Dynamic Buckling of a Filament Impacted by a Falling Droplet.

We investigate the buckling dynamics of an elastic filament impacted axially by a falling liquid droplet, and identify the buckling modes through a combination of experimental and theoretical analyses. A phase diagram is constructed on a plane defined by two primary parameters: the falling height and the filament length. Two transition boundaries are observed, with one marking the occurrence of dynamic buckling and the other separating the buckling regime into two distinct modes. Notably, the hydrodynamic viscous force of the liquid dominates during the impact, with the dynamic buckling instability predicted by a single elastoviscous number. The critical load is twice the critical static load, which is, however, lower for the deformable droplet utilized in our study, as compared to a solid object. An additional time-dependent simulation on a longer filament exhibits a higher buckling mode, succeeded by a more distinct coarsening process than our experimental observations.

Symmetry Breaking of Tail-Clamped Filaments in Stokes Flow.

Symmetry breaking (SB) of fluid-structure interaction problems plays an important role in our understanding of animals' locomotive and sensing behaviors. In this Letter, we study the SB of flexible filaments clamped at one end and placed in a spanwise periodic array in Stokes flow. The equilibrium state of the filament along the streamwise direction loses stability and experiences two-dimensional and then three-dimensional SBs as the spanwise distance increases, or as the filament rigidity reduces. For slightly deformed filaments, the viscous and pressure forces are commensurate, while for extremely deformed filaments the viscous force becomes dominant.

Quasi-water layer sandwiched between hexagonal ice and wall and its influences on the ice tensile stress.

The presence of a quasi-water/premelting layer at the interface between wall and ice when the temperature (T) is below the melting point was extensively observed in experiments. In this work, molecular dynamics simulations are performed to analyze the underlying physics of the quasi-water layer and the effects of the layer on the ice tensile stress. The results indicate that each molecule and its four nearest neighbours in the quasi-water layer representing an equilibrium structure gradually form a tetrahedral ice-like arrangement from an unstructured liquid-like structure along the direction away from the wall. The average density of the quasi-water layer is equal to or higher than the bulk density of water at T ≥ 240 K or T ≤ 240 K respectively, and reaches 1.155 g cm-3 at T = 210 K, suggesting a structural correlation with the high-density liquid phase of water. Depending on the temperature and wall wettability, the thickness of the quasi-water layer (Hq) ranges from ∼2 Å to ∼25 Å. For prescribed hydrophilic walls, Hq increases monotonically with temperature, and is almost proportional to(Tm - T)-1/3, where Tm is the melting temperature of ice. Hq keeps an almost constant value (2 Å) as the temperature increases and rises sharply after passing a threshold temperature of T ≈ 250 K. In the joint effects of the wall wettability and quasi-water layer's thickness, the ice tensile stress decreasing monotonically at a larger temperature shows an upward trend and then reduces to almost a constant value as the wall changes from a hydrophobic to a hydrophilic one. The results reveal the potential development of anti-icing/de-icing techniques by heating the wall or modifying its surface to increase Hq.

Dynamics of two-dimensional flow around a circular cylinder with flexible filaments attached.

A direction adaptive approach for the reduction of drag and the suppression of lift fluctuation in flow passing a circular cylinder is developed. Flexible filaments are attached to the surface of the cylinder, and different configurations, including the number, lengths, and angles of attachment of the filaments, as well as their tension and bending features, are investigated. In this comprehensive numerical study, the configuration with two filaments 180^{o} apart is found to be optimal for drag reduction and lift fluctuation suppression and is adaptive to the direction of the incoming flow. A drag reduction of 10.8% and a lift fluctuation suppression of 34.6% can be achieved as one filament is attached to the rear stagnation point and the other to the front stagnation point. The hairy coating resembled by 12 evenly attached filaments is also considered. Though marked drag reduction has not been found for this configuration, we leave it an open question for future studies to explore various properties of the filaments in turbulent flow, whose interaction with the filaments would be significant.

Numerical prediction of aerodynamic performance for a flying fish during gliding flight.

Flying fish is a family of unique aerial-aquatic animals, which can both swim in the water and glide over the sea surface. Most previous studies on their aerodynamic characteristics were based on field observations or measurements of their morphometric parameters. In the present study, we consider three different flying fish models, of which the preliminary one mimics the Cypselurus hiraii in the pectoral fin morphology, following a previous wind tunnel experiment (Park and Choi 2010 J. Exp. Biol. 213 3269-79). Their aerodynamic performances are numerically studied by the computational fluid dynamics (CFD) method. The maximum lift force coefficient of 1.03 is reached at the angle of attack [Formula: see text], and the maximum lift-to-drag ratio of 4.7 is achieved at [Formula: see text]. By choosing appropriately the center of gravity, the flying fish model is proved to be longitudinally stable, according to the negative slope of pitching moment profile. Furthermore, we build a three-degrees-of-freedom (3-DOF) dynamic model in the longitudinal plane based on the aerodynamic coefficients obtained in our simulations, to predict its gliding performance. The results show that the flying fish can achieve a distance up to 45.4 m, and reach a height of 13.2 m, indicating an extraordinary gliding performance. Our numerical simulations are consistent with previous experimental results and theoretical prediction, which can be taken as the basis of further research on robotic flying fish.

An electrochemical anodization strategy towards high-activity porous MoS2 electrodes for the hydrogen evolution reaction.

Molybdenum disulfide (MoS2) is a promising non-precious metal electrocatalyst for the hydrogen evolution reaction (HER). Herein, we have described an anodization route for the fabrication of porous MoS2 electrodes. The active porous MoS2 layer was directly formed on the surface of a Mo metal sheet when it was subjected to anodization in a sulfide-containing electrolyte. The Mo sheet served as both a supporter for MoS2 electrocatalysts and a conductive substrate for electron transport. After optimizing the anodization parameters, the anodized MoS2 electrode showed a high electrocatalytic activity with an onset potential of -0.18 V (vs. RHE) for the HER, a Tafel slope of ∼101 mV per decade and an overpotential of 0.23 V at a current density of 10 mA cm-2 for the HER. These results indicate that our facile anodization strategy is an efficient route towards a high-activity MoS2 electrode.

Instabilities of interacting vortex rings generated by an oscillating disk.

We propose a natural model to probe in a controlled fashion the instability of interacting vortex rings shed from the edge of an oblate spheroid disk of major diameter c, undergoing oscillations of frequency f_{0} and amplitude A. We perform a Floquet stability analysis to determine the characteristics of the instability modes, which depend strongly on the azimuthal (integer) wave number m. We vary two key control parameters, the Keulegan-Carpenter number K_{C}=2πA/c and the Stokes number ß=f_{0}c^{2}/ν, where ν is the kinematic viscosity of the fluid. We observe two distinct flow regimes. First, for sufficiently small ß, and hence low frequency of oscillation corresponding to relatively weak interaction between sequentially shedding vortex rings, symmetry breaking occurs directly to a single unstable mode with m=1. Second, for sufficiently large yet fixed values of ß, corresponding to a higher oscillation frequency and hence stronger ring-ring interaction, the onset of asymmetry is predicted to occur due to two branches of high m instabilities as the amplitude is increased, with m=1 structures being dominant only for sufficiently large values of K_{C}. These two branches can be distinguished by the phase properties of the vortical structures above and below the disk. The region in (K_{C},ß) parameter space where these two high m instability branches arise can be described accurately in terms of naturally defined Reynolds numbers, using appropriately chosen characteristic length scales. We subsequently carry out direct numerical simulations of the fully three-dimensional flow to verify the principal characteristics of the Floquet analysis, in particular demonstrating that high wave-number symmetry-breaking generically occurs when vortex rings sequentially interact sufficiently strongly.