This proposal focuses on an undeveloped subject, how fast colloidal particles rotate. The recent development of optically anisotropic MOON particles (modulated optical nanoparticles) by Kopelman and coworkers sets the stage for much interesting work predicated on this conceptual breakthrough. Specifically: (a) Task 1: Translation-rotation decoupling in concentrated colloidal suspensions. Brownian motion, manifested as rotation, will be studied at the single-particle level.; (b) Task 2: New and interesting anisotropic colloidal structures with MOON particles as building blocks will show dynamical rearrangements proposed to study here; (c) Task 3: Magnetic fields will be used to drive rotation, thus enabling not only linear and nonlinear viscoelastic studies, but also, when a yield strength is exceeded, studies of particle-particle friction. This opens the door to study microrheology/nanorheology at the level of individual colloidal particles. Particle tracking has had a huge impact on the colloid field, yet has dwelt nearly exclusively on translational motion. This proposal argues that the time is right to seek to do the same concerning particle rotation.

The intellectual merit is twofold. First, this will develop new understanding of colloidal dynamics, which is fundamental in many scientific and technical applications, including paints, slurries, powders, raw material extraction, the mechanical behavior of composite and nanocomposite materials -- as well as applications of these materials in other fields such as catalysis and energy harvesting. This research will result in new experimental tools to quantify particle dynamics that is impossible to characterize by conventional methods. Second, the methods developed may find general application in other research groups, as they should also find wider utility after protocols have been developed and their value has been demonstrated. This work will import, into the field of multiphase flow, imaging methods with single-particle sensitivity that hold exceptional promise for this field. This will improve research infrastructure in the multiphase flow field by developing new techniques not previously used in this discipline.

The broader impact is to integrate research and education. As these methods are new to the field of multiphase flow, their use will accomplish a significant intellectual transfer between disciplines. Furthermore, the graduate students will be part of an interdisciplinary research group with other students from chemistry, chemical engineering, physics, and materials science. Efforts will be made to recruit graduate students from underrepresented groups. An REU supplement will be requested so that undergraduates can be involved. This laboratory has an excellent track record of involving women and Hispanic undergraduate researchers starting in their freshman year, continuing until receiving their undergraduate degree, and writing honors theses based on this research. This proposal discusses at length the broader impact activities that will be enabled by this proposal, as well as this laboratory?s track record in this area.

Project Report

The primary objective of this research was to develop the subject of how fast colloidal particles rotate. Particles have historically been considered to be homogeneous in their chemical and optical makeup, but here we the different class of Janus particles. Janus particles studied in this research consist of micron-sized spheres whose two hemispheres differ. When only one hemisphere is metallic, we could watch particles turn and tumble, and like "moons" watch the particle rotation in complex environments. When one hemisphere is hydrophobic, we could watch particles assemble into clusters akin to soap micelles, except that the constituents were just spheres. Specifically, optically anisotropic particles were used to understand problems of translation-rotation coupling in concentrated colloidal suspensions, problems of self-assembled structures of Janus particles, and the rotation of colloidal particles by magnetic fields. New image analysis methods were developed to enable the experiments and novel self-assembled microstructures were developed, thus providing guidance to researchers in industry who try to create new applications of Janus materials. Several PhD graduate students participated in this research and have subsequently taken up research and development positions in US research enterprises. Major emphasis was placed on developing their communication and presentation skills. All Ph.D. students presented their work multiple times at the national March Meeting of the American Physical Society as well as at related meetings. Based on the research results reported above, interactions developed with various industrial concerns that were interested to exploit the results of this research.

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University of Illinois Urbana-Champaign
United States
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