Force Measurements Using Optical Tweezers

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Force Measurements Using Optical Tweezers

• Ranga Koswatta
• PHY615 Student Presentation

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Force due to Light??
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Review of Optical Tweezers

• Optical Tweezers have nanometer scale spatial
  resolution and piconewton forces.
• It can exert forces on the order of 100pN.

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Review of Optical Tweezers
Image from lecture notes.
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Two Modes of Operating Optical Tweezers

• If the size of the particle is significantly
  bigger than the wavelength of the laser, we can
  apply ray optics to explain the force exerted on
  the particle.
• If the particle is comparable in size to the
  wavelength, the particle should be viewed as a
  dielectric material in an electromagnetic field.

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Forces due to Light

• Light, have a momentum that is given my the de
  Broglie equation,
• F (n /c)??(Sin - Sout)dA
• It can exert forces on the order of 100pN.
• When measuring the external force, the force is
  usually proportional to the distance of the bead
  center from the focus of the laser1.
• F-kX

The force due to the refraction and reflection of
light. The net force is upward. Image from the
lecture notes
1. Robert Simmons et al, Biophysical Journal, 1996
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Force Measurement

• The distance could be measured with a 10nm
  accuracy1.
• The force could be measured up to a 0.5pN
  resolution1.
• The relative force calibrated using multiple
  methods.

1. Williams, Biophysics
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Applications

• Studying the unfolding and refolding of E. coli
  RNase H1.
• Study adhesion of Helicobacter pylori proteins
  with receptors2.
• Study, properties of the force exerted by
  filopodia and lamellipodia and the involvement of
  cytoskeletal components3.

• Cecconi et al, Science, 2005
• Oscar Björnham et al, Journal of Biomedical
  Optics, 2005
• Cojoc et al, PLoS ONE, 2007

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Thank you!