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ACS Appl Mater Interfaces ; 11(35): 32481-32488, 2019 Sep 04.
Artigo em Inglês | MEDLINE | ID: mdl-31408315


Water slip at solid surfaces is important for a wide range of micro-/nanofluidic applications. While it is known that water slip behavior depends on surface functionalization, how it impacts the molecular level dynamics and mass transport at the interface is still not thoroughly understood. In this paper, we use nonequilibrium molecular dynamics simulations to investigate the slip behavior of water confined between gold surfaces functionalized by self-assembled monolayer (SAM) molecules with different polar functional groups. We observe a positive-to-negative slip transition from hydrophobic to hydrophilic SAM functionalizations, which is found to be related to the stronger interfacial interaction between water molecules and more hydrophilic SAM molecules. The stronger interaction increases the surface friction and local viscosity, making water slip more difficult. More hydrophilic functionalization also slows down the interfacial water relaxation and leads to more pronounced water trapping inside the SAM layer, both of which impede water slip. The results from this work will provide useful insights into the understanding of the water slip at functionalized surfaces and design guidelines for various applications.

ACS Nano ; 13(2): 1097-1106, 2019 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-30633498


Polymers with superior mechanical properties are desirable in many applications. In this work, polyethylene (PE) films reinforced with exfoliated thermally reduced graphene oxide (TrGO) fabricated using a roll-to-roll hot-drawing process are shown to have outstanding mechanical properties. The specific ultimate tensile strength and Young's modulus of PE/TrGO films increased monotonically with the drawing ratio and TrGO filler fraction, reaching up to 3.2 ± 0.5 and 109.3 ± 12.7 GPa, respectively, with a drawing ratio of 60× and a very low TrGO weight fraction of 1%. These values represent by far the highest reported to date for a polymer/graphene composite. Experimental characterizations indicate that as the polymer films are drawn, TrGO fillers are exfoliated, which is further confirmed by molecular dynamics (MD) simulations. Exfoliation increases the specific area of the TrGO fillers in contact with the PE matrix molecules. Molecular dynamics simulations show that the PE-TrGO interaction is stronger than the PE-PE intermolecular van der Waals interaction, which enhances load transfer from PE to TrGO and leverages the ultrahigh mechanical properties of TrGO.

ACS Appl Mater Interfaces ; 10(33): 28159-28165, 2018 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-30056700


Thermal transport across solid-water interfaces is critical for a wide range of applications such as solar thermal evaporation, nanoparticle-assisted hyperthermia therapeutics, and nanofluids. Surface functionalization using self-assembled monolayers (SAMs) to change the hydrophilicity of the solid surface is a common strategy to improve the thermal conductance of solid-water interfaces. Although it is known that hydrophilic interfaces increase the interfacial bonding, how it impacts the molecular level energy transport across the interface is still not clear. In this paper, we perform molecular dynamics simulations to calculate the thermal conductance of differently functionalized gold (Au)-water interfaces. Combining the heat flux decomposition to different interatomic interactions across interfaces and analyses of water structures close to the functionalized surfaces, we found that there is a collaborative effect from the electrostatic interactions and the Lennard-Jones (L-J) interactions (especially the repulsive part). The electrostatic interactions, which are between the polar functional groups of SAMs and water, will attract water molecules closer to the SAM surface, leading both the electrostatic and L-J interactions to have larger effective forces across the interfaces. This increases the power exchanged between solid and water atoms, enhancing the thermal energy transport. The results from this work will provide new insights to the understanding of thermal transport across solid-water interfaces.