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A method is described for measuring the potential energy of nano-particles in an optical trap by trapping an ensemble of particles with a focused laser beam. The force balance between repulsive osmotic and confining gradient-force pressures determines the single-particle trapping potential independent of interactions between the particles. The ensemble nature of the measurement permits evaluation of single-particle trapping energies much smaller than kBT. Energies obtained by this method are compared to those of single-particle methods as well as to theoretical calculations based on classical electromagnetic optics.
(Right)Schematic depicting the force balance of radiation and osmotic pressures experienced by the colloidal suspension. (Left) Experimental and calculated values of U0 per milliwatt of laser power versus particle size. The refractive-index mismatch between the coverslip and the water was accounted for in the calculation.
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