Molecular dynamic simulations study of nano bubble attachment at hydrophobic surfaces
Jiaqi Jin 1,   Liem X. Dang 2,   Jan D. Miller 1  
 
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1
University of Utah, Department of Metallurgical Engineering, 135 South 1460 East, Rm 412, Salt Lake City, UT 84112
2
Physical Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352
CORRESPONDING AUTHOR
Jan D. Miller   

Metallurgical Engineering, University of Utah, 135 S 1460 E, Rm 412 WBB, 84112-0114 Salt Lake City, United States
 
Physicochem. Probl. Miner. Process. 2018;54(1):89–101
 
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ABSTRACT
Bubble attachment phenomena are examined using Molecular Dynamics Simulations (MDS) for the first time. The simulation involves a nitrogen nano bubble containing 906 nitrogen molecules in a water phase with 74,000 water molecules at molybdenite surfaces. During a simulation period of 1 ns, film rupture and displacement occurs. The attached nanobubble at the hydrophobic molybdenite face surface, results in a contact angle of about 90º. This spontaneous attachment is due to a “water exclusion zone” at the molybdenite face surface and can be explained by a van der Waals (vdW) attractive force, as discussed in the literature (Wang, Yin, Nalaskowski, Du, Miller, 2015, Molecular features of water films created with bubbles at silica surfaces, Surface Innovations, 3(SI1), 20-26). In contrast, the film is stable at the hydrophilic quartz (001) surface and the bubble does not attach. Contact angles, determined from MD simulation are reported and these results agree well with experimental and MDS sessile drop results. In this way, film stability and bubble attachment are described with respect to interfacial water structure for surfaces of different polarity. Interfacial water molecules at the hydrophobic molybdenite face surface have relatively weak interactions with the surface when compared to the hydrophilic quartz (001) surface, as revealed by the presence of a 3 Å “water exclusion zone” at the molybdenite/water interface. The molybdenite armchair-edge and zigzag-edge surfaces show a comparably slow process for film rupture and displacement when compared to the molybdenite face surface, which is consistent with their relatively weak hydrophobic character.
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