Title: Measurement of Surface Tension and Other Rheological Properties of Liquids

Field of the Invention

This invention relates to apparatus for monitoring the surface tension of a liquid, and to a method of monitoring such properties.

Some embodiments of the invention may also be used to monitor other rheological properties of a liquid.

Background to the Invention

In many production processes, involving the use of a liquid, surface tension can be a very sensitive property, especially where surfactants are used, which may be affected directly by relatively small changes in the constitution of the liquid being used in the production process. Traditionally, surface tension is measured by techniques which involve taking a sample of a liquid to be analysed and bringing the surface of the liquid sample into contact with a surface, for example the interior of a capillary tube or a microscope slide, which exerts adhesion forces on the molecules of the liquid. These techniques require that the surface of the sample liquid is in a relatively settled state, are time consuming and are therefore not ideally suited to continuous monitoring of surface tension properties.

Summary of the Invention

According to a first aspect of the invention, there is provided apparatus for monitoring the surface tension properties of a liquid, the apparatus comprising a moveable member having a surface which carries a number of liquid accumulation sites, application means for applying said liquid to said surface, to be retained at said sites, at least in part, by surface tension forces in the liquid, drive means for accelerating the surface so as to exert release forces on the liquid at the accumulation sites, wherein the relative magnitudes of the release and

surface tension forces vary in a predetermined fashion across the surface so that the release of accumulations from the accumulation sites defines a pattern related to the surface tension of the liquid, the apparatus further comprising monitoring means for monitoring said pattern.

This pattern may be any distribution of retained liquid or areas from which liquid has been reduced that can provide an indication of surface tension properties.

Since the monitoring of the surface tension properties is achieved using a member which is accelerated so as to release accumulations of liquid from multiple accumulation sites, the liquid can be applied to different parts of the surface at different times so that accumulations (or sites from which accumulations have been discharged) on one part of the surface can be monitored whilst liquid is being reapplied to another part of the surface. Thus the apparatus lends itself to the continuous monitoring of surface tension properties.

Preferably, the sites are discrete.

The relative magnitudes of the surface tension forces retaining the liquid at the sites and the release forces may vary by virtue of variations in the properties of the accumulation sites, and thus in the amount of force, per unit volume of liquid, with which the surface tension retains the accumulations, i.e. the force that has to be overcome to release the accumulations of liquid from the liquid accumulation sites.

For example, the accumulation sites may be such that the sizes of the accumulations vary across the surface, so that there is a corresponding variation in the ratio of wetted area to the volume of accumulated liquid at the sites. This may be achieved by providing accumulation sites of varying size.

Additionally or alternatively, said variation in relative magnitudes of surface tension and release forces may be achieved by varying the magnitude of the acceleration of the surface or applying different magnitudes of acceleration to different parts of the surface. In this case, the sites may be of a uniform size or may be constituted by different areas of a continuous surface to which the liquid is applied.

Preferably, the member is rotatable and the drive means is operable to rotate the member so that the release forces arise from centripetal acceleration of the surface.

With such an arrangement, continuous monitoring can be achieved using application means and monitoring means which interact with the surface at angularly spaced positions about the axis of rotation.

The application means conveniently comprises a spray head for spraying liquid onto the surface of the member.

Preferably, the drive means is adjustable so as to vary, in a controlled manner, the rotational speed of the member.

The range of surface tensions to which the apparatus is sensitive can thus be adjusted by adjusting the rotational speed of the member.

The surface is preferably easily wetted by the liquid, for example in the case of an aqueous liquid the surface is preferably hydrophilic, for example gold or fused silica.

The member may be rotationally symmetric. Such a member may conveniently comprise a cylinder, said accumulation sites being provided on the cylinder's circumferential surface.

In this case, the accumulation sites preferably comprise surface formations of different sizes on the cylinder.

Preferably, each formation comprises a respective pit, the size of pits varying across the surface. The size of pits may vary in diameter or depth or both.

The size of each pit will have a bearing on the surface area of the pit wetted by the liquid, and hence on the surface tension forces, and on the volume of the accumulation of liquid at that site, and hence on the magnitude of the release force. Larger pits will acquire larger

accumulations which will tend to be shed at lower rotational speeds than the accumulations in the smaller pits.

Preferably, the size of the pits varies progressively along the axis of the cylinder.

Consequently, the pattern caused by the release of liquid may comprise an area of filled pits separated by an annular interface, concentric with the axis of rotation, from an area of pits from which liquid has been discharged. Accordingly, the axial position of the interface provides an indication of surface tension.

Additionally, or alternatively, the rotatable member may have a radius that varies with position along the axis of rotation. For example, the member may be in the shape of a cone, cone frustum or may be bell-shaped.

It will be appreciated that a given rotational speed will subject the liquid in the sites to release forces that vary with position along the axis of rotation.

Accordingly, in this case, the pits may be of a uniform size.

The monitoring means may to advantage comprise a camera for capturing images of the surface.

Preferably, the apparatus includes a barrier shielding the camera from any liquid ejected from the surface.

Preferably, the apparatus includes a strobe illumination source for illuminating the surface being imaged in such a way as to facilitate the capture of clear images of the surface by the camera.

By matching the frequency of the strobing illumination to the rotational speed, it is possible to make the accumulation sites appear stationary.

If the accumulation sites are arranged in a pattern that repeats itself around the rotatable member multiple times it is possible to use a strobe illumination rate higher than the frequency of rotation of the member. This has the advantage of allowing a faster measurement to be made and/or a lower intensity and lower cost light source to be used for illumination.

Preferably, the pits around the rotatable member at a given point along the axis all have the same size. In addition the spacing between the centres of adjacent pits is preferably constant over the surface of the member so that there are the same number of pits of each size on the member. These features facilitate the imaging of the pits using a camera and strobe light source.

According to a second aspect of the invention, there is provided a method of measuring the surface tension of properties of a liquid, a method comprising the steps of:-

a) applying the liquid to a surface having a plurality of liquid accumulation sites at which the liquid is retained, at least in part, by surface tension forces;

b) accelerating the surface so as to apply release forces to the accumulations of liquid at said sites, the relative magnitude of the release and surface tension forces varying in a predetermined manner across the surface so that some accumulations are released whilst others remain; and

c) detecting the distribution of sites from which release has occurred and/or the distribution of sites at which the accumulations have been retained.

Preferably, the detection occurs in a zone through which the surface is swept whilst liquid is applied at another zone in a continuous monitoring process.

Preferably, the member is rotatable, said surface being of circular cross-section, and the liquid is applied to the surface at one angular position, whilst the monitoring means monitors the surface at another angular position.

According to a third aspect of the invention, there is provided apparatus comprising multiple liquid accumulation sites of more than one size on the surface of an accelerating member, application means for delivering a liquid to said surface and monitoring means operable to measure and/or observe the level and/or shape of the said liquid at the said liquid accumulation sites.

The accelerating member is preferably a rotating member, the surface of which experiences centripetal acceleration due to the speed or rotation of the rotating member.

The acceleration of the accelerating member may be variable so that the acceleration may be set to a value corresponding to a desired range of surface tensions or other properties to be measured.

The liquid accumulation sites may be constituted by pits projecting into the surface of the accelerating member.

Where the sites are pits projecting into the surface of the accelerating member the pits are preferably circular.

The depth of each pit is preferably less than the diameter of the pit.

Preferably the diameter of the largest pit is between 500 microns and 20 microns.

Alternatively, the sites may comprise a plurality of grooves projecting into the surface of the accelerating member and ridges projecting out of the surface of the accelerating member.

The grooves may be of constant width.

Alternatively, the grooves are of a continuously varying width. In this case each groove plays the same role as two or more discrete sites of fixed size. Thus, if surface tension is being monitored, the transition from filled to empty occurring along the length of the groove

at a particular width corresponds to a given surface tension. Alternatively, a single helical groove may define the liquid accumulation sites.

The application means may comprise means for directing a jet or a spray of liquid onto the surface of the accelerating member.

In the latter case, the application means may comprise means for delivering an air-spray of liquid such as from an air-brush onto the surface of the accelerating member.

Alternatively, the application means may be operable to provide a point of direct contact between the supply of liquid and the surface of the accelerating member.

Preferably monitoring means operable to measure and/or observe comprises a stroboscopic illumination device and an imaging device.

The stroboscopic illumination device may for example comprise a spark-gap or Xenon flash tube type illumination device.

Alternatively the stroboscopic illumination device may comprise a high intensity Light Emitting Diode illumination source.

A further example of the stroboscopic illumination device comprises a pulsed laser illumination source.

The liquid accumulation sites may be disposed in a pattern that repeats itself around the circumference of the rotating member and the strobe illumination flashes more than once per revolution of the rotating member.

Each basic unit of the repeating pattern of liquid accumulation sites may to advantage correspond to the field of view of the imaging system such that a fixed position imaging device can capture and image all liquid accumulation site sizes in a single fixed field of view.

The imaging device may be operable to accumulate light from more than one flash of the stroboscopic light source.

The liquid accumulation sites may be disposed in a symmetric arrangement such that constant illumination may be used and the imaging device can determine the level or shape of the liquid at the liquid accumulation sites from the motion blurred image.

For example, if the moveable member is a rotary member the sites corresponding to the same magnitude of surface tension force are arranged at the same position along the axis of rotation.

The wavelength of the illumination source may be selected or filtered to provide good contrast of image of the liquid and accelerating member for the imaging device.

The inclination of the illumination source may be set to create a specular reflection from the surface of the liquid in such a way as to increase the sensitivity of the image system to determine the level or angle of liquid at a liquid accumulation site.

The imaging device may comprise one or several photo-detectors, for example photodiodes or phototransistors.

The imaging device may comprise a linear Charge Coupled Device (CCD) array.

The imaging device conveniently comprises a video camera with an appropriate optical attachment such as a microscope lens system.

The imaging device may be sensitive to a particular range of wavelengths of light that provide a good image of the liquid and accelerating member, for example by placing a coloured filter before the imaging device.

The imaging device may be shuttered to provide a short exposure, in which case, the need to provide a stroboscopic illumination source is avoided.

The liquid accumulation sites on the accelerating member may be continuously wetted by the liquid.

The liquid accumulation sites on the accelerating member may be hydrophilic, the liquid being aqueous. The liquid accumulation sites on the accelerating member may be lipophilic, the liquid being lipid based, or the liquid accumulation sites on the accelerating member may be oleophilic, the liquid being oil based.

The surface of the accelerating member may have liquid attracting and liquid repelling areas wherein the liquid attracting areas are the liquid accumulation sites and the liquid repelling areas are areas between the liquid accumulation sites.

The rotating member may form a cone, cone frustum or bell shape and liquid is applied to the small radius of the rotating member, which then flows from the small radius to the larger radius from where the liquid is ejected, wherein the liquid accumulation sites are formed along the central section of the rotating member.

The liquid accumulation sites may be of the same or similar size, but in this case the acceleration experienced at the sites varies due to the variation in the radius of the rotating member.

The means for delivering liquid may be operable to control the flow rate of liquid onto the rotating member to produce a constant rate of flow of liquid over the rotating member.

The surface of the cylinder may contain one or more obstructions to the flow of liquid over the rotating member and wherein the shape or level of liquid around these obstructions is measured by the imaging device to obtain an indication of the viscosity of the liquid.

The obstructions may consist of ridges or grooves.

Such ridges or grooves may be of varying height or width and/or of varing orientations.

The invention also lies in a method of measuring surface tension and/or other rheological properties of a liquid by supplying the liquid to apparatus in accordance with the third aspect of the invention. Preferably the liquid is a newly created liquid from a production process and is constantly supplied to the rotating member.

In one example of the invention the liquid on the rotating member may be constantly removed after measurement so that the liquid being measured is not mixed with existing liquid on the rotating member.

The liquid may be is constantly removed from the rotating member by, for example, a jet of neutral liquid or gas, e.g. compressed air.

The image or other measured data may be constantly and automatically interpreted by a computer to provide a continuously updated value for the surface tension or other rheological properties

The method may include the step of generating an alarm signal when the surface tension or other interpreted property of the liquid goes outside pre-determined parameters.

The apparatus may be used within a closed-loop production process such that the formulation parameters of the production process are varied dynamically to maintain the measured properties of the liquid, for example by the increased addition of surfactants to the liquid production process if the measured surface tension of the liquid falls below a threshold value.

The method may also include directing the flow of a liquid based on the current measured value of surface tension or other property, for example to provides parameter-graded output in a continuous liquid production process.

Brief Description of the Drawings

The invention will now be described by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a diagrammatic view of apparatus in accordance with the invention;

Figure 2 shows a moveable member in the form of a cylinder, forming part of the apparatus of Figure 1; and

Figure 3 is a sectional view of part of the cylinder, showing the interaction between a liquid applied to the cylinder and a number of pits in the cylinder surface.

Detailed Description

With reference to Figure 1, the apparatus according to the invention is for use in monitoring the surface tension properties of a liquid, which is used in a production process, and which flows through a supply tube 1. The apparatus has a wide range of application. For example, the liquid may form one or more ingredients of a food product, such as ice cream, or the production process may be the manufacture of printing ink, in which case the liquid may constitute an ingredient of the ink or the final ink product.

A feed tube 2 leads off from part of the supply line 1 and is terminated in a spray nozzle 4 which is adjacent to a cylinder 6. The spray nozzle 4 is elongated in a direction perpendicular to the plane of Figure 1 so that it extends along substantially the entire length of the cylinder 6.

The cylinder 6 has a radius of 10mm and a length of 10mm, and has a circumferential outer surface (8 in Figure 2) formed from gold or fused silica. For example, the cylinder may be made from brass with a gold coating or comprise a fused silica tube over an aluminium inner cylinder. The cylinder is mounted for rotation about its axis 10 and is rotated in a controlled manner by drive means in the form of an electric motor (Figure 2) connected via gearing (not

shown) to an axle 15 concentric with axis 10. This motor may for example be a 50,000 rpm brushless motor having a 3mm shaft.

The axle 15 rotatably supports the cylinder 6 on a support frame (now shown) through ball race bearing (not shown).

With reference to Figure 2, the circumferential surface of the cylinder carries as^ array of pits which are circular in cross-section and extend radially relative to the axis 10. Reference numbers 12, 14, 16, 18, 20, 22, 24, 26, 28 and 30 denote some of these pits. There are other pits in the array which would be apparent from the area of surface shown in Figure 2, but these have been omitted for the sake of clarity. The pits constitute a series of groups, each of which is annularly arranged at a respective axial position along the cylinder. Each group contains pits of the same size, whilst the size of the pits vary progressively from one group to another, with the smallest pits being at the left hand end of the cylinder as shown in Figure 2, the largest pits being at the opposite end. Thus, for example, the pits 16 and 18 are members of the same group of pits which are annularly arranged around the axis 10, and situated towards the left hand end of the cylinder 6, whilst pits 28 and 30 are part of the group of largest pits, which are annularly arranged around the axis 10, and situated towards the right hand end of the cylinder as viewed in Figure 2.

The diameters of the pits vary monotonically along the axial length of the cylinder. The spacing between the centres of neighbouring pits is uniform so that each group has the same number of pits as the other group, and the pits in each group are equiangularly arranged around the axis 10. In the present example the radii of the pits vary from 20 microns to 200 microns and all of the pits are 10 microns deep. Such pits and other features may be formed on the surface of the cylinder by, for example, direct laser ablation of the surface or the use of ion bean ablation or by the use of a photo-mask and etching process or direct stamping or other similar means to create surface structures.

Corresponding pits in the groups are at the same angular position as each other, so that the groups, between them, define a series of linear arrays of pits which are arranged at regular angular intervals around the cylinder 6 and extend in the direction of the axis 10. The size of

the pits in each linear array progressively increases from left to right as viewed in Figure 2, and each linear array constitutes a respective unit of a pattern that is therefore also repeated at regular angular intervals about the cylinder 6. Pits 18, 20, 22, 24, 26 and 28 are some of the pits in one such linear array.

The angular position of the cylinder 6 is monitored by a sensor 32, for example, an optical or magnetic rotary encoder. The output of the sensor is connected to a computer system 34 which is also connected to a stroboscopic light source (in this case an LED light source) 36 which directs illuminating light onto the surface of the cylinder 6. The system 34 is also connected to a camera microscope 38 that captures images of the surface illuminated by light from the source 36.

A barrier 40 extends radially with respect of the cylinder 6 from a position in close proximity to the surface of the cylinder 6. The barrier 40 is positioned adjacent to the light source 36 and imaging means comprising camera microscope 38, but upstream of these two components in relation to the path of movement of surface 6. Accordingly, the barrier 40 can block droplets of liquid that may be ejected from the cylinder 6 so as to prevent these from impinging on the source 36 or camera microscope 38.

The computer system 34 controls the motor that drives the cylinder 6, so as to control the speed of rotation of the latter in response to signals received from the sensor 32. The system also operates the light source 36 in synchronism with signals received from the sensor 32 so that the interval between successive flashes of the source corresponds to the time taken for the cylinder to rotate through an angle which is the same as the angular separation between adjacent pits in any given group (and hence the angle between adjacent linear arrays). Consequently, the surface of the cylinder as illuminated by the source 36 will appear stationary. The microscope camera 38 monitors the illuminated image of the portion of the surface of the cylinder under the microscope, along the entire axis of the cylinder.

In operation, some of the liquid, which is being fed through the line 1 under pressure passes through the feeder tube 2 to the nozzle 4 from which it is sprayed onto the surface of the cylinder 6. Surface tension of the liquid will tend to cause the pits to fill under capillary

action to minimise the surface area of the liquid on title cylinder surface. However, the surface acceleration (i.e.centripetal acceleration) of the cylinder 6 creates a counteracting force which tends to expel the liquid from the pits. A pit of a given size will be sensitive to a specific surface tension of liquid, such that when the surface tension is above a threshold level the pit is filled and if the surface tension is below the threshold, the pit is emptied (through the expulsion of the liquid from the pit as the result of the surface acceleration of the cylinder). It will be appreciated that the expulsion force is related to the mass of liquid in the pit which in turn depends upon the surface area of the pit, whilst the surface tension forces will be related to the circumference of the pit. Accordingly, the expulsion forces will tend to vary in relation to the square of radius of the pit, whilst the surface tension forces will tend to vary in relation to the radius, so that the above mentioned threshold surface tension will increase with pit radius.

The effect of this is illustrated in Figure 3, in which reference numeral 40 denotes the film that the liquid forms on the surface of the cylinder 6. This film was formed as the result of spraying liquid from the nozzle 4 onto the surface of the cylinder 6, said surface being hydrophilic. Initially, the liquid goes into all of the pits 18, 20, 22 and 24 shown in Figure 3, and all the corresponding pits in the other groups. However, as the cylinder surface rotates towards the barrier 40 (in a clockwise direction as shown in Figure 1) some of the pits will be emptied as a result of the centripetal acceleration of the cylinder surface leaving a relatively thin film of liquid, while other pits will be filled (to roughly the level of the surrounding area or above) by the action of surface tension acting to minimise the surface area of the liquid

Figure 3 shows the pits, 18, 20, 22 and 24 when they have passed the barrier 40 and are in register with the camera microscope 38. Because the pits 18 and 20 are of a smaller diameter than the pits 22 and 24, they have retained their charge of liquid, whereas the weight of the liquid in the filled pits 22 and 24 was sufficiently high for the centripetal acceleration to eject that liquid from those pits, leaving a relatively thin film.

If the surface tension forces in the liquid had been higher, it is possible that the ink would not have been ejected from the pit 22, whereas lower surface tension forces may have

resulted in the ejection of ink from the pit 20, in addition to pits 22 and 24. Thus, for a given rotational speed of the cylinder, the image captured by the camera microscope 38 of the filled and emptied pits provides an indication of the surface tension properties of the liquid.

The computer system 34 is operable to vary the rotational speed of the cylinder 6 in a controlled manner, and to use a measurement of the rotational speed in the determination of the surface tension properties.

Typically, in operation the system can be tuned to be sensitive to a specific range of surface tensions for a given range of liquid formulations. This can be accomplished by changing the rotational speed of the cylinder 6 until the current surface tension of the liquid forms an observable transition in the centre of the image formed by the microscope camera 38. Any change in the surface tension of the liquid supplied to the cylinder can now be observed as a movement of the point of transition (i.e the interface between filled and emptied wells) from the central position. This observation can be monitored automatically using image processing software on the computer control system 34 and can for example raise an alarm when the surface tension moves outside set allowable parameters. Alternatively, the surface tension information can be used dynamically in a feedback process automatically to adjust the formulation of the liquid to maintain its surface tension properties within specific parameters when some parts of the input formulation may be changing, for example due to use of natural formulation components that may have inherent surface tension variability.

Many types of liquid formulation, for example inks, paints, food stuffs, chemical and medicines can be sensitive to changes in surface tension during the manufacturing process. For example the introduction of small amounts of surfactants or small changes in the concentration of surfactants can lead to significant changes in the properties of liquid. These changes can be measured as change in surface tension by the present apparatus which is suited to the constant in-line monitoring of surface tension for process control.

It will be appreciated that the above apparatus may be modified without departing from the scope of the invention. For example, the stroboscope light source 36 could be replaced by a

source of constant illumination, and the image of empty/filled pits may be detected in the radially motion blurred image formed by the camera microscope 38.

New liquid is constantly mixing with the liquid already present on the surface of the cylinder 6, and this mixing generally occurs during each rotation of the cylinder (which may be rotating at a rate of several hundred revolutions per second. In some circumstances, very small amounts of liquid formulation components, eg surfactants, may cause significant effects on surface tension even at very low concentrations. In such circumstances it may be preferable to clean the surface of the cylinder during rotation. This could be achieved by, for example, directing a neutral fluid or gas onto the cylinder. The fluid could for example be deionized water or a jet of high pressure gas, for example air. This may be done either constantly or intermittently to ensure that the liquid being measured has the same formulation as that emerging from the nozzle 4.

The system can be calibrated with respect to known liquid formulation and may operate accurately over a wide range of surface tensions due to the ability to vary the speed of rotation of the cylinder to bring the surface tension of the liquid to be tested within the variance scope of the pits on the cylinder. It may be necessary to adjust the interpretation of the information from the system with respect to the relative density of a calibration liquid under test liquid to give absolute value to surface tension.

As an alternative to pits or other features of varying sizes, the pits or other features may all be of the same dimensions and the radial diameter of the "cylinder" may vary, i.e. forming a shallow cone or bell-shape. The surface acceleration is related to the radial distance from the axis of rotation so a cone or bell-shape produces a range of accelerations on its surface for the same rotational speed. Thus the same transition effects as with pits of varying sizes may be achieved: for a given surface tension and speed of rotation there will be a transition from filled pits to empty pits as the radius of the cone increases.

If a cone (or bell-shape) rotating member is used then liquid may be applied to the smaller radius of the rotating member, e.g. as a jet of liquid, and the liquid will then flow across the rotating member to the higher radius portion of the rotating member where it will then be

ejected. In such an arrangement liquid is not required to be applied to the whole surface of the rotating member as it will naturally flow over the liquid accumulation sites present on the surface of the rotating member. By controlling the flow rate of liquid over such a rotating member other rheological properties such as the viscosity of the liquid may also be measured alongside the surface tension. For example viscosity may be measured by placing obstructions, e.g. ridges, on the surface of the rotating member and observing the relative shape and size of the bulges formed by the liquid on each side of the ridge, the difference in the shape and size of the bulges being related to the viscosity of the liquid as it passes over the obstruction formed by the said ridges.