Optical Tweezers rolf
Project Goals • We will calibrate the strength of an optical trap (Optical Tweezer) • Optical Tweezers may be used to measure very small forces (femtoNewton, 10-15N) • Applications include Biophysics
Description • A laser beam is expanded and collimated. This collimated beam is directed through a microscope objective into a flow cell. Spheres with a higher index of refraction than the medium in the cell (water) will be trapped at the focus of the beam.
Optical trapping of dielectric spheres • Force due to refraction is always toward the focus
Dual-beam Tweezers are nice • But we aren’t going to make one. • Dual beam instruments are more complicated and difficult to align and have at least twice the equipment investment (2 objectives, 2 lasers, etc. • So we are building a single-beam tweezer.
Laser line mirror Laser Beam expander Cell White Light Source CCD CCD Objective Laser line mirror Color Filter Tip Schematic diagram
In the flow cell • We apply a force to the trapped sphere by flowing water through the cell. This force is dependent on radius r, viscosity η, and velocity v of the water. • Within the limits of the strength of the trap, the sphere remains trapped, but undergoes a displacement under the influence of this external force just like a mass on a spring.
Apply a known force • If a known force is applied, and the displacement is measured, the ‘stiffness’ of the optical trap may be determined.
Viscosity, velocity • Viscosity is a function of temperature, which we will measure. • Velocity of the fluid flow through the cell will be derived by dimensions of the cell, and may also be directly measured by displacement vs. time of spheres traveling through the flow cell with the trap inactive.
Velocity as a function of Δh • We will take measurements of flow rate and displacement as a function of time at a range of heights in order to determine v as a function of Δh.
Putting it all together • With the data we will collect, we can determine the stiffness of the trap. • This determined, we could, in future experiments, determine the tiny forces involved in biological processes. For example, the overstretchng transition of DNA:
Overstretching transition of DNA • http://www.atsweb.neu.edu/mark/opticaltweezersmovies.html
Team/Resources • Our team: • People: Rolf Karlstad and Joe Peterson • Equipment: 633 nm laser, microscope objective, CCD camera, dichroic mirrors, white light source, optical table and various optical elements • Where: Physics 66 • Advisor: Kurt Wick • Cell created in student shop
Current Status • High-level overview of progress against schedule • On-track ! • Leak fixing cell • Apparatus built, flow cell built, working out minor issues
Project Goals repeated • We will calibrate the strength of an optical trap (Optical Tweezer) • Optical Tweezers may be used to measure very small forces (femtoNewton, 10-15N)
References • K. Dholakia, P. Reece. Optical micromanipulation takes hold. Nano Today, Volume 1, Number 1. February 2006. • Mark C. Williams. Optical Tweezers: Measuring Piconewton Forces. Previously published in Biophysics Textbook Online. Available at: http://www.biophysics.org/education/williams.pdf • K. Dholakia, G. Spalding, M. MacDonald. Optical tweezers: the next generation. Physics World, October 2002. • B. Tuominen, R .Hoglund. Optical Tweezers. May 2005. At the time of writing available at the MXP website: http://mxp.physics.umn.edu/s05/Projects/S05Tweezer/ • Kurt Wick. University of Minnesota. Minneapolis, MN. February 2006. Private Conversation. • Handbook of Chemistry and Physics, 80th edition. CRC Press, Florida. Pg 6-3. 1999. • Mark C. Williams. Northeastern University, Boston, MA. January 2006. Private correspondence.