Method and apparatus for particle detection

Abstract

The invention relates to a method for particle detection in liquid medium, wherein an excitation light is applied to the liquid medium, the excitation liquid generating a respective bubble around one or more particles. An ultrasonic wave is used in order to expand the size of the bubbles. The bubbles are then detected. The invention relates further to a corresponding apparatus.

Description

[0001] Method and apparatus for particle detection

[0002] The invention relates to a method and an apparatus for particle detection in a liquid medium.

[0003] Liquid media, for example water, are commonly used for different purposes. For example, highly purified water can be used in order to clean laboratory or production equipment, or in order to be used in a production process. In many cases it is required to ensure that the water or another liquid medium has a certain degree of cleanliness, for example that a particle concentration is below a threshold.

[0004] It is therefore an object of the invention to provide for a method and a corresponding apparatus for particle detection in a liquid medium. This is achieved by a method and an apparatus according to the respective main claims. Preferred embodiments can, for example, be derived from the respective dependent claims.

[0005] The invention relates to a method for particle detection in a liquid medium. The method comprises the following steps: applying an excitation light to the liquid medium, the excitation light generating a respective bubble around one or more particles, applying an ultrasonic wave to the liquid medium after having started applying the excitation light, the ultrasonic wave expanding the one or more bubbles, and detecting one or more particles by detecting one or more bubbles after having started applying the ultrasonic wave.

[0006] With such a method a specifically reliable and efficient particle detection in a liquid medium can be performed. The method can especially observe particles that are present in the liquid medium, but which would as such not be observable with a human eye or with a camera, even with specific optics, because it is below the resolution limit of visible light.

[0007] Particle detection may not only comprise detecting a particle or particles, but may also comprise quantification of a particle or of particles and / or size detection. Especially, the bubble generation using the excitation light can be used in order to increase the size of the detectable volume. With the ultrasonic wave, the detectable volume around each particle can be increased even further. Then, the particle detection can be performed with much more certainty, because even particles that are too small to be observed directly can now be identified, as they have generated a respective bubble.

[0008] It should be noted that especially the first step, namely applying an excitation light, can also be omitted, especially if bubbles are already present in the liquid medium. The method can then be regarded as a method for detection of bubbles that are already present in the liquid medium.

[0009] In general, particles with a diameter of, for example, more than 1 nm or more than 5 nm, or comparably small particles can be detected. Of course, also larger particles can be detected.

[0010] Particle detection especially means that particles are detected such that they can be identified separately, or at least that an indication of a number of particles can be observed. Especially, the presence of a particle can be identified by detecting its respective bubble. This means, that even if the particle itself remains invisible for a detecting entity, the generated bubble can be observed.

[0011] For example, the liquid medium may be water, especially purified or highly purified water. However, the method can be used also for other liquid media, especially for those that tend to generate bubbles around particles when an excitation light is applied.

[0012] When the excitation light is applied, a particle being present in the liquid medium typically serves as an incubator for a respective bubble. The bubble is typically generated by evaporating particles of the liquid medium, so that a local gas phase is generated. Typically, the bubble is generated around or at least in the vicinity of the particle that has served as an incubator. The ultrasonic wave expands the respective bubble, especially by lowering the pressure at the location of the bubble. Thus, the bubble’s volume increases above the detection limit.

[0013] The ultrasonic wave may be applied after the excitation light was applied, for example after a pulse of excitation light, or the ultrasonic wave may be applied while the excitation light is still active.

[0014] The detecting can especially be performed using a camera or another optical detecting means. Especially, it can be done automatically. For example, an image can be taken of the liquid medium or at least a part of it and especially an image recognition step can be performed on such an image.

[0015] Applying an ultrasonic wave to the liquid medium after having started applying the excitation light especially means that the ultrasonic wave may be applied at a time when the excitation light is no longer applied, partially concurrently with the excitation light, or fully concurrently with the excitation light. The step of applying an ultrasonic wave may even start before applying the excitation light is started. The typically relevant issue is that there is some ultrasonic wave present after having started applying the excitation light, because a bubble induced by the excitation light should be expanded by the ultrasonic pulse. Such a consideration is especially applicable for a specific combination of pulses of excitation light and ultrasonic wave, wherein such combinations may be repeated with a specific repetition rate. The same consideration can be applied to the step of detection with its reference to the start of applying the ultrasonic wave.

[0016] Especially, the excitation light may be applied collimated. Especially, a beam diameter of the excitation light may be at least 1 mm, at least 5 mm, or at least 1 cm. Thus, focusing of the excitation light can be omitted. A high energy concentration in a focus can be omitted. Thus, it can be prevented that the excitation light potentially produces bubbles already due to its energy without the presence of a particle that serves as an incubator. With an excitation light that is not collimated, the energy density is more evenly distributed. A beam diameter can be defined based on an area in cross section where the beam intensity exceeds a certain threshold, e.g. 90 %, 75 %, or 1 / e (corresponding to 0.37) of the maximum intensity. Especially, the excitation light may be applied as one single pulse. The pulse may have a certain length, wherein only during that pulse an excitation light is present. This allows for a specified timing of the process.

[0017] Especially, the ultrasonic wave may be applied after the end of the pulse. Thus, the ultrasonic wave is only applied when the pulse has already finished. The bubbles which have been generated will then expand.

[0018] Alternatively, it is possible to start the ultrasonic wave already during a light pulse. The ultrasonic wave and the light pulse can even be started concurrently or the ultrasonic wave may even be applied before starting of the light pulse. This is true irrespective of how may pulses are applied.

[0019] Especially, the excitation light may be applied as a series of pulses. Each such pulse of a series of pulses can be regarded as a single pulse as discussed above. Especially, a repetition rate of the series of pulses may be at least 1 kHz and / or at most 100 kHz. Thus, a nearly continuous observation is possible.

[0020] A respective ultrasonic wave pulse may especially be applied between consecutive pulses of excitation light. Thus, a respective ultrasonic wave pulse may always be applied when there is no excitation light present. After an end of the respective ultrasonic wave pulse or one ultrasonic wave pulse, a detection step can be performed. Then, the next pulse of the series of pulses of excitation light can be applied. Especially, there may be exactly one pulse of ultrasonic wave between two consecutive pulses of excitation light.

[0021] In an alternative implementation, the ultrasonic wave may be applied continuously. Any bubble generated by excitation light can thus instantly be expanded by the ultrasonic wave.

[0022] Especially, a pulse length of excitation light may be at least 0.1 ns and / or at most 100 ns. Such lengths have been proven to yield suitable bubbles that can be expanded afterward by the ultrasonic wave pulse. Especially, a time span between an end of a pulse of excitation light and start of an ultrasonic wave may be up to 10 ps and / or up to the time of the next pulse of excitation light. This allows the bubbles to expand already due to the excitation light, and then a further expansion of the bubbles is performed by the ultrasonic wave.

[0023] Especially, a laser light may be used as the excitation light. Such a laser light has been shown to yield good results, especially because it has comparatively high energy. Especially, the excitation light may have a wavelength in the visible range and / or in the near infrared range. Especially, it may have a wavelength of at least 300 nm and / or at most 1 ,200 nm. Especially, energy and / or wavelength of the laser light may be chosen such that the laser light is below an ionization threshold of the liquid medium. This prevents an inadvertent generation of bubbles without the presence of particles.

[0024] Especially, an ultrasonic wave pulse may comprise only a few cycle pulses, for example at least one and / or at most ten cycles. A minimum pressure amplitude may be a 0.1 MPa negative amplitude, so the pressure may in fact be a negative value smaller than 0.1 MPa with an absolute value of more than 0.1 MPa. The frequency of the ultrasonic wave may be at least 10 kHz or at least 100 kHz. It may be at most 1 MHz or at most 10 MHz. Such values have been proven suitable for typical applications. However, also other values can be used depending on the specific application.

[0025] The ultrasonic wave can be a continuous wave of a given frequency and / or it can be given by one or more acoustic pulses, like for instance a rarefaction pulse.

[0026] According to an implementation, the liquid medium may be supplied in a continuous flow. With such an implementation, a continuous measurement can be performed on a flowing medium. This allows, for example, a continuous observation and evaluation of a flow, for example a supply flow or a flow of a medium that is generated and / or used in some process.

[0027] According to an implementation, the liquid medium may be supplied static. This allows for a static measurement of some medium that does not flow. A static medium is especially not flowing. Especially, the ultrasonic wave may have a center frequency of at least 10 kHz and / or at most 10 MHz. Such values have been proven suitable for typical applications. However, also other values can be used.

[0028] Especially, the step of detecting may comprise taking at least one image of the one or more bubbles by at least one two-dimensional camera. With such a step, an image can be taken, which can then be evaluated. For example, automatic image recognition can be used. Alternatively, the image may also be shown to a human being who can evaluate the image.

[0029] A two-dimensional camera is a camera taking a two-dimensional picture. Instead of such a two-dimensional camera, also a three-dimensional camera can be used, which can also determine a distance to a detected object.

[0030] Especially, the step of detecting may comprise taking at least one first image of the one or more bubbles by at least one first two-dimensional camera and taking at least one second image of the one or more bubbles by at least one second two-dimensional camera. The first image and the second image may at least partially cover one identical volume of the liquid medium from different directions. Especially, these directions may be perpendicular to each other. This allows taking account of possible shadow effects, wherein it can happen that a particle and its corresponding bubble are located such that they cannot be viewed when viewing just from one direction, because another bubble is between a certain bubble and the camera. By taking at least two images from different directions, these effects can be omitted. A localization of the bubbles in a three- dimensional space can be performed. However, in certain situations, also taking only one image may be sufficient. This may especially be the case if there are not so many particles, so that the probability of shadow effects is low, and / or when only an indication of the number of particles and not an exact number of particles is required.

[0031] Especially, the method may further comprise a step of illuminating the liquid medium while taking an image. This can improve the detectability of the bubbles. Especially, the bubbles may significantly deflect and / or attenuate the light which is used for illuminating. This can increase the contrast. Especially, the step of detecting may comprise measuring an intensity of the ultrasonic wave after having traversed the liquid medium. This can give a further indication of the number of particles present. While the number of particles may not be directly observed using such an intensity measurement, it may be stated that a drop in the ultrasonic intensity is an indication for the number of particles because each particle typically generates a bubble as discussed above, and the bubbles attenuate the intensity which traverses the liquid medium. It should be noted that the measuring step and the corresponding evaluation can also be regarded as separate inventive principles.

[0032] According to an implementation, the step of detecting may comprise applying a probe light to the liquid medium after having started applying, or after the end of applying, an ultrasonic wave, and measuring an intensity of the probe light after having traversed the liquid medium.

[0033] Such an intensity of a probe light may also give an indication of the number of particles present. As discussed above with regard to the ultrasonic wave, the more particles are present, the more attenuated is the probe light. This can be regarded as a separate inventive principle.

[0034] Especially, an indication of a number of particles may be obtained based on a temporal attenuation of the intensity of the ultrasonic wave and / or of the intensity of the probe light. Such an indication may be obtained automatically. Especially, the intensity can be measured and there may be a known correlation between the intensity of the traversed ultrasonic wave and / or the intensity of the probe light and the number of particles. For example, such a correlation can be measured or calculated.

[0035] According to an implementation, a propagation direction of the excitation light may be perpendicular to a propagation direction of the ultrasonic wave. This allows placing the corresponding emitters in a convenient manner. A common volume may be defined where a volume traversed by the excitation light and a volume traversed by the ultrasonic wave overlap. This common volume can be used for detecting. According to an implementation, a propagation direction of the excitation light may be parallel to a propagation direction of the ultrasonic wave. Also in this case a common volume can be defined, where both the ultrasonic wave and the excitation light are present.

[0036] Especially, the excitation light may have an intensity of at least 1 MW / cm2With such an intensity, it has been shown that in typical situations of particles being present in a liquid medium like water, bubbles are generated with a sufficient probability that they can be detected, at least after expanding their size by using the ultrasonic wave.

[0037] Especially, the step of detecting may comprise counting a number of bubbles and / or determining sizes of bubbles around particles. By counting a number of bubbles an indication about the number of particles can be obtained. Counting a number of bubbles may also yield the number of particles present or observed. When the bubbles around particles are determined, this can give an indication about a size of the respective particle, as it is to be expected that a larger particle generates a larger bubble.

[0038] The invention relates further to an apparatus for particle detection in a liquid medium. The apparatus comprises a reservoir for the liquid medium. The apparatus comprises an excitation light source for applying an excitation light to liquid medium being present in the reservoir. The apparatus comprises an ultrasonic emitter for applying an ultrasonic wave to the liquid medium. The apparatus comprises a particle detection arrangement, and it further comprises a control unit. The control unit is preferably configured for performing a method as disclosed herein and / or to control some or all electric elements of the apparatus, especially the excitation light source, the ultrasonic emitter and / or the particle detection arrangement. With regard to the method, all disclosed embodiments and variations can be applied.

[0039] With such an apparatus, the method as disclosed above can preferably be performed.

[0040] Especially, bubbles can be generated around one or more particles, which can especially be present in the liquid medium in the reservoir. Especially the ultrasonic wave can extend the bubbles. In addition to the mentioned electric elements which the control unit may control, i.e. the excitation light source, the ultrasonic emitter, and / or the particle detection arrangement, it can also be configured to control other electric elements like a probe light source, an ultrasonic detector, a probe light detector, and / or one or more cameras.

[0041] The reservoir may be a volume that is enclosed such that the liquid medium may not flow out and / or that the liquid medium is kept inside. The excitation light source may be embodied to emit an excitation light. This excitation light may especially serve the purpose of generating bubbles, as already discussed above. Parameters already disclosed can be applied for the excitation light source. The same is true for the ultrasonic emitter.

[0042] The control unit may especially be a programmed or programmable entity like a processor or computer.

[0043] According to an implementation, the apparatus may further comprise an auxiliary reservoir being filled with a fluid. The reservoir may partially be submerged in the auxiliary reservoir. The ultrasonic emitter may emit its ultrasonic wave into the fluid in the auxiliary reservoir. Preferably, the fluid is de-ionized water. With such an implementation, it can be omitted that the ultrasonic emitter emits its ultrasonic waves directly into the reservoir for the liquid medium. Especially, it is possible to have a reduced portion of liquid with a sample to be analyzed, while wave generation and propagation can take place in a larger media. Deterioration of an ultrasonic transducer may be prevented.

[0044] According to an implementation, the ultrasonic emitter may directly emit ultrasonic waves into the reservoir, for example through an acoustically transparent window.

[0045] According to an implementation, the particle detection arrangement may comprise one or more two-dimensional cameras. With such cameras, images can be taken, for example as discussed above. The particle detection arrangement may further comprise a detector for detecting an intensity of the ultrasonic wave after having traversed the reservoir. The particle detection arrangement may also comprise a probe light source for applying a probe light to the liquid medium in the reservoir, and a detector for detecting an intensity of the probe light after having traversed the reservoir.

[0046] With such implementations, the detection methods as disclosed above can be used. For example, with the cameras, a direct detection of the bubbles that have been generated by particles can be performed. With a detector for detecting an intensity of the ultrasonic wave, the traversed ultrasonic wave can be used as an indicator of the number of particles. With a probe light source and a corresponding detector, the traversed probe light can be used as an indication of the number of particles.

[0047] It should especially be noted that the probe light is a light that traverses the liquid medium in bulk form and only the intensity of the probe light after traversal is relevant. There is typically no direct observation of single particles when using the probe light.

[0048] It is possible to describe a concept as follows. A reduced volume of a liquid sample doped with nanoparticles may be placed in a tailored, e.g. 3D printed, cuvette. The cuvette should have at least one side permeable to the sound waves. The sample may be exposed to a high-power laser pulse and bulk nanobubbles are produced on the particles. After a given time At, a piezoelectric ultrasonic transducer, for example being medical grade, may focus a shock / tension-wave on the cuvette and the nanobubbles may become visible, revealing the location of the particles. This kind of device can produce a pressure wave with high repeatability, leading to almost indistinguishable waves from one wave emission to the following. The pressure variation in the transducer (i.e. the shock wave generator) may be originated in a cubical water reservoir by the movement of a parabolic plate, and later focused on a reduced region of the liquid media at a fixed distance above the plate (close to the parabola focus).

[0049] For example, a tailored cuvette of 1 .5 cm of size may be held from its top from four points and may be partially submerged in a de-ionized water which transmits the pressure wave generated by a transducer. The particles may be suspended in the liquid sample. The bubbles may be observed from a direction perpendicular to the seeding laser beam. Then, a shock wave focusing may be performed. The seeding laser beam position may match the pressure focal region in the test cuvette. The setup may be designed to allow a microscope objective to get close enough to the cuvette in order to have its focal plane where the nanobubbles may be generated by the laser beam. The illumination in the shadowgraph images may be performed with a green pulsed laser. A cuvette design may be such that an acoustically permeable bottom of the cuvette may be given by a nylon film which lets the shock wave pass through the liquid column above without significant attenuation or distortion. The walls of the cuvette may be made from glass windows, while the bottom may be sealed with a threaded cap and a flat rubber “0” ring. The cuvette may be equipped with inlets to allow liquid circulation.

[0050] The bubbles may be induced by the laser beam on the nanoparticles. They may be sensitive to the specific shape of the shock / tension-waves used to expand the gas cavities, e.g., bubbles of different sizes may be expanded by different pressure amplitudes. That is why the acoustic wave can be perfectly characterized to have a correct calibration of the device. This can be done by an amplitude sweep.

[0051] For example, a laser pulse for generating bubbles may have an energy of at least 0.5 mJ and / or at most 50 mJ.

[0052] The method disclosed herein may especially allow direct observation down to a single particle in the observation volume. Dynamic light scattering may rely on the light scattered from many particles to obtain statistical averages. Compared to dynamic light scattering, the method disclosed herein has a significantly higher accuracy. Utilizing a laser together with a shock wave source may offer the detection of hydrophilic particles, which is impossible for a pure acoustic detection method. Hydrophilic silica and glass may be common particles found in ultra-clean processing of water in the semiconductor industry and thus may be relevant but undetectable when using concepts known in the prior art in ultra-clean processing of water at low particle counts. Utilizing specific laser wavelengths, energies, and pulse durations may make the device sensitive to certain particle types and sizes. Combining these laser parameters in a single device can allow distinguishing between those particle types and may work for a wider range of particle sizes and materials. The device may be used for nanoparticles or nanobubbles. It allows even to discriminate between both of these particle types by selectively switching on or off the laser beam. Especially, the method and the apparatus disclosed herein can be used in order to control or evaluate ultra-clean water. This is used, for example, in the pharmaceutical industry and in the semiconductor industry. It is possible to perform a detection even down to single particles. It is also possible to evaluate ultra-clean process water with regard to hydrophilic silica or glass particles, which have not been detectable in implementations known in the prior art. Such particles are especially relevant in the semiconductor industry. Different types of particles and particle sizes can be evaluated, for example by using different laser parameters.

[0053] It can be regarded as a function of the implementation disclosed herein to detect and count nanoparticles and / or nanobubbles suspended in a liquid solution. The implementation can be regarded as a hybrid system that can precisely detect both nanoparticles and nanobubbles, especially by using a high-power laser pulse as an optical probe and a high-intensity acoustic rarefaction pulse to reveal the particles' position. The measuring method is optical (which provides accuracy), but it does not require high image magnifications, making it possible to analyze higher volumes on a single-shot measurement compared with similar devices discussed above.

[0054] Especially, the ultrasonic pulse as discussed above may be regarded as a rarefaction pulse.

[0055] For example, the system may work following two basic steps. In a first step, a high power collimated laser beam passes through the sample. The particles in suspension would absorb the laser energy causing a sudden increase in the temperature and inducing the transition to a gas phase in their surface. The duration of the high power laser pulse is extremely short, i.e. in the order of nanoseconds, inducing bubbles on the nanoparticles which have also a nanometric scale. In a second step, immediately after the production of the nanobubbles on the particles, a rarefaction wave is focused on the same volume of fluid exposed to the laser light, and as a consequence the nanobubbles expand, revealing the position of the nanoparticles. Considering that the expanded nanobubbles now take a micrometric size, it is possible to visualize them with a standard photographic objective, for example with a magnification of three times or five times, covering a volume of several millimeters squared with each image. The rarefaction wave can be produced in a very repetitive way by a piezoceramic element focusing the sound wave in the liquid volume representing the region of interest (ROI). With a collimated laser beam, thousands of particles or any other number of particles can be heated simultaneously.

[0056] The two steps measuring concept can, for example, be applied to sealed cuvettes or cuvettes equipped with inlets that allow liquid circulation, for example, meaning that it supports both batch and inline measurements. The liquid container can be made from glass, quartz, or acrylic. The sound driving can be incorporated to the container or be placed outside as long as one of the cuvette boundaries has such acoustic properties to allow the transmission of the sound wave through it. Such an acoustic wave can be produced in a repetitive way using a piezoceramic element driven by an amplified voltage signal. The image acquisition could be performed by one or more standard CCDs triggered synchronically to the passage of their rarefaction wave and the illumination can be made with a low intensity and diffused pulsed laser, which would produce very defined shadowgraphic images of the expanded bubbles. The last component of the dual nanoparticle / nanobubble counter is given by a tailored software that would automatically calibrate the specific parameters of the system, for instance the measurement frequency, the laser energy, the ultrasound amplitude, the illumination laser intensity, etc., to obtain optimized measurements. Specifically, the software may analyze the images captured by the CCDs and perform statistics on the bubble count.

[0057] The implementation can be used in the whole spectrum of particle sizes, concentrations, and materials. This is because even if different particle sizes and materials would absorb the laser light differently, the laser pulse energy can always be adjusted to produce the tiny gas pocket required to force its expansion with the rarefaction wave, revealing the nanoparticle position. This is not the case for other particle detectors, in which the illumination light intensity needs to be adjusted to avoid saturation coming from the bigger particles, and consequently those systems pose a reduced dynamic range compared with the implementation described herein. Regarding the concentration range, the implementation disclosed herein can be used for measuring very low particle concentrations, especially compared with other systems known in the prior art. The implementation can be used as a nanobubble concentration analyzer, even when no particles are present in the sample. For example, if the sample has some nanobubbles in suspension, the implementation can be used without the need of firing the laser pulse, but using the rarefaction wave to expand the nanobubbles as described before. Furthermore, the size of the nanobubble distribution can be estimated by performing an acoustic probing consisting of sending a series of rarefaction waves of expanding intensity and evaluating the change in the number of bubbles observed in each case.

[0058] Further features and advantages will be apparent from the following description of embodiments, which is given with respect to the accompanying drawings.

[0059] Fig. 1 : shows a top view of an apparatus according to a first embodiment, fig. 2: shows a side view of the apparatus according to the first embodiment, fig. 3: shows a side view of an apparatus according to a second embodiment, and fig. 4: shows a side view of an apparatus according to a third embodiment.

[0060] Fig. 1 shows an apparatus 10 for particle detection in a liquid medium. The apparatus 10 comprises a reservoir 20 for liquid medium, wherein the reservoir is embodied as a flow cell through which a liquid medium 22 can flow, especially in the direction shown by arrows. The apparatus 10 comprises an excitation light source 30 for applying an excitation light 32, wherein the excitation light source 30 is embodied as a laser. The excitation light 32 is a laser light, which is first directed to a beam expansion optics 34, so that the excitation light 32 is widened, but is still collimated. The excitation light 32 is then passed through an aperture 36, and then on an optical window 24 in the reservoir 20. Through the optical window 24, the excitation light 32 can enter the reservoir 20 and can generate bubbles around particles that are present in the liquid medium 22.

[0061] The apparatus 10 further comprises an ultrasonic emitter 40, which is coupled to an acoustic window 26 of the reservoir 20. Thus, an ultrasonic wave 42 can be emitted into the liquid medium 22. Especially, such an ultrasonic wave 42 can be applied immediately after a pulse of the excitation light 32 has passed through the liquid medium 22. Bubbles generated by the excitation light 32 can thus be expanded, so that they can be better viewed by optical means.

[0062] The apparatus 10 further comprises a control unit 15, which is adapted to control all electric elements, especially the excitation light source 30 and the ultrasonic emitter 40. Furthermore, also the optic and acoustic means which will be described further below with reference to figs. 2 and 3 can be controlled by the control unit 15.

[0063] Fig. 2 shows a side view of the apparatus 10 according to the first embodiment. It is seen that the excitation light 32 has a circular cross-section, and that bubbles 23 are generated inside the region of the excitation light 32.

[0064] In order to detect the bubbles 23, the apparatus 10 further comprises a particle detection arrangement 50. The particle detection arrangement 50 comprises a light source 52, which emits light that enters the reservoir 20 through an optical window 28 in the reservoir 20. This light is used in order to illuminate the bubbles 23. In order to view the bubbles 23, the particle detection arrangement 50 further comprises a camera 54, which has an associated optics 56. The camera 54 views through an optical window 29 in the reservoir 20, wherein between the optical window 29 and the optic 56 there is positioned a filter 57 in order to block out the excitation light 32. The camera 54 can thus take pictures of the bubbles 23, which can further be evaluated, for example by using image recognition. Especially, the number of bubbles 23 can be counted.

[0065] Fig. 3 shows an alternative embodiment, wherein instead of the optical means shown in fig. 2 the particle detection arrangement 50 comprises an ultrasonic detector 58. This ultrasonic detector 58 can detect ultrasonic waves emitted by the ultrasonic emitter 40 that have transmitted the liquid medium 22 and reached the ultrasonic detector 58. It can be said that the more bubbles 23 are present in the liquid medium 22, the more attenuation of the ultrasonic wave can be expected. Thus, the intensity measured by the ultrasonic detector 58 is an indication of the number of bubbles 23.

[0066] Fig. 4 shows an alternative embodiment in a schematical side view, in which the ultrasonic emitter 40 is not directly attached to the reservoir 20, but to an auxiliary reservoir 27 which is filled with a fluid. The reservoir 20 is partially submerged in the auxiliary reservoir 27. Thus, an ultrasonic wave 42 is first emitted into the auxiliary reservoir 27 and then enters the reservoir 20. This allows for using a larger volume of fluid for generation and propagation of the ultrasonic wave, which is more suitable for certain types of ultrasonic transducers. The excitation light source 30 is positioned above the reservoir 20 and emits light downwards to the reservoir 20. This is done comparable to the implementation shown in fig. 1 , but vertically instead of horizontally.

[0067] In the implementation shown in fig. 4, the reservoir 20 supplies the liquid medium 22 static, i.e. , without flow. A camera, which is not shown in fig. 4, may view generated bubbles with a horizonal viewing direction.

[0068] It should be noted that the implementations described with respect to the first, second and third embodiments can also be combined.

[0069] The apparatus 10 according to at least one of the embodiments thus provides for a reliable and efficient detection of small particles in the liquid medium 22, which can, for example, be used in order to evaluate ultra-clean water or other liquid substances.

[0070] Mentioned steps of the inventive method can be performed in the given order. However, they can also be performed in another order, as long as this is technically reasonable.

[0071] The inventive method can, in an embodiment, for example with a certain combination of steps, be performed in such a way that no further steps are performed. However, also other steps may be performed, including steps that are not mentioned.

[0072] It is to be noted that features may be described in combination in the claims and in the description, for example in order to provide for better understandability, despite the fact that these features may be used or implemented independent from each other. The person skilled in the art will note that such features can be combined with other features or feature combinations independent from each other.

[0073] References in dependent claims may indicate preferred combinations of the respective features, but do not exclude other feature combinations. List of reference signs

[0074] 10 apparatus

[0075] 15 control unit

[0076] 20 reservoir

[0077] 22 liquid medium

[0078] 23 bubbles

[0079] 24 optical window

[0080] 26 acoustic window

[0081] 27 auxiliary reservoir

[0082] 28 optical window

[0083] 29 optical window

[0084] 30 excitation light source

[0085] 32 excitation light

[0086] 34 beam expansion optics

[0087] 36 aperture

[0088] 40 ultrasonic emitter

[0089] 42 ultrasonic wave

[0090] 50 particle detection arrangement

[0091] 52 light source

[0092] 54 camera

[0093] 56 optics

[0094] 57 filter

[0095] 58 ultrasonic detector

Claims

Patent Claims1. Method for particle detection in a liquid medium (22), the method comprising the following steps: applying an excitation light (32) to the liquid medium (22), the excitation light (32) generating a respective bubble (23) around one or more particles, applying an ultrasonic wave (42) to the liquid medium (22) after having started applying the excitation light (32), the ultrasonic wave (42) expanding the one or more bubbles (23), and detecting one or more particles by detecting one or more bubbles (23) after having started applying the ultrasonic wave (42).

2. Method according to claim 1 , wherein the excitation light (32) is applied collimated.

3. Method according to one of the preceding claims, wherein the excitation light (32) is applied as one single pulse, and / or wherein the ultrasonic wave (42) is applied after the end of the pulse.

4. Method according to one of claims 1 or 2, wherein the excitation light (32) is applied as a series of pulses, and / or wherein a respective ultrasonic wave pulse is applied between consecutive pulses of excitation light (32).

5. Method according to one of claims 3 or 4, wherein a pulse length of the excitation light (32) is at least 0.1 ns and / or at most 100 ns, and / or wherein a time span between end of a pulse of excitation light (32) and start of an ultrasonic wave (42) is up to 10 ps.

6. Method according to one of the preceding claims, wherein the step of detecting comprises taking at least one image of the one or more bubbles (23) by at least one two-dimensional camera (54).

7. Method according to one of the preceding claims, wherein the step of detecting comprises taking at least one first image of the one or more bubbles (23) by at least one first two-dimensional camera, and taking at least one second image of the one or more bubbles (23) by at least one second two-dimensional camera, wherein the first image and the second image at least partially cover one identical volume of the liquid medium (22) from different directions.

8. Method according to one of claims 6 or 7, further comprising illuminating the liquid medium (22) while taking an image.

9. Method according to one of the preceding claims, wherein the step of detecting comprises measuring an intensity of the ultrasonic wave (42) after having traversed the liquid medium (22).

10. Method according to one of the preceding claims, wherein the step of detecting comprises applying a probe light to the liquid medium (22) after having started applying, or after the end of applying, an ultrasonic wave (42), and measuring an intensity of the probe light after having traversed the liquid medium (22).11 . Method according to one of claims 9 or 10, wherein an indication of a number of particles is obtained based on a temporal attenuation of the intensity of the ultrasonic wave (42) and / or of the intensity of the probe light.

12. Method according to one of the preceding claims, wherein the step of detecting comprises counting a number of bubbles (23) and / or determining sizes of bubbles (23) around particles.

13. Apparatus (10) for particle detection in a liquid medium (22), the apparatus (10) comprising: a reservoir (20) for the liquid medium (22), an excitation light source (30) for applying an excitation light (32) to liquid medium (22) being present in the reservoir (20), an ultrasonic emitter (40) for applying an ultrasonic wave (42) to the liquid medium (22), a particle detection arrangement (50), and a control unit (15) being configured for performing a method according to one of the preceding claims and / or to control some or all electric elements of the apparatus (10).

14. Apparatus (10) according to claim 13, further comprising an auxiliary reservoir (27) being filled with a fluid, wherein the reservoir (20) is partially submerged in the auxiliary reservoir (27), and wherein the ultrasonic emitter (40) emits the ultrasonic wave (42) into the fluid in the auxiliary reservoir (27).

15. Apparatus (10) according to one of claims 13 or 14, wherein the particle detection arrangement (50) comprises one or more of the following items: one or more two-dimensional cameras (54), and / or an ultrasonic detector (58) for detecting an intensity of the ultrasonic wave (42) after having traversed the reservoir (20), and / or a probe light source for applying a probe light to the liquid medium (22) in the reservoir (20), and a detector for detecting an intensity of the probe light after having traversed the reservoir (20).

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