Dokument: Computer Simulations Of Hard-Core Yukawa System In Confinement
| Titel: | Computer Simulations Of Hard-Core Yukawa System In Confinement | |||||||
| URL für Lesezeichen: | https://docserv.uni-duesseldorf.de/servlets/DocumentServlet?id=49718 | |||||||
| URN (NBN): | urn:nbn:de:hbz:061-20190531-105610-3 | |||||||
| Kollektion: | Dissertationen | |||||||
| Sprache: | Englisch | |||||||
| Dokumententyp: | Wissenschaftliche Abschlussarbeiten » Dissertation | |||||||
| Medientyp: | Text | |||||||
| Autor: | Eshraghi, Mojtaba [Autor] | |||||||
| Dateien: |
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| Beitragende: | Prof. Dr. Horbach, Jürgen [Gutachter] Prof. Dr. Hartmut Löwen [Gutachter] | |||||||
| Stichwörter: | charged colloids, molecular dynamics, hard-core Yukawa, interfacial free energy, thermodynamic integration, phase behavior, interface, pre-freezing, pre-melting, hard wall, confinement, charged wall, local bond order parameter | |||||||
| Dewey Dezimal-Klassifikation: | 500 Naturwissenschaften und Mathematik » 530 Physik | |||||||
| Beschreibung: | The hard-core Yukawa potential has been widely used to describe the effective interaction between like-charged colloids. The phase diagram of bulk hard-core Yukawa systems is well studied. It shows the emergence of three different phases, namely body-centered cubic (BCC) crystal, face-centered cubic (FCC) crystal and fluid, and lines of first-order transitions BCC-fluid, FCC-fluid and BCC-FCC that intersect in a triple point In this work, we investigate the phase behavior of hard-core Yukawa crystals and fluid in confinement via extensive molecular dynamics (MD) simulations in the NVT ensemble, i.e., with constant number of particles N, in fixed volume V, and at constant temperature T. We have studied these systems in two different types of confinements. First, we have studied the behavior of hard-core Yukawa systems in presence of pair of short-range structureless flat hard walls and then the same process has been performed to observe the effect of a pair of relatively long-range flat charged walls on the hard-core Yukawa systems.
The FCC(111) crystal for different densities from the coexistence to slightly higher than coexistence density is investigated at the confinement of a pair of structureless flat hard walls. In this part due to the long-range interaction of hard-core Yukawa particles and short-range interaction of flat hard walls the multiple time step scheme for MD simulation is employed. We have observed that the FCC crystals in confinement of flat hard walls are stable. By using a thermodynamic integration scheme, we were able to calculate the free energy contribution of the system to form an FCC-wall interface. We have observed that the calculated values for FCC-wall interfacial free energies are negative and this negativity is increasing by an increment of the density. This negativity means that the hard walls are attractive for the bulk particles. The fluids at coexistence with FCC and densities lower than coexistence density were examined in the presence of the flat hard walls. These density profile analyses of these systems show that some layers are forming in the vicinity of the flat hard walls. An analysis of the structure shows the formation of hexagonal layers (pre-freezing) at coexistence and slightly below the coexistence density. Moreover, calculation of the 2D packing fraction for the first formed layer close to the flat wall confirms that their density is close to the density of the first layers in FCC crystals. The free energy of the fluid-wall interface is calculated via thermodynamic integration scheme which shows the negative values. However, the values for the systems with phase transitions are not reliable. The flat hard wall confinement was also examined on the BCC(111) crystals at coexistence and densities higher than the coexistence density. Our simulations and structure analysis show that a confined configuration with a BCC structure is divided into three regions. First one can observe the formation of hexagonal layers in the vicinity of the walls. Next, we see an area which consists of melted particles from the BCC crystal. The width of this region is decreasing by an increment of density. And the last part is a stable BCC(111) crystal which is located in the middle of the systems. By investigating the behavior of the fluid which is at coexistence with the BCC for coexistence density and some low dense systems, we observe that same as FCC-fluid coexistence, the pre-freezing happens even for densities lower than coexistence. Moreover, a thermodynamic integration scheme is employed to calculate the fluid-wall interfacial free energy for systems at sufficiently low densities where no pre-freezing is observed. The calculated values are negative. The next confinement is studied by inserting a pair of flat charged walls into a system with FCC(111) structure. As in the hard wall case, the crystals are stable; however, the calculated FCC-wall interfacial free energies are positive, which mean that the charged walls are not attractive for the bulk particles. The confinement behavior of fluid hard-core Yukawa system at coexistence with FCC for coexistence and densities lower than coexistence at the presence of flat charged walls are studied. The structure analysis shows that the formed layers in the vicinity of the charged walls are just amorphous layers and there is no evidence for a phase transition. We have calculated the fluid-wall interfacial free energies, and it turned out that they are positive. In addition, we have investigated the BCC(111) crystal in the presence of flat charged walls. Our simulation and structure analysis show that some amorphous layers are formed in the vicinity of the charged walls. Also, between these layers and bulk BCC, there is a fluid area which is made by melted BCC particles. For the fluid case in the presence of charged walls, there is no evidence of prefreezing and the layers which are formed in the vicinity of the charged walls are just amorphous layers. By performing a thermodynamic integration scheme, we were able to calculate the fluid-wall interfacial free energies which are positive. These values are growing with respect to the densities but will saturate at densities close to the coexistence. The results of this work open the door for targeted experimental and further theoretical studies on charged colloids in confinement. | |||||||
| Lizenz: |  Urheberrechtsschutz | |||||||
| Fachbereich / Einrichtung: | Mathematisch- Naturwissenschaftliche Fakultät » WE Physik » Theoretische Physik | |||||||
| Dokument erstellt am: | 31.05.2019 | |||||||
| Dateien geändert am: | 31.05.2019 | |||||||
| Promotionsantrag am: | 12.03.2019 | |||||||
| Datum der Promotion: | 17.05.2019 |
