Dynamics of Stratification in Multicomponent Colloidal Films During Evaporative Drying

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Nanoparticle films have wide applications, including antibacterial paints, corrosion-resistant coatings, and battery separator membranes. Often, the locations of certain components in the film must be carefully controlled. This has traditionally been achieved with expensive multi-step fabrication processes. Recent mathematical models of nanoparticle film formation have identified a means to create complex, vertically-structured coatings in a single step, a process referred to as stratification. This process has the potential to decrease production time and costs of multifunctional nanoparticle films. However, there are few experimental studies of the stratification process. In this project, a new experimental technique that involves scanning through the film with a narrowly-focused X-ray beam will be performed during the stratification process. These experiments will follow motion of particles throughout the film formation process, and will ultimately provide information on how stratification depends on the nanoparticle concentration, nanoparticle size, evaporation rate, and other processing parameters. The research will help improve manufacturing processes for multicomponent films used in applications that will benefit human welfare and health, such as inorganic-polymer films for next-generation batteries, drug-loaded coatings for medical implants, and mold-resistant paints for humid environments. Impacts related to STEM workforce development include training of a graduate student and undergraduate students in novel experimental techniques for colloidal materials. The investigator will also aim to recruit students from underrepresented populations for the research. Finally, concepts from the research will be incorporated into new curriculum materials and video lectures on fluid mechanics for high school audiences. Recent research has identified a means to create vertically-structured coatings in a single step during stratification. Theoretical descriptions of binary colloidal mixtures show a variety of stratification regimes as the volume fraction and Peclet number of small and large particles are varied. However, comprehensive comparison of model predictions with experiments has not yet been achieved because of difficulties in (i) quantifying the vertical concentration profile of particles in the film, and (ii) following motion of particles during the film formation process. This project will use small-angle X-ray scattering (SAXS) with a narrowly-focused beam, performed at varying vertical positions in the film, to fully explore the complex stratification behavior predicted by recent models. The objectives of the work are to: (1) create an experimental process diagram of stratification behavior as a function of particle volume fraction and Peclet number; (2) conduct the first measurements of stratification during film formation by performing time-resolved microbeam SAXS; (3) conduct X-ray photon correlation spectroscopy (XPCS) studies to yield knowledge of how particle diffusivity and dynamics vary during film formation; and (4) integrate research into new curricular materials and video lectures on fluid mechanics for high school audiences. The work will provide the first in situ measurements of particle concentration during film formation in multicomponent systems and the first direct experimental test of recent stratification models in both final dried films and during the film formation process. The project will also help establish new advanced scattering techniques for characterization of colloidal materials. Finally, the data and insight gained will aid in driving improvements to theoretical descriptions of stratification. The research will broadly impact development of single-step processes to create multicomponent films with complex concentration profiles, decreasing production time and costs of multifunctional films.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.