Microfluidic system

1. A method of introducing a fluid into a first droplet, comprising:

providing a first droplet comprising a first plug fluid in an immiscible carrier fluid within a microchannel; and

contacting the first droplet with a fluid stream continuously flowing from a side channel that is in fluidic communication with the microchannel, thereby introducing a portion of the fluid stream into the first droplet.

2. The method of claim 1 , wherein the first plug fluid and the fluid stream comprise hydrophilic liquids.

3. The method of claim 2 , wherein the first plug fluid and the fluid stream comprise aqueous solutions.

4. The method of claim 1 , wherein the carrier fluid comprises a hydrophobic liquid.

5. The method of claim 1 , wherein the first plug fluid and the fluid stream comprise a chemical, biological, or biochemical entity.

6. The method of claim 5 , wherein the first plug fluid and the fluid stream comprise different chemical, biological, or biochemical entities.

7. The method of claim 1 , wherein the microchannel has a maximum cross-sectional dimension of less than about 500 micrometers.

8. The method of claim 1 , further comprising mixing the portion of the fluid stream introduced into the first droplet with the first plug fluid of the first droplet.

9. The method of claim 1 , further comprising:

providing a sensor for sensing a characteristic of the first droplet after a portion of the fluid stream has been introduced into the first droplet, wherein the characteristic is fluorescence, spectroscopic signature, radioactivity, mass, volume, density, temperature, viscosity, pH, or concentration of a substance.

10. The method of claim 9 , wherein the characteristic indicates the presence of a nucleic acid.

11. The method of claim 1 , further comprising providing a second droplet comprising a second plug fluid in the immiscible carrier fluid within the microchannel before the contacting step.

12. The method of claim 11 , wherein in the contacting step, a portion of the fluid stream is not introduced to the second droplet.

13. The method of claim 12 , wherein the second plug fluid is different from the first plug fluid.

14. The method of claim 12 , further comprising adjusting spacing between the first droplet and the second droplet before the contacting step.

15. The method of claim 12 , wherein the second plug fluid comprises a surfactant.

16. The method of claim 12 , wherein the second plug fluid comprises a chemical, biological, or biochemical entity.

17. The method of claim 12 , wherein the side channel has a smaller cross-sectional dimension than the microchannel.

18. The method of claim 1 , wherein the side channel has a same cross-sectional dimension as the microchannel.

19. The method of claim 1 , wherein the microchannel comprises a plurality of side channels each containing a continuously flowing stream to enter into the first droplet.

20. The method of claim 1 , wherein the first droplet flows at a first flow rate and the fluid stream continuously flows at a second flow rate, and the ratio of the first flow rate and the second flow rate is about 1:5 to about 5:1.

21. The method of claim 1 , wherein the portion of the fluid stream introduced into the first droplet is of about nanoliter volume.

22. The method of claim 1 , wherein the portion of the fluid stream introduced into the first droplet is of about picoliter volume.

23. The method of claim 1 , wherein the portion of the fluid stream introduced into the first droplet is of about femtoliter volume.