Photonic Crystals for Optofluidic Transport
It is a well known fact that eletromagnetic waves (light) can exert radiation pressure on objects that it is incident upon. In certain situations where these objects have low mass and are in vacuum or fluid solution (such that frictional forces are low) these optical forces can actually be used to manipulate particles. By using a loosely focussed laser beam it is possible to push particles in the direction of laser propagation. On the other hand if the laser is tightly focussed the particles tend to experience a gradient force that pushes them towards this region of high optical intensity by a strong gradient force. This phenomena is what has been exploited very effectively over the last decade in the form of optical tweezers.
The reason I was interested in using optical forces for transport of particles in a fluidic medium is because the scattering force on particles has a 6th power dependence on the particle size making it an extremely size sensitive force. It is possible to envision exploiting this phenomena to perform ultra-sensitive size based particle separations. The drawback with traditional techniques at the time is that people used a loosely focussed gaussian laser beam to move particles and the distance over which particles could be transported was limitted by the depth of focus of the objective (on the order of a few hundred microns to a few milimeters). To overcome this limitation Prof. Erickson and I decided to work on developing a liquid-core waveguiding structure to confine the laser light in the liquid core over long distances so that we could exert these optical forces over centimeters-meters. The figure below helps explain this idea. As you can see, with a typical loosely focused gaussian laser beam, the intensity drops off very rapidly preventing transport over large distances. However a liquid-core waveguiding structure overcomes this limitation.
To create this liquid-core waveguiding structure we used Hollow Core Photonic Crystal Bandgap Fibers (PCF). We modified the structure of these fibers by fusing the outer capillaries thus filling only the central core with liquid (water) while keeping the all the surrounding capillaries air-filled. In this manner we assured light guidance via total internal reflection (since the effective refractive index of the liquid-core is larger than that of the surrounding air filled capillary cladding). Our experimental setup is shown in the figure above. We dipped the end of a liquid-core PCF (LCPCF) in an aqueous solution of fluorescent polystyrene particles which we imaged using a microscope objective and a CCD camera. The laser light was coupled vertically into the liquid-core thus pushing the polystyrene particles up through the liquid fiber core.
The videos below show some experimental results. In the first one we see a steady stream of polystyrene particles being transported up through the liquid-core fiber. They appear to have a band like appearance due to diffraction of the fluorescent light at the outter air filled capillaries in the cladding region of the fiber. The second video shows a single particle being pushed up the fiber. On lowering the laser power we observe the particle to come to rest at a height where the scattering force pushing it up is balanced by its weight. When the laser power is lowered further, the particle begins to sink down since its weight overcomes the upward acting scattering force. The last video shows an interesting “Band formation” effect that we observed wherein different 3 micron polystyrene particles all traveled up the fiber and came to rest at an equilibrium position thus forming a band.
For more details regarding this work please refer to my publication in Applied Physics Letters: Mandal, S., Erickson, D., “Nanoscale Optofluidic Sensor Arrays” Optics Express, 16, 1623-1631 (2008).
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