WO2010149887A1

WO2010149887A1 - Improved method of particle size measurement

Info

Publication number: WO2010149887A1
Application number: PCT/FR2010/000473
Authority: WO
Inventors: Didier Frot; David Jacob
Original assignee: IFP Energies Nouvelles; Cordouan Technologies
Priority date: 2009-06-26
Filing date: 2010-06-24
Publication date: 2010-12-29
Also published as: FR2947338B1; FR2947338A1

Abstract

Method for measuring the intensity of the light scattered by a thin film of a colloidal medium, in which the following steps are carried out: a device comprising: a monochromatic light source; a convergent optical system focusing said light source onto said thin film to be analyzed and comprising a dioptric element (7), one of the faces of which constitutes a first wall delimiting the thin film; at least one photosensitive detector reacting to the light scattered or backscattered by the thin film at a measurement point; and means for processing the signal from said photodetector is operated; and said thin film is formed by means of a second wall consisting of a plane surface (3) at the end of a rod (2), means for positioning said plane surface at a defined distance from said first wall and means for maintaining parallelism between said two walls.

Description

IMPROVED METHOD OF GRANULOMETRIC MEASUREMENT

The present invention relates to an improved method for measuring the intensity of light scattered by high concentrations of particles or macromolecules of size between a few nanometers and several hundred nanometers. It applies more particularly to the correlation of photons in liquid media.

It is a well-known industrial demand to want to follow, continuously and in different stages of a process, the evolution of the size of the particles circulating therein. Methods capable of continuous measurements on fluids are those sensitive to the optical contrast between a particle and its external environment. This is the case of optical microscopy and diffraction measurements. In the particle size range of the order of 200 nanometers or less, these techniques no longer give results. The dynamic light scattering technique then takes over. This last technique, which is based on the analysis of pure Brownian motion, does not make it possible to measure flowing liquids, or in the presence of convection. This clearly shows that the measurement in a flow, however slow, is impossible since it leads to measuring artifacts.

Measurements of the intensity of light scattered by thin films, especially colloidal media, is the basis of the object of the present invention.

Document EP-0654661A1 discloses light diffusion measurements performed on a thin film produced by means of a first dioptric interface and a measuring finger. Figure 1 shows in section the device according to this prior document. It comprises a prism P, of angle A = 90 °, responsible for totally reflecting the laser beam L. The different faces of the prism each form a diopter, that is to say an optical surface separating two mediums of index of different refraction. The laser beam enters through the face E, is totally reflected by the secant face F and leaves by the normal of the face S. The face S is surmounted by a pierced piece N and delimiting a receiving tank of a certain volume sample containing the M objects to analyze. The device further comprises a micro positioner G holding a bar H carrying at its end a bar D of black glass to limit the reflected or scattered light intensity due to the laser beam transmitted by the sample, parasitic intensity which must not reach the I. Advantageously, the bar D has a radius of curvature (convex) so as to punctually guarantee a film as thin as possible and to create a very punctual analysis zone of the order of 1 mm ² .

However, this device of the prior art has a number of disadvantages:

damage to the surface of the bar and / or the prism, after several uses likely to lead to artefacts due to the intensity diffused by these micro scratches which appear over time,

the manufacture of the convex bar is delicate. Indeed, the polishing operation (tolerance Lambda / 4) is difficult on curved shapes and significantly increases the cost of this part,

the black bar may have residual reflectivities which interfere with the measurement,

- It is impossible to perform a measurement under flow without it disturbs the measurement due to the low contact surface of the bar.

Thus, the present invention improves the measurement method by using an improved device which notably provides a new geometry of finger and bar allowing, among other things, to perform a measurement under liquid flow in the cell, to facilitate the manufacture of the bar, to allow a simplified mode of implementation of the measurements, and not to damage the diopter by scratching its surface.

The present invention relates to a method for measuring the intensity of light scattered by a thin film of a colloidal medium in which the following steps are carried out:

a device comprising:

a monochromatic light source, a convergent optical system focusing the source on the thin film to be analyzed and comprising a dioptric element, one of the faces of which constitutes a first wall delimiting the thin film; at least one photosensitive detector reacting to the diffused or retro-diffused light by the thin film; at a measuring point,

means for processing the signal coming from the photo detector,

the thin film is formed by means of a second wall constituted by a flat surface at the end of a bar, means for positioning the flat surface at a determined distance from the first wall and means for maintaining the parallelism between the two walls.

The medium around the bar can be circulated to the measuring position on the thin film.

The flat surface can be lifted, the medium being circulated to renew the medium to be measured.

The bar may be carried by a body which comprises at its end sealing means so as to isolate the thin film present under the bar.

It is possible to determine, for given conditions of circulation of the medium, the minimum area of the plane surface so that at least one portion of the film is static at the measurement point.

Direct measurements can be made through a transparent finger using an optical lens and a CCD sensor.

Zeta potential measurements can be made on the thin film by means of electrodes arranged at the end of the bar.

The bar can be mounted in a body assembled on a casing which encloses the dioptric element, said casing comprising an inlet orifice and a outlet orifice of the colloidal medium allowing circulation in the housing of said medium.

The light source can be transported to the optical fiber optical system, and the diffused intensity signal can be fed to the measuring means also by optical fiber.

The flat surface may have an area of at least 10 mm ² .

The present invention will be better understood and its advantages will appear more clearly on reading the following description of embodiments and embodiments, which are in no way limitative, illustrated by the appended figures, of which: FIG. 1 schematically represents a device according to the prior art,

FIG. 2a schematically illustrates, in section, the finger of the device implemented by the method according to the invention,

FIG. 2b illustrates a variant of the assembly of the finger, FIGS. 3a and 3b show a variant of the end of the bar,

FIG. 4 schematically shows another variant of the finger which carries the bar,

FIG. 5 shows an additional device for carrying out another type of measurement, FIG. 6 illustrates the device according to the invention for measurements in circulation,

FIGS. 7, 8 and 9 show measurement results obtained respectively for a sampling of standard media: - without flow, - with flow and the finger raised, - with flow and the finger in the contact position. The present invention is based on the implementation of a finger capable of producing a thin film between two flat surfaces having a high level of parallelism, said thin film of colloidal media for measurements of the scattered intensity.

Figure 2a shows schematically the means of realization of the device according to the invention. The finger 1 consists of a bar 2 glass, or equivalent, which defines at its end a flat surface 3. The bar is held in a body 4. The optical specifications of the faces of this cylindrical bar are standard, a state Lambda / 4 surface area and Lambda / 2 flatness.

The body 4 is mechanically connected to an upper part, or cover, of a casing 5 by fastening and positioning means 6 whose functions are: to seal between the body and the casing, to provide means for adjusting the the thickness of the thin film, that is to say the distance between the lower face 3 of the bar 2, and ensure the parallelism between the face 3 of the bar and the diopter 7.

The parallelism between the underside of the finger and the upper face of the prism (diopter) constituting the bottom of the tank 8, is provided by self adjustment through a mechanical clearance provided by the fixing means 6. This game can be realized by a low guide length (eg twice the diameter of the finger) of the body of the finger in the housing cover. This self adjustment can also be achieved by adjusting the machining tolerances of the bore of the sleeve guide. Unexpectedly, it has been found that it is the drainage of the liquid film under the action of the thrust exerted on the finger which ensures perfect parallelism of the lower face 3 of the finger with the upper face 7 of the tank, insofar as the bar, or the holding body of the bar, has a degree of freedom sufficient to self-align. Thus, it can be considered that the bar is set up on a liquid cushion.

The fastening means 6 can also advantageously comprise an axial clearance which allows, for example under a tapping action, for example by a finger of an operator, the elimination of the fluid film ("flushing"). This movement which decreases or cancels the thickness of the film, can be antagonistic to the action of a return spring. The upper face 9 of the bar is preferably inclined at an angle α

(about 5 degrees) from the underside so as not to disturb the back scattered light. In addition, the internal volume 10 to the body 4 contains a liquid of refractive index determined to minimize this disturbance. The volume is closed by a plug 11. The incident laser beam 12 is brought to the measurement point by the known optical means, in particular those described in the document EP-0654661A1, cited here by reference. It is the same for the means of measurements on the diffused ray 13.

Thus, the implementation according to the invention has the following advantages.

- When the finger is in position in contact with the prism, a liquid film trapped between the two flat faces (diopter and bar) is immobile and insensitive to the environment of the bar, including flow;

- One can approach the finger quickly on the diopter without worrying about the effect of impact on the surface, because the thin film provides a buffer function that mutually protects each of the surfaces of shocks and scratches.

This configuration is exploited to carry out the "flushing" function which allows the renewal of the thin film of the sample and the optimization of the homogeneity in temperature, concentration and statistical distribution of the constituents of the sample in the case of a sample. mixed.

- The elimination in the thin film of analysis of possible aggregates or undesirable dust.

- Achieving a shearing effect in the thin film that can separate loosely bound aggregates. - The parallelism of the underside of the finger with the upper face of the tank makes it possible to produce concentric interference rings (no air wedge effects) and therefore to follow by optical interferometry precisely the step of draining the film. It is a relevant indicator of the stationary state of the liquid film that one wishes to analyze.

FIG. 2b illustrates a variant of the finger of the measuring device according to the invention in which the body 14 comprises end sealing means 15 arranged so as to isolate a portion of the medium around the thin film trapped beneath the bar. This variant is particularly suitable for highly diluted media, thus weakly diffusing.

FIGS. 3a and 3b illustrate another variant of a bar 16 held in a body 4. The cylindrical bar carries, on at least half of the surface of the face, a cleaning assembly 17, the other part of the surface remaining available to be the measuring point. The bar can be rotated about its longitudinal axis so as to clean the entire surface of the face of the diopter, in particular the measurement point, by means of partially flexible micro rods made of a material compatible with a large number of solvents organic acids or basic. Industrial environments are often concentrated and sometimes contain aggregates when the dispersion is not homogeneous. To overcome the difficulties of analysis on such systems and rather than filter solutions, we can use this solution. Figure 3b shows the face of the bar in view from below.

FIG. 4 shows a finger variant combined with a set of optical components 18 equivalent to a microscope objective, to which a CCD camera 19 is added. The bar 20 may have a thin thickness and parallel faces. In this case, it acts as a protective window for the purpose of microscope 18, which makes it possible not to have to use an immersion lens which is very expensive. The focus of the thin film image on the CCD sensor is provided by an extension ring (variable draw) or an optical lens system with variable focus which can be, for example, electrically controlled. FIG. 5 illustrates a possibility of measurement complementary to photon counting. This is to give access to a measure of the Zeta potential for a broad particle distribution through the combination of scattered intensity analysis (small particle domain generally less than one micron) with analysis of the electrophoretic mobility by means of image processing (see above), or the wavelength shift of the intensity diffused by particles in forced movement (unidirectional displacement).

One of the embodiments is a cylindrical bar 21 reamed about one millimeter and having two metal electrodes 22 and 23 on the walls of the bore zone, and arranged parallel to one another. In the volume defined by the bore when the finger is in contact, it is possible to measure the Zeta potential.

FIG. 6 shows a system integrating the device implemented according to the invention for on-line measurements. The device 30 comprises a support 31 pierced with a cavity 32 closed by a prism 33 and a housing 34. The casing 34 is in hydraulic communication via ducts 36 and 37. The assembly of the measuring finger 40 is constituted by a bar 41 and a body 42 according to the present description. The laser beam 35 can be transported to the prism 33 by an optical fiber 43 and a collimator 44a, and the diffused intensity signal 45 collected by the collimator 44b can be sent to the measuring means and to the PC by an optical fiber. 46. In general, the optical signals can be "deported" by means of optical fibers, whether in the "online measurement" version according to FIG. 6, or not.

The system according to Figure 6 allows a measurement method based on the functionality of the finger device capable of delimiting a thin film between two walls. This finger has two positions: one remote, the other called "in contact" with the prism to measure. When the finger is moved away from the wall, the fluid circulates in the cavity 32 and renews itself as a function of the circulation of the medium in the conduits 35 and 36. When the bar of the finger is forced to come into contact with the prism, by maintaining a determined pressure on the latter, it immobilizes and isolates at least a portion of a liquid film trapped between the two interfaces, while allowing the flow to bypass the finger without disturbing the measurement. This is possible thanks to the sufficiently wide flat surface of the bar which allows to isolate the measuring point from its environment. The bar has, for example, a diameter at least greater than 5 mm so as to ensure sufficient isolation of at least a portion of the liquid film, combined with the self-adjustment of the parallelism of the faces at the measuring point. It is noted that in this measurement domain under flow of the scattered intensity, the parallelism of the faces is an essential condition for the measurement. It is also possible, according to the variant described in FIG. 2b, to immobilize the liquid film by integrating a seal at the end of the body. The seal is chosen sufficiently flexible, so that once compressed, the surface of the finger is in contact with the lower diopter insulating a film.

It is also possible, according to the variant described in FIG. 4, to equip the finger with a microscope objective and a camera. Measuring on a liquid film at rest does not require the use of a fast camera to image the particles. Under these conditions, the use of a camera with a high spatial resolution (large number of small pixel) has the advantage of being able to make measurements of particles of smaller sizes.

The measurement method therefore has many advantages:

- It is not necessary to take a sample, which eliminates doubt about the representativeness of the sample and simplifies the procedure of the measurement. Indeed, the liquid medium resulting from a process can be conducted directly into the measuring cavity 32 by the pipes 36 or 37.

- The measurement can be repeated at will, finger away and finger contact, without loss of load on the line. - This is a classic measure of dynamic light scattering without having to make additional assumptions about particle motion. Figures 7, 8 and 9 illustrate the effectiveness of the device and the measurement method under flow.

These graphs represent the autocorrelation function of the scattered intensity with the abscissa time and the ordinate the amplitude. It is from these curves that we extract the size of the objects responsible for the scattered intensity. FIG. 7 represents the autocorrelation curve of a reference standard, here a monomodal latex standard of 160 nanometers in hydrodynamic diameter. It is known that there is a relationship of proportionality between the size of the objects and the slope at the origin of the autocorrelation curve.

Figure 8 gives the autocorrelation curve of the same standard in the presence of a flow (with a system of the type shown in Figure 6), with the finger away from the surface of the prism. It is noted that a flow modifies the slope at the origin since it is approximately 30 times smaller here than that obtained by measuring the standard (FIG. 7). This has the effect of reducing the apparent size of the same factor producing as result 6-nanometer pseudo objects instead of 160 nm. FIG. 9 gives the autocorrelation curve of the same standard in the presence of a flow, but with the finger in contact with the surface. The measurement made with the contact finger gives a size measurement result in agreement with that obtained for the standard in the absence of flow, which shows that at least a portion of the film trapped under the bar is immobile. .

Claims

1) Method for measuring the intensity of the light diffused by a thin film of a colloidal medium in which the following steps are carried out:

a device comprising:

a monochromatic light source,

a convergent optical system focusing the light of said source on said thin film to be analyzed and comprising a dioptric element (7) one of whose faces constitutes a first wall delimiting the thin film,

at least one photosensitive detector reacting to the diffused or retro-scattered light diffused by the thin film at a measurement point; means for processing the signal issuing from said photo detector;

said thin film is formed by means of a second wall constituted by a flat surface (3) at the end of a bar (2), means for positioning said flat surface at a determined distance from said first wall and means for maintaining the parallelism between said two walls.

2) Method according to claim 1, wherein said medium is circulated around the bar in measuring position on the thin film.

3) Method according to claim 2, wherein said flat surface is raised, said medium being circulated to renew the medium to be measured.

4) Method according to one of the preceding claims, wherein the bar is carried by a body (4) which comprises at its end sealing means (15) so as to isolate the thin film present under the bar. 5) Method according to one of the preceding claims, wherein is determined for the given conditions of circulation of the medium, the minimum area of the flat surface so that at least a portion of the film is static at the measuring point.

6) Method according to one of the preceding claims, wherein direct measurements are made through a transparent finger (20) using an optical objective (18) and a CCD sensor (19).

7) Method according to one of the preceding claims, wherein one carries out Zeta potential measurements on the thin film with the help of electrodes (22, 23) arranged at the end of the bar.

8) Method according to one of the preceding claims, wherein the bar is mounted in a body assembled on a housing (34) which encloses the dioptric element, said housing having an inlet port (36) and an outlet port (37) Colloidal medium for circulation in the housing of said medium.

9) Method according to one of the preceding claims, wherein the light source is transported to the optical fiber optical system, and wherein the diffused intensity signal is fed to the measuring means also by optical fiber.

10) Method according to one of the preceding claims, wherein said flat surface has an area of at least 10 mm ² .