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Reducing the contact time of impacting nanodroplets is of fundamental interest and importance due to its promising potential in various engineering applications, such as self-cleaning and anti-icing. In this work, nanodroplets impacting superhydrophobic surfaces decorated by a rectangular ridge are studied over a wide range of Weber number (We) and different sizes of the ridge via molecular dynamics. Six bouncing modes have been distinguished. The usage of the rectangular ridge significantly enhances the bouncing performance of nanodroplets, including enlarging the We range for bouncing and reducing the contact time. It is especially found that the rectangular ridge leads to an increased contact time compared with the flat surface in the moderate We region. The underlying mechanism of the differential effect of the rectangular ridge on the contact time is compressively elucidated by the retraction dynamics of distinct bouncing modes. Subsequently, the retraction models considering different retraction characteristics are developed for the unsplit case. To promote a universal understanding of the effect of the ridge on the retraction dynamics of nanodroplets, several typical ridge shapes (triangular, cylinder, and semi-circular) are further considered. The retraction models are extended and verified to consider the effect of the ridge shape by introducing the equivalent structure parameters. Based on such a universal understanding, a newly designed shoot-shaped ridge is proposed and can considerably reduce the contact time of unsplit nanodroplets in the moderate We region. [ABSTRACT FROM AUTHOR]

For turbine blade internal cooling, the performance of turbulent flow and convective heat transfer in the cooling channel is numerically investigated via the computational fluid dynamics method, where the cooling channel adopts the combination of transverse rib arrangements and round-edged rib configurations or the combination of oblique rib arrangements and round-edged rib configurations. Under these two combination conditions, the simulation result shows that in the wide Reynolds number range of Re = 20 000–80 000, the heat transfer enhancement and the flow loss reduction, such as the larger normalized Nusselt number of Nu/Nu0 and smaller normalized friction factor of f/f0, are simultaneously realized by the oblique rather than transverse rib arrangement. In the oblique rib channel, the relationship between turbulent flow and convective heat transfer is for the first time revealed by the relationship between secondary vortices and turbulent kinetic energies. Based on the relation between secondary vortices and turbulent kinetic energies, the overall performance of turbulent flow and convective heat transfer for the oblique rib channel is first optimized by various normalized rib height, e/Dh, and rib spacing, p/e, and then evaluated by both overall performance factors of OPT1 and OPT2. Both OPT1 and OPT2 are larger at e/Dh = 0.062 and p/e = 15.00 in the wide range of e/Dh = 0.047–0.101 and p/e = 10.00–15.00. As a result of the comprehensive evaluation of OPT1 and OPT2, the combination of e/Dh = 0.062 and p/e = 15.00 is eventually employed by the oblique rib channel. [ABSTRACT FROM AUTHOR]

The present study investigates impact of nanodroplets on liquid films with varied film thicknesses over hydrophilic to hydrophobic surfaces via molecular dynamics simulations. A new crown pattern, hollow crown, is observed on medium liquid films for nanodroplets, whereas it is not reported for millimeter-sized droplets. Holes (or hollows) are attributed to capillary vibration of a nanometer-thick liquid film. The occurrence of holes is inhibited effectively for deep liquid films because the cavity thickness is much larger than the amplitude of capillary vibration. Surface wettability has a significant influence on thin and medium liquid films for nanodroplets, whereas it only affects thin liquid films for macrodroplets because the range of solid–liquid intermolecular forces is comparable to the size of droplets when the size of droplets reduces to the nanoscale. Subsequently, the accuracies of the existing macroscale models for predicting the time-dependent crown radius of nanodroplets are tested by simulated results. The results show that the macroscale models cannot capture the time-dependent crown radius of nanodroplets. On the basis of energetics analysis, we prove that the failure of macroscale models is attributed to the distinct difference in viscous dissipation mechanisms between macroscale and nanoscale droplets. [ABSTRACT FROM AUTHOR]

On nanopillar-arrayed superhydrophobic surfaces, the contact time of oblique nanodroplet impacts is for the first time investigated via molecular dynamics simulations. Here, oblique nanodroplet impacts are triggered by nanodroplets impacting superhydrophobic surfaces under various impact angles, α. The simulation results show that owing to the non-axisymmetry of spreading factors on nanopillar-arrayed superhydrophobic surfaces, the contact time of oblique nanodroplet impacts is always less on nanopillar-arrayed rather than smooth superhydrophobic surfaces under same impact angles. As the impact angle is increased from 5° to 65°, that is, under different impact conditions, the non-axisymmetry is more remarkable as α > 35° instead of α < 35° at the low, medium, and high normal Weber numbers, Wen. Hence, the contact time is sharp as α > 35° and then slowly reduced as α < 35° at the low, medium, and high Wen, at which the drastically increased sliding length as α > 35° further promotes the rapid reduction in contact time. As the impact angle is constant at 35°, the non-axisymmetry is more remarkable as h/w < 1 instead of h/w > 1 as the aspect ratio of nanopillars, h/w, is increased, that is, under different surface conditions. Hence, the contact time is sharp as h/w < 1 and then slowly reduced as h/w > 1 at the low, medium, and high Wen, at which the drastically reduced sliding length as h/w > 1 further hinders the rapid reduction in contact time. [ABSTRACT FROM AUTHOR]

Nanodroplet impacts have attracted significant attention, while the effect of surface wettability on contact time is evaluated poorly. Utilizing molecular dynamics simulations, the current work with a special focus on the contact time studies nanodroplets impacting solid surfaces in a wide range of static contact angles (θ0 = 105°–175°) and the Weber number (We = 0.1–200). The complete trends in contact time and restitution coefficient with surface wettability are analyzed and reported for the first time. For surfaces with θ0 > 160°, four different regimes are identified for the contact time and restitution coefficient as a function of the Weber number. For surfaces with 110° < θ0 < 160°, the fourth regime is not observed. The restitution coefficient is employed to analyze the contact time of distinct rebound patterns in the individual wettability range. Intriguingly, surface wettability has a remarkable influence on the contact time of nanodroplets even for superhydrophobic surfaces. The main reason for the difference between the macroscale and nanoscale is attributed to the significantly enhanced viscous effect and interfacial effect of the nanoscale impact. Considering the different effects of surface wettability on spreading and retraction dynamics, the theoretical models for the maximum spreading factor, spreading velocity, and retraction velocity are established. Finally, scaling laws of the spreading time τspr ∼ (R0/Vi)We2/3Re−1/3 and retraction time τret ∼ (R0/Vi)We2/3Re−1/3(1 − cos θ0)−1/2 are proposed. An excellent agreement with both the current data sources and the results in the literature verifies the universality of the current scaling law from hydrophobic to superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

Under parallel electric fields and free evaporation conditions, the statics and dynamics of spreading–evaporating nanodroplets are investigated on an isothermally heated surface via molecular dynamics (MD) simulations. The simulation results show that at the substrate temperature of Ts = 320 K, the static and dynamic contact angles on the left and right edges are initially asymmetric and then symmetric with increasing field strengths of E = 0.00–0.06 V Å−1, resulting in the asymmetric-to-symmetric spreading transition of spreading–evaporating nanodroplets. Under weak evaporation condition, that is, at Ts = 320 K, the asymmetric-to-symmetric spreading transition is triggered by enhancing the intrinsic surface wettability θ0 = 49°–80° at a constant field strength of E = 0.03 V Å−1. However, at the substrate temperature of Ts = 350 K, the symmetric-to-asymmetric spreading transition first appears for the static and dynamic contact angles on the left and right edges, and then the asymmetric-to-symmetric spreading transition appears with increasing field strength. Under strong evaporation condition, that is, at Ts = 350 K, as the field strength is constant at E = 0.03 V Å−1, the asymmetric-to-symmetric spreading transition also appears with increasing surface wettability. [ABSTRACT FROM AUTHOR]

On a rectangular-ridged superhydrophobic surface, the contact time of the rebounding-coalescing droplet is for the first time investigated via lattice Boltzmann method simulations, where the rebounding-coalescing droplet is caused by an impinging droplet coalescing with an adhesive droplet. The simulation results show that at constant initial radii of impinging droplets, R0, and various initial radii of adhesive droplets, R1, the contact time of rebounding–coalescing droplets depends not only on the impact condition but also on the surface condition. Under various impact conditions, that is, with increased Weber numbers of We = 1–30, the contact time is gradually reduced, and then nearly constant, and eventually constant after slightly reduced at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is gradually reduced, then increased, and eventually constant. It indicates that the contact time of rebounding-coalescing droplets is affected by the initial radii of adhesive droplets. Under different surface conditions, that is, with increased spacing distances between adhesive droplets and ridges of L = 3–17, the contact time is reduced at the low Weber number of We = 3, constant at the moderate Weber number of We = 12, and increased at the high Weber number of We = 28 at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is reduced at both low and moderate Weber numbers of We = 3 and 12, and constant at the high Weber number of We = 28. It indicates that under different surface conditions, the contact time of rebounding-coalescing droplets is also affected by the initial radii of adhesive droplets. [ABSTRACT FROM AUTHOR]

The Cassie–Wenzel (C–W) wetting transition has been extensively investigated; however, the wetting transition of water films on textured surfaces with different topologies, together with underlining mechanisms, is unsatisfactorily explored. In this study, the C–W wetting transition of water films on pillar-arrayed and striped surfaces is studied. The results show that, on pillar-arrayed surfaces, the free energy variation during the C–W wetting transition follows the classical wetting pathway. The free energy first increases with the intrusion of water into the asperities and then decreases after a water film touches the basal surface. However, on striped surfaces, there exist multiple partial wetting states with each one occupying a local energy-minimization configuration. Accordingly, the water film needs to overcome multiple energy barriers to realize the C–W wetting transition. Moreover, the effects of aspect ratio and intrinsic wettability of the two textured surfaces on the C–W wetting transition are discussed. [ABSTRACT FROM AUTHOR]

Aiming at the stress concentration phenomenon and the fracture problem in the practical work in the transitional structure of crosshead link, the varus type of double-toggle clamping mechanism was regarded to analyze and a design method with surface in the transitional structure was proposed. The dynamics simulation and the static finite element analysis were carried out on the crosshead link, the results showed that the stress concentration was obvious, the maximal stress exceeded the safety factor-1.5 of allowable stress and it was the cause of the crosshead link fracture. Based on the relationship between radius and position of maximal stress in circle surface design method, a transitional curve equation was presented to construct the surface to reduce the maximum stress in the transitional structure. The comparison was carried on with the circle surface design method and wedge surface design method, the results indicate that the method verify the effectiveness of reducing the maximal stress of proposed design. [ABSTRACT FROM AUTHOR]