Classifying – separating – and assorting solids – Plural – diverse separating operations – Aqueous suspension and stratifying
Reexamination Certificate
2000-01-20
2001-12-25
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Plural, diverse separating operations
Aqueous suspension and stratifying
C209S160000
Reexamination Certificate
active
06332541
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for performing the manipulation of particles suspended in a fluid, using an acoustic standing wave field.
2. State of the Art
When particles suspended in a fluid are subjected to an acoustic standing wave field, the particles displace to the location of the standing wave nodes. The effectiveness of this process varies with the relative densities and compressibilities of the particles and the suspending fluid.
A number of techniques have been proposed, using this phenomenon, to separate particles from a liquid or other fluid. Typically, the fluid is caused to flow through a duct in which an acoustic standing wave field is established, transverse to the length of the duct. The particles accordingly displace to form a series of parallel bands, and then a number of outlet passages are provided to lead the individual bands of particles away from the main flow duct. Because there are engineering difficulties involved in providing a parallel array of narrow outlet passages to collect the particle bands, the tendency is to operate at relatively low frequencies, so that the wavelength of the standing wave field is sufficiently large to provide an adequate spacing between the particle bands (half-wavelength spacing).
The primary acoustic radiation force on a single particle in an acoustic standing wave field is proportional to the operating frequency. Also the distance which a particle needs to move to reach a node decreases with increasing frequency (because the wavelength is smaller and hence the spacing between nodes is smaller). It is therefore easier to concentrate particles (including biological cells) at higher operating frequencies. Ultrasonic cavitation is also less likely to limit the applicable acoustic pressure at higher frequencies. However, the use of higher frequencies, and therefore smaller wavelengths, increases the engineering difficulties involved in providing outlet passages for the individual particle bands. In some cases, instead of separating particles from the suspending fluid, it may be required to form the particles into their bands for the purpose of observation (e.g. for the purpose of an immuno-agglutination assay, as described in GB-2265004). For such cases, it will be appreciated that the particle bands are closer together at the higher frequencies, and therefore even more difficult to observe.
We have now devised an apparatus and method which overcome the difficulties noted above, and can be used whether the particles are to be separated from the suspending fluid or whether they are to be formed into their bands for observation purposes.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an apparatus for performing the manipulation of particles suspended in a fluid, the apparatus comprising a duct for the flow of a fluid in which particles are suspended, and means for establishing an acoustic standing wave field across the width of the duct, the duct being formed with an expansion in width downstream of the standing wave field.
In use of this apparatus, the particles in the fluid are displaced into a series of parallel bands by the acoustic standing wave field. The particles remain in these bands as the fluid flows downstream from the section in which the standing wave field is present. When the fluid reaches the expansion of the duct, the stream of fluid expands accordingly in width and in so doing the bands of particles are spread further apart, so increasing the spacing between adjacent bands.
In passing further along the duct, the particle bands retain their increased spacing. The bands can now either be observed, or they can be separated from the duct.
Preferably the duct includes an initial elongate section in which a laminar flow of the fluid is established, before the fluid enters the acoustic standing wave field.
The expansion of the duct is preferably formed by a section of the duct which progressively increases in width over the length of that section. Downstream of the expansion section, the duct preferably comprises an elongate section of uniform cross-section, in which the laminar flow of fluid is maintained.
It will be appreciated that by expanding the separation of the particle bands, these bands become substantially easier to observe and/or to separate from the fluid. Ultrasound of higher frequencies than hitherto can be used, preferably greater than 2 MHz. Preferably means are provided downstream of the expansion section for observing the particle bands, or for separating them from the main duct.
The duct may be formed with at least one outlet passage which extends outwardly at an inclined angle, thus forming the expansion in width of the duct. The fluid adjacent the side of the duct from which the outlet passage diverges, and outwardly of the adjacent band of particles, now passes out of the duct along the outlet passage.
Preferably each outlet passage is provided with a valve. When the valve is closed, all of the fluid passes along the duct, without widthwise expansion and corresponding increased separation of the particle bands. As the valve is progressively opened, progressively more of the fluid is able to flow through the outlet passage, so effectively increasing the expansion of the fluid stream and passing an increasing portion of that stream out from the main flow duct. Alternatively or in addition, each outlet passage may be provided with a pump the flow rate of which is independently adjustable.
Also in accordance with the present invention, there is provided a method of performing the manipulation of particles suspended in a fluid, the method comprising causing the fluid to flow along a flow duct, establishing an acoustic standing wave field transversely of the duct, and providing a widthwise expansion of the stream of fluid downstream of the standing wave field.
Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings.
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IBM Technical Disclosure vol. 25, No. 1, Jun. 1982 discloses ultrasonic continuous flow plasmapheresis separator, p. 192-193.*
Article entitled “Enhanced synchronized ultrasonic and flow-field fractionation of suspensions” in Ultrasonics 1994, vol. 32, No.2, pp. 113-121.
Barrow David Anthony
Cefai Joseph
Coakley William Terence
Hawkes Jeremy John
Gallagher Thomas A
Gordon David P.
Jacobson David S
Rodriguez Joseph C
University College Cardiff Consultants Ltd
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