Optical Tweezers

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Optical Tweezers

• rolf

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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

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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.

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Trapping a particle with light
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Optical trapping of dielectric spheres

• Force due to refraction is always toward the focus

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What about reflection?
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Dual beam tweezer design
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Dual-beam Tweezers are nice

• But we arent 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 diagram
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Full view
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Side view
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Top view
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Room light
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Laser light
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Flow cell
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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 ?, 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.

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Apply a known force

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

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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.

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Velocity as a function of ?h

• 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
  ?h.

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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

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Overstretching transition of DNA

• http//www.atsweb.neu.edu/mark/opticaltweezersmovi
  es.html

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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

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Schedule
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 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 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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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/S
  05Tweezer/
• 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.