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Force Measurements Using Optical Tweezers

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Title: Force Measurements Using Optical Tweezers


1
Force Measurements Using Optical Tweezers
  • Ranga Koswatta
  • PHY615 Student Presentation

2
Force due to Light??
3
Review of Optical Tweezers
  • Optical Tweezers have nanometer scale spatial
    resolution and piconewton forces.
  • It can exert forces on the order of 100pN.

4
Review of Optical Tweezers
Image from lecture notes.
5
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.

6
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
7
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
8
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

9
Thank you!
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