Droplet generator

1. A method of controlling the formation of droplets of a droplet fluid composition from a jet of the droplet fluid composition, the method comprising providing a microfluidic device having at least one channel for the passage of the droplet fluid composition leading via an orifice to a droplet receiving space, providing a perturbing means within the channel that passively creates a periodic flow instability within the channel that controls the formation of droplets in the fluid composition, causing the droplet fluid composition to pass through the channel at sufficient velocity to form a jet of said fluid emanating from the orifice, and controlling the formation of droplets received in the droplet receiving space by creating a passive periodic flow instability in the droplet fluid composition by causing the droplet fluid composition to flow by the perturbation means as the droplet fluid composition passes through the channel.

2. The method as claimed in claim 1 , the method controlling the formation of droplets of a droplet fluid composition in a vacuum or a carrier phase, wherein the vacuum or carrier fluid phase are contained within the droplet receiving space.

3. The method as claimed in claim 1 , wherein the carrier phase is air and the droplet composition is a liquid.

4. The method as claimed in claim 1 , wherein the droplet fluid composition is a single droplet fluid phase.

5. The method as claimed in claim 1 , wherein the flow instability is such that vortices are periodically shed.

6. The method as claimed in claim 1 , wherein the perturbation means for passively causing a flow instability within the channel is a bluff body.

7. The method as claimed in claim 6 , wherein the bluff body is a pillar formed within the channel.

8. The method as claimed in claim 6 , wherein the bluff body is capable of oscillating within the channel in response to fluid flow.

9. The method as claimed in claim 1 , wherein the droplet fluid composition is an aqueous phase composition.

10. The method as claimed in claim 1 , wherein the droplet fluid composition has particles, reagents or components dissolved and/or dispersed therein.

11. The method as claimed in claim 1 , which is a method for generating droplets of a droplet fluid composition, wherein the range of size dispersity of the droplets formed is, at half height on the distribution curve, +/−5% based on the mean droplet size.

12. The method as claimed in claim 1 , wherein the droplet fluid composition comprises at least two phases, an outer fluid in contact with the inner surface of the channel and an inner fluid which populates interior portion of the channel, and wherein the perturbing means is provided such as to cause flow instability primarily in the outer fluid whereby the inner fluid remains relatively unperturbed until drop formation occurs on passing through the orifice when the flow instability induced in the outer fluid takes effect in influencing drop formation.

13. The method as claimed in claim 1 , which is for generating droplets for continuous inkjet printing.

14. The method as claimed in claim 1 , which is for generating droplets for spray drying.

15. The method as claimed in claim 1 , which is for generating droplets for crop spraying.

16. The method as claimed in claim 1 , which is for generating droplets for nebulising inhalable medicines.

17. The method as claimed in claim 1 , for use in the manufacture of pharmaceuticals.

18. The method as claimed in claim 1 , the periodic flow instability of the fluid flow having a frequency, wherein the frequency of the periodic flow instability is within an order of magnitude of the Rayleigh frequency for the jet of the fluid emanating from the orifice.

19. A microfluidic device for forming droplets of a droplet fluid composition, the device comprising at least one channel for the passage of said droplet fluid composition, at least one outlet orifice leading to a droplet receiving space and a means for creating a flow velocity of the droplet fluid within the channel, wherein a perturbation means is provided within the at least one channel that passively creates a periodic flow instability of fluid passing through the channel as the fluid flows by the perturbation means to control in a regular manner formation of droplets of fluid from a jet of said fluid exiting the orifice into the droplet receiving space.

20. A microfluidic device as claimed in claim 19 , wherein the perturbation means is provided by a geometric arrangement of two or more channels within the device.

21. A microfluidic device as claimed in claim 19 , wherein the perturbation means is provided by at least one bluff body positioned within the at least one channel.

22. A microfluidic device as claimed in claim 21 , wherein the perturbation means is provided by a single bluff body positioned within the at least one channel.

23. A microfluidic device as claimed in claim 21 , wherein the bluff body is a pillar.

24. A microfluidic device as claimed in claim 21 , wherein the bluff body is capable of oscillating within the channel in response to the fluid flow.

25. A microfluidic device as claimed in claim 19 , which further comprises a locking means for providing a locking perturbation to phase lock one or more parallel flow instabilities.

26. A microfluidic device as claimed in claim 25 , wherein the locking means is an active perturbation means.

27. A microfluidic device as claimed in claim 19 , wherein the perturbation means for passively creating flow instability is positioned fifteen channel widths or less from the orifice.

28. A microfluidic device as claimed in claim 27 , wherein the perturbation means is positioned ten channel widths or less, preferably five channel widths or less from the orifice.

29. A microfluidic device as claimed in claim 19 , wherein the perturbation means comprises a bluff body protruding part way into the channel from a channel wall whereby it is capable of inducing a flow instability primarily in an outer portion of a droplet fluid composition.

30. A microfluidic device as claimed in claim 19 , the perturbation means being,nrovided by at least one bluff body positioned within the at least one channel, wherein the bluff body is positioned such that at the flow velocity it causes the formation of a vortex street.

31. A microfluidic device assembly comprising a plurality of microfluidic devices as defined in claim 19 arranged in parallel or in series or a combination of both.

32. A continuous inkjet printhead comprising a microfluidic device for generating droplets of an inkjet ink, said microfluidic device being as defined in claim 19 .

33. A nebuliser comprising at least one microfluidic device as defined in claim 19 .

34. Use of a microfluidic device, comprising at least one channel for the passage of fluid, at least one outlet orifice and a perturbation means positioned within the channel that passively creates a periodic flow instability within the channel that controls the formation of droplets of a droplet fluid phase into a droplet receiving space, by passing the droplet fluid phase through the device at a velocity sufficient to cause a jet of said fluid to emanate from the outlet orifice and to induce the perturbation means to create fluid flow instability in the channel by causing the droplet fluid phase to flow by the perturbation means as the droplet fluid phase passes through the channel.

35. A use as claimed in claim 34 , in which the fluid flow instability involves the shedding of vortices from the perturbing means.

36. A use as claimed in claim 35 , in which the vortices shed from the perturbing means cascade through the orifice.

37. A method of controlling the formation of droplets of a droplet fluid composition from a jet of the droplet fluid composition, the method comprising providing a microfluidic device having at least one channel for the passage of the droplet fluid composition leading via an orifice to a droplet receiving space, providing a perturbing means for passively causing a flow instability within the channel, and causing the droplet fluid composition to pass through the channel at sufficient velocity to form a jet of said fluid emanating from the orifice whereby the fluid flow may be perturbed by the perturbation means for passively causing a flow instability thereby influencing the formation of droplets received in the droplet receiving space, wherein the droplet fluid composition comprises at least two phases, an outer fluid in contact with the inner surface of the channel and an inner fluid which populates interior portion of the channel, and wherein the perturbing means is provided such as to cause flow instability primarily in the outer fluid whereby the inner fluid remains relatively unperturbed until drop formation occurs on passing through the orifice when the flow instability induced in the outer fluid takes effect in influencing drop formation.

38. A microfluidic device for forming droplets of a droplet fluid composition the device comprising at least one channel for the passage of said droplet fluid composition, at least one outlet orifice leading to a droplet receiving space and a means for creating a flow velocity of the droplet fluid within the channel, wherein the at least one channel is provided with a perturbation means for passively creating flow instability of fluid passing through the channel whereby droplets of fluid are formed from a jet of said fluid exiting the orifice into the droplet receiving space in a regular manner that is influenced by creation of flow instability in the fluid, wherein the microfluidic device further comprises a locking means for providing a locking perturbation to phase lock one or more parallel flow instabilities.