Pancake Jumping of Sessile Droplets.

Rapid droplet shedding from surfaces is fundamentally interesting and important in numerous applications such as anti-icing, anti-fouling, dropwise condensation, and electricity generation. Recent efforts have demonstrated the complete rebound or pancake bouncing of impinging droplets by tuning the physicochemical properties of surfaces and applying external control, however, enabling sessile droplets to jump off surfaces in a bottom-to-up manner is challenging. Here, the rapid jumping of sessile droplets, even cold droplets, in a pancake shape is reported by engineering superhydrophobic magnetically responsive blades arrays. This largely unexplored droplet behavior, termed as pancake jumping, exhibits many advantages such as short interaction time and high energy conversion efficiency. The critical conditions for the occurrence of this new phenomenon are also identified. This work provides a new toolkit for the attainment of well-controlled and active steering of both sessile and impacting droplets for a wide range of applications.

Freezing of Nanofluid Droplets on Superhydrophobic Surfaces.

Droplet freezing on cold superhydrophobic surfaces has been studied extensively in recent years. However, previous works are mainly focused on studying water droplet freezing behavior; little work has been conducted to investigate the freezing dynamics of nanofluid droplets on superhydrophobic surfaces. In this work, freezing morphologies of water and nanofluid droplets on superhydrophobic surfaces with different roughnesses were compared and studied. The nanofluid droplets underwent a shape transition from spherical to flat plateau morphology, different from the frozen water droplets that exhibit a sharp cusp. The size of the flat plateau for the frozen nanofluid droplet increases with increasing nanoparticle concentration. The underlying mechanism of the morphology change during the freezing process was elucidated using COMSOL Multiphysics. Compared to the frozen water droplets, more air bubbles are trapped inside the frozen nanofluid droplets, which might be ascribed to the fast freezing speed of the nanofluid droplets. These results can provide important insights for many applications that require freezing of nanofluid droplets, such as material solidification, three-dimensional (3D) printing, as well as phase change enhancement.

Volume evolution of small sessile droplets evaporating in stick-slip mode.

This work is aimed at developing a simple and efficient predictive model and revealing the volume evolution behavior of small sessile droplets evaporating in stick-slip (SS) mode, covering a wide range of initial (θ_{0}) and receding (θ_{rec}) contact angles (0^{∘}<θ_{rec}≤θ_{0}<180^{∘}). For this purpose, simple closed-form expressions for the evolutions of the contact angle and contact radius of droplets evaporating in SS mode are first developed, and are then validated with the experimental data in the literature. With the proposed expressions, the volume evolutions of evaporating droplets with different θ_{0} and θ_{rec} are predicted and analyzed. The results show that on a hydrophilic surface (θ_{0}≤90^{∘}), the volume evolution of droplets in SS mode always occurs in a way between the constant contact radius (CCR) and constant contact angle (CCA) modes (i.e., two extreme modes); however, on a hydrophobic surface (θ_{0}>90^{∘}), as θ_{rec} increases from 0 to θ_{0}, the volume evolution of droplets in SS mode first departs from the CCR mode, then approaches the SS mode with θ_{rec}=90^{∘}, and finally returns to the CCA mode. More interestingly, the initially identical droplets evaporating on hydrophobic surfaces with different receding contact angles (θ_{rec}=90^{∘}±Δθ) can even have approximately the same volume evolution behavior. It is also found that the widely accepted 2/3 law is applicable only for volume evolution of droplets in SS mode with large initial contact angle or small contact angle hysteresis, with the predicted deviation less than 1.5% for θ_{0}≥150^{∘} or θ_{0}≥50^{∘} when θ_{0}-θ_{rec}≤15^{∘}. Finally, a physical interpretation for the volume evolution behaviors of droplets in SS mode is proposed.