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Optical TweezersOptical Tweezers

rolf

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Project GoalsProject 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

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DescriptionDescription

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.

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Trapping a particle with lightTrapping a particle with light

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Optical trapping of dielectric spheresOptical trapping of dielectric spheres

Force due to refraction is always toward the focus

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What about reflection?What about reflection?

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Dual beam tweezer designDual beam tweezer design

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Dual-beam Tweezers are niceDual-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.

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Schematic diagramSchematic diagram

Laser

Beam expander

White Light Source

Cell

CCDCCD

Tip

ObjectiveColor Filter

Laser line mirror

Laser line mirror

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Full viewFull view

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Side viewSide view

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Top viewTop view

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Room lightRoom light

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Laser lightLaser light

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Flow cellFlow cell

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In the flow cellIn 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.

rvFdrag 6

kxF

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Apply a known forceApply a known force

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

x

rvk

6

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Viscosity, velocityViscosity, 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.

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Velocity as a function of Velocity as a function of ΔΔhh

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.

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Putting it all togetherPutting 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:

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Overstretching transition of DNAOverstretching transition of DNA

http://www.atsweb.neu.edu/mark/opticaltweezersmovies.html

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Team/ResourcesTeam/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

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ScheduleSchedule

Week 1 2/20-2/24 Begin set-up of tweezers apparatus. Determine how to construct flow cell.

Week 2 2/27-3/3 Finish set-up of tweezers. Continue constructing flow cell.

Week 3 3/6-3/10 Finish flow cell construction and integrate into the rest of the experimental set-up. Try to trap particles.

Week 4 3/13-3/17 Spring Break

Week 5 3/20-3/24 Measure height dependant flow rate of water through cell.

Week 6 3/27-3/31 Finish flow rate measurements. Begin measuring position changes of trapped particles under viscous drag forces.

Week 7 4/3-4/7 Continue to measure position changes of trapped particles.

Week 8 4/10-4/14 Finish data taking, begin data analysis

Week 9 4/17-4/21 Finish data analysis, begin final report.

Week 10 4/24-4/28 Finish final report.

Week 11 5/1-5/5 Final presentations.

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Current StatusCurrent Status

High-level overview of progress against schedule– On-track !– Leak fixing cell – Apparatus built, flow cell built, working

out minor issues

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Project Goals repeatedProject 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)

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ReferencesReferences 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.