We report a novel method, the optical bottle that was used to directly measure the osmotic bulk modulus for a colloid
suspension. We determined the bulk modulus by optically trapping multiple nanoparticles and considered a mechanical
balance between the compressive laser gradient force pressure and the resulting resistive osmotic pressure. Osmotic
bulk moduli results measured with the optical bottle are presented for aqueous suspensions of latex particles as a
function of solution ionic strength; and are compared to results from identical samples measured using turbidity spectra.
With the right proportions, a binary suspension of different sized particles may be subject to
entropic effects that can generate a depletion-induced attraction between large particles. A
manifestation of the induced attraction is the enhanced osmotic compressibility of the larger species
in the presence of the smaller species. We conducted an experimental study on how such an
enhancement is affected for 190 nm polystyrene spheres in the presence of polyethelyne-oxide in
aqueous solutions. Using the gradient force from a tightly focused laser, we can locally concentrate
the polystyrene nanoparticles in suspension, and from the changes of local particle density under the
known gradient force, we deduce a quantitative measure of the isothermal compressibility of the
particles. We report the analysis of these compressibilities and their enhancement by the added
polymers for a broad range of particle and polymer concentrations.
This paper reports an experimental study of the interparticle interactions present in a model colloid system composed of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. By independently measuring the fluorescence intensity as a function of particle number density, we were able to determine the relationship between the radiation pressure generated by the optical trap and the resulting number density increase, yielding the calculation of the isothermal compressibility of the colloid system. Optical trapping was made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linearly with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio required to detect the changes in local particle density induced by optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from the diffraction-limited focal region of the two laser beams.
This paper reports an experimental study of the low laser intensity Kerr Effect produced by optical trapping
of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. Optical trapping was
made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linear
with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles
trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio
required to detect the very minute (sub-thermal fluctuation) changes in local particle density induced by
optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from
the diffraction-limited focal region of the two laser beams. By independently measuring the fluorescence
intensity as a function of particle concentration and dn/dC (the change in refractive index due to change in
concentration), we were able to determine the Kerr coefficients for laser trap powers in the range of 10.6
mW to 85 mW. Non-linear behavior in the refractive index vs. laser intensity relationship indicates that
higher order Kerr coefficients are needed to describe the Kerr effect. Kerr coefficients obtained by using a
circularly polarized IR laser were similar to those obtained by a linearly polarized laser, indicating that the
induced electric dipole-dipole interactions did not contribute to the electric field-induced concentration
changes giving rise to the Kerr effect.
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