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Efficient droplet shedding from surfaces is fundamentally interesting and important due to its promising potential in numerous applications, such as anti-erosion, anti-icing, and self-cleaning. In this work, the bouncing dynamics of the head-on impact of two nanodroplets on superhydrophobic surfaces are investigated through molecular dynamics simulations. Three bouncing patterns, including regular-coalescence-bouncing, coalescence-hole-bouncing, and ring-bouncing, are identified at a wide range of impacting Weber numbers. For three bouncing patterns, the time evolutions of the spreading factors and the vertical velocity components are employed to analyze the particular dynamic behaviors and elucidate the underlying physics. As a counter-intuitive bouncing pattern, the ring-bouncing that two impact nanodroplets coalesce, spread, and then leave the surface in a ring shape without retracting exhibits a remarkable reduction in contact time by up to 60%. Considering four typical states for the ring-bouncing pattern, the comparison of the velocity distribution within the droplet clearly reveals that the ring-shaped droplet reshapes interfaces, which leads to a special hydrodynamics distribution. As a result, the internal flows at the inner and outer edges along the opposite direction collide with each other, leading to a sudden increase in the upward velocity. Combining the largely decreased contact area between solid and liquid with the small surface adhesion, the ring-shaped droplet rapidly bounces off the surface at the maximum spreading state. Finally, it is significantly highlighted that the ring-bouncing pattern offers a new avenue to break the contact time limit for efficient droplet shedding.

This work investigates the retraction and bouncing dynamics of an impacting low-viscosity nanodroplet on superhydrophobic surfaces via molecular dynamics simulations, aiming to reveal the scaling laws of retraction and bouncing velocities and to establish the relationship between them. The retraction velocity, Vre, is found to scale as Vre ∼ Dmax/ τc,n, where Dmax is the maximum spreading diameter, τc,n = ( D0/ V0) We1/2 Oh1/3 is the inertial-capillary-viscous time, and We and Oh are the Weber number and Ohnesorge number, respectively. The bouncing stems from the collision of the retracting rim at the center of the nanodroplet, leading to the bouncing velocity scaling as the retraction velocity. Combining the relationship of Vre ∼ Dmax/ τc,n with the scaling law of Dmax ∼ We1/2 Oh1/3 D0 yields both the retraction and bouncing velocities scaling as the impact velocity, indicating that both the retraction and bouncing velocities of low-viscosity nanodroplets on a superhydrophobic surface depend merely on the impact velocity. An energy analysis shows that the proportion of the surface energy at the maximum spreading state ( Es,max) to the initial kinetic energy ( Ek,ini) follows Es,max/ Ek,ini ∼ Oh2/3, whereas the proportion of the bouncing kinetic energy ( Ek,b) to the surface energy at the maximum spreading state follows Ek,b/ Es,max ∼ Oh−2/3, leading to constant Ek,b/ Ek,ini and also constant restitution coefficient for low-viscosity nanodroplets impacting superhydrophobic surfaces.

Direct numerical simulations are conducted to gain insight into the blowing-based active control in a spatially developing turbulent boundary layer at a low Reynolds number. The drag reduction properties and mechanisms of different blowing velocity distribution forms under constant wall-normal mass flux are comparatively studied, including uniform blowing and blowing-only opposition control (BOOC). After the application of blowing control, the self-similarity of the Reynolds shear stress is influenced. The property of drag reduction and control gain of the blowing-based active control schemes in the turbulent boundary layer is similar to that in turbulent channel flow, i.e., the BOOC scheme can achieve higher drag reduction than uniform blowing, but the control gain reduces. Due to the coexistence of the opposition effect and the induction effect, the negative wall-normal velocity fluctuations accompanied by the sweep motion are induced to form small-scale flow structures in the near-wall region. The decomposition of the skin-friction drag coefficient shows that the changes of each contribution term are basically the same for different blowing schemes, except that the BOOC scheme has a more substantial influence on mean convection and spatial development. According to the property that the drag reduction of the BOOC scheme with additional threshold limitation is equivalent to that without the restriction, it can be determined that the effect of blowing-based active control is mainly based on the temporal and spatial averaging effects of blowing, including the opposition effect and the induction effect.

A droplet impacting hydrophobic surfaces is explored via lattice Boltzmann method simulations. Dynamics of the spreading and retraction processes of the impacting droplet with different surface wettability ( θ = 106°–145°) at various Weber numbers ( We = 15–35) and Reynolds numbers ( Re = 235–359) are investigated. It indicates that the spreading time, the time interval from a droplet just touching the surface to reaching the maximum spreading diameter, is a fixed value that is not related to the above factors. The retraction time, the time interval from a droplet reaching the maximum spreading diameter to rebounding the surface, is just closely dependent on the surface wettability. Therefore, the contact time, the sum of the spreading time and the retraction time, is mainly related to the retraction process and dependent on the surface wettability. The time is commonly normalized as the non-dimensional time, τ = t/( ρR03/ σ)1/2, where R0 is the initial droplet radius and σ is the surface tension. The emphasis of this work is placed on establishing a relationship of the contact time with the surface wettability. Based on simulation research and theoretical modeling, the surface wettability is incorporated into the scaling law of contact time, and a modified scaling law, tc ∼ ( R0/ V0) We1/2(1 − cos θ)−1/2, where V0 denotes the initial impact velocity, is established for a droplet impacting hydrophobic surfaces of θ = 106°–145°and We = 15–35. It is demonstrated that the scale relationship can be well applied to calculate the contact time of a droplet impacting hydrophobic surfaces for moderate Weber numbers.