Henry J. Castejon from Wilkes College is supported by a RUI award from the Chemical Structures, Dynamics and Mechanism program in the Chemistry division to develop a phenomenological approach to study transport properties and interfacial behavior of complex liquids by combining non-equilibrium molecular dynamics (NEMD) and visualization techniques. Dr. Castejon and his students study the transport properties of complex liquids by finding stationary non-equilibrium states in the thermodynamic systems and determining the transport coefficients from the relation between the flux and the conjugate driving forces. The molecular response of the system to the appropriate field gradients is analyzed visually in order to correlate the transport properties with the structural changes during the process.
Professor Castejon and his research group study transport phenomena in ionic liquids which are important environmentally friendly "green" solvents as well as being suitable for a variety of industrial applications such as fuel cells, solar cells, capacitors and batteries. They also study transport processes in biological systems, for example the mobility of macromolecules within a cell.
This program integrates undergraduate research and education into a single hybrid activity. The extensive use of visualization techniques not only allows researchers to analyze the results with exquisite detail but also helps to introduce complex concept. Students participate in multidisciplinary projects and learn how, when and why to use computers in scientific research.
Henry J. Castejon Homepa Transport Properties and Phase Behavior of Complex Liquids This project implemented a phenomenological approach to study the tribological properties and wetting capabilities of ionic liquids. Computational simulations were performed in systems subjected to the exact same conditions used in the experiments and the transport properties were obtained as the response of the system to the perturbations introduced. Wetting Capabilities The wetting capabilities of the liquids were studied by simulating the morphological transformation undergone by a drop of the liquid when going from being isolated in the vacuum to its equilibrium state resting on top of a quartz surface. The figure below shows the equilibration process of the liqui drop on a quartz surface: Figure 1 Once the equilibrium state was achieved the wetting capabilities of the liquid were determined by measuring the contact angles with the surface as depicted in the figure below. Figure 2 The table below shows the magnitude of the contact angles for the ionic liquids investigated. Liquid Silanol Surface Silane Surface ______________________________________ [bmp][pf2] 40.13+/-5.17 45.38+/-4.52 [cpy][tbp] 47.38+/-11.66 56.0+/-7.8 [nba][pf2] 50.13+/-19.12 55.13+/-6.24 [nbt][pf2] 37.63+/-9.39 42.88+/-4.85 [nbt][tf2] 43.63+/-6.28 41.38+/-5.40 ____________________________________ All liquids have contact angles lower than 90 degrees. These values indicate a favorable solid-liquid interaction which in turn suggests good lubrication properties. The hydrophilicity difference between the two surfaces is also demonstrated by the relative magnitude of the contact angles on each of the surfaces. Contact angles on the hydrophilic silanol surface are slightly lower than those on the hydrophobic silane surface. Tribological Properties The tribological properties of the liquids were studied by simulating the lubrication process. The liquid was confined between two quartz surfaces and equilibrated. Once the thermodynamic equilibrium was attained. The lubrication process was simulated by sliding the top surface at a constant velocity while keeping the bottom surface stationary. The animation below demonstrates the computational experiment performed. Figure 3 It can be observed how the top surface "drags" the upper layers of the liquid as it slides toward the right. The moving surface transfers momentum to the liquid and creates a velocity gradient in the direction perpendicular to the surface. This velocity gradient is directly proportional to the viscosity of the liquid which represents the resistance of the fluid to be deformed. Thus, by relating the velocity gradient to the transfer of momentum the viscosity of the liquid can be determined. The table below shows the viscosity obtained from this simulation for all the investigated liquids. ____________________________ Liquid Viscosity (Pa s ) ____________________________ [bmp][pf2] 0.001867 [cpy][tbp] 0.002442 [nba][pf2] 0.002125 [nbt][pf2] 0.001098 [nbt][tf2] 0.001918 _____________________________ Viscosities shown in the table above agree with the experimental values for similar liquids. This demonstrates that the phenomenological approach used in these simulations is a more robust way to calculate transport properties than the traditional equilibrium techniques. Pedagogical Component Since the research described here was carried out with undergraduate students, the pedagogical component of the project included using animations to illustrate the physical processes being investigated. The use of animations demonstrated to be a powerful tool to convey abstract concepts to the students.
