is immersed in an enclosure (3) for thermal regulation, comprising in particular a block heated by resistors (8), and equipped with a thermal probe (7). The temperature of this block is managed by a central thermal regulation unit (10). The temperature within the enclosure (3) is standardized by means of a fan (9) pulsating the air thus heated by the block (8).

 There is shown in Figure 2 a schematic sectional view of such a measurement cell. One can in particular observe on it the location of the piezoelectric sensors (5, 6). These open out at the level of the two cavities (1, 2) through an orifice (18) of corresponding diameter or dimensions formed within the cell, the said sensors being held in place on the one hand, by cooperation an annular projection (14) with the periphery of said orifice by means of a seal (15), for example made of FIFE, and on the other hand by screwing a suitable nut (16) cooperating with a thread (17), made for this purpose in the extension of the orifice (18), and coming to bear on said annular projection (14) .We have materialized by the reference (19) one of the orifices of filling of one of the cavities.

 The implementation of this device makes it possible to arrive at the determination of the complex viscosity or of the complex elastic modulus.

 By way of example, a description will now be given of how to access these quantities.

 As already specified, the physical operating conditions of the device of the invention are those of a Poiseuille flow oscillating in the capillary tube.

The velocity averaged over the section of the capillary tube, U (jo), is related to the pressure difference applied at its ends, and measured by the sensors (5,6) AP (jo) = P1 (j #) - P2 ((j #) according to the relation:
U (j) = K (jo) AP (jw) # L where K (joe) represents the dynamic permeability of the capillary, rl the dynamic viscosity of the fluid and L the length of the capillary tube.

où p représente la masse volumique du fluide, R le rayon du tube capillaire, Jo et J1 les fonctions de Bessel d'ordre 0 et 1
Cavité 2
Le réservoir (2) joue le rôle d'une capacité hydraulique de capacité effective C. L'équation de conservation du débit permet de relier U(j#) à
P2 (j), selon la relation: Cj#) P2(j#)) = #RȖ(j#)
capteurs piézoélectriques
Sur le domaine d'étude mis en oeuvre, les capteurs se comportent comme des filtres "passe-haut" du premier ordre.Ces capteurs interviennent par le rapport de leur sensibilité complexe S(jo), selon la relation
V1(jw)= α l+#2j# P1(j#)=S(j(#) P1(j#) V2(j#) l+#1j# P2(j#) P2(j#)
Modèle complet de la cellule
La fonction de transfert globale de la cellule prend alors la forme:
V1(j#) = S(j#)/l - j# où b= #R4
V2(j#) bk(j#) 8C#L
Outre la viscosité complexe Tl(jto) = Tl'(j) + j #"(j#) qui constitue la grandeur recherchée, la réponse de la cellule fait intervenir les paramètres de cellule α, #1, T2 et C.Pour la détermination de ces paramètres, on utilise une cellule de calibrage représentée sur la figure 3, telle que par construction, pl(t) = p2(t).In the case of a circular cross-section tube, the reduced permeability k (j) is expressed:

where p represents the density of the fluid, R the radius of the capillary tube, Jo and J1 the Bessel functions of order 0 and 1
Cavity 2
The reservoir (2) plays the role of a hydraulic capacity of effective capacity C. The flow conservation equation makes it possible to link U (j #) to
P2 (j), according to the relation: Cj #) P2 (j #)) = # RȖ (j #)
piezoelectric sensors
On the field of study implemented, the sensors behave like first-order "high-pass" filters. These sensors intervene by the ratio of their complex sensitivity S (jo), according to the relation
V1 (jw) = α l + # 2j # P1 (j #) = S (j (#) P1 (j #) V2 (j #) l + # 1j # P2 (j #) P2 (j #)
Full cell model
The cell's global transfer function then takes the form:
V1 (j #) = S (j #) / l - j # where b = # R4
V2 (j #) bk (j #) 8C # L
In addition to the complex viscosity Tl (jto) = Tl '(j) + j # "(j #) which constitutes the quantity sought, the response of the cell involves the parameters of cell α,# 1, T2 and C. For the determination of these parameters, a calibration cell shown in FIG. 3 is used, such that by construction, pl (t) = p2 (t).

 This calibration cell is symmetrical in order to achieve the desired result, namely pl (t) = p2 (t). The piezoelectric sensors are put in place in the same way as for the cell described in connection with FIG. 2. The excitation of the fluid is ensured, for example by a vibrating pot associated with an elastic membrane sealing the pipe (20).

 Adjusting the sensor model to the experimental results gives access to the cell parameters a, # 1, 12 sought.

 With the sensors used (DYTRAN 2013 V), the following results are obtained: CX = 1.389, Ti = 1.141 and set T2 = 0.9208 s.

Tank 2 calibration
This calibration gives access to C. For this determination, the measuring cell is used which is filled with the solvent which will be used subsequently to make the solution to be studied. It is also the adjustment of the model to the experimental results which reveals the effective capacity C.

 The solvent must be a Newtonian fluid. In practice, it will most often be water.

 Furthermore, when filling the cell, care is taken not to introduce air bubbles. The presence of a bubble would modify the value of C (a degassed solvent will be used).

 For water and the geometry indicated in FIG. 3, on which the filling and excitation nozzles are not represented, the following value is obtained for C: C = 2. 10-10 cgs units.

Test results
A series of experiments, carried out with Newtonian fluids, made it possible to test the validity of the model over the frequency range between 1 and 3000 Hz. A particular system was tested, for which the behavior at low frequency is experimentally well known , and whose behavior with respect to high frequencies is the subject of theoretical predictions. This system is an aqueous solution of living polymers in a semi-diluted regime (solution obtained from the mixture of a surfactant (CTAB) (cethyltrichloro ammonium bromide) and a salt (KBr).

 Figures 4 and 5 represent the changes in the real and imaginary parts of the complex viscosity, T '(jo) = Tl' (jCo) + j Ii "(jo), and of the complex elastic modulus, G (jo) = G '(jo) + j G "(jo) with frequency.

 In FIG. 6 has been represented the corresponding Cole-Cole diagram: G "(jo) f (G '(jo)). For sufficiently low frequencies, the behavior of the solution is perfectly Maxwellian and managed by the characteristic times Tr and Tv. For frequencies exceeding 1 / 2v, there appears a clear deviation from the Maxwell model (solicitation of the high frequency modes of the system) characterized by a rise in the dissipation module These experimental results confirm the theoretical predictions.

 We thus observe the possibility of achieving with the device of the invention the analysis and obtaining the viscoelastic characteristics of a fluid for a wide frequency range, and this by means of a dose cell, therefore in the absence of any concentration drift.

In addition, the device used is relatively low cost, in particular compared to the rotary rheometer.

 The applications of this device are relatively numerous.

Among these, we can cite the food industry. In this area, it is most often a question of comparing the product to be checked with a reference whose characteristics have been defined by analysis of consumer reactions (texture). However, for manufacturing problems, it is desirable to know better the main absolute rheological characteristics of the products. Thus, these properties are used to control the raw materials purchased entering into the composition of the products. It is important that the analysis methods do not cause damage to the products, which is the case with the device of the invention, because the measurement then no longer makes sense if the product is modified during analysis.

 We can also mention the cosmetics industry. Indeed, it is important to know the rheological behavior of materials in this area, because consistency and viscosity are essential elements for the consumer.

 In addition, this device is also applicable for the petrochemical industry. With regard to motor oils, the industry has defined very specific international standards and test procedures. Specialized technical centers, such as the French Petroleum Institute, are interested in the fundamental properties of these fluids and more particularly in the additives that are used in the composition of modern oils. In this case, the possibility of carrying out measurements at high frequency is of paramount importance.

 In the polymer industry, there is a need to perform measurements at high frequency which has not been met to date. The device according to the invention thus makes it possible to respond to this request.

 It is thus observed that the possibility of obtaining measurements of the visco-elastic characteristics of fluids at high frequencies thus meets a definite need in most fields of application of rheometers, in particular in the manufacture and transformation of polymers, in the food industry, cosmetics and paints and varnishes.

 Furthermore, taking into account the technique used, the fluid whose characteristics one seeks to determine is not destroyed or modified during the measurement. This result proves to be of very particular importance, in particular in the fields where the texture of the fluid is an essential parameter, such as the food industry, where these fluids are often in the gel state.

 As already said, the device is simple to build and does not include complex or delicate mechanisms. The use, cleaning and maintenance are thus greatly facilitated.

Claims (8)

 1 / Device for determining the visco-elastic characteristics of a fluid, comprising a measuring cell consisting of two cavities (1, 2) communicating with each other by means of a rectilinear capillary tube (4), characterized :  - in that the measuring cell (1,2,4) is hermetically dose;  - in that said cavities (1, 2) are each provided with a pressure sensor (5, 6), said sensors being electrically connected to a central unit (11) intended to process the signals picked up and to restore the visco characteristics -elastic of the fluid;  - And, in that one of the cavities (1) is associated with a vibrating member (12), capable of setting in motion the fluid contained in the measurement cell thus formed.  2 / Device for determining the visco-elastic characteristics of a fluid according to claim 1, characterized in that the device operates on a frequency interval, the upper limit of which exceeds 1000 Hertz.  3 / Device for determining the visco-elastic characteristics of a fluid according to one of claims 1 and 2, characterized in that the two cavities (1, 2) and the capillary tube (4) are completely filled with the fluid to analyze.  4 / Device for determining the visco-elastic characteristics of a fluid according to one of claims 1 to 3, characterized in that the pressure sensors are piezoelectric sensors.  5 / Device for determining the visco-elastic characteristics of a fluid according to one of claims 1 to 4, characterized in that the movement imparted to the fluid to be analyzed is an oscillating movement.  6 / Device for determining the visco-elastic characteristics of a fluid according to claim 5, characterized in that the oscillating movement, communicated to the fluid to be analyzed, is of low amplitude.  7 / A device for determining the visco-elastic characteristics of a fluid according to one of claims 1 to 6, characterized in that the oscillating fluid setting member consists of a vibrating pot (12) associated with a elastic membrane (13) closing one (1) of the two cavities (1, 2), or by a piezoelectric exciter immersed in one of the two cavities (1, 2).  8 / Device for determining the visco-elastic characteristics of a fluid according to one of claims 1 to 7, characterized in that the measuring cell is immersed in a thermostatically controlled enclosure (3), so as to keep the temperature constant during the duration of the experiment, according to a determined value.