Agglomeration of gold nanoparticles under microgravity conditions
Nanoparticles attract interests in research and industry because their small size implies properties that deviate from those of the bulk material and can be exploited in materials such as nanocomposites.[Jeevanandam et al.] Agglomeration is a very common process during processing that changes the effective particle size and can therefore affect material properties.[Ranga Reddy et al.] A deeper understanding of the agglomeration process is essential to prevent or promote agglomeration in order to produce materials with desired properties. The in-situ observation of agglomeration in low-viscosity liquids is often challenging, because agglomeration leads to gravitational sedimentation that alters overall structure and impeding measurements. Microgravity prevents sedimentation.
We investigated the agglomeration of metal nanoparticles under microgravity conditions to study how gravity affects agglomerate structure. Initial experiments were performed in the drop tower of ZARM in Bremen (146 m) that provides a microgravity interval of 9.3 seconds. We had to create a rapidly agglomerating particle system that can be induced at the beginning of the fall and observed at sufficient temporal evolution. Dynamic light scattering (DLS) measurements were repeated at a short interval of 1 s in order to follow agglomerate growth. Such measurements are comparatively robust and provide robust signals on the movement of particles inside the liquid during and after the drop. Gold nanoparticles with diameters of 4-10 nm were stabilized with a organic ligand shells such that they reversibly agglomerated upon a temperature change of 20-60 K.[Kister et al.] Fast cooling was achieved by inject a hot particle dispersion into a cold measurement cell. We analyzed the light scattering and analyzed the slowing of particle dynamics after injection.
We will present initial results of three falling tower campaigns that indicate a reproducible and considerable change in the time-dependent scattering signals when comparing ground experiments to those performed under microgravity. Extensive reference measurements will be discussed to confirm that the observed change is due to gravitational effects. We will introduce several hypotheses on mechanisms that could explain the change.