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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.

Phys Chem Chem Phys ; 19(44): 29855-29861, 2017 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-29104997


In this work, the droplet size in a water-in-oil emulsion obtained by supersaturation is studied. The emulsion is obtained by cooling down a saturated water/oil solution by a certain temperature difference. The effects of the cooling rate and temperature difference on the produced droplet size are experimentally investigated. The average size of water droplets in the emulsion is found to be proportional to the square root of the cooling rate. By analyzing the time scales of three different steps, including nucleation, droplet growth due to diffusion and coarsening, involved in the emulsification process, it is found that the time scales of nucleation and droplet growth due to mass diffusion are much smaller than the cooling time constant, which is much shorter than the coarsening time scale. A mechanism that links the cooling rate and supersaturation temperature to droplet size is proposed: the cooling rate influences the nucleation and thus droplet density, while the temperature difference, which is linearly proportional to the total volume of precipitated water from the saturated water-in-oil solution, determines the size of each droplet. The droplet size data were found to support this proposed mechanism well. The results obtained from this work may provide useful guidance on controlling the droplet size in the supersaturation-based emulsification process, which has a lot of practical relevance to many applications.

ACS Nano ; 11(6): 5510-5518, 2017 06 27.
Artigo em Inglês | MEDLINE | ID: mdl-28511003


The ability to efficiently utilize solar thermal energy to enable liquid-to-vapor phase transition has great technological implications for a wide variety of applications, such as water treatment and chemical fractionation. Here, we demonstrate that functionalizing graphene using hydrophilic groups can greatly enhance the solar thermal steam generation efficiency. Our results show that specially functionalized graphene can improve the overall solar-to-vapor efficiency from 38% to 48% at one sun conditions compared to chemically reduced graphene oxide. Our experiments show that such an improvement is a surface effect mainly attributed to the more hydrophilic feature of functionalized graphene, which influences the water meniscus profile at the vapor-liquid interface due to capillary effect. This will lead to thinner water films close to the three-phase contact line, where the water surface temperature is higher since the resistance of thinner water film is smaller, leading to more efficient evaporation. This strategy of functionalizing graphene to make it more hydrophilic can be potentially integrated with the existing macroscopic heat isolation strategies to further improve the overall solar-to-vapor conversion efficiency.

Phys Chem Chem Phys ; 18(43): 29786-29796, 2016 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-27747354


Electric field assisted coalescence is one of the most efficient methods for water-in-oil emulsion separation. In this paper, we experimentally study water droplet evolution in an oil phase under different electric field configurations. We determine that non-uniform fields can enhance the performance of electrocoalescence compared to uniform fields. The analysis indicates that the enhanced coalescence is due to the combined effects of dipole-dipole interaction between droplets and dielectrophoresis between individual droplets and the applied non-uniform field. The present study shows that a non-uniform electric field and the induced dielectrophoretic effect can accelerate the coalescence and phase separation of micro-emulsions. These results may provide useful guidance in designing an optimum electrode configuration for efficient electrocoalescence.