Suppression of Sedimentation in Polystyrene Microsphere Suspensions via Femtosecond Laser Irradiation
Micrometer-sized particles denser than their medium settle out of suspension within hours, and Brownian motion is too weak to prevent it. We report that femtosecond laser irradiation at 1030 nm slows this settling in suspensions of 8 um polystyrene microspheres in water, without altering the particles themselves. Suspensions at three concentrations were treated for up to 3 h under weak acoustic agitation or vigorous magnetic stirring, each with and without simultaneous irradiation, and the particles remaining in the probe volume were quantified optically by angle-resolved Mie scattering. Under acoustic agitation, irradiation raised retention in eleven of twelve paired comparisons, at every concentration, by 0.17 on average against a measured aliquot-to-aliquot scatter of 0.02; the twelfth comparison lies within that scatter. Under stirring, which already keeps most particles suspended, it produces no resolvable change. The angular positions of the Mie maxima are unchanged to within about 1% over 3 h, so the particles are neither fragmented nor aggregated and the effect arises from particle transport rather than from changes in the particles. An order-of-magnitude analysis rules out optical forces, the optical Kerr effect, and transient photochemistry, and leaves laser-driven convection: the power absorbed by the water sustains flows orders of magnitude faster than the 1.8 um/s settling velocity. Femtosecond irradiation therefore acts as a non-contact, supplementary agitation source, most effective where mechanical mixing is weak.