Nathan B. Terry and Dr. Justin B. Peatross, Physics and Astronomy
The laser trapping of microscopic particles has many applications, especially in chemistry, biology and physics (1). It is of special interest to utilize optical trapping to levitate small objects for interferometry work. The levitation of a glass microsphere requires first that the Van der Waals forces between the sphere and the surface on which it rests (the contact surface) must be overcome. The first stage of this experiment examines overcoming these Van der Waal forces by mechanically vibrating the contact surface.
A collimated laser beam was created with a two lenses telescope and sent through a series of positioning mirrors and lenses to an objective lens. This objective lens focused the beam to a diameter of 26.8 microns. The laser beam was then imaged onto a piece of white paper. This provided a screen on which the motion of the glass beads could be measured. The beam was approximately one inch from the paper, a distance which allowed the beam to expand, thus magnifying any images contained therein.
Glass spheres were obtained from Minnesota Mining and Manufacturing Co (3M). The beads ranged in size from 10-170 microns. When examined under a light microscope, the beads appeared spherical. It has been proposed that the aforementioned Van der Waals forces can be overcome by placing the spheres on a glass microscope slide, which can be vibrated at its resonance frequency2. This will cause the beads to be forced upward with a force greater than that of the Van der Waals forces.
Two piezoelectric buzzers were obtained from a commercial electronics vendor. The two buzzers were wired together out of phase. A glass microscope slide was sandwiched between the two buzzers with superglue. Since the resonance frequency of the glass could not be readily calculated, the buzzers were connected to a frequency generator.
For the first test, some of the larger beads were placed on the microscope slide. Once the buzzers were turned on the entire range of frequencies from 10 Hz to 10 MHz was scanned. At certain frequencies, most notably 815Hz and 1218Hz, the glass particles were observed to move greatly. Thus at those frequencies, the Van der Waals forces of the microscope slide were overcome. The particles moved upward into the focus of the laser beam. This technique will therefore be used to overcome Van der Waals forces, allowing the particles to be trapped and used for further interferometric experiments.
References
- K. Svoboda, S. M. Block. Opt Lett 19 930 (1994)
- Sranek, Ibor, Jonas. Poc. of SPIE v 80 p 82-90 (1998)