When 3:30 PM - 5:00 PM Oct 17, 2014
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Engineering desirable metastable states at the nanoscale: droplets and polymers

Fernando Escobedo
Cornell University, School of Chemical & Biomolecular Engineering

Metastable states are important in numerous natural and man-made processes and materials. While sometimes the goal is to allow a system to attain a thermodynamic state without trapping the system in metastable states, on occasion it is a metastable state which is of primary interest. In such cases, it is important to understand how both molecular interactions and external conditions affect the mean residence time of the system in such a metastable state – or conversely – the rate at which the system transitions to the stable state. In this context, measuring (and possibly “engineering”) the free energy barriers separating stable and metastable states is key to predict and modify the behavior of a system. In this talk I will discuss our work using molecular simulation to study the metastable behavior of two systems designed to: (i) retard the wetting of a phillic rough surface by a fluid, and (ii) enhance the toughness of polymer networks.


The wetting kinetics of a rough surface by a fluid was simulated to elucidate the transition mechanism of a small droplet on a rough surface formed by model nails. The nails provide the re-entrant geometry necessary to keep the droplet in the metastable, non-wetted state. Since the droplet must touch the bottom surface in order to transition, increasing the nail height is an effective way to increase the barrier to wetting for both phobic and slightly phillic drops, but not for very phillic fluids. Overall, our results suggest that non-wettability could be practically enhanced by promoting the "kinetic" trapping of the system in the non-wetted state.


Some natural super-tough materials such as the adhesive in abalone shells maximize toughness by exhibiting a saw-tooth elastic response where the force periodically goes up but drops down each time a threshold is reached, which prevents the breakage of chemical bonds. While each stress “tooth” is the free-energy barrier associated with the loss of a folded domain in nacre, it can also be associated with the creation of a new ordered domain in nematic polydomain-forming elastomers. In the latter case (and contrary to nacre) each stress drop is entropy-driven as stretching induces the rearrangement of chains into more numerous smectic domains. We show that the toughness of regular end-chain crosslinked networks can be optimized by using building blocks that allow control the height and number of the saw-teeth (free-energy barriers). This is done by synergistically leveraging the self-assembling properties of chains that are capable of forming entropy-driven liquid crystalline order (like semiflexible chains) and enthalpy-driven micro-segregated ordered phases (like block copolymers).

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