Measurements of charged colloidal bulk moduli using optical bottles

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.