This study numerically investigates the effects of the Weber number (We) and cylinder-to-droplet radius ratio (R*) on the impact dynamics of a lowviscosity droplet on a hydrophobic cylinder by the lattice Boltzmann method. The intrinsic contact angle of the surface is chosen as θ 0 = 122°± 2°, which ensures a representative hydrophobicity. The regime diagram of the impact dynamics in the parameter space of We versus R* is established with categories of split and nonsplit regimes. The droplet would split during impact as α = We/R* exceeds a critical value. In the nonsplit regime, the droplet bounces off the cylinder at most Weber numbers unless the impact velocity is minuscule (We < 2). The contact time of the droplet on the cylinder surface decreases with increasing R* or decreasing We, indicating bouncing is facilitated under such conditions. This can be explained by the suppressed adhesion dissipation between the droplet and surface due to a reduction in the contact area. In the split regime, sufficient kinetic energy inside the impacting droplet determines whether the whole droplet could detach from the surface. With a small cylinder (R* < 0.83) and large We (>25), the adhesion effect is weakened for the side fragments because of the small contact area, and it facilitates the dripping of fragments. For other conditions, the detachment, especially for the tiny droplet on the cylinder top, only occurs if the deformation is prominent at We > 35. Moreover, the spreading dynamics of the impacting droplet are also highlighted in this work.

No abstract

No abstract

Reducing the contact time of a droplet hitting a solid surface is crucial for many situations. In this work, the dynamic behavior of a low-viscosity droplet on a superhydrophobic surface, which consists of a cylindrical substrate and a macro ridge placed axially on the peak, was numerically investigated via the lattice Boltzmann method. The focus was given to the spreading and the detaching morphology of the droplet at the Weber number We = 0.84–37.8 and the cylinder-to-droplet radius ratio R* = 0.57–5.71. The ridge is found to redistribute the droplet mass and affect the impact outcomes, as well as the contact time. For each R*, a jug rebound, a stretched rebound straddling the ridge, and a split detachment occur sequentially with the increasing We. When R* does not exceed 1.71, the contact time decreases continuously with the increase in We. With R* being taken between 1.71 and 5.14, the contact time initially reduces with We and plateaus after We reaches 10.3. Once R* exceeds 5.14, the split droplets may present as a bestriding shape at We > 30.3 rather than the regular jug shape with a small We. The contact time would be decreased to a second plateau in this case. In most cases, the contact time can be shortened effectively for the droplet on a ridged cylinder compared with that of a smooth cylinder.

The rebound behaviors of multiple droplets simultaneously impacting a superhydrophobic surface were investigated via lattice Boltzmann method (LBM) simulations. Three rebound regions were identified, i.e., an edge-dominating region, a center-dominating region, and an independent rebound region. The occurrence of the rebound regions strongly depends on the droplet spacing and the associated Weber and Reynolds numbers. Three new rebound morphologies, i.e., a pin-shaped morphology, a downward comb-shaped morphology, and an upward comb-shaped morphology, were presented. Intriguingly, in the edge-dominating region, the central droplets experience a secondary wetting process to significantly prolong the contact time. However, in the center-dominating region, the contact time is dramatically shortened because of the strong interactions generated by the central droplets and the central ridges. These findings provide useful information for practical applications such as self-cleaning, anticorrosion, anti-icing, and so forth.