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45303-G7
Micro Wax Disks: Fabrication and Characterization
Colloidal disks are important but have not been investigated sufficiently. In petroleum engineering, disk-like asphaltenes in oil production and refining are nano-colloidally suspended in crude oil causing a myriad of problems. Synthetic platelet particles are widely utilized in cosmetic and painting products. In the 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, incomplete statistics on the research papers in colloidal science indicated that sphere: rod: disk = 80: 20: 1. There is a plenty of space to fill in the colloidal disk behavior studies. Therefore, the PI proposed a systematic plan to explore the colloidal disk dispersions, including an experimental platform producing uniform disk particle, and a set of probe techniques for rheological, hydrodynamic and self-assembly behavior study. He aims to contribute this knowledge to both the academic and industrial communities.
The PI has led his group to establish an experimental platform to produce uniform colloidal disk particles using phase-changing eicosene wax emulsions. The entire process includes drop formation via electrospray of melt wax, and following microdisk formation via drop entrancement and stabilization in the ethanol aqueous solution with Tergitol and SDS as surfactants. The emulsion droplets were further converted into disks by keeping the sample at low temperature (4 degree C). The microdisk suspensions can be mass-produced at the rate of a few milliliters per hour having a narrower size distribution than other current synthesis methods for discotic particles. This new emulsification method is a nature extension to the microfluidic emulsification for monodispersed emulsion droplets of tens of microns, and the knowledge of the effects of surface tension and density match on the mono-dispersion formation is also crucial to other techniques, e.g. biomaterials development and drug delivery. Two students in the PI's group have obtained their master degrees in the topics with regards to this experimental platform and continue their career in relevant industries. One exchange undergraduate student from Columbia has conducted the continuing research together with another graduate student on this grant to further improve the uniformity and reach larger size.
Using the wax microdisks as a model system, the PI and his group discovered a novel morphology of disk particles by surfactant control, as well as the disk-to-column transition induced by concentration and depletion interaction. By tuning the concentration of surfactants different aspect ratio colloidal particles could be obtained. The disk particles can further assemble into column phase or irregular aggregates, showing some evidence of crystalline packing at high concentration, and displaying liquid crystal characteristics of eicosene when dried out. The PI has years of interest and extensive research in colloidal crystallization and interactions. With this novel research results, the PI has extended his scientific research to phase-transition materials, liquid crystals and soft condensed matter physics. Additionally, the PI utilized his expertise in microfluidics and laser tweezer, and gave guidance to his students on discotic particles behavior. A few preliminary experiments have been performed in microfluidic channels to study the hydrodynamics of colloidal disks in laminar flow. In the future, besides the hydrodynamics study, a series of experiments will also be conducted using laser tweezer on the particle-particle interaction and self-assembling of microdisks.
In summary, the PI and his group have successfully established a proposed mass-production experiment method of uniform colloidal wax disks using electrospray emulsification, and discovered the shape-changing emulsions via surface control. Based on these fulfillments, the study of columnar assembling of colloidal disks has been launched and a series of techniques, like laser tweezers and microfluidic chips, will be employed to search for the deeper understanding of colloidal interactions and hydrodynamics of disk-shape particles. The PI believes that this knowledge will be helpful to solving the asphaltene aggregates problem in petroleum engineering, and be inspiring to colloidal disk and liquid crystal study, providing a profitable gaining on the grant.
