Retraction and bouncing dynamics of nanodroplets upon impact on superhydrophobic surfaces

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)We1/2Oh1/3 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 ∼ We1/2Oh1/3D0 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.