1. 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
Course : Sensing and Actuation in Miniaturized Systems
By : Prof. Cheng-Hsien Liu
Presentation by :
Kumar Avinash
Student ID-101063422
Date : 8th January 2013

2. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

3. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

4.  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.

5.  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.

6.  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.

7. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

8.  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.

9.  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.

10. 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.

11. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

12. Simulation
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.

13. 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.

14. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

15. Experimental Setup
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.

16. 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.

17. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

18. Results And Discussion
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).

19. 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.

20. 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.

21. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

22. CONCLUSION
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

23. OUTLINE
 INTRODUCTION
 THEORY
 SIMULATION
 EXPERIMENTAL SETUP
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES

24. References
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[10] A. S. Basu and Y. B. Gianchandani, “Virtual microfluidic traps, filters, channels and pumps using Marangoni
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25. Thank you for your attention