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.
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