Running Droplet Optical Multiplexer

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Jan 30, 2015 - crosensors, -acuators and -systems / Microsystems Center Bremen ... Bereitgestellt von | Staats- und Universitätsbibliothek SuUB Bremen.
Optofluid. Microfluid. Nanofluid. 2014; 1:62–68

Research Article

Open Access

Lukas Brandhoff*, Mahmuda Akhtar, Mike Bülters, Ralf B. Bergmann, and Michael J. Vellekoop

Running Droplet Optical Multiplexer Abstract: We present an optofluidic device for switching light from multiple inputs to one common output. The device uses a microfluidic channel filled with high index of refraction oil as a waveguide, and moves low refractive index interruptions in the form of aqueous droplets through the channel. Whenever a droplet passes one of the optical inputs, this specific input is switched through to the output. This produces a running switching of one output following the other creating a 8x1 multiplexer. Keywords: optofluidics; optics; microfluidics; droplet; digital-microfluidics; optical-switch DOI 10.2478/optof-2014-0007 Received July 14, 2014; accepted October 5, 2014

1 Introduction Today, optical sensing is one of the most versatile, flexible and accurate physical measurement methods known. Examples of such systems are found in biology and medicine, with fluorescent measurements of cell interiors [1], high throughput DNA-sequencing, flow cytometry [2] and cancer cell detection [3]. In material-sciences optical methods are for example applied for thin film analysis and they can be found in process monitoring as well as chemical analysis [4]. Compared to electrical circuits, optical setups unfortunatly have one disadvantage: When high performance is needed, they are often large, unwieldy, precision manufactured mechnanical assemblies, with need for damped

low vibration tables, filtered air, climate control, and precise adjustment. Industrial environments, for example chemical plants, would benefit from a more widespread use of the aforementioned sensing methods, which are until now prohibited by the environmental conditions in such a plant. Additionally, taking measurements at different points in such a plant would need multiple setups which can result in a large increase of costs. Integrated optics and micro-opto-electro-mechanicalsystems (MOEMS) are one way to solve the issues described above. Optical integration repeats the path that electronics took 50 years ago, away from huge setups, to rugged and cheap single chip solutions. Since some optical setups are currently unintegratable, we chose a different path, similar to a development that took place in Lab-on-Chip (LoC) Technology. While the final goal in LoC is to integrate a complete analytical lab on a single chip, today the research is more focussed towards a "Chip in a Lab", enhancing exisiting analytical methods, and using technological advantages to drive applications forward. In our case optical and optofludical chips are combined with traditional optical setups, creating a hybrid system with the advantages of both technologies. Industrial Environment

Optical Laboratory

Optical table with high precision setup

Running Droplet Optical Multiplexer

Figure 1: Application example *Corresponding Author: Lukas Brandhoff: Institute for Microsensors, -acuators and -systems / Microsystems Center Bremen (IMSAS/MCB), University of Bremen, Bremen, Germany, E-mail: [email protected] Mahmuda Akhtar, Michael J. Vellekoop: Institute for Microsensors, -acuators and -systems / Microsystems Center Bremen (IMSAS/MCB), University of Bremen, Bremen, Germany Mike Bülters, Ralf B. Bergmann: BIAS - Bremer Institut für angewandte Strahltechnik, Bremen, Germany

Our running droplet optical multiplexer presented here is a possible interface between the integrated and non-integrated optical world. In the scenario of a chemical plant as in Fig. 1, optical signals can be collected from various points of interest, such as chemical reactors, exhausts or storage facilities, via optical fibers, and joined

© 2014 L. Brandhoff et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

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Running Droplet Optical Multiplexer |

together in one optical laboratory. To connect all of the incoming signals to the expensive measurement apparatus, an optical switch or multiplexer is needed. Traditionally, light can be switched by flipping or turning mirrors manually or with the help of solenoids. In the last years, a large amount of different optical switches have been investigated and developed. MEMS-based switches, which are integrated versions of the mechanical mirror light switches, present the most mature technology today. Fig. 3.

Figure 2: Microphotograph of Device

Our device, displayed in Fig. 2 uses liquids of different optical properties to manipulate - i.e. switch - light subsequentially from multiple inputs to one common output. It is based on our "Optofluidic Multiplexing and Switching Device" [5], but with reversed mode of operation - combining 8 inputs into one single output instead of one input into two outputs, with refined optical design and an improved microfluidic structure. It is composed of a high refractive index liquid core waveguide, and a lower refractive index movable interruption in form of a liquid droplet as seen in Fig. 3.

n=1.52

63

Liquids as optical elements can change their shape, refractive index, and position to manipulate light according to the needs of the device. With traditional solid materials, this is, apart from exceptions impossible.

2 Theory An optical waveguide is composed of a medium of higher index of refraction, embedded in a low refractive index environment. It confines the light striking the boundary between the two materials at an angle greater than the critical angle by total internal reflection according to Snell’s law. In a liquid core waveguide, the core, i.e. the light confining medium, is composed of a high refractive index liquid, surrounded by the channel walls, which have to be of lower index of refraction. Since most aqueous liquids have low refractive indices compared to most polymers and glasses, only special combinations confine the light in a liquid core waveguide. In our device, this material system is immersion oil (n = 1.52) and Polydimethylsiloxane (PDMS) (n = 1.43) as channel material. To understand the principle behind the switchable waveguide, we have to distinguish two cases and apply Snells law n1 sin(Θ1 ) = n2 sin(Θ2 ) (1) to both situations as shown in Fig. 4. First, the bare channel, without a droplet in front of the optical input will be discussed. When the collimated light beam hits the front-surface of the channel, it is refracted according to Snells law: Θ t = arcsin(n cladding sin(Θ i )/n core )

(2)

If we choose our input fiber placement to 15 ∘ relative to the channel - which equals to 75 ∘ relative to the normal - the light will not be guided. When entering the waveguide, it will be refracted to 65.3 ∘ according to Eq. 2 which is lower than the critical angle Θ c , which can be determined as:

n=1.43

n=1.43

Figure 3: Moving Droplet

Θ c = arcsin(n cladding /n core ) = arcsin(1.43/1.52) = 70.2 ∘ (3)

By excerting positive or negative pressure on the liquid inlet, the droplet can be moved through the channel, passing one optical input after the other, switching them seperately to the output.

Next we look at the case where a lower refractive index (n=1.43) droplet is present. An optical break is present at the position of the droplet in the waveguide, enabling the light to penetrate the waveguide and enter the high index liquid from the side.

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64 | L. Brandhoff et al.

a)

75° n=1.52

65.3°

PDMS n=1.43

Droplet n=1.43

65.3°

Oil n=1.52

15° 75° n=1.43

Air-Lens n=1.0

a) No droplet present, optical input off PDMS n=1.43

b) n=1.52 n=1.43

Droplet n=1.43

75° 15°

Oil n=1.52

76.0° 76.0° 14.0°

15° 75°

Air-Lens n=1.0

n=1.43

b) Droplet present, optical input on

Figure 4: Theroretical analysis behind the droplet switching

c)

PDMS n=1.43

Droplet n=1.43 Oil n=1.52

The entrance angle relative to the normal will consequently now be 15 ∘ and our liquid will be refracted accoring to Eq. 2 to 14.0 ∘ in the oil. When it strikes the back side of the waveguide, its angle of incidence will now be Θ =90 ∘ −14.0 ∘ =76 ∘ , which exceed the critical angle. Total internal reflection, and with it wave guidance, will take place. The light the follows the waveguide towards the end of the channel, where it hits another perpendicular wall and exits the channel. To confirm this theoretical approach for our specific design, the geometry used was simulated with the nonsequential ray-tracer of Radiant ZEMAX, and the propagation of the light for different droplet positions was analyzed. The results are displayed in Fig. 5. Visible are the input fibers on the right side, angled at 15 ∘ , the corrosponding air-lenses, the channel filled with high index oil and the droplet, here approximated in rectangular form. This simulation confirms the analytical approach. In Fig. 5a) no input is currently connected to the output, since the droplet is not in front of any input. As it moves down the channel, the first input is connected to the output, first partially (5b) and then fully (5c). The same is repeated for input number two (5d). Finally, as the droplet leaves both inputs, they are not connected to the output anymore. To determine how sensitive the device will react to changes in incidence angles or refractive indices, calculations based on Snells law were made. For a working device two conditions - as described above - have to be fullfilled:

Air-Lens n=1.0

d)

PDMS n=1.43

Droplet n=1.43 Oil n=1.52

Air-Lens n=1.0

Figure 5: 2-dimensional ray-tracing results

No total internal reflection should occur when no droplet is present, and the light must be reflected when a droplet is present. The results of those calculations, as seen in Fig. 6, show that an angle error through misalignment or imperfect collimation of the light beam up to 6 ∘ is acceptable without disturbing the funcionality of the device. A maximum increase of the refractive index ratio n cladding /n core of 3% is acceptable as well. The expected tolerances of the real device fall well within those limits, as distances and angles in the system are defined through photo-lithography with sub-micrometer resolution and the change of refractive index through temperature can generally be observed in the ppm range.

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Running Droplet Optical Multiplexer |

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30 Operating Range

Angle of Incidence @°D

25 Design Operating Point

20

Deviation from Operating Point

+6°

15

+3%

10

Figure 8: Simulation of the droplet creation Blue=Water, Red=Oil 5 0 0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

Refractive Index Ratio

Figure 6: Theoretical operating range(blue) in dependence on incidence angle and refractive index ratio

3 Materials & Methods 3.1 Design of Optofluidic Chip The mask design used to fabricate the optofluidic device is shown in Fig. 7 together with annotations explaining the different features. The device consists of a main channel (1), acting as the liquid waveguide, connected to the liquid outlet (2) on one side, and the droplet generation area on the other side. Both are placed at a 90 ∘ angle, as not to interfere with the optical path.

5

4

Aqueous Inlet

Oil Inlet

2 Outlet

1

3

8 Output Fiber

Main Channel

7

T-Junction

Air Lens

6 Input Fibers

7

9

Air Lenses Air Mirror

Figure 7: Annotated photolithographic mask design

Droplets [6] are created with the help of a T-Junction (3) [7], with the continuous phase coming from the top (4), and the aqueous phase from the side inlet (5). Continuous flow into both inlets produces aqueous droplets inside of the oil through surface tension. A simulation of the droplet creation, done with Comsol Multiphysics 4.4 can be seen in Fig. 8. The introduction of this T-Junction simplifies handling of the device compared to a creation through a single inlet as used in previous experiments.

As mentioned in section 2, and verified by the raytracing simulation, all 8 optical inputs (6) are placed at 15 ∘ angle to the main channel (1). Air lenses (7) are placed in front of the fiber inlets to collimate the light before coupling into the main waveguide (1), and to focus the light again into the output fiber (8). Additionally, an air mirror (9), also working through total internal reflection is placed next to the end of the main channel (1), to direct the light towards the output. This mirror is needed, since the light can exit the waveguide directed towards the output fiber, as well as directed towards the opposite site. The air-mirror reflects the light traveling away from the output back towards the output.

3.2 Fabrication of Optofluidic Chip The device is fabricated with a microfluidic PDMS-on-SU-8 based imprint lithographic technology [8] as seen in Fig. 9 and described below. A mold with a height of h = 100 µm is fabricated on a silicon wafer in SU-8 3000 (Micro-Chem) by photolithography. 100 µm of SU-8 3050 are spun onto the wafer in two steps with 3000 rpm, soft-baked at 95 °C for 45 minutes and subsequently exposed with a negative mask as described in 3.1. The following post-bake at 65 °C for 1 min and 95 °C for 5 min cross-link the photo-initiated reaction and prepare the photo-resist for developing as described in the datasheet [9] of the resist. After 20 minutes of development the mold is finished. A release agent, BGL-GZ-83 by Profactor, is now applied to the mold to ease demolding of the PDMS. Dow Corning Sylgard 184 PDMS is mixed with its curing agent in a 10:1 ratio, defining the refractive index in the devices fully cured bulk material to n = 1.43 [10]. An approximately 5 mm thick layer is degassed and cured on top the mold at 95°C for one hour, forming the optofluidic device. After release of the chips, they are diced, fluidic in- and outlets are punched and the chips

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66 | L. Brandhoff et al. Exposure+Development

Spincoating

SU-8 Silicon

Silicon

Molding+Curing

Structured SU-8 Silicon

Demolding+Dicing Structured PDMS Structured SU-8

Silicon

Punching of In-/Outlets

O2-Plasma Bonding Structured PDMS

Structured PDMS

Finished Device

O2-Plasma Bonding Liquid Inlet Structured PDMS PDMS

larger than the devices channel-height, the fibers will be hold in place by the flexible PDMS around it. The output fiber is connected to an amplified photodiode (PDA36A-EC,Thorlabs) and the input fibers to different sources such as a blue (λ = 485 nm), and a green (λ = 532 nm) fiber-coupled laser diode.

Liquid Outlet

Structured PDMS Channel

Fiber Input

PDMS Glass

Figure 9: Processflow for creation of the optofluidic device

Figure 10: Connection diagram

are bonded by plasma activation to a second, blank PDMS layer and then a glass microscope slide for stabilization.

The connections are schematically displayed in Fig. 10.

3.3 Liquids

3.5 Measurements

Low viscosity (η = 30 – 60 mPas, Sigma-Aldrich) microscopy immersion oil is used for the continuous phase, high refractive index (n = 1.52) waveguide liquid. The immiscible disperse phase is composed of an aqueous CaCl2 solution. It is prepared by solution of Calciumchloride-dihydrate (Sigma-Aldrich) in deinonized (DI)-water to a concentration of 65% w/w, resulting in a refractive index [11] of n =1.43 matching the refractive index of PDMS. The refractive index of the liquids is confirmed with a handheld refractometer (PAL-RI by Atago).

To prove the practical functionality of the device, and evaluate its performance, it was connected as descirbed in section 3.4. The completed setup is shown in Fig. 11 for additional clarification.

3.4 Connections and Setup Liquid inlets are connected to Hamilton 1000 series glass syringes, 1 ml for the disperse phase, and 2 ml for the high index oil. The continuous phase syringe is actuated by a precision syringe-pump (KD-Scientific). The disperse phase syringe can be either hand actuated or driven by a second syringe-pump. Optical connections to the device are made with 124 µm coating, 110 µm cladding, and 100 µm core diameter step index multimode optical fiber (UM22100,Thorlabs). Since the fibers outer diameter is slightly

Figure 11: Measurement setup used for the verification of the chips functionality

The laser light sources were turned on and let to warm up, to reach stable operating conditions. The channel was primed with oil, and a constant oil flow with a flow rate of 5 µl/min was established. The flowrate was set as slow as possible, as to observe all phases of the movement correctly and to confirm the theoretical expectations.

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Running Droplet Optical Multiplexer |

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The droplet length, which is controllable through the flowrates of the liquids has to be at least 1.2 mm. This minimum can be calculated by trigonometry from the size of the light beam hitting the channel, as in Fig. 12 or through ray-tracing simulation.

Channel Wall

Beam Width / min. Droplet Size 1.2mm

0.11mm 8.85°

Fiber

0.6mm 15°

Figure 12: Parameters of the minimum droplet size

Figure 13: Switching of a single source to the output

Since the light will only be coupled into the waveguide at the beginning of the droplet, longer droplets do not influence the path of the light. Hence there is no maximum droplet length. A single droplet of about 5 mm length adheres to those conditions and was then created. The creation of the droplet was observed under a microscope. Alternatively a string of droplets can be created by a 250 nl, 150 µl/min pulse with a second syringe pump. The signal from the photodiode was recorded through a Tectronix DSO-X 2014A Oscilloscope. Amplification of the photodiode was set to 70 dB.

4. The switch is turning off. A spike in light can be observed, since the fringes of the droplet now concentrate the light into the channel. This takes about 0.3 s 5. The droplet has moved past the input fiber. Since the droplet interrupts the waveguide now at a place before the fiber, even the stray light can not reach the output anymore. This consitutes a drop in output voltage almost to zero. 6. Once the droplet has left the channel, the stray light comes back to normal level.

4 Results & Discussion From the measurement results, as seen in Fig. 13, the following observations can be made: The switching can be differentiated in 5 phases annotated in the diagram: 1. The switch is turned off, no droplet is present, and the light arriving at the photodiode is stray light, reflected off the top and bottom of the chip and coupled back into the fiber. In the measurement taken, this equals to about 15 mV of output voltage. 2. The switch is turning on and the droplet is moving into place. A drop in stray light can be observed since it is deflected away from the fringes of the droplet. This takes about 1 s at the set flowrate. 3. The switch is turned on. An output voltage of 18 mV is measured for 1.3 s.

The turn-on and turn-off time is connected to the time that the droplet needs to travel, and because of this depends on the time each input is turned on. With higher droplet speeds, the time it takes to turn on and off is reduced in the same way as the time that the signal is valid. According to the simulation, the on/off ratio of the mulitplexer should be high, since theoretically all light is coupled into the waveguide. However, as the measurements show, the system posseses a on/off ratio of 1.2 or 1.6 dB. Two reasons can be attributed to this: Since the current technology only permits the use of two-dimensional lenses, not all light is collimated before entering the waveguide. This light travels over and under the lenses, as the ray-trace simulation in Fig. 14 shows. Further, the light is scattered from minor imperfection in the chips material and the top and bottom surface of the device. This light hits the channel in different angles, some of which are then guided in the channel. Positive results were established for the light that was collimated successfully, proving the feasability of our idea

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References [1]

Figure 14: 3D-Ray trace with cylinder lenses, view perpendicular to optical plane

as well as device. This light is exhibiting behaviour like predicted. The droplet creating T-Junction not only increases the ease of setup and use of our device, but eliminates stray droplets that were a problem before.

5 Conclusion We have shown the design and the experimental performance of new (8x1) running droplet optical multiplexer driven by a movable droplet in a microfluidic channel. The experiments show that with this device the signal form different inputs can be detected by the output, depending on the position of the droplet. An improved microfluidic structure with a T-Junction for droplet generation simplified handling and provided more control. The simulation and theoretical calculations further prove the feasibily and provide an incentive for further research.

J. Pawley, Handbook of biological confocal microscopy. Springer, 2010. [2] M. Rosenauer, M. J. Vellekoop, Characterization of a Microflow Cytometer with an Integrated 3D Optofluidic Lens System, Biomicrofluidics 4, 043005 (12 pages), 2010. [3] V. Fioravanti, E. Weber, S. van den Driesche, M. J. Vellekoop, D. Pucciarelli, H. Breiteneder, and C. Hafner, Biopsy analysis using a quadruple infrared sensor, IEEE SENSORS, 2013 pp.1,4, 3-6 Nov. 2013. [4] E. Weber, M. J. Vellekoop, Optofluidic micro-sensors for the determination of liquid concentrations, Lab Chip. 2012 (19):37549. [5] L. Brandhoff, E. Weber, S. van den Driesche, M. Bülters, R. B. Bergmann, and M. J. Vellekoop, Optofluidic multiplexing and switching device, 2013 Transducers & Eurosensors XXVII: The 17th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers & Eurosensors XXVII), pp.2329,2332, 16-20 June 2013. [6] E. Weber, D. Puchberger-Enengl, F. Keplinger, et al., In-line characterization and identification of micro-droplets on-chip. Optofluidics, Microfluidics and Nanofluidics, (2013). [7] P. Garstecki, M. J. Fuerstman, H. A. Stone, and G. M. Whitesides, Formation of droplets and bubbles in a microfluidic T-junction— scaling and mechanism of break-up, Lab on a Chip, Bd. 6, No. 3, S. 437, 2006. [8] D. C. Duffy, J. C. McDonald, O. J. A. Schueller, and G. M. Whitesides, Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane), Analytical Chemistry, Bd. 70, No. 23, S. 4974–4984, Dec. 1998. [9] Microchem, SU-8 3000 Permanent Epoxy Negative Photoresist Datasheet, http://www.microchem.com/pdf/SU8{%}203000{%}20Data{%}20Sheet.pdf. [10] F. Schneider, J. Draheim, R. Kamberger, and U. Wallrabe, Process and material properties of polydimethylsiloxane (PDMS) for Optical MEMS, Sensors and Actuators A: Physical, Bd. 151, No. 2, S. 95–99, Apr. 2009. [11] D. R. Lide, CRC Handbook of Chemistry and Physics, CRC Press, 2007.

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