RESUMEN
The current study focuses on non-equilibrium molecular dynamics (NEMD) simulations to investigate the slip properties of water flowing over different nanostructured surfaces. A simulation protocol is developed that applies constant shear stress throughout the fluid before measuring the slip length. Using pseudo-data, the reliability of this protocol in terms of both accuracy and noise of the results for high-slip and multiphase systems is demonstrated. In contrast to the NEMD techniques available in the literature, the protocol also enables a convenient way to compare the slip lengths of different surface coatings. The fluid slip lengths of surface coatings comprising carbon nanotubes on platinum are predicted using the proposed protocol with nitrogen gas trapped in the interstitial gaps. The role of these gas pockets in determining surface slip properties is investigated. The NEMD results from the proposed model compare well with a macroscopic theoretical model for nano-patterned surfaces. Finally, it is concluded that entrapped gas within nanostructures may offer significant drag reduction only if the gas surface coverage is above 95%.
RESUMEN
Predictions have shown that our demand for oil and gas will continue to grow in the next decade, and future supply will become more reliant on tertiary recovery and from nonconventional resources. However, current reservoir characterization methodologies, such as well logs, cross-well electromagnetic imaging and seismic methods, have their individual limitations on detection range and resolution. Here we propose a pioneering way to use carbon quantum dots (CQDs) as nanoparticle tracers, which can be transported through a reservoir functioning as conventional tracers, while acting as sensors to obtain useful information. These hydrothermally produced CQDs from Xylose possess excellent stability in high ionic strength solutions, durable absorbance and fluorescence ability due to multi high-polarity functional group on their surfaces. Consistency between our on-line ultraviolet-visible (UV-Vis) spectroscopy and off-line Confocal laser scanning microscopy (CLSM) measurements confirms that CQDs have the tracer-like migration capability in glass beads-packed columns and sandstone cores, regardless of particle concentration and ionic strength. However, their migration ability is undermined in the column packed with crushed calcite grains with positive charge. We also demonstrate that quantitative oil saturation detection in unknown sandstone core samples can be achieved by such CQDs based on its breakthrough properties influenced by the presence of oil phase.
RESUMEN
The spatial distribution of regions that lie above contours of constant height through a self-affine surface is studied as a function of the Hurst exponent H. If the surface represents a landscape, these regions correspond to islands. When the surface represents the height difference for contacting surfaces, the regions correspond to mechanical contacts in the common bearing area model. The autocorrelation function C(Δr) is defined as the probability that points separated by Δr are both within islands. The scaling of C has important implications for the stiffness and conductance of mechanical contacts. We find that its Fourier transform C(q) scales as a power of the wavevector magnitude q: [Formula: see text] with µ = 2 + H rather than the value µ = 2 + 2H reported previously. An analytic argument for µ = 2 + H is presented using the distribution of areas contained in disconnected islands.