Method of forming an aggregate of objects

1. A method of forming an aggregate of objects in a channel comprising a liquid, said method comprising:

a) providing objects in at least a region of the channel, and

b) forming an aggregate of said objects by submitting them to a modulated pulsed acoustic field,

wherein the modulated pulsed acoustic field applied at step b) is modulated in amplitude.

2. A method according to claim 1 , wherein the modulated pulsed acoustic field comprises a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency, and said groups being separated between each other by a period having a non-zero duration wherein no acoustic wave is applied.

3. A method according to claim 1 , wherein the modulated pulsed acoustic field comprises a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency, and said groups being separated between each other by a period having a non-zero duration wherein an acoustic wave is applied, said acoustic wave having at least one extremum of the absolute value of its amplitude that is different from the highest amplitude of the acoustic wave pulses belonging to the group just preceding said period.

4. A method according to claim 1 , wherein a standing acoustic wave is created along a transverse dimension of the channel at step b) during the application of the modulated pulsed acoustic field.

5. A method according to claim 4 , wherein said transverse dimension is the thickness of the channel.

6. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency, and each of said groups of acoustic wave pulses comprising 10 or more acoustic wave pulses.

7. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a group, the same amplitude and frequency, at least one group of acoustic wave pulses lasting a duration t 1 that is greater than or equal to 0.01 ms.

8. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency and step b) comprising submitting the objects to at least 100 groups of acoustic wave pulses.

9. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency and at least one period separating two successive groups of acoustic wave pulses having a duration t 2 that is greater than or equal to 0.05 ms.

10. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency and at least one period separating two successive groups of acoustic wave pulses having a duration t 2 that is less than or equal to 0.5 s.

11. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency and wherein at least a couple of consecutive group of acoustic wave pulses of duration t 1 and period separating two successive groups of acoustic wave pulses of duration t 2 have a pulse mode factor

P

mf

=

t

1

t

1

+

t

2

that is greater than or equal to 0.01.

12. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency and wherein at least a couple of consecutive group of acoustic wave pulses of duration t 1 and period separating two successive groups of acoustic wave pulses of duration t 2 have a pulse mode factor

P

mf

=

t

1

t

1

+

t

2

that is less than or equal to 0.95.

13. A method according to claim 1 , at least two couples of consecutive groups of acoustic wave pulses of duration t 1 and period separating two successive groups of acoustic wave pulses of duration t 2 having a different pulse mode factor

P

mf

=

t

1

t

1

+

t

2

.

14. A method according to claim 1 , all the couples of consecutive groups of acoustic wave pulses of duration t 1 and period separating two successive groups of acoustic wave pulses of duration t 2 having substantially the same pulse mode factor

P

mf

=

t

1

t

1

+

t

2

.

15. A method according to claim 1 , the modulated pulsed acoustic field comprising a repetition of a plurality of groups of acoustic wave pulses, said pulses having, among a given group, the same amplitude and frequency and said groups of acoustic wave pulses being periodically spaced between each other by a period having a non-zero duration.

16. A method according to claim 1 , wherein the created aggregate is a 2D-aggregate.

17. A method according to claim 1 , wherein the channel comprises at least a first and a second wall defining an inner volume wherein said liquid is present and wherein the created aggregate is not in contact with said walls during step b).

18. A method according to claim 1 wherein the channel has, over at least a portion of its length, a ratio width/thickness and/or length/thickness that is greater than or equal to 10.

19. A method according to a claim 1 , wherein the upper and/or lower surface of the created aggregate has a temporal average speed of growth during step b) that is less than or equal to 0.1 mm 2 /s.

20. A method according to claim 1 , wherein, during step b), at least one feature of the aggregate is measured and at least one feature of the modulated pulsed acoustic field is modified in function of this measure.

21. A method according to claim 20 , the measured feature being the instantaneous temporal speed of growth of the upper and/or lower surface of the created aggregate.

22. A method according to claim 1 , wherein the formed aggregate of objects has a size of 1 nm or more.

23. A method according to claim 1 , the objects being mono or poly disperse biological cells.

24. A method according to claim 1 , the average size of the objects being less than or equal to 100 μm.

25. A method according to claim 1 , wherein the Reynolds number of the flow is 10 or less during step b).

26. A method according to claim 1 , the liquid not be flowing during step b).