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When a droplet off-center impacts a macro-ridge, it may split into two asymmetric fragments, i.e., the smaller part (SP) and larger part (LP), which then exhibit asymmetric spreading and receding phenomena. Using lattice Boltzmann method simulations, the dynamics of a droplet off-center impacting a hydrophobic surface are explored for the first time. Influences of the off-center distance, the impact Weber number, and the surface wettability on the impact outcomes and the contact time are elucidated, and the impact regime is established. Six impact outcomes are discovered as off-center distance changes, which are dependent on the interaction force between the droplet and the surface. The effect of the off-center distance and the Weber numbers on the spreading and the receding dynamics of the SP is more obvious than that of the LP, due to the fact that adhesion of the ridge on the LP can be negligible compared to the inertial force and the surface tension. The contact time is closely related to the impact outcomes, that is, when the impact outcome is type 1 (SP and LP separate rebound), the contact time can be significantly reduced compared with that on the flat hydrophobic surface, due to the fact that the ridge can reduce the asymmetry of a liquid film and the receding distance in this case; while when the impact outcome is type 6 (SP and LP completely coalescence and rebound), the contact time cannot be reduced. This work extends the fundamental knowledge of off-center impact and provides inspiration for the efficient reduction in the contact time. [ABSTRACT FROM AUTHOR]

In this study, we experimentally explored the impact outcomes of droplets under a wide range of Weber numbers (0.2 ≤ We ≤ 200) and contact angles (91° ≤ θ ≤ 162°). Five impact outcomes were identified: deposition, rebound, partial rebound, receding breakup, and prompt splash. Compared to the literature, we gathered more comprehensive data on the impact outcomes at various contact angles, which were then organized into a complete phase diagram. Furthermore, we corroborated the accuracy of these outcomes through comparisons with other studies. We conducted a comprehensive analysis of the associated phenomena and underlying mechanisms of these outcomes. By introducing the concept of surface hysteresis energy, we also proposed identification criteria for partial rebound. This innovative approach provides an important reference for further understanding of droplet impact behavior and provides guidance for future research in this field. [ABSTRACT FROM AUTHOR]

This study investigates the heat transfer enhancement by electrohydrodynamic (EHD) in a rectangular channel with various geometric and operational parameters in a wide NEHD range of 0.4–5. Here, NEHD is a dimensionless number, describing the ratio of electrostatic, inertial, and viscous forces. Two working regimes in EHD are identified: inertial and electrostatic regimes. The results show that the barrier effect exists only in the inertial regime; however, the interference effect occurs in the electrostatic regime. The barrier effect relies heavily on NEHD rather than the dimensionless distance between electrodes l* and the dimensionless channel width H* because the thermal boundary layer cannot be disturbed sufficiently by a small electrostatic force; however, the interference effect depends on l* and H* rather than NEHD due to the stagnant area initiated only by a strong interaction between adjacent emitting electrodes. A new effect, the "blocking effect," is found in the electrostatic regime. The mechanism of the blocking effect is different from that of the barrier and interference effects. The blocking effect is initiated by a giant vortex, which "blocks" the airflow flowing toward the downstream channel. The average Nusselt number of channels can be reduced by at least 8%, 13%, and 5% for the barrier, interference, and blocking effects, respectively. A working spectrum of EHD-induced heat transfer enhancement in a rectangular channel is provided under NEHD and channel area coupling conditions. We believe the spectrum can help in designing EHD-induced heat transfer enhancement because it provides theoretical guidance for avoiding the three effects. [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]

In this work, we investigate microscale electrohydrodynamic (EHD) conduction pumps of dielectric liquids in a wide working regime range from the Ohmic to the saturation regime. We show that the electric force of microscale EHD conduction pumps differs from that of macroscale EHD conduction pumps owing to the scale effect. We reveal that the scale effect of microscale EHD conduction pumps is triggered by the enhanced ionic diffusion. When the characteristic length of the system reduces from millimeter to micrometer, the maximum order of magnitude of α increases from O(10−5) to O(10−1), resulting in significantly enhanced ionic diffusion; however, the ionic diffusion can be neglected for macroscale EHD conduction pumps, where α denotes the ratio of ionic diffusion velocity to ionic migration velocity. We found that the existing macroscale theoretical models do not work for the microscale EHD conduction pumps due to the rough estimation of heterocharge layer thickness. We show that the heterocharge layer thickness of microscale EHD conduction pumps not only depends on ionic migration but also relies on ionic diffusion. By modifying the expression of the heterocharge layer thickness with respect to ionic migration and diffusion, we develop a theoretical model of dimensionless electric force for the microscale EHD conduction pumps. [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]

This work presents a study of a droplet impacting an inclined hydrophobic surface using lattice Boltzmann method (LBM) simulation. The influence of surface inclination, surface wettability, and the Weber number on the dynamic of spreading and receding is elucidated. Intriguingly, the contact time is independent of the surface inclination. The surface wettability and Weber number do not affect the spreading time, while they significantly influence the receding dynamic. To further quantitatively describe the influence of surface wettability and Weber number, scaling laws of the receding rate are established as Vret ∼ (1 − cos θ)−0.25 and Vret ∼ Wen0.19, and, thus, scaling laws of the receding time are established as tr ∼ (1 − cos θ)−0.5 and tr ∼ Wen0.1, respectively, where Wen is the normal Weber numbers and θ is the contact angle. Based on this, a relationship of the contact time for a droplet impacting an inclined hydrophobic surface is ultimately established as tc = 3.1(ρR03/σ)1/2 (1 − cos θ)−1/2Wen0.1, where ρ, R0, and σ denote the droplet density, radius, and surface tension, respectively. This study provides a quantitative relationship to calculate the contact time of a droplet impacting an inclined hydrophobic surface, which can simultaneously efficiently evaluate the anti-freezing, anti-icing, and self-cleaning performance of hydrophobic surfaces employed in practical applications. [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]