The Hyperloop system is a new concept that allows a train to travel through a near-vacuum tunnel at transonic speeds. Aerodynamic drag is one of the most important factors in analyzing such systems. The blockage ratio (BR), pod speed/length, tube pressure, and temperature affect the aerodynamic drag, but the specific relationships between the drag and these parameters have not yet been comprehensively examined. In this study, we investigated the flow phenomena of a Hyperloop system, focusing on the effects of changes in the above parameters. Two-dimensional axisymmetric simulations were performed in a large parameter space covering various BR values (0.25, 0.36), pod lengths (10.75–86 m), pod speeds (50–350 m/s), tube pressures (~100–1000 Pa), and tube temperatures (275–325 K). As BR increased, the pressure drag was significantly affected. This is because of the smaller critical Mach number for a larger BR. As the pod length increased, the total drag and pressure drag did not change significantly, but there was a considerable influence on the friction drag. As the pod speed increased, strong shock waves occurred near the end of the pod. At this point, the flows around the pod were severely choked at both BR values, and the ratio of the pressure drag to the total drag converged to its saturation level. At tube pressures above 500 Pa, the friction drag increased significantly under the rapidly increased turbulence intensity near the pod surface. High tube temperatures increase the speed of sound, and this reduces the Mach number for the same pod speed, consequently delaying the onset of choking and reducing the aerodynamic drag. The results presented in this study are applicable to the fundamental design of the proposed Hyperloop system.

Driven by a magnetic field, the rotation of a particle near a wall can be rectified into a net translation. The particles thus actuated, or surface walkers, are a kind of active colloid that finds application in biology and microfluidics. Here, we investigate the motion of spherical surface walkers confined between two walls using simulations based on the immersed-boundary lattice Boltzmann method. The degree of confinement and the nature of the confining walls (slip vs no-slip) significantly affect a particle's translational speed and can even reverse its translational direction. When the rotational Reynolds number Re ω is larger than 1, inertia effects reduce the critical frequency of the magnetic field, beyond which the sphere can no longer follow the external rotating field. The reduction of the critical frequency is especially pronounced when the sphere is confined near a no-slip wall. As Re ω increases beyond 1, even when the sphere can still rotate in the synchronous regime, its translational Reynolds number Re T no longer increases linearly with Re ω and even decreases when Re ω exceeds ∼10.

The adsorption of analyte molecules on nano-optoelectrodes (e.g., a combined nanoantenna and nanoelectrode device) significantly affects the signal characteristics in Surface-Enhanced Raman Scattering (SERS) measurements. Understanding how different molecules adsorb on electrodes and their modulation of electrical potential help better interpret SERS measurements. This work investigates the adsorption of prototypical analyte molecules, Rhodamine 6G and choline, on gold electrodes with negative, neutral, and positive surface charges using molecular dynamics simulations. We show that both molecules can readily adsorb on gold surfaces at all surface charge densities considered here. Nevertheless, the configurations of adsorbed molecules can differ for different surface charge densities, and adsorption can also change a molecule’s conformation. Rhodamine 6G molecules adsorb more strongly than choline molecules, and the adsorption of both molecules is affected by electrode charge in 0.25 M NaCl solutions. The mechanisms of these observations are elucidated, and their implications for SERS measurements are discussed.

Fluid transport involving brine–oil interfaces plays an important role in applications including enhanced oil recovery and oil–brine separation and can be affected markedly by the slippage at these interfaces. The slippage at brine–oil interfaces, however, is not well understood, especially in the presence of surfactants, which are ubiquitous in natural and engineering systems. Here, we report molecular dynamics studies of the slippage at brine–decane interfaces in the presence of two surfactants, nonylphenol and phenol. They share essentially the same head but nonylphenol has a nine-carbon alkyl tail and phenol has no clear tail. At zero surfactant density, a slip length of 1.2 nm exists at the brine–decane interface. As either surfactant is introduced to brine–decane interfaces, the slip length initially decreases linearly, with nonylphenol being more effective in reducing the slip length. As more surfactants are introduced, the decrease in slip length slows down and eventually, the slip length plateaus at −1.4 and −0.5 nm for interfaces populated with nonylphenol and phenol, respectively. The mechanisms of the observed slip length vs surfactant density relations and the effects of tail length on the interfacial slippage are elucidated by analyzing the molecular structure and transport of interfacial fluids and surfactants.

Molecular transport across liquid–vapor interfaces covered by surfactant monolayers plays a key role in applications such as fire suppression by foams. The molecular understanding of such transport, however, remains incomplete. This work uses molecular dynamics simulations to investigate the heptane transport across water–vapor interfaces populated with sodium dodecyl sulfate (SDS) surfactants. Heptane molecules’ potential of mean force (PMF) and local diffusivity profiles across SDS monolayers with different SDS densities are calculated to obtain heptane’s transport resistance. We show that a heptane molecule experiences a finite resistance as it crosses water–vapor interfaces covered by SDS. Such interfacial transport resistance is contributed significantly by heptane molecules’ high PMF in the SDS headgroup region and their slow diffusion there. This resistance increases linearly as the SDS density rises from zero but jumps as the density approaches saturation when its value is equivalent to that afforded by a 5 nm thick layer of bulk water. These results are understood by analyzing the micro-environment experienced by a heptane molecule crossing SDS monolayers and the local perturbation it brings to the monolayers. The implications of these findings for the design of surfactants to suppress heptane transport through water–vapor interfaces are discussed.