Measurement of the optical trapping force on micro-particles immersed in fluids

Monica Nadasan*, Revati Kalkami, Enrico Ferrari, Valeria Garbin, Dan Cojoc, Enzo Di Fabrizio

*Corresponding author for this work

Research output: Contribution to journalConference articlepeer-review

1 Scopus citations


A strongly focused laser beam through an objective microscope with high NA allows the trapping of dielectric particles with micrometric sizes. The trapping force is proportional to the power of the laser, the relative refractive index (the ratio between the refractive index of the particle and the refractive index of the medium surrounding it) and the trapping geometry (shape of the laser beam, shape of the particle, transmission and reflection coefficients). Numerical models to evaluate the trapping force can be developed for simple geometrical shapes of the trapped particle. For particles with complicated shapes the trapping force should be measured experimentally. The goal of this paper is to evaluate a measurement method based on the equilibrium between the drag force in a fluid with knows viscosity and the transversal trapping force. A particle with a known size is first trapped in a cell filled with water. After stable trapping, the cell is shifted with controlled velocities using piezoelectric actuators. If the velocity exceeds a certain threshold, the particle escapes from the trap. This threshold allows to determine the trapping force. Experimental results obtained with high and low index particles are presented and discussed.

Original languageEnglish (US)
Article number597209
JournalProceedings of SPIE - The International Society for Optical Engineering
StatePublished - 2005
Externally publishedYes
EventAdvanced Topics in Optoelectronics, Microelectronics, and Nanotechnologies II - Bucharest, Romania
Duration: Nov 24 2004Nov 26 2004


  • Micromanipulation
  • Optical tweezers
  • Stoke's force
  • Trapping force

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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