SCL Seminar by Miljan Dasic

SCL seminar of the Center for the Study of Complex Systems, will be held on Thursday, 18 May 2017 at 12:00 in the library reading room “Dr. Dragan Popović" of the Institute of Physics Belgrade. The talk entitled

"Modelling the behaviour of confined dipolar helices and ionic liquids"

will be given by Miljan Dašić (Scientific Computing Laboratory, Center for the Study of Complex Systems, Institute of Physics Belgrade).

Abstract of the talk:

This talk has two distinct parts related to my research until now. The first part is concerned with dipolar particles in cylindrical confinement. We have been analyzing self-assembly of hard shperes with permanent dipole moment under cylindrical confinement, i.e., dipolar helices made up of hard spheres [1]. Such structures are relevant objects in biological systems. We observe a complex dependence of cohesive energy on surface packing fraction and dipole moment distribution. As far as single helices are concerned, the lowest cohesive energy is achieved at the highest surface packing fraction. Besides, a striking non-monotonic behavior is reported for the cohesive energy as a function of the surface packing fraction. For multiple helices, we discover a new phase, markedly having higher cohesive energy. This phase is referred to as ZZ tube consisting of stacked crown rings, resulting in a local triangular arrangement with densely packed filaments parallel to the tube axis.

The second part of the talk is dealing with ionic liquids under confinement and shear. We present an approach for simulating mesoscopic phenomena related to lubrication with ionic liquids [2]. We have implemented a coarse grain molecular dynamics (MD) description of an ionic liquid as a lubricant which can expand into lateral reservoirs. Ionic liquid (IL) consists of an equal number of cations and anions, represented as coarse-grained particles charged with elementary charge, which are placed between fcc (111) plates. Interactions in our system include non-bonded Lennard-Jones and Coulombic potentials. Our geometry allows a variable confinement gap and a variable amount of lubricant in the gap. Ions are initially randomly placed inside the gap and they are equilibrated. We observe that IL confined between solid surfaces features alternating positive and negative ionic layers. We observe nonmonotonic dependence of the normal force (acting on the top plate) on plate-to-plate distance. At small plate-to-plate distances we observe a steep rise of that normal force. This is an interesting behaviour that could be exploited for preventing solid–solid contact and wear. The shear simulation results have revealed two distinct regimes of lubrication: elasto-hydrodynamic regime under low loads and lubrication regime with low and velocity-independent specific friction, under high loads.

[1] I. Stanković, M. Dašić and R. Messina, “Structure and Cohesive Energy of Dipolar Helices”, Soft Matter (2016)
[2] K. Gkagkas, V. Ponnuchamy, M. Dašić, I. Stanković, Molecular dynamics investigation of model ionic liquid lubricant for automotive applications, Tribology International (2016)