OPTOFLUIDIC TWEEZERS: MANIPULATION OF OIL DROPLETS WITH 105 GREATER FORCE THAN OPTICAL TWEEZERSG.K. Kurup1 and Amar S. Basu1,21Electrical 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
Optical techniques for droplet manipulation have always been more important than mechanical techniques because : provide dynamic control needed for programmable real time manipulation. it doesnt 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. Optoelectronic tweezers (OET), originally designed to manipulate dielectric particles in an aqueous phase , have been adapted to manipulate oil-in-water droplets with nN forces ; 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 ,[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.
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 , optofluidic tweezers are 100 stronger than OET, and 105-106 times stronger than optical tweezers.
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.
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  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.
SimulationMultiphase CFD simulations (Figure 2) illustrate the effect of a local reduction in IFT actingon a 200 μm oil-in-water droplet.In the trapping simulation (part A), the vortex flows induced by the IFT profile pull thedroplet toward the substrate.
SimulationIf the laser is scanned (part B), the illumination becomes laterally non-uniform, and theresulting vortices pull the droplet toward the axis of the laser.The maximum scanning velocity of the droplet is determined by the droplet’shydrodynamic drag (proportional to drop radius) and the magnitude of IFT reduction,which is proportional to the heating from the laser.
Experimental SetupThe experimental setup (Figure 3) is compatible with astandard inverted fluorescence microscope.A 150 mW, 405 nm diode laser is aligned in thefluorescence port, and is directed to the sample througha filter cube.A 10X objective focuses the laser to a spot size of a few10’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 with10 parts water, and sonicated to produce droplets ofvarious diameters.
Experimental SetupIn some experiments, fluorescent particles(Magnaflux) were also added to the oil phasefor visualization.The oil/water emulsion was pipetted onto aglass slide containing a plastic ring to containthe fluid.In droplet translation experiments, themechanical stage of the microscope is movedlaterally so that the droplet moves relative tothe surrounding fluid, but the droplet itselfremains aligned to the laser.
Results And DiscussionTrapping of 50 and 200 μm diameter oildroplets is shown in Figure 4.A laser positioned near the edge of a dropletgenerates asymmetric thermocapillary flowswhich pull the droplet toward the laser’s focalpoint.When the droplet and laser are aligned, theflow is symmetric, leading to balanced lateralforces which trap the droplet .The flows also pull the droplet vertically downfrom the surface to the glass substrate (Figure1).
Results And DiscussionThe apparent increase in radius after trapping (C)is due to the drop deforming once it reaches theglass substrate.The time varying flow patterns are visualizedusing fluorescent tracers (D-F).During the trapping process, the flows areasymmetric, leading to imbalanced forces whichpull the drop toward the laser.Once trapped, the flows are axisymmetric,yielding zero net lateral force on the droplet.
Results And DiscussionTrapped droplets can be translated in twodimensions, by either moving the stage orscanning the laser (Figure 5).We obtain translational velocities up to 10 dropdiameters/ second and a maximum speed >10mm/s, corresponding to holding forces in theμN range.The large force allows optofluidic tweezers toaccommodate 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 oildroplets because their low thermal conductivity(1/5th of water) forms sharp temperaturegradients, leading to larger thermocapillaryforces.
CONCLUSIONThis paper demonstrates the concept of an optofluidic tweezers, whichtransduces focused light to thermocapillary flows which trap droplets.The large forces allow the trapping, manipulation, and merging of droplets aslarge as 1 mm at speeds of several mm/s.To maintain high temperature gradients, the droplet should have a low thermalconductivity, making this method well suited for oil droplets.The flow localization provides a high spatial resolution and single-dropletaddressability.One advantage of utilizing the liquid-liquid interface compared to a liquid-solid interface (as in OEW based approaches) is the reduced possibility ofsurface contamination
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