Reduction in the contact time of droplet impact on superhydrophobic surface with protrusions

Abstract

Previous studies have pointed out that adding a single protrusion on the superhydrophobic surface can effectively reduce the contact time of droplets, which is of practical importance in applications like anti-icing, self-cleaning, and anti-frost. However, the droplet impact dynamics and the mechanism for contact time reduction are still far from completely understood. Therefore, in this paper, via a three-dimensional pseudopotential lattice Boltzmann model coupled with a modified curved boundary scheme (which satisfies mass conservation), the droplet impact dynamics is simulated in a wide range of Weber numbers (0 < We≤ 67.7), protrusion size (10 ≤ W ≤ 30), and protrusion shape (triangle, square, and circle), with particular interest in understanding their effects on the contact time. We demonstrate that the variation of contact time depends on the bouncing type as well as the retraction way. Among three possible bouncing types observed in the simulation, including non-break bouncing, two droplets bouncing, and three droplets bouncing, two droplets bouncing has less contact time and could be facilitated by increasing We and using triangle protrusion. However, with increasing of W or using square protrusion, the bouncing type may change from two droplets bouncing to three droplets bouncing, and the deformation of fragments may become definitely different, leading to a long contact time. In addition, a transition from twice-retraction to once-retraction can be obtained by increasing We or making the protrusion shape sharp. When the fragments resulting from droplet splitting only retract once on the bottom wall, the contact time can be effectively reduced.

Enhanced Surface Icephobicity on an Elastic Substrate

“…33 Mazloomi Moqaddam et al for the first time realized accurate three-dimensional simulations involving realistic macrotextured surfaces and demonstrated that the reduction in contact time occurs entirely as a result of the increase in the droplet surface area which acts as storage for kinetic energy during the impacting process. 56,57 Shen et al studied a limited contact time of a bouncing droplet on a rationally designed TiO 2 nanowire-decorated Ti6Al4 V substrate (i.e., three-forked and cross-shaped macro-textures) and achieved contact times of 5.8 and 5.6 ms on the corresponding SHSs, which is very close to the theoretically calculated time (5.5 ms). 45 Gauthier et al illustrated that the contact time of drops impacting a nickel-wire-based macrotexture exhibits discrete values when varying the impact speed.…”

“…Guo et al experimentally investigated the droplet impact dynamics on submillimeter-sized (or millimeter-sized) cross-scale cylindrical ridges of Cu/CuO-based SHSs, and the maximum reduction in contact time can be realized when the ridge diameter is close to that of the droplets . Mazloomi Moqaddam et al for the first time realized accurate three-dimensional simulations involving realistic macrotextured surfaces and demonstrated that the reduction in contact time occurs entirely as a result of the increase in the droplet surface area which acts as storage for kinetic energy during the impacting process. , Shen et al studied a limited contact time of a bouncing droplet on a rationally designed TiO 2 nanowire-decorated Ti6Al4 V substrate (i.e., three-forked and cross-shaped macro-textures) and achieved contact times of 5.8 and 5.6 ms on the corresponding SHSs, which is very close to the theoretically calculated time (5.5 ms) . Gauthier et al.…”

Dynamics of droplet impacting on a cone

“…It was found that liquid possessed a faster retraction along the ridge than that on the flat surface, leading to a 37% reduction in contact time. 21 Following this work, studies focused on contact time reduction by asymmetric dynamics have been carried out, including the inclined surface, 22,23 curved surface, [24][25][26][27][28][29][30] surfaces with macro structures of different geometries, [31][32][33][34][35][36][37][38][39][40] moving surfaces, 41,42 and off-center impact. 43,44 Besides the aforementioned asymmetric approaches, symmetric bouncing by involving the droplet center to retraction with point-like structure to reduce contact time has also been proposed.…”

Contact Time of a Droplet Off-Centered Impacting a Superhydrophobic Cylinder

“…For a droplet with low viscosity, the impact velocity and the cylinder size, represented by the Weber numbers ( We = 2ρ R 0 u 0 2 /σ, with u 0 referring to the initial velocity of the droplet) and the size ratio of the cylinder to the droplet ( R * = R cy / R 0 , where R cy refers to the cylinder radius), respectively, are two key factors affecting the dynamic behaviors of the droplet. For example, as We increases or R * decreases, the uneven distribution of the momentum becomes more pronounced, leading to a continuous reduction in contact time. ,, This feature is distinct from the constant value observed on smooth horizontal surfaces (τ c ≈ 2.6), or the discrete forms on flat substrates patterned with single-ridge (exhibiting a reduction rate of 25 and 50%) − or submillimeter-scale posts (showing a reduction rate up to 80%) . In an extreme case where a much higher We is applied or R * is much smaller, the droplet will directly break into smaller fragments, splash from the cylinder, or pass through the cylinder and fall off the surface, undergoing almost no retraction process. , When disregarding the aforementioned particular situations, considerable efforts have been made to correlate the contact time with the given conditions.…”