Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Article Citation - WoS: 22Citation - Scopus: 24Size Dependent Influence of Contact Line Pinning on Wetting of Nano-textured/Patterned Silica Surfaces(Royal Society of Chemistry, 2020) Özçelik, H. Gökberk; Barışık, Murat; Satıroğlu, Ezgi; Barışık, Murat; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyWetting behavior on a heterogeneous surface undergoes contact angle hysteresis as the droplet stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning. However, there is a lack of consensus on how to calculate the influence of pinning forces. In general, the pinning effect can be characterized as (i) microscopic behavior when a droplet is pinned and the contact angle increases/decreases as the droplet volume increases/decreases and (ii) macroscopic behavior as the pinning effects decrease and ultimately, disappear with the increase of the droplet size. The current work studied both behaviors using molecular dynamics (MD) simulation with more than 300 different size water droplets on silica surfaces with three different patterns across two different wetting conditions. Results showed that the contact angle increases linearly with increasing droplet volume through the microscopic behavior, while the droplet is pinned on top of a certain number of patterns. When we normalized the droplet size with the corresponding pattern size, we observed a "wetting similarity" that linear microscopic contact angle variations over different size heterogeneities continuously line up. This shows that the pinning force remains constant and the resulting pinning effects are scalable by the size ratio between the droplet and pattern, independent of the size-scale. The slope of these microscopic linear variations decreases with an increase in the droplet size as observed through the macroscopic behavior. We further found a universal behavior in the variation of the corresponding pinning forces, independent of the wetting condition. In macroscopic behavior, pinning effects become negligible and the contact angle reaches the equilibrium value of the corresponding surface when the diameter of the free-standing droplet is approximately equal to 24 times the size of the surface structure. We found that the pinning effect is scalable with the droplet volume, not the size of the droplet base.Article Citation - WoS: 26Citation - Scopus: 27Electric Charge of Nanopatterned Silica Surfaces(Royal Society of Chemistry, 2019) Özçelik, H. Gökberk; Barışık, Murat; Barışık, Murat; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyThe most recent technologies employ nanoscale surface patterning or roughening in order to engineer desired properties on a surface. Electrokinetic properties at the interface of such surfaces and ionic liquids show different behavior to the well-known theoretical descriptions. Basically, the ionic distribution on the surface differs due to electrical double layer overlap effects in the pits and curvature effects at the tips of surface structures. Generally, the charge density of a surface is assumed to be a material property and surface roughness effects are overlooked in most of the literature. In contrast, we properly calculated the local surface charges based on surface chemistry at the corresponding local ionic concentration (charge regulation) for various surface roughness and solution conditions. The results showed that the surface charge density of silica decreased at the pits but increased at the tips of surface patterns. Even for the simplest case of self-repeating surface structures, the average of local surface charges becomes lower than the theoretical predictions. Based on numerical calculations, a phenomenological model was developed as an extension to the existing flat surface theory, which can successfully predict the average surface charge on a nano patterned surface as a function of the surface pattern size, ionic concentration and pH.Article Citation - WoS: 14Citation - Scopus: 14Size Dependent Surface Charge Properties of Silica Nano-Channels: Double Layer Overlap and Inlet/Outlet Effects(Royal Society of Chemistry, 2018) Şen, Tümcan; Barışık, Murat; Barışık, Murat; Şen, Tümcan; 01. Izmir Institute of Technology; 03.10. Department of Mechanical Engineering; 03. Faculty of EngineeringTransport inside nano-channels and tubes is highly dependent on their surface charge properties. While previous studies assume that the charge density of a surface is a material property and independent of confinement size, this study properly characterized the surface charge of a nanochannel as a function of channel height and length under various solution conditions. By calculating the local surface charge based on local ionic concentrations, the surface charge of a nano-channel was studied by considering the effects of both overlapping electrical double layers (EDLs) and inlet/outlet regions. First, the surface charge of silica decreased with the increase in EDL overlap, which is characterized by the ratio of EDL thickness to channel height. Second, the local surface charge showed variation at the inlet/outlet regions where the channel’s electrokinetics was in development. We defined a general entrance length as a function of EDL thickness for the electrokinetically developing part of different cases, after which the surface charge reached its equilibrium value and remained constant. Based on such length scales, we extended the existing theory to include nano-effects. A phenomenological model was developed, which can predict the average nano-channel surface charge as a function of EDL thickness, pH, channel length and channel height.