Reducing the contact time of a bouncing drop

Reduction in the contact time of impacting droplets by decorating a rectangular ridge on superhydrophobic surfaces

Drop impact on superhydrophobic surfaces has received significant attention because of the advantages of self-cleaning and anti-icing attained by minimum contact time with the surface. Drop hydrodynamics is generally assumed to be axisymmetric, and the contact time is still bounded below by a theoretical Rayleigh limit. In this study, we report an ellipsoidal drop impact on a superhydrophobic surface to demonstrate an efficient way to reduce the contact time and suppress the bounce magnitude by breaking the symmetry. The outcome of the bounce is characterized in terms of a geometric aspect ratio (AR) and Weber number of the drop by comparing the dynamics with a spherical drop. The experimental result shows that the bouncing of the ellipsoidal drop can reduce the contact time and maximum bounce height below the spherical one by at least 30% and 60%, respectively. The exceptional rim dynamics at high AR produces a liquid alignment along the principal direction, leading to the symmetry breaking in the mass and momentum distribution and the subsequent fast drop detachment, which is quantitatively rationalized by the numerical study. The distinct features of the ellipsoidal drop impact will provide an insight into shape-dependent dynamics and open up new opportunities for self-cleaning and anti-icing strategies.

Reducing the contact time of an impacting droplet is highly desirable in various industrial fields including anti-icing. With the straightforward upscaling advantage, singularities on superhydrophobic surfaces can induce an annular rebound with a limited reduction in contact time. To break this limitation and further reduce contact time, this study focuses on optimizing the singularity number and arrangement. The effects of the singularity number and dimensionless spacing (l* scaled by the droplet diameter) on the dynamic and contact time characteristics of a droplet impacting the superhydrophobic surface are experimentally studied under varying Weber numbers (We). The experimental results indicate that in comparison to the single singularity, two singularities with l* < 1.0 can generate two liquid rings with four lateral liquid subunits due to the impalement at the high We region. Owing to the reduced equivalent diameter of the subunit, increasing We results in a gradually decreased contact time and accordingly breaks the limitation. However, the liquid film cannot be pierced at l* > 1.0 with a limited reduction. Considering the further reducing potential at l* < 1.0, four singularities are explored without a further reduced contact time due to the formed central liquid film. Using an additional central singularity, the central liquid film is pierced promoting its annular rebound. In consequence, five singularities significantly break the limitation in contact time, particularly a 61.7% reduction to the superhydrophobic flat surface at l* < 1.0.

Dynamic behavior of water droplets impacting on the superhydrophobic surface: Both experimental study and molecular dynamics simulation study

The impact dynamics of double unequal-sized droplets impacting superhydrophobic surfaces are investigated via lattice Boltzmann method (LBM) simulations. The simulations capture the spreading and receding dynamics of unequal-sized droplets on superhydrophobic surfaces, and reveal the influences of the radius ratio of the droplets (K), the droplet spacing (L*), and the Weber number. The radius ratio and the droplet spacing significantly affect the contact time. The contact time increases with the radius ratio when 1.2 < K < 2.1, as the increasing radius ratio leads to the intensification of the asymmetry in the impact processes that causes more intense internal collisions and energy dissipation; while it remains unchanged when 2.1 ≤ K ≤ 3.0, since the larger droplet dominates the morphological evolution and the contact time is equivalent to that of a single droplet. Besides, the contact time decreases with the droplet spacing when 1.2 ≤ L* ≤ 1.6, as the coalescence strength decreases with the increase of droplet spacing, leading to less viscous dissipation; while the contact time increases when 1.6 < L* ≤ 2.0, since the increasing droplet spacing causes the increase of the degree of separation of droplets during rebound, which reduces the kinetic energy for rebound provided by the smaller droplet. Our work demonstrates the key physics governing the impact dynamics of unequal-sized droplets impacting superhydrophobic surfaces, key for enhanced antifrosting/icing, self-cleaning, and water/energy harvesting.

Inspired by the array microstructure of natural superhydrophobic surfaces (lotus leaf and cicada wing), an array microstructure was successfully constructed by high speed wire electrical discharge machining (HS-WEDM) on the surfaces of a 7075 aluminum alloy without any chemical treatment. The artificial surfaces had a high apparent contact angle of 153° ± 1° with a contact angle hysteresis less than 5° and showed a good superhydrophobic property. Wettability, contact time, and the corresponding superhydrophobic mechanism of artificial superhydrophobic surface were investigated. The results indicated that the micro-scale array microstructure was an important factor for the superhydrophobic surface, while different array microstructures exhibited different effects on the wettability and contact time of the artificial superhydrophobic surface. The length (L), interval (S), and height (H) of the array microstructure are the main influential factors on the wettability and contact time. The order of importance of these factors is H > S > L for increasing the apparent contact angle and reducing the contact time. The method, using HS-WEDM to fabricate superhydrophobic surface, is simple, low-cost, and environmentally friendly and can easily control the wettability and contact time on the artificial surfaces by changing the array microstructure.

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 &amp;lt; 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.

Superhydrophobic surfaces with a low liquid–solid contact time have huge application prospects in anti-icing, corrosion-resistant, self-cleaning, etc. Significant attempts have been devoted to reducing the contact time through altering the hydrodynamics of the process through which the droplet contacts the superhydrophobic surface. However, these works are rarely considered to be related to the influence of environmental conditions (e.g., the pH of the droplet, salinity of the droplet, droplet viscosity, and supercooled droplet impact). Here, we report various superhydrophobic cone arrays (SCAs) with low droplet impact contact times under various conditions (pH of the droplet, salinity of the droplet, droplet viscosity, droplet temperature, etc.). We demonstrate that the low contact time of the droplet impacting cone-arrays can be optimized via the critical Weber number, pillar-to-pillar spacing, and pillar height (e.g., 11.1, 350 μm, and 300 μm, respectively). The lowest droplet contact time of ~6 ms, which is reduced by more than 60% compared to conventional bouncing, can be achieved. In addition, directional pancake bouncing behaviors can achieve the largest horizontal displacement (85% of the droplet size, ~3 mm) on a tilted SCA with optimal tilt angles. These findings offer insights into the interface effect for controlling wetting that would extend the practical applications, e.g., liquid repellency, anti-corrosion, anti-icing, heat transfer, etc.

Adverse impact of macro-textured superhydrophobicity on contact time reduction at high Weber numbers

Lattice Boltzmann simulation of droplet impact dynamics on superhydrophobic surface decorated with triangular ridges

When a water drop impinges on a flat superhydrophobic surface, it bounces off the surface after a certain dwelling time, which is determined by the Rayleigh inertial-capillary timescale. Recent works have demonstrated that this dwelling time (i.e., contact time) is modified on curved superhydrophobic surfaces, as the drop asymmetrically spreads over the surface. However, the contact time on the curved surfaces still remains poorly understood, while no successful physical model for the contact time has been proposed. Here, we propose that the asymmetric spreading on the curved surface is driven by either the Coanda effect or inertia depending on the ratio of the drop diameter to the curvature diameter. Then, based on scaling analysis, we develop the contact time model that successfully predicts the contact time measured under a wide range of experiment conditions such as different impact velocities and curvature diameters. We believe that our results illuminate the underlying mechanism for the asymmetric spreading over the curved surface, while the proposed contact time model can be utilized for the design of superhydrophobic surfaces for various thermal applications, where the thermal exchange between the surface and the water drop occurs via a direct physical contact.

A numerical investigation of droplet bouncing behaviors on the superhydrophobic surfaces with different micro-structures

Impacting-bouncing nanodroplets on superhydrophobic surfaces under electric fields

Surfaces designed to reduce the contact time of impacting droplets are potentially of great importance for fundamental science and technological applications, for example, anti-icing, self-cleaning and heating transfer applications. Previous studies have shown that the contact time can be reduced via introducing one or several crossing macroscale wires on superhydrophobic surfaces (SHSs). However, the impacts that strike far from the wires (off-center impacts) have contact times that are equal to those obtained on SHSs. Here we demonstrate that this problem can be largely solved by using macro anisotropic SHSs (macro-aniso-SHSs)—in which the wires are parallel and macroscaled. The droplet contact time depends on the spacing between the macrostripes and is remarkably reduced by 40–50% when the spacing is comparable to the droplet size. Obvious differences in the contact time are not observed for impacts that are centered on the stripe and in the groove. The impacts centered in the groove produce new hydrodynamics that are characterized by extended spreading, easy break up and bouncing in a flying-eagle configuration. The study discusses the underlying mechanisms of the impact processes. Moreover, the effect of parallel wires on the contact time is discussed by comparing the droplet impact data for grooved rice leaves and non-grooved cabbage leaves. The enhanced drop mobility associated with the macro-aniso-SHSs should be very useful in many industrial applications.

Fast droplet bouncing induced by asymmetric spreading on concave superhydrophobic surfaces