1 Optical Tweezers rolf

2 Project Goals We will calibrate the strength of an optical trap (Optical Tweezer) Optical Tweezers may be used to measure very small forces (femtoNewton, N) Applications include Biophysics

3 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.

4 Trapping a particle with light

5 Optical trapping of dielectric spheres Force due to refraction is always toward the focus

6 What about reflection?

7 Dual beam tweezer design

8 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.

9 Schematic diagram Laser Beam expander White Light Source Cell CCD Tip Objective Color Filter Laser line mirror

10 Full view

11 Side view

12 Top view

13 Room light

14 Laser light

15 Flow cell

16 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.

17 Apply a known force If a known force is applied, and the displacement is measured, the ‘stiffness’ of the optical trap may be determined.

18 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.

19 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.

20 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:

21 Overstretching transition of DNA

22 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

23 Schedule Week 12/20-2/24Begin set-up of tweezers apparatus. Determine how to construct flow cell. Week 22/27-3/3Finish set-up of tweezers. Continue constructing flow cell. Week 33/6-3/10Finish flow cell construction and integrate into the rest of the experimental set-up. Try to trap particles. Week 43/13-3/17Spring Break Week 53/20-3/24Measure height dependant flow rate of water through cell. Week 63/27-3/31Finish flow rate measurements. Begin measuring position changes of trapped particles under viscous drag forces. Week 74/3-4/7Continue to measure position changes of trapped particles. Week 84/10-4/14Finish data taking, begin data analysis Week 94/17-4/21Finish data analysis, begin final report. Week 104/24-4/28Finish final report. Week 115/1-5/5Final presentations.

24 Current Status High-level overview of progress against schedule –On-track ! –Leak fixing cell –Apparatus built, flow cell built, working out minor issues

25 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, N)

26 References K. Dholakia, P. Reece. Optical micromanipulation takes hold. Nano Today, Volume 1, Number 1. February Mark C. Williams. Optical Tweezers: Measuring Piconewton Forces. Previously published in Biophysics Textbook Online. Available at: K. Dholakia, G. Spalding, M. MacDonald. Optical tweezers: the next generation. Physics World, October B. Tuominen, R.Hoglund. Optical Tweezers. May At the time of writing available at the MXP website: Kurt Wick. University of Minnesota. Minneapolis, MN. February Private Conversation. Handbook of Chemistry and Physics, 80th edition. CRC Press, Florida. Pg Mark C. Williams. Northeastern University, Boston, MA. January Private correspondence.