Speaker: Ludovico Cademartiri, Assistant Professor of Materials Science and Engineering at Iowa State University
Title:Peering into “black boxes” with materials science: growth of ensembles of crystals and water foraging by plants
Abstract: In this seminar I will discuss two fundamental questions that my students have recently addressed and resolved.
1. How do ensembles of crystals in solution grow in the absence of supersaturation?
The development of ligand-controlled syntheses of ensembles of nanocrystals was considered a milestone towards understanding the limitations of the Gibbsian framework in describing the nucleation and growth of ensembles of crystals. Nonetheless, while more than 2 decades have passed since their inception, these syntheses are still mostly conducted through a trial and error approach, detailed studies of their kinetics have been limited by technical challenges, and their nucleation has spurred an ongoing controversy about the thermodynamic nature of nuclei.
I will describe our 5-year experimental, theoretical, and computational effort in resolving, at least in part, these issues.
Our newly developed model system allowed us to peer deeper than ever before into the kinetics of activated coalescence processes between post-critical nuclei that are neglected by the Gibbsian framework (and are thereby considered “non-classical”). We establish that coalescence between nanocrystals proceeds with rate constants comparable to those between reacting molecules, and show that these rate constants follow an Arrhenius dependence on temperature that is likely determined by two activated processes. On the other hand, we show how the inaccuracies of the the commonly used Smoluchowski’s equation in describing the collision frequencies in crowded colloidal systems can explain how the coalescence of fully formed crystals could have similar rate constants than bimolecular reactions. This glaring problem had been left unexplained till now.
Lastly we show how our model allows, for the first time, to quantitatively predict the growth of nanocrystals as a function of ligand length in non-saturated conditions.
2. Do plants need an external potential gradient to optimize their access to water?
Reliable access to water is the most significant factor determining plant health. Most water is uptaken by plants through their root system. Nonetheless, roots must accomplish a number of other tasks beside foraging for water: stabilizing the plant, interacting with other organisms in soil, etc… Therefore, the development of a root system that secures the survival of a plant in variable environmental conditions is a genetically-encoded, highly complex optimization process.
While foraging by organisms is conducted by strategies that include collective behavior, foraging of water by plants is thought to occur mostly by the growth of roots towards water in response to a water potential gradient or the formation of new roots in the presence of water.
We developed a microfluidic device to answer the question whether plant root really need an external water potential gradient to optimize their access to water. By careful design we were able to reduce the water potential gradients experienced by the plant by 4 orders of magnitude compared to traditional assays.
In spite of the minuscule water potential gradients, we find plants successfully increase the access to water and they do so increasingly well the more water becomes scarce. I will discuss two equally fascinating potential hypotheses that could explain the observations.
Bio: Ludovico Cademartiri obtained a Laurea degree in Materials Science from the University of Parma in 2002 and a PhD in Chemistry from the University of Toronto in 2008. He was then a Postdoctoral Fellow at Harvard University. He is at Iowa State since 2012.
Seminar Host: Xiaoli Tan