Summary

Nucleation dominates the remaining crystallization, crystal growth and the structure and properties of final crystals. Despite in excess of 100 years of study, the mechanism of nucleation is still far from understood mainly because nucleation is a (sub)nanoscale stochastic process. Recent advancements in characterization techniques provided much more information of nucleation but revealed unexpected complexity simultaneously. A number of materials were found to nucleate in multiple steps and through multiple pathways.

It has also been found that an undersaturated electrolyte solutions form clusters of ions, so called prenucleation clusters (PNCs). PNCs grow in size and population with increasing solution concentration. Some molecular dynamics simulation predicted nucleation happened within PNCs. The role of PNCs in complex nucleation is important. However, the nautre of PNCs are still in veil primarily because of their low volume fraction (~ 1%) and small size (~1-2 nm). Such barriers often result in inconclusive results from X-ray or neutron scattering experiments.

To increase the volume fraction of PNCs, a solution must be highly supersaturated, which is typically hindered by heterogeneous nucleation at container walls which may also disturb the intrinsic solution structure. We propose to overcome these barriers by integrating SEL with neutron scattering at ORNL (NeuSEL) and synchrotron X-ray scattering at ANL (X-SEL). Quiescent levitation using SEL enables attaining extremely high supersaturation by solvent evaporation where the high-volume fraction of PNCs significantly improves the sensitivity and quality of scattering data without being disturbed by container walls.

The results of the research will answer the following scientific questions:

1) How do PNCs form?

2) How does the structure of PNCs evolve with concentration?

3) How does dehydration affect the structure and stability of PNCs?

4) How does the breakage of hydration structure due to water depletion at extreme supersaturation affect the PNC structure and nucleation?

5) Are PNCs thermodynamically stable?

Impacts

The properties and performance of many industrial crystal products, including pharmaceuticals, foods, and chemicals such as fertilizers, depend significantly on their morphology and microstructure, which are often determined in the stage of nucleation and crystal growth. For instance, the surface area to volume ratio of a crystal product will affect its dissolution rate, which is important for functional drugs (for example 12-hour or 24- hour pills) or time release fertilizers. Revealing the formation mechanism and structure of PNCs and their role in crystal nucleation will make a meaningful contribution to understanding the mechanism of complex nucleation of a variety of material systems and potentially enable the innovative design and synthesis of advanced functional nanomaterials for diverse applications. Understanding the nucleation mechanism will also provide deeper insight in the prevention and remediation of urinary stones, oilfield scales, or heat exchanger fouling.

Sponsor

National Science Foundation

Summary

Although its large industrial and scietific impacts, the dynamics and stability of a drop has not been adequately understood mainly due to the experimental limitations. The size of drops typically in micron and nano scales which makes their experimental characterization difficult. We are studying the behavior of a charged drop in a strong electric field using the Soft matter Electrostatic Levitator.

A charged (~100 pC)water drop levitates in a strong electric field (~1 MV/m). The surface charge density increases as the drop evaporates and distance between surface charges gets closer. If the Coulomb force approaches to the surface tension force, the drop becomes unstable and elongates along the directon of gravity. Upon further evaporation, some of surface charge is ejected by jetting.

Impacts

Charged drops happen in nature (clouds and raindrops) and in a number of industrial processes (inkjet printing, water-oil separation, and more).

Summary

Continuing pressures for higher performance and efficiency in energy conversion and propulsion systems are driving ever more demanding needs for new materials which can survive high stresses at the elevated temperatures. In such severe environments, the characterization of creep properties becomes indispensable. Conventional techniques for the measurement of creep are limited to about 1,700 °C. A new method which can be applied at temperatures higher than 2,000 °C is strongly demanded. This research presents a non-contact method for the measurements of creep resistance of ultra-high-temperature materials. Using the electrostatic levitation (ESL) facility at NASA Marshall Space Flight Center, a spherical sample was rotated quickly enough to cause creep deformation due to the centripetal acceleration. The deformation of the sample was captured with a digital camera, and the images were then analyzed to measure creep deformation and to estimate the stress exponent in the constitutive equation of the power-law creep.

The containerless creep measurement had been utilized to characterize the creep properties of newly developed ultra-high-temperature materials developed by Aerojet Rocketdyne, General Electric, NASA, and U.S. Airforce.

Impacts

A conventional creep test of ultra-high-temperature materials may take days to weeks depending on their composition and operating conditions (stress and temperature). The containerless creep test allows for accelerated (within several hours) creep tests of ultra-high-temperature materials, which should reduce the leadtime of material development.