The coalescence-induced droplet jumping is a self-propelled water removal phenomenon on superhydrophobic surfaces, which has attracted considerable attention due to its potential in a wide range of applications such as self-cleaning and anti-icing/frosting. Improving the energy conversion efficiency, from the excessive surface energy to the kinetic energy, is pivotal to facilitate droplet jumping. In this study, we numerically investigated the dynamics of droplet coalescence on superhydrophobic surfaces with macro-stepped structures, with particular interest in understanding the role of the stepped structure on the droplet jumping process. Three-dimensional simulations were performed by using the lattice Boltzmann method, with the pseudopotential multiphase model and the multiple-relaxation-time collision operator being adopted to achieve high liquid–gas density/viscosity ratios. A wide range of nondimensional height difference of the stepped structure (0–1.5) and droplet radius ratio (0.5–2) was covered. Results show that adding macro-stepped structures can significantly enhance the droplet-wall interaction, thus yielding increased droplet velocity. The enhancement of droplet jumping is more remarkable for droplets of similar sizes, and the dimensionless height difference of the stepped structure is required to exceed a threshold of approximately 0.5. Among the present simulations, the maximum dimensionless droplet jumping velocity reaches 0.66, corresponding to an energy conversion efficiency of 35%. The present findings are helpful for the development of novel superhydrophobic surfaces that pursue efficient droplet removal.
The impact of droplets at increased environmental pressure is important in many industrial applications. Previous studies mainly considered the impact process at standard or reduced environmental pressure, and the effect of high environmental pressure is unclear. In this study, we experimentally investigate the impact of ethanol droplets on dry smooth surfaces at increased environmental pressure. The effects of the environmental pressure on the splashing and rupture of the crown during the impact process are analyzed. The results show that surrounding gas with high environmental pressure can lead to the splashing of the crown in a “thread rupture” mode and the sizes of the secondary droplets from the rim of the liquid crown increase with the environmental pressure. The threshold for the transition from spreading to splashing during the impact process is obtained based on the theory of aerodynamics analysis of the lamella. At increased environmental pressure, the threshold speed of the impact decreases with increasing the environmental pressure because the wedge of the lamella is prevented from moving forward and is driven to detach from the substrate by the air ahead, which has a higher density due to the higher environmental pressure.
Droplet coalescence is a common phenomenon and plays an important role in multidisciplinary applications. Previous studies mainly consider the coalescence of miscible liquids, even though the coalescence of immiscible droplets on a solid surface is a common process. In this study, we explore the coalescence of two immiscible droplets on a partial wetting surface experimentally and theoretically. We find that the coalescence process can be divided into three stages based on the time scales and force interactions involved, namely (I) the growth of a liquid bridge, (II) the oscillation of the coalescing sessile droplet and (III) the formation of a partially engulfed compound sessile droplet and the subsequent retraction. In stage I, the immiscible interface is found not to affect the scaling of the temporal evolution of the liquid bridge, which follows the same 2/3 power law as that of miscible droplets. In stage II, by developing a new capillary time scale considering both surface and interfacial tensions, we show that the interfacial tension between the two immiscible liquids functions as a non-negligible resistance to the oscillation which decreases the oscillation periods. In stage III, a modified Ohnesorge number is developed to characterize the visco-capillary and inertia-capillary time scales involved during the displacement of water by oil; a new model based on energy balance is proposed to analyse the maximum retraction velocity, highlighting that the viscous resistance is concentrated in a region close to the contact line.
Cavitation is a ubiquitous phenomenon in nature and bubble dynamics in open spaces have been widely studied, but the effects of the wall on the dynamics of cavitation bubbles in confined spaces are still unclear. Here, the dynamics of cavitation bubbles in small corners is studied experimentally, focusing on the interaction of the bubble with the wall. High-speed photography is used to visualize the temporal development of laser-induced cavitation bubbles in a small corner formed by two rigid walls, and the bubble spreading on the corner walls and the bubble migration is analyzed via digital image processing. We identify three distinct modes of bubble collapse, namely, collapse dominated by annular shrinkage, collapse dominated by wall attraction, and collapse governed by a combination of both annular shrinkage and wall attraction. The distribution of different collapse modes at different opening angles of the corner is also analyzed. The displacement and shrinkage of different parts of the bubble surface, as well as the direction and amount of bubble migration, are determined. The results show that the asymmetric structure of the corner leads to asymmetric bubble dynamics, including asymmetric bubble expansion, spreading, contraction, and migration.
The design of the intake port plays a critical role in the development of modern internal combustion (IC) engines. The traditional method of the intake port design is a time-consuming process including a huge amount of tests and the production of core box. Compared with the traditional methods, parametric approach attracts increasing attentions by virtue of its high-efficiency, traceability, and flexibility. Based on a tangential port model created by a three-dimensional (3D) computer aided design (cad) software, a new tangential port can be quickly generated with different sets of structure parameters, then computational fluid dynamics (CFD) was employed to explore the influence of structure parameters on the intake port performance. The results show that the flow capacity and the large-scale vortex intensity change regularly with the variations of structure parameters. Finally, the parametric approach was employed to design the intake port of a production four-valve direct-injection (DI) gasoline engine, and the good applicability this approach is well illustrated.
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