Surface Acoustic Waves to Control Droplet Impact onto Superhydrophobic and Slippery Liquid-Infused Porous Surfaces

Superhydrophobic coatings and slippery liquid-infused porous surfaces (SLIPS) have shown their potentials in self-cleaning, anti-icing, anti-erosion, and antibiofouling applications. Various studies have been done on controlling the droplet impact on such surfaces using passive methods such as modifying the lubricant layer thickness in SLIPS. Despite their effectiveness, passive methods lack on-demand control over the impact dynamics of droplets. This paper introduces a new method to actively control the droplet impact onto superhydrophobic and SLIPS surfaces using surface acoustic waves (SAWs). In this study, we designed and fabricated SLIPS on ZnO/aluminum thin-film SAW devices and investigated different scenarios of droplet impact on the surfaces compared to those on similar superhydrophobic-coated surfaces. Our results showed that SAWs have insignificant influences on the impact dynamics of a porous and superhydrophobic surface without an infused oil layer. However, after infusion with oil, SAW energy could be effectively transferred to the droplet, thus modifying its impact dynamics onto the superhydrophobic surface. Results showed that by applying SAWs, the spreading and retraction behaviors of the droplets are altered on the SLIPS surface, leading to a change in a droplet impact regime from deposition to complete rebound with altered rebounding angles. Moreover, the contact time was reduced up to 30% when applying SAWs on surfaces with an optimum oil lubricant thickness of ∼8 μm. Our work offers an effective way of applying SAW technology along with SLIPS to effectively reduce the contact time and alter the droplet rebound angles

Unchannelized granular flows: Effect of initial granular column geometry on fluid dynamics

In recent years, significant progress has been made in modelling granular flows using the μ(I)-rheology model, which connects the viscosity of a granular medium to the pressure and strain rate via a dimensionless quantity called the inertial number, I. This model allows treating the granular material as a non-Newtonian liquid with a yield stress, making it possible to model the flow using the continuum approach, which is less computationally expensive than discrete element methods. In this paper, we implement the μ(I)-rheology model in a computational fluid dynamics (CFD) code and couple it with the volume of fluid (VOF) interface tracking approach to model the three-dimensional (3D) flow of monodisperse granular materials. After validating the model using experimental data, we briefly describe a trial-and-error method for evaluating the material properties of powders via a simple collapse experiment. Then, employing the CFD model, we investigate the physics of the unchannelized collapse of granular materials and perform an energy budget analysis to demonstrate the different stages of the granular collapse. To further investigate the effect of the initial shape of the pile on the spreading dynamics, we run a campaign of 3D simulations. Our results show that the μ(I)-rheology model accurately reproduces the dynamics of the granular material during the collapse and can be used for risk assessment purposes in natural disasters. The findings from our simulations can also aid in developing preventative measures to minimize potential harm