Final presentation2

OPTOFLUIDIC TWEEZERS: MANIPULATION OF OIL DROPLETS WITH 105

GREATER FORCE THAN OPTICAL TWEEZERS

G.K. Kurup1 and Amar S. Basu1,2

1Electrical and Computer Engineering Department, 2Biomedical Engineering Department,Wayne State University, Detroit USA

Presentation by : Kumar AvinashStudent ID-101063422Date : 8th January 2013

Course : Sensing and Actuation in Miniaturized SystemsBy :  Prof. Cheng-Hsien Liu

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

Introduction Optical techniques for droplet manipulation have always been more

important than mechanical techniques because :

provide dynamic control needed for programmable real time manipulation.

it doesn't require on chip patterned structures so cheaper fabrication.

Optical Techniques for droplet manipulation

Optical Tweezers. Optoelectronic Tweezers. Optoelectrowetting. Optofluidic Tweezers.

Optical tweezers have been used for droplet manipulation, but they are not ideally suited because they have relatively low force (pN) , and the forces are typically repulsive[1].

Optoelectronic tweezers (OET), originally designed to manipulate dielectric particles in an aqueous phase [2], have been adapted to manipulate oil-in-water droplets with nN forces [3]; however, it requires on chip electrodes providing an in-plane AC electric field.

Optoelectrowetting is a powerful technique which relies on optically modulated wetting properties to transport, merge and split W/O droplets [6],[7but requires require electric field generators and opaque photoconductive substrates which can complicate microscope observation.

Optofluidic Tweezers are thermocapillary -based optical trap which can be used for droplet manipulation.

Introduction

Thermocapillary flow refers to capillary action actuated by temperature gradient.

Thermocapillary effect can generate attractive as well as repulsive forces.

Optofluidic tweezers can trap droplets, manipulate them in a 2-dimensional space, and also merge multiple droplets.

Since thermocapillary forces are in the .1-1μN range [8], optofluidic tweezers are 100 stronger than OET, and 105-106 times stronger than optical tweezers.

Optofluidic Tweezers

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

Optofluidic tweezers rely on optically-driven thermocapillary flow at the liquid- liquid interface of a droplet and the continuous phase.

Focused laser incident on the droplet surface (which contains an absorbing dye) locally increases the temperature on the interface.

The degree of heating depends on the laser intensity, absorptivity of the dye, and the thermal diffusivity of the two phases.

Due to the inverse relation between interfacial tension (IFT) and temperature, the IFT is reduced in the heated region, forming a local gradient.

The non-uniform surface stress generates interfacial Marangoni flow directed away from the heated region.

Theory

Inside the droplet, fluid flows in the opposite direction, forming a toroidal microvortex with axial symmetry.

The vortices exert a viscous shear force on continuous phase [11] which causes the droplet to migrate in the direction of the laser.

In addition, if the droplet is not aligned laterally to the axis of the laser, the asymmetry of the vortices create a net force which ultimately aligns the droplet with the laser.

Theory

We note that optofluidic tweezers are driven by a temperature gradient, not absolute temperature.

A thermal fluid simulation (Fig. 1B) shows that flow velocities several mm/s can be obtained with a 10K temperature differential provided a sharp gradient is formed.

This is possible if the fluid has low thermal conductivity and if the heating is highly localized.

Theory

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

Multiphase CFD simulations (Figure 2) illustrate the effect of a local reduction in IFT acting on a 200 μm oil-in-water droplet.

In the trapping simulation (part A), the vortex flows induced by the IFT profile pull the droplet toward the substrate.

Simulation

If the laser is scanned (part B), the illumination becomes laterally non-uniform, and the resulting vortices pull the droplet toward the axis of the laser. The maximum scanning velocity of the droplet is determined by the droplet’s hydrodynamic drag (proportional to drop radius) and the magnitude of IFT reduction, which is proportional to the heating from the laser.

Simulation

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

The experimental setup (Figure 3) is compatible with a standard inverted fluorescence microscope.

A 150 mW, 405 nm diode laser is aligned in the fluorescence port, and is directed to the sample through a filter cube.

A 10X objective focuses the laser to a spot size of a few 10’s of μm depending on the aperture of the diode laser.

Images are captured by a mounted CCD camera.

Oleic acid is dyed with solvent yellow #14, mixed with 10 parts water, and sonicated to produce droplets of various diameters.

Experimental Setup

In some experiments, fluorescent particles (Magnaflux) were also added to the oil phase for visualization.

The oil/water emulsion was pipetted onto a glass slide containing a plastic ring to contain the fluid.

In droplet translation experiments, the mechanical stage of the microscope is moved laterally so that the droplet moves relative to the surrounding fluid, but the droplet itself remains aligned to the laser.

Experimental Setup

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

Trapping of 50 and 200 μm diameter oil droplets is shown in Figure 4.

A laser positioned near the edge of a droplet generates asymmetric thermocapillary flows which pull the droplet toward the laser’s focal point.

When the droplet and laser are aligned, the flow is symmetric, leading to balanced lateral forces which trap the droplet [9].

The flows also pull the droplet vertically down from the surface to the glass substrate (Figure 1).

Results And Discussion

The apparent increase in radius after trapping (C) is due to the drop deforming once it reaches the glass substrate.

The time varying flow patterns are visualized using fluorescent tracers (D-F).

During the trapping process, the flows are asymmetric, leading to imbalanced forces which pull the drop toward the laser. Once trapped, the flows are axisymmetric, yielding zero net lateral force on the droplet.

Results And Discussion

Trapped droplets can be translated in two dimensions, by either moving the stage or scanning the laser (Figure 5).

We obtain translational velocities up to 10 drop diameters/ second and a maximum speed >10 mm/s, corresponding to holding forces in the μN range.

The large force allows optofluidic tweezers to accommodate a wide range of droplets (20-1000 μm). If a droplet is dragged toward a second droplet, they spontaneously merge.

Currently, this technique is well suited to oil droplets because their low thermal conductivity (1/5th of water) forms sharp temperature gradients, leading to larger thermocapillary forces.

Results And Discussion

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

This paper demonstrates the concept of an optofluidic tweezers, which transduces focused light to thermocapillary flows which trap droplets.

The large forces allow the trapping, manipulation, and merging of droplets as large as 1 mm at speeds of several mm/s.

To maintain high temperature gradients, the droplet should have a low thermal conductivity, making this method well suited for oil droplets.

The flow localization provides a high spatial resolution and single-droplet addressability.

One advantage of utilizing the liquid-liquid interface compared to a liquid-solid interface (as in OEW based approaches) is the reduced possibility of surface contamination

CONCLUSION

INTRODUCTION

THEORY

SIMULATION

EXPERIMENTAL SETUP

RESULTS AND

DISCUSSION

CONCLUSION

REFERENCES

OUTLINE

References

[1] R. M. Lorenz, J. S. Edgar, G. D. M. Jeffries, and D. T. Chiu, “Microfluidic and Optical Systems for the On-Demand Generation and Manipulation of Single Femtoliter-Volume Aqueous Droplets,” Analytical Chemistry, vol. 78, no. 18, pp. 6433-6439, 2006.[2] P. Y. Chiou, A. T. Ohta, and M. C. Wu, “Massively parallel manipulation of single cells and microparticles using optical images,” Nature, vol. 436, no. 7049, pp. 370–372, 2005.[3] S. Park et al., “Floating electrode optoelectronic tweezers: Light-driven dielectrophoretic droplet manipulation in electrically insulating oil medium,” Applied Physics Letters, vol. 92, no. 15, p. 151101, 2008.[4] S. K. Cho, H. Moon, and C. J. Kim, “Creating, transporting, cutting, and merging liquid droplets by electrowettingbased actuation for digital microfluidic circuits,” Journal of Microelectromechanical Systems, vol. 12, pp. 70-80, 2003.[5] M. G. Pollack, R. B. Fair, and A. D. Shenderov, “Electrowetting-based actuation of liquid droplets for microfluidic applications,” Applied Physics Letters, vol. 77, no. 11, p. 1725, 2000.[6] H.-S. Chuang, A. Kumar, and S. T. Wereley, “Open optoelectrowetting droplet actuation,” Applied Physics Letters, vol. 93, no. 6, p. 064104, 2008.[7] S.-Y. Park, M. A. Teitell, and E. P. Y. Chiou, “Single-sided continuous optoelectrowetting (SCOEW) for droplet manipulation with light patterns,” Lab on a Chip, vol. 10, no. 13, p. 1655, 2010.[8] C. Baroud, J.-P. Delville, F. Gallaire, and R. Wunenburger, “Thermocapillary valve for droplet production and sorting,” Physical Review E, vol. 75, no. 4, Apr. 2007.[9] G. K. Kurup and A. S. Basu, “Rolling, Aligning, and Trapping Droplets on a Laser Beam using Marangoni Optofluidic Tweezers,” in Proc. International Solid-State Sensors, Actuators and Microsystems Conference (Transducers), Beijing, China, 2011, pp. 266-269.[10] A. S. Basu and Y. B. Gianchandani, “Virtual microfluidic traps, filters, channels and pumps using Marangoni flows,” Journal of Micromechanics and Microengineering, vol. 18, no. 11, p. 115031, 2008.[11] R. Subramanian, The motion of bubbles and drops in reduced gravity. Cambridge: Cambridge University Press, 2005.[12] G. Chavepeyer, “Temperature Dependence of Interfacial Tension between Normal Organic Acids and Water,” Journal of Colloid and Interface Science, vol. 167, no. 2, pp. 464-466, Oct. 1994.

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