Microfluidic Device for Sorting Out Droplets

Abstract

A micro-fluidic device is provided to sort out objects from a liquid stream. The device comprises a first channel comprising a first liquid and a second channel comprising a second liquid and the first liquid, and a third channel. The second channel is connected to the first channel and the channels are positioned such that a jet flow coming from the second channel can deflect objects in the first liquid into the third channel. The first liquid is a liquid which has a higher viscosity than water and the second liquid may be the same as or different from the first liquid. The micro-fluidic device is adapted for generating the jet flow in the second liquid.

Claims

1. A micro-fluidic device to sort out objects from a liquid stream, the device comprising: a first channel comprising a first liquid; a second channel comprising a second liquid and the first liquid, and a third channel; wherein the second channel is connected to the first channel, wherein the first channel and the second channel are positioned such that a jet flow coming from the second channel can deflect objects in the first liquid into the third channel; wherein the first liquid has a higher viscosity than water, wherein the second liquid may be the same as or different from the first liquid, and wherein the micro-fluidic device is adapted for generating the jet flow in the second liquid.

2. The micro-fluidic device according to claim 1, further comprising a heater adapted for generating at least one microbubble in the second liquid for generating the jet flow from the second channel.

3. The micro-fluidic device according to claim 1, wherein the second liquid is the same as the first liquid.

4. The micro-fluidic device according to claim 1, further comprising a feedback loop, wherein the feedback loop comprises an optical detector for detecting an edge of an object in the first channel, and a feedback system for providing information of the presence of an object for determining the generation of the jet flow in the second liquid.

5. The micro-fluidic device according to claim 1, further comprising a controller for controlling actuation signals for generating a jet, wherein the actuation signals comprise a tapered leading edge and a tapered trailing edge.

6. The micro-fluidic device according to claim 1, further comprising a hydrophobic coating on one or more walls of the first channel.

7. The micro-fluidic device according to claim 6, wherein the hydrophobic coating is a perfluorodecyltrichlorosilane (PDTS) monolayer.

8. The micro-fluidic device according to claim 1, wherein the second liquid is different from the first liquid and wherein an interface between the first liquid and the second liquid is present in the second channel.

9. The microfluidic device according to claim 8, further comprising a monitor for monitoring the interface between the first liquid and the second liquid.

10. The microfluidic device according to claim 8, wherein the first liquid and the second liquid are separated by a gas plug in the second channel.

11. The microfluidic device according to claim 10, wherein the second channel comprises an additional chamber, and wherein the gas plug is captured in the additional chamber.

12. The microfluidic device according to claim 11, further comprising hydrophilic pillars in the chamber, wherein the hydrophobic pillars define an interface between the second liquid and the gas plug.

13. The microfluidic device according to claim 10, further comprising an additional channel adapted for controlling a size of the gas plug.

14. The microfluidic device according to claim 8, wherein the second liquid is water.

15. The microfluidic device according to claim 14, further comprising a stabilizer adapted for stabilizing the interface between the first liquid and the second liquid.

16. The microfluidic device according to claim 1, wherein the second channel is an elongated channel.

17. The microfluidic device according to claim 2, wherein the heater comprises a stack, the stack comprising a metal layer in between a first passivation layer and a second passivation layer, wherein the stack is on top of a semiconductor substrate layer or a glass layer.

18. The microfluidic device according to claim 17, wherein the first passivation layer has a lower thermal conductivity than the second passivation layer.

19. A diagnostic device for diagnosing a status of a patient, the diagnostic device comprising: the microfluidic device according to claim 1; and an output device, wherein the output device determines a quality or quantity of the objects and provides an output diagnosis based on the quality or quantity of the objects.

20. An industrial inspection device for inspecting a liquid flow comprising objects, the industrial inspection device comprising: the microfluidic device according to claim 1; and an output device, wherein the output device determines a quality or quantity of the objects and provides an output based on the quality or quantity of the objects, wherein the output characterizes the liquid flow.

21. A method for forming the micro-fluidic device according to claim 1, the method comprising: providing a substrate comprising at least one jet flow actuator for generating the jet flow in the second liquid; providing a structural layer over the substrate; patterning the structural layer such that the structural layer comprises at least one micro-fluidic channel exposing the at least one jet flow actuator; providing a cover over the structural layer, wherein the cover comprises at least one access to the at least one micro-fluidic channel; and rendering a surface of the at least one micro-fluidic channel hydrophobic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 schematically shows a micro-fluidic device, wherein the first liquid is the same as the second liquid in accordance with embodiments of the present invention.

[0056] FIG. 2 schematically shows a microfluidic device comprising a second channel which is adapted for containing a first and a second liquid in accordance with embodiments of the present invention.

[0057] FIG. 3 shows the interface position in the microfluidic device of FIG. 2 when the heater fires a jet flow, in accordance with embodiments of the present invention.

[0058] FIG. 4 schematically shows a microfluidic device comprising a gas plug in between the first and the second liquid in accordance with embodiments of the present invention.

[0059] FIG. 5 shows a similar microfluidic device as FIG. 4. The microfluidic device comprises an additional channel adapted for controlling the size of the gas plug, in accordance with embodiments of the present invention.

[0060] FIG. 6 schematically shows a microfluidic device comprising a chamber wherein the gas plug can be captured in the chamber in accordance with embodiments of the present invention.

[0061] FIG. 7 schematically shows a micro-fluidic device comprising two heaters in accordance with embodiments of the present invention.

[0062] FIG. 8 schematically shows an optical signal measured on a stream of droplets in accordance with embodiments of the present invention.

[0063] FIG. 9 schematically shows an actuation signals for actuating a heater in accordance with embodiments of the present invention.

[0064] FIG. 10 schematically shows different steps in a process for forming a micro-fluidic device to sort out objects from a liquid stream in accordance with embodiments of the present invention. Any reference signs in the claims shall not be construed as limiting the scope.

[0065] In the different drawings, the same reference signs refer to the same or analogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0066] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0067] The terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0068] Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

[0069] It is to be noticed that the term comprising, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression a device comprising means A and B should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[0070] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0071] Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[0072] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0073] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0074] Where in embodiments according to the present invention reference is made to a stroke, reference is made to a jet flow caused by the generation of at least one microbubble in the second liquid.

[0075] Embodiments of the present invention relate to a micro-fluidic device for isolating objects in a liquid stream. The micro-fluidic device therefore comprises a first channel comprising a first liquid. In embodiments of the present invention the objects are water droplets and the first liquid may be any immiscible liquid that presents higher viscosity than the water phase at the droplet generation point to create a fragmented flow. In embodiments according to the present invention the first liquid may be an oil. In embodiments of the present invention the flow rate of the first liquid (e.g. oil) may be the same or higher than the flow rate of the water phase in order to create a segmented/droplet flow. The flow rate of the oil may for example be twice the flow rate of the water. The droplets may have captured one or more cells or molecules. During operation the droplets pass through the first channel. Micro-fluidic devices, according to embodiments of the present invention, provide means for sorting out certain of these droplets which are passing through the first channel.

[0076] The micro-fluidic device, according to embodiments of the present invention comprises a second channel which is connected to the first channel and a third channel. The first channel and the third channel are positioned such that a jet flow coming from the second channel can deflect objects (e.g. droplets) which are present in the first channel, into the third channel.

[0077] For generating the jet flow, the micro-fluidic device comprises a heater. This heater is immersed in a second liquid in a chamber connected to the second channel. The heater is adapted for generating at least one microbubble in the second liquid. The microbubble(s) will induce the jet flow from the second channel. In embodiments according to the present invention a plurality of microbubbles may be required to generate an efficient jet flow.

[0078] In embodiments of the present invention the second liquid is the same as the first liquid and both are an oil. In these embodiments an oil vapor bubble is created by the heater. Therefore the oil must be heated above the boiling temperature of the oil which may for example be between 150-180 C. Therefore, in these embodiments, the heater must be adapted for generating an oil vapor bubble. The heater may comprise a number of hotspots which can be heated by applying a current pulse to the heater. The current pulse may for example have a duration of about half of the time that the droplet travels through the sorting junction. Thereby the sorting junction is the T junction between the first channel and the second channel. The current pulse may for example have a duration between 5 s and 1 ms. The current pulse height may for example be between 1 amp and 10 amp, for example between 1 amp and 5 amp. This heating of the hotspots consequently generates the vapor bubbles of the second liquid which on its turn induces the jet flow from the second channel towards the first channel. After the current pulse is turned off, the vapor bubbles collapse and the heater is restored to be ready for the next sorting.

[0079] In embodiments of the present invention the heater is adapted for generating at least one microbubble in the second liquid. The heater may therefore comprise a stack of layers wherein a metal layer is positioned between a first and a second passivation layer. The metal layer is thereby adapted for generating heat when sending a current through it. It may for example comprise aluminium or tungsten. The first passivation layer may be mounted on a substrate (e.g. glass or semiconductor substrate). In embodiments according to the present invention the first passivation layer is a SiO.sub.2 layer and/or the second passivation layer is a SiN+SiC layer. This may typically result in the following stack: silicon/SiO2/W/SiN/SiC. Other possible choices are: first passivation=SiN, second passivation=SiO2 or SiN (single material) or SiO2/SiC, SiO2/Ta, SiN/Ta.]

[0080] The second passivation layer may be closest to the second liquid during operation of the microfluidic device. The first passivation layer may have a lower thermal conductivity than the second passivation layer. Thus resulting in a heat flow towards the second liquid.

[0081] When the first liquid and the second liquid are the same oil, the oil preferably has a low boiling temperature. The following oil composition may for example be used as carrier for the objects which may be water droplets generated on a T-junction area at the first channel (before the T-junction area between the first channel and the second channel): [0082] FC-40 is fluorinated oil manufactured by 3-M company. The oil has a clear/transparent fluid and presents a boiling point at 165 C. Its liquid density is 1855 kg/m3 and present a refractive index of 1.29 and dielectric constant of 1.9. [0083] An alternative is the HFE7500 Novec fluorinated oil that presents a boiling point of 128 C. This oil is a clear fluid with liquid density of 1614 kg/m.sup.3. Its refractive index is 1.39 and dielectric constant is 5.8.

[0084] FIG. 1 schematically shows a micro-fluidic device 100, wherein the first liquid 115 is the same as the second liquid 125, in accordance with embodiments of the present invention. The first and second liquid may be an oil. The micro-fluidic device 100 comprises a first channel 110 comprising the first liquid 115. In this example the first liquid can flow through the first channel and comprises water droplets. Some of these are wanted water droplets 111 and others are unwanted water droplets 112. The micro-fluidic device comprises a second channel 120 connected to the first channel 110, and a heater 140. In this example the heater 140 is immersed in the second liquid 125 in a heating chamber 145 connected to the second channel 120. In this exemplary embodiment the heater 140 is adapted for heating the oil, being the second liquid. By heating the oil an oil vapor bubble can be generated. This vapor bubble causes a jet flow 127 from the second channel 120 and this jet flow 127 can deflect the wanted water droplet 111 into a third channel 130 which is also connected to the first channel 110.

[0085] In yet another embodiment of the present invention the micro-fluidic device 100 comprises a first channel 110 adapted for flowing a first liquid 115, a second channel 120 connected to the first channel 112 and a third channel 130, positioned such that, during operation, a jet flow 127 coming from the second channel 120 can deflect objects in the first liquid into the third channel 130. The second channel 120 is dimensioned such that it can contain a first 115 and a second liquid 125 such that the interface 150 between the first and second liquid is in the second channel 120 when both liquids are in rest and also when microbubbles generated in the second liquid 125 result in a jet flow from the second channel 120. The micro-fluidic device comprises a heater 140 adapted for generating microbubbles in the second liquid 125 during operation of the micro-fluidic device. During operation the heater 140 is immersed in the second liquid 125. For the design of the second channel 120 a tradeoff has to be made for the size of the channel between a smaller channel in order to minimize the disturbance of the interface 150 between the first and the second liquid during a sorting cycle, and a larger channel with a smaller resistance towards liquid flow in order to increase the jet flow power. Thereby, both the width and the height have to be designed. In embodiments according to the present invention the channel width and height of the second channel may for example be between 5 and 100 m.

[0086] FIG. 2 schematically shows a microfluidic device 100 comprising a second channel 120 which is adapted for containing a first 115 and a second 125 liquid in accordance with embodiments of the present invention.

[0087] FIG. 2 shows a heater 140 in a heating chamber 145 connected to a second channel 120 wherein the second channel is connected to a first channel 110. The second channel is dimensioned such that the interface 150 between the first and second liquid is present in the second channel 120 (in rest and also when a jet flow is generated). The dimensioning may comprise dimensioning the length and/or the width of the second channel. In this exemplary embodiment the second liquid 125 is water. The second liquid is filling up the heating chamber 145 and the second channel has both a water segment 125 and an oil segment 120. Vapor bubbles created in the heating chamber 145 lead to jet flow which propagates to the second channel and finally to the first sorting channel. During the entire jet flow resting and firing process, the water-oil interface 150 remains in the second channel 120 to prevent oil from entering the heating chamber 145 or water from entering the first channel 110. The advantage of a microfluidic device 100 as in FIG. 2 is the low heat amount required to produce the jet flow, compared with the oil vapor bubble nucleation in the embodiment illustrated in FIG. 1. The reason therefore being that the boiling temperature of water is usually much lower than that of oil. As a result, the entire device is less heated and thus the device is more durable and a higher sorting speed may be realized.

[0088] FIG. 3 shows the same microfluidic device 100 as in FIG. 2 when the heater heats the second liquid resulting in a jet flow. While FIG. 2 shows the interface position 150 when the heater is at rest, FIG. 3 shows the interface position 150 when the heater fires a jet flow 127.

[0089] In embodiments according to the present invention the second liquid 120 and the first liquid 110 are separated by a gas plug 410 in the second channel 120 (the interface between the first and second liquid is thereby formed by the air plug). This gas plug may be an air plug, for example an air bubble. The second channel 120 is thereby designed such that during the entire sorting process the gas plug 410 remains inside the second channel 120.

[0090] In embodiments of the present invention wherein the first and second liquid are separated by a gas plug, controlling of the microbubbles generation may take into account the dynamics of the gas plug. The dynamics of such a system may be different because the air is compressible. The interface dynamics may also be dependent on the temperature. This may also be taken into account when generating the microbubbles.

[0091] FIG. 4 schematically shows a microfluidic device 100 comprising a gas plug 410 in between the first 115 and the second liquid 125 in accordance with embodiments of the present invention.

[0092] FIG. 5 shows a similar microfluidic device 100 as FIG. 4. The microfluidic device comprises an additional channel 420 adapted for controlling the size of the gas plug. The additional channel 420 is connected with the second channel 120 at a position such that the second channel can contain a gas plug 410 in between the first liquid 115 and the second liquid 125.

[0093] In embodiments of the present invention the gas plug 410 may be blocked in a certain position in the second channel 120 by providing dedicated structures in the second channel which prevent the gas plug from entering such a dedicated structure. In embodiments of the present invention a slightly bigger chamber 510 may be introduced between two narrow channels.

[0094] FIG. 6 schematically shows a microfluidic device comprising an additional chamber 510 wherein the gas plug 410 can be captured in accordance with embodiments of the present invention. Additionally hydrophilic pillars 520 are present in the chamber. These hydrophilic pillars 520 may form a barrier for the second liquid 125 between the heater 140 and the hydrophilic pillars 520. A hydrophilic pillar allows additional surface tension to hold liquid 125 (e.g. water) and therefore to trap a gas plug 410. Additionally pillars 530 may be present at the first liquid side 115. The interaction between the first liquid 115 (e.g. oil) and the pillars 530 can be various, e.g. similar to water (e.g. oil-philic, hydrophobic) or the opposite, depending on the oil property.

[0095] Hydrophilic pillars may be used to trap a gas plug 410 (as illustrated in FIG. 6; see pillars 510, 520, 530) or they may be used when no gas plug 410 is used. In either case, the hydrophilic pillar surface only interacts with a liquid. In the first method, the surface tension holds the second liquid 125 and consequently keeps the interface between the second liquid 125 and the gas plug 410 in the pillar array. In the second method, the surface tension holds the interface 150 between the first and second liquid in the pillar array.

[0096] In embodiments of the present invention the pillar array may have much smaller attractive force on the first liquid or on the gas plug compared to the second liquid (because of surface hydrophilicity) and the pillar array may stop the second liquid up to a certain pressure level. This pressure level is depending on the design of the pillar array.

[0097] In embodiments of the present invention the length of the pillar array is related to the stroke size (i.e. the displacement of the second liquid during a stroke). This length may for example be the double of the stroke size. In embodiments of the present invention the interface between the second liquid and the gas plug or the interface between the first liquid and the gas plug is residing between the pillar array and the heater during an entire sorting cycle. If for some reason the interfaces move too much toward the first channel, the pillar array functions as a safety valve to hold one of the interfaces.

[0098] The microfluidic device 100 comprising the additional chamber 510 in the second channel may be filled first by the second liquid 125 until the second liquid is immersing the heater and until the second channel is filled up to the chamber (e.g. up to the hydrophilic pillars 520). When filling up the second channel 120 with the first liquid 115 from the other side, a gas plug 410 will be trapped in the additional chamber 510 between the first liquid 115 and the second liquid 125. In embodiments according to the present invention the gas plug is preferably as small as possible, in order to prevent that the gas plug decreases the efficient jet flow power. In embodiments according to the present invention the gas volume is no larger than 10 times the total volume of all the vapor bubbles which are generated during one stroke. By proper design of the second channel 120 and the additional chamber 510 the gas plug 410 will be captured inside the chamber 510 between the first liquid 115 and the second liquid 125. For this design also pillars 520, 530 may be introduced to improve the stability of the gas plug. In embodiments of the present invention the design is such that the gas plug 410 does not fill the complete additional chamber 510.

[0099] In embodiments according to the present invention a sorting rate of more than 500 objects per second, or even more than 1000 objects/second, or even more than 2000 objects/second or even more than 5000 objects/second can be obtained. Depending on the embodiment, the sorting rate may be higher or lower. The sorting rate may be lower in an embodiment comprising an air plug interface because of power dampening by the air bubble. This can, however, be compensated for by extra jet flow power. In an embodiment wherein the first and second liquid are the same (e.g. oil), more heating is required to produce bubbles and therefore also an additional cooling mechanism may be required to increase the sorting rate by decreasing the required cooling time per jet flow cycle (i.e. per stroke).

[0100] In embodiments of the present invention the time between droplets to be isolated may be below one second, or it may even be below 100 ms, or even below 10 ms, or even below 1 ms.

[0101] In embodiments of the present invention the microfluidic device 100 comprises a plurality of jet flow actuators 140 (e.g. a heater). A plurality of jet flow actuators 140 can advantageously be used in tandem in a multitude of ways. In embodiments of the present invention a plurality of jet flow actuators 140 can be present in a single second channel 120. In embodiments of the present invention the plurality of jet flow actuators 140 in the single second channel 120 can be actuated simultaneously. The plurality of jet flow actuators 140 can advantageously be used to form a combined jet flow which is more powerful compared to a jet flow from a single jet flow actuator 140. This may for example allow a jet flow of the desired strength to be obtained faster, allowing in turn to achieve a higher sorting rate.

[0102] In embodiments, the plurality of actuators can reside on the same side of the droplet-carrying channel 110 or can be arranged different. The first case is illustrated by FIG. 7. One possible purpose may be to compensate the push-pull flow. Putting the heaters on the same side can do the same job as well. Also, on the same or different side, multiple actuators, e.g. heaters, have the advantage of supporting sequential firing, or sequential jet flow, which can help smoothen the total jet flow and which can avoid over-using or over-heating a single heater that might lead to early heater failure (when multiple heaters are used alternatingly, each heater will have sufficient time to cool down).

[0103] In embodiments of the present invention, a second jet flow actuator 140 can be actuated out of phase with respect to a first jet flow actuator 140. A jet flow in accordance with the present invention may in embodiments be composed of two phases: a push phase, when microbubbles are created and eject an outbound jet flow, and a pull phase, when microbubbles collapse and retract an inbound jet flow. Either one of these phases may be used for sorting, dependent on the timing set by the user. A problem may arise when two objects in the first channel 110 are too close to each other. In such a situation, after pushing the first object, the second object may for example inevitably be caught in a pull phase. As a result, the second cell may be wrongfully sorted. To overcome this, the second jet flow actuator 140 may advantageously be positioned and actuated such that the force created by its push (or pull) phase cancels out the pull (or push) phase of the first jet flow actuator 140.

[0104] In embodiments of the present invention the micro-fluidic device 100 can comprise a plurality of second channels 120, each comprising at least one jet flow actuator 140. This situation is depicted in FIG. 7. The plurality of jet flow actuators 140 across the plurality of second channels 120 can for example advantageously be used to sort objects (e.g. wanted water droplets 111) across a plurality of third channels 130, e.g. depending on their content as determined from an optical measurement.

[0105] It will be clear that several or all of these uses for the plurality of jet flow actuators 140 can in embodiments be combined. For example, a micro-fluidic device 100 may comprise a plurality of second channels 120, each comprising a plurality of jet flow actuators 140. Some of these jet flow actuators may be used to sort objects across different third channels 130. Others jet flow actuators 140 may be combined to improve (e.g. speed up) a jet stream formation. Meanwhile, some jet flow actuators 140 may be used to counteract e.g. an inconvenient pull or push phase.

[0106] In embodiments of the present invention wanted 111 and unwanted 112 objects (e.g. droplets) are detected and/or counted by an optical measurement. In embodiments of the present invention the optical measurement may comprise an edge detection. Droplet sorting can conventionally be achieved by detecting a scatter signal; either forward scatter (FSC), side-scatter (SSC) or back scatter (BSC). Moreover, when in the present invention the first liquid is an oil and the object is a water droplet, since the refractive index contrast between oil and water across the oil-water interface is considerable, it is conveniently possible to detect droplets by simply measuring the double-edge of the droplet. This is also the case for other objects, provided the refractive index contrast is sufficiently high. Alternatively also electrical measurement can be performed for droplet detection. The electrical measurement may be adapted for detecting the droplets by their electrical impedance in contrast to oil, since water has a different conductivity and dielectric permittivity from oil. The edge detection may be performed as an alternative or as an additional detection technique. It is an advantage of embodiments of the present invention that optical edge detection can be performed close to the position where selection is to be performed, so that changes in speed have little or no effect on the selection. The edge detection is illustrated in FIG. 8, at the top of which a stream of droplets 111 being measured by a light source 600 is schematically depicted. The bottom of FIG. 8 shows the corresponding measured optical signal 601, said signal 601 being characterized by valleys 602 corresponding to the double-edge.

[0107] In some embodiments, it may be beneficial to also consider the droplet integrity during sorting. Unlike cells, which have a physical encapsulating membrane, droplets only preserve their physical integrity by their surface tension; therefore, a droplet's structural integrity is often weaker compared to cells. The forces to which the droplets are exposed during sorting may therefore advantageously be sufficiently mild in order not to damage or even break open the droplet during sorting. Referring to FIG. 9a, one way to moderate said forces may be to enlarge the duration T.sub.1 of the actuation signal, while lowering the amplitude V, thereby controlling the power of the jet stream. Alternatively, as shown in FIG. 9b, an actuation signal may be used with a leading edge which increases towards a maximum intensity over a duration T.sub.2. Likewise, an abrupt cut-off of the jet stream may also lead to the droplet being exposed to excessive forces. The actuation signal can therefore advantageously be such that its trailing edge tapers down over a duration T.sub.3. In embodiments of the present invention an actuation signal for generating a jet flow (e.g. to a jet flow actuator 140) can comprise a tapered leading edge, and optionally a tapered trailing edge. The microfluidic device may thus comprise a controller for controlling actuation signals for generating a jet, whereby the actuation signal may comprise a tapered leading edge and optionally a tapered trailing edge.

[0108] In some embodiments, the walls of the first channel may be rendered hydrophobic by applying a hydrophobic coating thereto. The walls thus may be coated with a hydrophobic coating. The hydrophobic coating may be for example a hydrophobic perfluorodecyltrichlorosilane (PDTS) monolayer. It is an advantage of embodiments of the present invention that it may prevent droplets from sticking to the wall, thus allowing a better selection of the droplets.

[0109] A method for forming a micro-fluidic device 100 in accordance with embodiments of the present invention is depicted in FIG. 10. Initially, a substrate 701 is provided (FIG. 10a). The substrate 701 may be a semiconductor substrate (e.g. a Si substrate). The substrate 701 may have been processed to comprise one or more jet flow actuators for generating a jet flow in the second liquid in the final micro-fluidic device. These processing steps may typically be semiconductor technology processing steps, e.g. CMOS compatible processing steps. Subsequently, a structural layer 702 is provided over the substrate 701 (FIG. 10b). The structural layer 702 may be a conformal polymer layer (e.g. polydimethylsiloxane, PDMS). The structural layer 702 is then patterned (FIG. 10c), e.g. using a photolithography, to form therein one or more channels, such as the first 110, second 120 and/or third 130 channels; at least one channel providing an access to the one or more jet flow actuators. A cover 703 is subsequently provided over the structural layer 702 (FIG. 10d). The cover 703 may for example comprise a glass substrate, optionally itself covered with a bonding layer. The bonding layer may facilitate bonding of the cover with the structural layer 702. The bonding layer may for example be made of a same material as the structural layer 702, e.g. both being made of PDMS. The cover comprises an access to the one or more channels in the structural layer 702. To this end, the cover 703 may be patterned with one or more openings 704; for example after bonding the cover 703 to the structural layer 702. Optionally, the method may further comprise modifying a hydrophobicity of exposed surfaces of the micro-fluidic device 100, such as by depositing a hydrophobic coating 705 over said surfaces (FIG. 10e). A hydrophobic perfluorodecyltrichlorosilane (PDTS) monolayer 705 may for example be deposited over the micro-fluidic device 100, e.g. by means of a vapor deposition in an oven (e.g. at 120 C. for 120 min). Surfaces obtained in semiconductor technology may be hydrophilic in nature (e.g. being characterized by a water contact angle of 30 or less). These hydrophilic surface are typically less compatible with objects, such as droplets. It is therefore advantageous to provide a hydrophobic coating (e.g. characterized by a water contact angle of 90 or more, such as 100) over these the exposed surfaces.