WO2008083431A1

WO2008083431A1 - Flow control of ferrofluidic pumps

Info

Publication number: WO2008083431A1
Application number: PCT/AU2008/000007
Authority: WO
Inventors: Cedric Robillot; Brett Thomas Kettle; Klaus Stefan Drese; Dalibor Dadic
Original assignee: Cleveland Biosensors Pty Ltd
Priority date: 2007-01-08
Filing date: 2008-01-07
Publication date: 2008-07-17

Abstract

A ferrofluidic pump incorporating flow control by varying cross-sectional area in the chamber containing the ferrofluid. Also a method of operating a ferrofluidic pump that gives good performance over a broad range of flow rates.

Description

FLOW CONTROL OF FERROFLUIDIC PUMPS

This invention relates to flow control in microassay devices. In particular, it relates to the design of a ferrofluidic pump for flow control.

BACKGROUND TO THE INVENTION

A ferrofluidic pump employed in a microassay device is described in our co-pending international application titled "Microfluidic Device" (publication number WO2006/034525). Ferrofluidic pumps are based on using an external magnetic actuator to move a slug of ferrofluid trapped inside a channel. The flow rate is determined by the cross-section of the pump channel and the speed of the external actuator. For practical reasons it is difficult to produce a pump which can achieve both high flow rates and low flow rates while retaining precision and smoothness.

One suitable type of magnetic actuator is a permanent magnet moved by a stepper motor. A stepper motor produces pulsed flow as the motor is stepped but the pulsed flow approaches continuous flow at high step frequency. At low step frequency the pulsing is significant and limits the usefulness of the pump.

Notwithstanding the limitations, a stepper motor is the most useful motor for microfluidic pump applications.

For maximum operating flexibility it is desirable to have continuous flow (not pulsed) at all flow rates. Various combinations of motors and gears can be designed to adjust the pumping rate but none of the solutions achieve the required flexibility and precision. The pump can be designed for good performance at high flow rate but it will have less than optimal performance at low flow rate. Conversely, a pump designed for good performance at low flow rate cannot achieve the required high flow rate performance.

Although the problem is exacerbated by stepper motor pumps other motors suffer from the same problem of being unable to provide flexibility while maintaining precision and smoothness across a wide operating range.

OBJECT OF THE INVENTION It is an object of the present invention to provide a ferrofluidic pump that is capable of acceptable performance at both high and low flow rate.

Further objects will be evident from the following description.

DISCLOSURE OF THE INVENTION In one form, although it need not be the only or indeed the broadest form, the invention resides in a ferrofluidic pump comprising: a chamber having a larger cross-sectional area portion, a lower cross- sectional area portion, and an intermediate transitional zone; a slug of ferrofluid movable in the chamber between the larger area portion and the smaller area portion; and an externally generated movable magnetic field to actuate the movement of the ferrofluid.

Preferably the transitional zone incorporates a trap able to retain a portion of the ferrofluid. The externally generated movable magnetic field is suitably generated by a permanent magnet or electromagnet moved by a motor.

In a further form the invention resides in a method of moving a fluid through a chamber with a ferrofluidic pump by: moving a ferrofluid through a larger cross-sectional area portion of a chamber under the influence of an externally generated magnet field to thereby cause the fluid to move at a first flow rate; moving the ferrofluid through a transitional zone; and moving the ferrofluid through a smaller cross-sectional area portion of the chamber under the influence of the externally generated magnet field to cause the fluid to move at a second flow rate; wherein the second flow rate is less than the first flow rate.

Suitably the external magnetic field is moved at a constant velocity.

It will be appreciated that in a further form the ferrofluid can be moved from the smaller cross-sectional area portion to the larger cross- sectional area portion so to increase the flow rate.

BRIEF DETAILS OF THE DRAWINGS

To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which: FIG 1 is a sketch of a first embodiment of a ferrofluidic pump;

FIG 2 shows the sequence of operation of the pump of FIG 1 ;

FIG 3 is a sketch of a second embodiment of a ferrofluidic pump;

FIG 4 is a sketch of a third embodiment of a ferrofluidic pump;

FIG 5 shows the sequence of operation of a fourth embodiment of a ferrofluidic pump; and

FIG 6 shows the sequence of operation of a fifth embodiment of a ferrofluidic pump.

DETAILED DESCRIPTION OF THE DRAWINGS In describing different embodiments of the present invention common reference numerals are used to describe like features.

Referring to FIG 1 there is shown a ferrofluidic pump generally indicated as 10. The ferrofluidic pump 10 may be part of a microassay device such as described in our co-pending application referenced above. The inventors envisage that the invention will find best application in closed loop devices but the invention is not limited to that application.

The pump 10 consists of a chamber 11 having a larger cross- sectional area portion 12 and a smaller cross-sectional area portion 13 with an intervening transition zone 14. A movable external magnet 15 drives a slug of ferrofluid 16 through the chamber to produce a motive force in channels connected to the pump. In a closed loop device, such as that described in our co-pending application, the pump generates a motive force that both pulls and pushes fluid through the connected channels.

It will be understood that the rate at which fluid flows through adjoining channels is dependent upon the volume displaced by the ferrofluid. If the external magnet is moved at a constant speed it is clear that the volume displaced when the ferrofluid is in the larger cross- sectional area portion 12 will be greater than the volume displaced when the ferrofluid is in the smaller cross-sectional area portion 13.

The chamber 11 is normally formed with a constant circular cross- section in each portion, although it is not essentially circular. In some applications it may be appropriate for the chamber to have an elliptical or rectangular cross-section. It may even be appropriate for the chamber to change cross-section from one end to the other.

The ferrofluid 16 is moved under the influence of the magnetic field of an external magnet 15 which is in turn moved by a motor (not shown). As will become clear from the following description, the motor can operate at a relatively high speed so as to provide a smooth and precise motive force. Other sources of motive force, such as an electromagnet, will also be suitable.

The operation of the pump 10 is best explained by reference to FIG 2. In FIG 2(a) the ferrofluid 16 is located in the larger cross-sectional area portion 12 of the chamber 11 and the magnet 15 is moving towards the smaller cross-sectional area portion 13. As the ferrofluid enters the transitional zone 14 as shown in FIG 2(b) it is divided by a wall 17. As the magnet 15 continues to move the ferrofluid 16 a portion of the ferrofluid is trapped in an pocket 18 and the remainder enters the smaller cross- sectional area portion 13, as seen in FIG 2(c). In FIG 2(d) it is seen that the magnet 15 continues to move so that a small amount of the ferrofluid 16 moves through the smaller cross-sectional area portion 13 while the remainder is trapped in the pocket 18.

It is useful if the wall 17 extends beyond the length necessary to capture the excess ferrofluid in the pocket 18. This is because the ferrofluid can jump out of the pocket due to attractive magnetic forces. The arrangement shown in FIG 2 reduces this tendency.

It is clear from a comparison of FIG 2(a) and FIG 2(d) that if the pump 10 is coupled to a microassay device it will generate a larger flow rate when the ferrofluid is in the larger cross-sectional area portion 12 than when it is in the smaller cross-sectional area portion 13, for a fixed speed of movement of the magnet 15. It is therefore possible to smoothly and precisely control the rate of flow in a microassay device by design of the geometry of the chamber 11 of the pump 10.

The particular geometry shown in FIG 2 is merely one possible configuration. Persons skilled in the art will be able to devise other geometries that operate on the same inventive principle. For example, two other geometries are shown in FIG 3 and FIG 4. The geometry of FIG 3 operates in similar manner to FIG 2. The geometry of FIG 4 is useful when a significant reduction in flow rate is required. As shown in FIG 4, the transition zone may include a first pocket 41 that retains a portion 42 of ferrofluid 44 and a second pocket 43 that retains a further portion 44 of ferrofluid 45.

The pump can also be configured for an increase in flow rate. Referring to FIG 5 there is shown a pump 50 comprising a chamber 51 having a smaller cross-sectional area portion 52, a larger cross-sectional area portion 53 and an intervening transition zone 54. A movable external magnet 55 drives a slug of ferrofluid 56 through the chamber to produce a motive force in channels connected to the pump. It will be noted that the ferrofluid 56 occupies a larger part of the smaller cross-sectional area portion 52 than is covered by the magnet 55 (FIG 5(a)). However, as the magnet 55 is moved through the transitional zone 54 (FIG 5(b)) to the larger cross-sectional area portion 53 (FIG 5(c)) the ferrofluid 56 occupies a space above the magnet 55. The transitional zone in FIG 5 is shown as a simple expansion but it will be appreciated that the transition zone structure could be the same as the structure shown in FIG 2 (or indeed any of the figures). The pocket 18 could be pre-loaded with ferrofluid so that movement of the magnet from the smaller cross-sectional area portion to the larger cross-sectional area portion would pick up the extra ferrofluid from the pocket.

More complex pumps can also be constructed, such as the pump shown in FIG 6. The pump 60 combines a "flow rate reducing" pump 10 of the form described above with reference to FIG 2 with a "flow rate increasing" pump 50 of the form described above with reference to FIG 5.

In FIG 6 the pump 60 comprises a chamber 61 having a larger cross-sectional area portion 62, a smaller cross-sectional area portion 63 and an intermediate cross-sectional area portion 64. A first transitional zone 65 links the larger cross-sectional area portion 62 and a smaller cross-sectional area portion 63. A second transitional zone 66 links the smaller cross-sectional area portion 63 and the intermediate cross- sectional area portion 64. An external magnet 67 moves a slug of ferrofluid 68 through the chamber 61.

Note that in FIG 6(d) the slug of ferrofluid that is being moved through the smaller cross-sectional area portion is longer than the width of the magnet. This is necessary so that the ferrofluid can span the intermediate cross-sectional area portion 64, as shown in FIG 6(T).

It should be appreciated that the ferrofluidic pump is completely reversible. It can be seen, particularly by reference to FIG 6, that reversing the pump will move the slug of ferrofluid from the portion 64 through the second transition zone 66 into smaller cross-sectional area portion 63. As the ferrofluid moves through the first transition zone 65 the portion of ferrofluid trapped in the pocket is picked up to return the pump 60 to the starting position shown in FIG 6(a). Reversibility is not limited to the embodiment of FIG 6 but applies to each of the embodiments described above as well as variations falling within the scope of the broad description. The ferrofluid pumps described above offer a level of flexibility not currently available. Careful design of the pump chamber allows for increase and decrease of the flow rate of a fluid in an associated microassay device.

Throughout the specification the aim has been to describe the invention without limiting the invention to any particular combination of alternate features.

Claims

I . A ferrofluidic pump comprising: a chamber having a larger cross-sectional area portion, a lower cross- sectional area portion, and an intermediate transitional zone; a slug of ferrofluid movable in the chamber between the larger cross- sectional area portion and the smaller cross-sectional area portion; and an externally generated movable magnetic field to actuate the movement of the ferrofluid.

2. The ferrofluidic pump of claim 1 wherein the transitional zone incorporates a trap able to retain a portion of the ferrofluid.

3. The ferrofluidic pump of claim 2 wherein the trap comprises a pocket.

4. The ferrofluidic pump of claim 2 wherein the trap is bound by a wall that defines a pocket with a wall of the chamber.

5. The ferrofluidic pump of claim 2 wherein the trap comprises two or more pockets.

6. The ferrofluidic pump of claim 5 wherein each pocket is bound by a wall.

7. The ferrofluidic pump of claim 2 wherein the trap is pre-loaded with ferrofluid.

8. The ferrofluidic pump of claim 1 comprising a permanent magnet moved by a motor that generates the movable magnetic field.

9. The ferrofluidic pump of claim 1 comprising an electromagnet that generates the movable magnetic field.

10. The ferrofluidic pump of claim 1 having a varying cross-section in each portion.

I 1. The ferrofluidic pump of claim 1 wherein the externally generated movable magnetic field is reversible to actuate the movement of the ferrofluid from the larger cross-sectional area portion to the lower cross- sectional area portion or from the lower cross-sectional area portion to the larger cross-sectional area portion.

12. A method of moving a fluid through a chamber with a ferrofluidic pump by: moving a ferrofluid through a first cross-sectional area portion of the chamber under the influence of an externally generated magnet field to thereby cause the fluid to move at a first flow rate; moving the ferrofluid through a transitional zone; and moving the ferrofluid through a second cross-sectional area portion of the chamber under the influence of the externally generated magnet field to cause the fluid to move at a second flow rate; wherein the second flow rate is different to the first flow rate.

13. The method of claim 12 wherein the first cross-sectional area is larger than the second cross-sectional area so that the second flow rate is slower than the first flow rate.

14. The method of claim 12 wherein the first cross-sectional area is smaller than the second cross-sectional area so that the second flow rate is faster than the first flow rate.

15. The method of claim 12 wherein a portion of the ferrofluid is trapped in the transitional zone.

16. The method of claim 15 wherein the portion of the ferrofluid is trapped in an pocket.

17. The method of claim 15 wherein the portion of the ferrofluid is trapped in two or more pockets.

18. The method of claim 12 wherein the external magnetic field is moved at a constant velocity.

19. The method of claim 12 wherein the externally generated magnetic field is generated by a permanent magnet moved by a motor.

20. The method of claim 12 wherein the externally generated magnetic field is generated by an electromagnet.

21. A ferrofluidic pump comprising: a chamber having a larger cross-sectional area portion, an intermediate cross-sectional area portion and a lower cross-sectional area portion; a first transitional zone between the larger cross-sectional area portion and the lower cross-sectional area portion; a second transitional zone between the lower cross-sectional area portion and the intermediate cross-sectional area portion; a slug of ferrofluid movable in the chamber through the larger cross- sectional area portion, the smaller cross-sectional area portion, and the intermediate cross-sectional area portion; and an externally generated movable magnetic field to actuate the movement of the ferrofluid.

22. The ferrofluidic pump of claim 21 wherein the first transitional zone incorporates a trap able to retain a portion of the ferrofluid.

23. The ferrofluidic pump of claim 21 wherein the second transitional zone incorporates a trap able to retain a portion of the ferrofluid.

24. The ferrofluidic pump of claim 21 wherein the externally generated movable magnetic field is reversible to actuate the movement of the ferrofluid from the larger cross-sectional area portion to the intermediate cross-sectional area portion or from the intermediate cross-sectional area portion to the larger cross-sectional area portion.