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Considerable efforts had been devoted to investigating numerically the droplet impact dynamics on a superhydrophobic surface, whereas most of these numerical simulations were restricted to the two-dimensional (2D) axisymmetric coordinate system with the one-dimensional (1D) substrate surface. In this work, a three-dimensional (3D) computational fluid dynamics (CFD) model, which intergrew a 2D random rough surface, was proposed to investigate the droplet impact dynamics, and the multi-phase flow issue was solved by the Navier–Stokes equations. It is remarkable that the 3D CFD model revealed several significant dynamic details that were not easily captured in a 2D axisymmetric coordinate system or practical experiments. For instance, the 3D CFD model provided a unique perspective to understand the varying dynamic behaviors of impinged droplet in terms of the velocity streamline and dynamic viscosity analyses. Herein, the dynamic viscosity diagram revealed that the sprawl droplet on the 2D random rough surface was classified as the Cassie state, while as the Wenzel state for the smooth surface, which also explained the better bouncing behaviors of the droplet from the random rough surface. Accordingly, we suggested a visual way to evaluate the solid–liquid contact area surrounded by the triple-phase contact line. The effects of finger protrusion and central cavity growth from the sprawl droplet on the vortex generation were further analyzed on the ground of the velocity amplitude distribution and streamline data. The present work can provide early guidance to inquire into the impact dynamics of droplets on the random rough surface. [ABSTRACT FROM AUTHOR]

• A 3-D numerical model including a 2-D random rough surface was established. • Providing a unique perspective in terms of a solidified cloud map. • The mechanical mechanism such as solidified resistance was suggested systematically. The spontaneous spreading of water droplets on a low-temperature solid surface is ubiquitous in nature and critical to numerous industrial applications, but its mechanism remains elusive. In this study, a three-dimensional (3-D) computational fluid dynamics (CFD) model, which intergrows a random rough surface, is proposed to investigate the effects of droplet impact dynamics on hydromechanical solidification. The 3-D CFD model reveals important physical details that cannot be captured easily by a 2-D axisymmetric model or by practical experiments. For instance, the established model provides a unique perspective on the complex hydromechanical solidification behaviours of a single falling droplet in terms of impact dynamic parameters and a solidified cloud map. In particular, the two-phase interfacial resistance derived from the liquid-to-solid transition is systematically discussed, demonstrating different wetting states for spreading droplets on a random rough surface. Moreover, the solidified cloud map suggests that surface wettability, substrate temperature, and droplet impact velocity have nonnegligible effects on phase transition solidification. This work may expand our understanding of droplet dynamic solidification on cold superhydrophobic surfaces and provide a great reference for related subsystems referring to anti-icing/frosting or self-cleaning technologies. [ABSTRACT FROM AUTHOR]

Icing is a multiphase/multiscale/multiparameter physical process, and is of frequent occurrence when suitable conditions with temperature, pressure and humidity are met. In the present work, we prepared a series of PDMS-matrix nanocomposite films with different SWCNT contents, which were endowed with hydrophobicity based on the low-surface-energy PDMS matrix and the conductivity on the SWCNT filler. Furthermore, by etching the pillar-textured structure on its surface, the nanocomposite with 5.0 wt% SWCNT was given the superhydrophobicity. These nanocomposites can be easily switched from a hydrophobic anti-icing mode to an electro-thermal deicing mode by supplying a low voltage. Using non-contact infrared thermometry, we presented an analysis of the freezing phase transition process of a single water droplet on cooling surfaces with different wettability, and investigated their ice nucleation rate and macroscopic growth velocity on these surfaces. The ice-retarding capability of superhydrophobic nanocomposite surface subjected to lots of condensed droplets was also confirmed, and understanding in light of weak contact interaction with droplets. Also under consideration is the icephobicity after freezing in terms of ice shear strength. In addition, we performed a statistical analysis about the Joule heat distribution on nanocomposite surface, the results of which demonstrated that the nanocomposite could supply a suitable heating function for active deicing, demonstrating with an energy-input deicing experiment subsequently. [ABSTRACT FROM AUTHOR]

Lotus leaves and rose petals are the two typical natural examples with superhydrophobicity, also known as the lotus effect and petal effect, respectively. It is generally recognized that the hierarchical structures on their surfaces are responsible for the lotus effect and petal effect. However, the relations between surface geometries and the corresponding wetting states have not been understood completely. In this paper, a 3-D model is built to thermodynamically analyze the wetting behavior of hierarchical structured surfaces. The free energy (FE) and free energy barrier (FEB) of different wetting states are analyzed in detail. Furthermore, equilibrium contact angle (CA), receding and advancing CAs, as well as contact angle hysteresis (CAH) can be predicted by this approach. More importantly, the critical Nano pillar height used for characterizing the transitions between the two wetting states is clearly defined in terms of geometric parameters of solid surfaces, and the final stable wetting state dominated by surface roughness and chemical composition can be determined based on the obtained results. The approach presented in this study can also be used as a guidance for fabricating superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

Shape memory polymer (SMP) surfaces can achieve wettability transition by controlling microstructure have attracted great attention. However, attempts at achieving in situ reversible switching capability remain largely challenging. This paper aims to elaborate on the preparation of a shape memory superhydrophobic surface with switchable wettability and electrothermal performance. The surface microstructure array is regulated by the shape memory effect based on the electrothermal response, and the reversible switching of the wetting state is realized under the action of voltage and external force. Different microstructural morphologies exhibit different solid-liquid contact states, as well as large/small contact angles and low/high adhesion. In addition, the rapid thermal response, excellent thermal conductivity, and satisfactory durability provide the possibility for anti-icing/deicing. The structure can prevent droplet aggregation, greatly delay the nucleation of droplet crystallization, and remove ice by Joule heat. In short, this work successfully realized the in-situ control of surface wettability which also provided a significant application potential in droplet manipulation, droplet writing, anti-icing/deicing, and other fields. [Display omitted] • In-situ electrothermal response through composite structure. • The solid-liquid contact forms under different micropillar states were observed. • The structure has excellent electrothermal performance and anti-icing ability. • Regulates surface wettability for droplet manipulation, writing and screening. [ABSTRACT FROM AUTHOR]