45303-G7
Micro Wax Disks: Fabrication and Characterization
Colloidal disks are significant anisotropic particles. In petroleum engineering, disk-like asphaltenes in oil production and refining are suspended in crude oils causing a myriad of problems. Synthetic platelet particles are widely utilized in cosmetic and painting products. In nature, the human red blood cells and the clay particles are also disk-like particles, and the knowledge of their self-assembly and rheological characteristics is vital for people to conquer certain diseases and environmental problems. However, colloidal disk behaviors are not sufficient studied. Therefore, the PI proposed a systematic plan to explore the colloidal disk dispersions, including an experimental platform to produce uniform disk particles, and a set of probe techniques to characterize the rheological, hydrodynamic and self-assembly behaviors. He aimed to contribute this knowledge to both the academic and industrial communities.
The PI has conducted extensive research to establish an experimental platform for the production of uniform colloidal disk particles as well as high aspect ratio inorganic nanoplatelets. Phase-changing α-eicosene wax emulsions were fabricated via the electrospray of melt wax and followed by drop entrancement and stabilization in the ethanol aqueous solution with Tergitol and SDS as surfactants. The emulsion droplets were then transformed into microdisks when kept at low temperature (4oC). Using co-surfactants, the aspect ratio of microdisks (diameter/thickness = 1 to 8) could be controlled. The PI confirmed the hypothesis that the flat-disk structure of the disks arose when the cooled wax entered the rotator phase (similar to smectic phase). The lamellar planes lie parallel to the flat faces, and the in-plane orientational disorder of wax molecules facilitates an in-plane isotropic surface energy permitting a circular perimeter. The knowledge of the effects of surface tension and density-match on the mono-dispersion formation will be also crucial to other techniques, e.g., biomaterials development and drug delivery. Inorganic zirconium phosphate (ZrP) nanoplatelets has been synthesized by the hydro-thermo method. The thickness of the nanoplatelets could be precisely obtained by exfoliation using Thiobarbituric acid (TBA), which results in a large aspect ratio (diameter/thickness = 10 to 300). The inorganic nanoplatelets provide different morphological, mechanical and surface properties, thus has brought in comparison and generality to the proposed colloidal disk system.
The PI has given guidance to three PhD students on the research of discotic collids. They have discovered a novel fluorescent confocal microscopy technique to study the disk-to-column self-assembly induced by concentration and depletion interaction. Different shapes of the colloidal particles, either flat-surface disk or red-blood-cell-like disk, could also be characterized using confocal microscopy. The disk particles assemble into columnar structure and show some evidence of crystalline packing at high concentration. The PI has years of interest and extensive research in colloidal crystallization and interactions. With these novel research results, the PI plans to extend to the study of field induced (shear, electric or magnetic field) self-assembly, rheology characterization, and sphere-disk separation in microchannels. Additionally, utilizing his expertise in rheology, the PI gave guidance to his students on sedimentation of discotic particles. A few experiments have been performed in an analytical ultracentrifuge to measure the hindrance functions in sedimentation of nanoplatelets and wax microdisks. A semi-dilute region in volume fraction of disks regarding sedimentation has been revealed, which does not exist for spheres, and the hindrance functions are revealed to be affected by aspect ratio. Phase transition has also been observed in the ZrP nanoplatelets of identical thickness. Hydrodynamics of colloidal disks in laminar flow has been studied by one undergraduate student in a summer project. The translation and flipping of disks in microchannels were recorded by fast camera and the effect of channel geometry is analyzed. In the future, besides the hydrodynamics study, a series of experiments will also be conducted using laser tweezers on the particle-particle interaction and self-assembling of microdisks.
