Method and apparatus to produce ions and nanodrops from taylor cones of volatile liquids at reduced pressures

• US 20030066969A1
• Filed: 10/04/2002
• Published: 04/10/2003
• Est. Priority Date: 10/05/2001
• Status: Active Grant

• Current Assignee: Yale University
• Original Assignee: Yale University
• Inventors: De La Mora, Juan Fernandez
• Granted Patent: US 6,825,464 B2
• Time in Patent Office: Days
• US Class Current: 250/423R
• CPC Class Codes: B64G 1/402 Propellant tanks; Feeding propellants
• CPC Class Codes: B64G 1/405 Ion or plasma engines per se F03H1/00
• CPC Class Codes: B64G 1/413 Ion or plasma engines
• CPC Class Codes: F03H 1/0012 Means for supplying the propellant
• CPC Class Codes: H01J 49/165 Electrospray ionisation

Abstract

The present invention concerns the formation of a stable electrospray of a volatile liquid at reduced pressures by limiting its tendency to evaporate. In a first approach, multi-component capillary jets are produced, comprising a thin layer of low volatility liquid, which is not necessarily a good conductor, surrounding a core of volatile and conducting liquid such as water, so as to minimize direct exposure of the volatile liquid to the region of low pressure. In a second approach, the diameter of the meniscus of a Taylor cone is reduced to a critical diameter, below which no evaporative freezing occurs.

Claims

1. A method of forming a stable electrospray of a volatile liquid in a low pressure environment comprising the steps of:

a. supplying one or more liquids into an emitter electrode located in a low pressure environment, wherein one of said liquids is a volatile liquid, and

b. establishing a voltage difference between said emitter electrode and one or more surrounding electrodes or grids, wherein a meniscus formed by said one or more liquids supplied to said emitter electrode forms one or more Taylor cones, from whose tip region at least one of drops and ions of said one or more liquids are ejected,

wherein a tendency of said volatile liquid to freeze, boil, or evaporate is diminished by a suitable evaporation-reduction means.

2. A method according to claim 1, wherein two or more liquids are supplied into said emitter electrode, wherein at least one of said liquids is a volatile liquid and another of said liquids is an involatile liquid, and said evaporation-reduction means comprises covering almost all free surfaces of said volatile liquid with one or more layers of said involatile liquids, so as to minimize direct exposure of said volatile liquid to said low pressure environment.

3. A method according to claim 2, wherein said involatile liquid is supplied to said emitting electrode by being fed through one or more outer capillary tubes, and said one or more outer capillary tubes surround one or more inner capillary tubes, and said volatile liquid is supplied to said emitting electrode by being fed though said one or more inner capillary tubes.

4. A method according to claim 3, wherein said one or more inner capillary tubes are centered in said one or more outer capillary tubes.

5. A method according to claim 4, wherein the exit region of the outermost capillary tube or at least one of said inner capillary tubes is non-cylindrical, such that a gap between said outermost capillary tube and said inner capillary tube can be almost closed by axially moving one tube with respect to the other to center said inner capillary tube in said outer capillary tube.

6. A method according to claim 1, wherein one liquid is supplied into said emitter electrode, said one liquid consisting of a volatile liquid, and said evaporation-reduction means comprises reducing all free surface area of said volatile liquid exposed to said low pressure environment to below a critical value, such that evaporative cooling does not cause said Taylor cone to freeze or boil.

7. A method according to claim 6, wherein said free surface area of said volatile liquid is less than about 100 μ

m².

8. A method according to claim 7, wherein said free surface area of said volatile liquid is less than about 40 μ

m².

9. A method according to claim 1, wherein said voltage difference between said emitter electrode and said surrounding electrodes or grids is controlled such that part or all of said at least one of drops or ions issuing from said Taylor cone pass through one or several openings in said surrounding electrodes or grids, or directly through the pores of said grid, to form a beam of said at least one of drops or ions.

10. A method according to claim 1, wherein said volatile liquid is selected from the group consisting of water and solvents having a boiling point of less than about 240°

C.

11. A method according to claim 10, wherein said volatile liquid is water.

12. A method according to claim 2, wherein said involatile liquid is oil.

13. A method according to claim 1, wherein said emissions of said at least one of drops and ions are used for electrical propulsion.

14. A method according to claim 13, wherein several stable electrosprays of said volatile liquid are used in parallel to increase output of said electrical propulsion.

15. A method according to claim 1, wherein said low-pressure environment is maintained by means of a vacuum pump.

16. A method according to claim 1, wherein said low-pressure environment is maintained at a pressure of less than about 0.1 torr.

17. An apparatus for forming a stable electrospray of a volatile liquid in a low pressure environment comprising:

a) two or more electrodes or grids, including at least one emitter electrode and at least one extractor electrode or grid

b) means to establish a difference of electrical potential between at least two of said electrodes or grids

c) means for supplying a flow of one or several liquids from one or several reservoirs to said emitter electrode

d) means for maintaining a low pressure environment around said emitter electrode;

wherein said difference of electrical potential is controlled to form a Taylor cone jet in said emitter electrode from whose tip region at least one of drops and ions are ejected into said low-pressure environment.

18. An apparatus according to claim 17, wherein said liquid supply means delivers at least one volatile liquid and at least one involatile liquid.

19. An apparatus according to claim 18, wherein said liquid supply means includes a capillary assembly comprising one or more outer capillary tubes and one or more inner capillary tubes, wherein the one or more outer capillary tubes surround the one or more inner capillary tubes, said outer capillary tubes for transporting said involatile liquid and said inner capillary tubes for transporting said volatile liquid.

20. An apparatus according to claim 19, wherein said one or more inner capillary tubes enter a first branch of a T and exit a second branch of a T, and said one or more outer capillary tubes enter a third branch of said T and exit said second branch of said T, wherein said at least one or more inner capillary tubes are assembled inside of said one or more outer capillary tubes in said T as said inner capillary tubes and said outer capillary tubes exit said T.

21. An apparatus according to claim 19, wherein said one or more inner capillary tubes are centered in said one or more outer capillary tubes.

22. An apparatus according to claim 21, wherein the exit region of the outermost capillary tube or at least one of said inner capillary tubes is non-cylindrical, such that a gap between said outermost capillary tube and said inner capillary tube can be almost closed by axially moving one tube with respect to the other to center said inner capillary tube in said outer capillary tube.