Rotational varifocal moiré metalens made of single-crystal silicon meta-atoms for visible wavelengths

Metasurface lenses (metalenses) offer an ultrathin and simple optical system with dynamic functions that include focal length tuning. In this study, a rotational varifocal (i.e., moiré) metalens based on octagonal single-crystal silicon pillars was designed and fabricated to realize a high transmittance, whole 2π phase coverage, and polarization insensitivity for visible wavelengths. The moiré metalens consists of a pair of cascaded metasurface-based phase lattices and the focal length can be adjusted from negative to positive by mutual rotation. The fabricated moiré metalens demonstrated a focal length that can be tuned from −36 mm to −2 mm and from 2 to 12 mm by mutual rotation from −90° to 90°, and the experimental measurements agreed well with theoretical values at the design wavelength of 633 nm. Imaging was demonstrated at three distinct wavelengths of 633, 532, and 440 nm.

Relative Permeability Behavior of Oil- Water Systems in Wolfcamp and Eagle Ford Fractures

Presently, two-phase flow behavior through propped and unpropped fractures is poorly understood, and due to this fact, reservoir modeling using numerical simulation for the domain that contains fractures typically assumes straight-line relative permeability curves and zero capillary pressure in the fractures. However, there have been several studies demonstrating that both viscous and capillary dominated flow can be expected in fractured reservoirs, where non-linear fracture relative permeabilities must be used to accurately model these reservoirs. The objective of this study is to develop an understanding of the relative permeability of oil-water systems in fractures through experimental study. The experimental measurements conducted in this study were done using downhole cores from the Wolfcamp and the Eagle Ford Shale formations. The cores were cut to 1.5-in diameter and 6-in length testing samples. The specimens are saw-cut to generate a fracture along each sample first, and then conditioned in the reservoir fluid at the reservoir temperature for a minimum of 30 days prior to any testing. Wolfcamp and Eagle Ford formation oil and reconstituted brine with and without surfactants are used as the test fluids. The measurements were recorded at effective fracture closure stress and reservoir temperature. Also, real-time measurements of density, pressure, and flow rate are recorded throughout the duration of each test. Fluid saturation within the fracture was calculated using the mass continuity equation. The oil-water relative permeability was measured using the steady-state method. All measurements were conducted at reservoir temperature and at representative effective fracture closure stress. The data from the experimental measurements was analyzed using Darcy's law, and a clear relationship between relative permeability and saturation was observed. The calculated relative permeability curves closely follow the generalized Brooks-Corey correlation for oil-water systems. Furthermore, there was a significant difference in the relative permeability curves between the oil-water only systems and the oil-water surfactant systems. The result of this study is useful for estimating the expected oil production more realistically. It also provides information about the effect of surfactants on oil-water relative permeability for optimal design of fracture fluids.

Adaptive obstacle climbing and hydrodynamic performance analyses of the amphibious robot with wheels and flexible undulating fins

In this paper, an amphibious robot with flexible undulating fins and self-adaptive climbing wheels are proposed for satisfying the needs of industrial applications. The structure of the climbing mechanism and undulating fin are firstly designed. Then, the adaptive obstacle climbing and the hydrodynamic characteristics are investigated through numerical simulations by using the Adams and Fluent, respectively. Finally, the experimental measurements of the land walking and underwater propulsion are studied. The numerical results illustrate that the amphibious robot could climb the vertical obstacle adaptively. In the underwater marching pattern, the underwater velocity could reach 1 m/s. In the rotating and yawing patterns, the angular velocity increases to the certain value while the rotating angle keeps increasing. The robot moves forward and turns around with the difference frequency of the undulating fins. The underwater propulsion and land-walking experiments show good swimming performance and the obstacle crossing ability of the amphibious robot, respectively, which verify the numerical simulation.

An integrated XFEM modeling with experimental measurements for optimizing thermal conductivity in carbon nanotube reinforced polyethylene

The present paper investigates the thermal properties of carbon nanotube reinforced polyethylene and specifically its potential as highly conductive material. To this end, an integrated approach is proposed combining both numerical and experimental procedures. First, in order to study conductive heat transfer in two-phase materials with imperfect interfaces, a detailed numerical model is developed based on the extended finite element method (XFEM), where material interfaces are modeled using the level set method. The thermal conductance at the interface of the carbon nanotubes and the polymer matrix is considered to be an unknown model parameter, the value of which is obtained by utilizing a series of experimental measurements of the composite material’s effective conductivity. The interfacial thermal conductance parameter value is inferred by calibrating the numerically predicted effective conductivity to the series of the corresponding experimental measurements. Once this parameter is estimated, the data-informed model is subsequently employed to provide reliable predictions of the effective conductivity of the composite for various weight fractions and configurations of carbon nanotubes in the parent material. Furthermore, microstructural morphologies that provide upper limits on the effective conductivity of the composite are identified via sensitivity analysis, demonstrating its potential as a highly conductive material.