(727h) An Extended Nanoparticle Haloing Study in Microgravity

Negligibly charged colloid microspheres, which flocculate when suspended alone in aqueous solution, form a stabilized homogenous fluid upon the addition of a critical volume fraction of highly charged nanoparticles. It has been previously reported that nanoparticles are not absorbed on the microspheres in the suspension yet contribute towards the effective charge buildup of the microspheres. This colloid stabilizing phenomenon has been termed nanoparticle haloing as it is proposed that the charged nanoparticles form a â??haloâ? structure around the larger microparticles. Previous work conducted nanoparticle stability experiments through the observation of colloidal structures after the particles have settled. On earth, the presence of gravitational forces prevents long-term observations of nanoparticle haloing stabilization mechanisms. In our set of microgravity experiments, conducted on the International Space Station (ISS), silica spheres (600 nm) at 1.0% volume fraction were suspended along with different concentrations (0.1%, 0.05% & 0.01%) of charged zirconia nanoparticles (8 nm). The media was an aqueous solution of nitric acid (pH ~2) to reach the larger colloidsâ?? isoelectric point.. Fifteen samples (five of each nanoparticle concentration) were sealed in 2.5 mm x 0.2 mm disc-shaped sample chambers circular wells with a stainless steel minirod. Prior to observation, an astronaut manually mixed each sample well for two minutes before loading the sample into the Light Microscopy Module (LMM). Samples were illuminated from the side to induce scattering and a mounted digital camera acquired images at 2.5X magnification. Acquired images from uncompromised samples were analyzed by time depended structure factor S(q,t) analysis. Qualitatively, three of the 0.1% nanoparticle samples exhibited long-term stability for the duration of observation (five weeks), whereas the other samples formed gel-like structures.