ISS: GOALI: Nonequilibrium Processing of Particle Suspensions with Thermal and Electrical Field Gradients

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Colloidal particles are particles with diameters between approximately 1 nanometer and 1 micron. They can be considered as blocks for building materials, like atoms for forming molecules and crystals. In colloidal crystals, the particles are organized in periodic structures with ordering on a range of length scales from nanometers to millimeters or larger. Research on colloidal crystals is motivated by several factors. They can be used to answer fundamental questions related to the assembly of materials, and they have many potential applications in electronics, photonics, and life sciences. However, the rich variety of colloidal crystal structures observed on earth are influenced by the effects of gravity, which leads to particles settling and may lead to motion of the surrounding fluid. In this study, a team of researchers will study colloidal crystal formation in both normal gravity in Earth-based experiments and in microgravity on the International Space Station (ISS). These experiments will reveal the important roles that gravity and external forces play on particle assembly. Ultimately, the researchers will obtain a better understanding of how to manipulate external forces to create colloidal crystals for high resolution 3D printing on Earth and in space. Mechanisms for formation of metastable and glassy phases in particle suspensions will be studied in the ISS and for comparison on Earth. Colloidal particle suspensions are the logical candidates to take advantage of long duration microgravity because (i) they are important for a wide range of terrestrial and space applications, (ii) they share the common feature of coupling between macroscopic particle transport and structure transformations at a particle level and (iii) an adequate explanation for basic features of structure formation in suspensions observed in Earth experiments has not been made to date. Although the time and equipment available for both ground and microgravity experiments is limited, the team of researchers plans to have an impact in many of these areas due to the novel use of field gradients to manipulate the particle density, which is the control parameter for suspensions. Thus, instead of using many different samples to access the interesting range of particle densities, a single sample will be arranged in a field gradient in the sample cell to cover this range. As the particle density is directly measured by microscopy in suspension experiments, a priori knowledge of the gradient profile is not required. The experiments involve setting up the field gradients and observing the resulting structures and then locally mixing a region of known density to watch it glassify or crystallize. Some knowledge of the suspension rheology will come from observations of mixing processes, but quantitative data will come from microrheology measurements through tracking particle thermal motion. The use of colloidal suspensions with real space-time microscopic observations in these studies makes them particularly attractive for teaching students at all levels. We have and will continue to participate in education programs that use space themes to improve interest and skills in STEM, to work with educational professionals to translate the project work to lesson plans for the benefit of students in middle and high school grades and to engage high school and undergraduate students in laboratory work as well as to train and educate undergraduate and graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.