Claims

1. A microfluidic device for determining at least one property of a gas-containing fluid, the device comprising:a substrate;a freestanding tube portion supported above a surface of the substrate so as to be capable of vibrating in a plane normal to the surface of the substrate, the freestanding tube portion having a continuous internal passage, a fluid inlet to the internal passage, and a fluid outlet to the internal passage;means for flowing the gas-containing fluid through the internal passage of the freestanding tube portion;means for generating a drive signal to vibrate the freestanding tube portion at a resonant frequency thereof, wherein the resonant frequency is proportional to the density of the gas-containing fluid, the Coriolis effect causes the freestanding tube portion to twist about an axis of symmetry thereof while being vibrated at the resonant frequency, and the freestanding tube portion exhibits a degree of twist that varies with the mass flow rate of the gas-containing fluid;means for sensing deflections of the freestanding tube portion relative to the substrate when vibrated by the generating means, the sensing means producing an output corresponding to the sensed deflections; andmeans for assessing the drive signal of the generating means and the output of the sensing means to determine at least one of the volume, density and flow rate of the gas of the gas-containing fluid.

2. The microfluidic device according to claim 1, wherein the gas-containing fluid consists of at least one gas in a liquid, and the assessing means is operable to correlate the resonant frequency of the freestanding tube portion to the volume of the at least one gas in the liquid.

3. The microfluidic device according to claim 1, wherein the gas-containing fluid consists of at least one gas in a liquid, the generating means controls the drive signal to maintain a substantially constant vibration amplitude of the freestanding tube portion, and the assessing means is operable to correlate the drive signal of the generating means to the volume of the at least one gas in the liquid.

4. The microfluidic device according to claim 1, wherein the gas-containing fluid consists of at least one gas dissolved in a liquid, and the assessing means is operable to detect nucleation of bubbles of the at least one gas in the liquid within the freestanding tube portion.

5. The microfluidic device according to claim 1, wherein the gas-containing fluid consists of at least one gas dissolved in a liquid, and the vibration of the freestanding tube portion causes nucleation of bubbles of the at least one gas in the liquid.

6. The microfluidic device according to claim 1, wherein the device is installed in a fuel cell system, the gas-containing fluid is a fuel cell solution, and the assessing means is operable to detect nucleation of gas bubbles in the fuel cell solution within the freestanding tube portion.

7. The microfluidic device according to claim 1, wherein the gas-containing fluid consists of one or more gases, and the assessing means is operable to correlate the resonant frequency of the freestanding tube portion to the density of the gas-containing fluid.

8. The microfluidic device according to claim 1, wherein the gas-containing fluid consists of two gases, and the assessing means is operable to correlate the resonant frequency of the freestanding tube portion to the relative amounts of the two gases in the gas-containing fluid.

9. The microfluidic device according to claim 1, wherein the freestanding tube portion is cantilevered and C-shaped and comprises a crossbar on the axis of symmetry of the freestanding tube portion.

10. The microfluidic device according to claim 9, wherein the crossbar is solid.

11. The microfluidic device according to claim 9, wherein the crossbar is hollow.

12. The microfluidic device according to claim 1, wherein the freestanding tube portion is cantilevered and C-shaped and comprises a crossbar oriented transverse to the axis of symmetry of the freestanding tube portion.

13. The microfluidic device according to claim 1, wherein the freestanding tube portion is cantilevered and C-shaped and comprises a first crossbar on the axis of symmetry of the freestanding tube portion and a second crossbar oriented transverse to the axis of symmetry of the freestanding tube portion.

14. The microfluidic device according to claim 1, wherein the freestanding tube portion is not cantilevered and is S-shaped.

15. The microfluidic device according to claim 1, wherein the freestanding tube portion is cantilevered and has a cantilevered length about three times or more greater than a transverse width thereof.

16. The microfluidic device according to claim 1, further comprising means for applying external damping to the freestanding tube portion.

17. The microfluidic device according to claim 1, wherein the internal passage has a maximum internal volume of about 100 microliters.

18. A method of determining at least one property of a gas-containing fluid, the method comprising:providing a freestanding tube portion supported above a surface of a substrate so as to be capable of vibrating in a plane normal to the surface of the substrate, the freestanding tube portion having a continuous internal passage, a fluid inlet to the internal passage, and a fluid outlet to the internal passage, the internal passage having a maximum internal volume of about 100 microliters;flowing the gas-containing fluid through the internal passage of the freestanding tube portion;generating a drive signal to vibrate the freestanding tube portion at a resonant frequency thereof, wherein the resonant frequency is proportional to the density of the gas-containing fluid, the Coriolis effect causes the freestanding tube portion to twist about an axis of symmetry thereof while being vibrated at the resonant frequency, and the freestanding tube portion exhibits a degree of twist that varies with the mass flow rate of the gas-containing fluid;sensing deflections of the freestanding tube portion relative to the substrate and producing an output corresponding to the sensed deflections; andassessing the drive signal and the output to determine at least one of the volume, density and flow rate of the gas of the gas-containing fluid.

19. The method according to claim 18, wherein the gas-containing fluid consists of at least one gas in a liquid, and the assessing step comprises correlating the resonant frequency of the freestanding tube portion to the volume of the at least one gas in the liquid.

20. The method according to claim 18, wherein the gas-containing fluid consists of at least one gas in a liquid, the drive signal is controlled to maintain a substantially constant vibration amplitude of the freestanding tube portion, and the assessing step comprises correlating the drive signal of the generating means to the volume of the at least one gas in the liquid.

21. The method according to claim 18, wherein the gas-containing fluid consists of at least one gas dissolved in a liquid, and the assessing step comprises detecting nucleation of bubbles of the at least one gas in the liquid within the freestanding tube portion.

22. The method according to claim 18, wherein the gas-containing fluid consists of at least one gas dissolved in a liquid, and the vibration of the freestanding tube portion causes nucleation of bubbles of the at least one gas in the liquid.

23. The method according to claim 18, wherein the device is installed in a fuel cell system, the gas-containing fluid is a fuel cell solution, and the assessing step comprises detecting nucleation of gas bubbles in the fuel cell solution within the freestanding tube portion.

24. The method according to claim 18, wherein the gas-containing fluid consists of one or more gases, and the assessing step comprises correlating the resonant frequency of the freestanding tube portion to the density of the gas-containing fluid.

25. The method according to claim 18, wherein the gas-containing fluid consists of two gases, and the assessing step comprises correlating the resonant frequency of the freestanding tube portion to the relative amounts of the two gases in the gas-containing fluid.

26. The method according to claim 18, wherein the freestanding tube portion is cantilevered and C-shaped and comprises a crossbar on the axis of symmetry of the freestanding tube portion.

27. The method according to claim 18, wherein the freestanding tube portion is cantilevered and C-shaped and comprises a crossbar oriented transverse to the axis of symmetry of the freestanding tube portion.

28. The method according to claim 18, wherein the freestanding tube portion is cantilevered and C-shaped and comprises a first crossbar on the axis of symmetry of the freestanding tube portion and a second crossbar oriented transverse to the axis of symmetry of the freestanding tube portion.

29. The method according to claim 18, wherein the freestanding tube portion is not cantilevered and is S-shaped.

30. The method according to claim 18, wherein the freestanding tube portion is cantilevered and has a cantilevered length about three times or more greater than a transverse width thereof.

31. The method according to claim 18, further comprising applying external damping to the freestanding tube portion.

32. The method according to claim 18, wherein the internal passage has a maximum internal volume of about 100 microliters.