Numerical and experimental investigations of interdigital transducer configurations for efficient droplet streaming and jetting induced by surface acoustic waves

Surface acoustic wave (SAW) based technologies have recently been explored for various sensing and microfluidic applications, and numerous experimental studies and numerical modelling of SAW streaming and liquid-solid interactions have been performed. However, the large deformation of droplet interface actuated by SAWs has not been widely explored, mainly due to the complex physics of SAW-droplet interactions and interfacial phenomena. In this paper, a computational interface tracking method is developed based on the couple level set the volume of fluid (CLSVOF) approach to simulate the interactions between liquid and acoustic waves and deformation of the liquid-air surface. A dynamic contact angle boundary condition is developed and validated by experimental results to simulate the three-phase contact line dynamics. The modified CLSVOF method is then used to study the droplet jetting and internal streaming behaviours by analyzing the energy terms within the liquid medium. Furthermore, by applying the numerical model, effects of configurations and positions of two interdigital transducers (IDTs) on droplet actuation have been investigated to achieve efficient mixing, separation, and jetting. Results show that two perfectly aligned IDTs are optimal for mixing applications. In contrast, two offset IDTs are optimal for concentration and separation applications. The maximum jetting velocity and minimum jetting time are achieved by using a pair of aligned IDTs, whereas by using the two offset IDTs, effective liquid mixing and jetting are observed which can be used in bioprinting applications

Dynamic Behavior of Droplet Impact on Inclined Surfaces with Acoustic Waves

Droplet impact on arbitrary inclined surfaces is of great interest for applications such as antifreezing, self-cleaning, and anti-infection. Research has been focused on texturing the surfaces to alter the contact time and rebouncing angle upon droplet impact. In this paper, using propagating surface acoustic waves (SAWs) along the inclined surfaces, we present a novel technique to modify and control key droplet impact parameters, such as impact regime, contact time, and rebouncing direction. A high-fidelity finite volume method was developed to explore the mechanisms of droplet impact on the inclined surfaces assisted by SAWs. Numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium, leading to different impact behaviors. We then systematically investigated the effects of inclination angle, droplet impact velocity, SAW propagation direction, and applied SAW power on the impact dynamics and showed that by using SAWs, droplet impact on the nontextured hydrophobic and inclined surface is effectively changed from deposition to complete rebound. Moreover, the maximum contact time reduction up to ∼50% can be achieved, along with an alteration of droplet spreading and movement along the inclined surfaces. Finally, we showed that the rebouncing angle along the inclined surface could be adjusted within a wide range

Acoustic waves for active reduction of droplet impact contact time

Minimizing droplet impact contact time is critical for applications such as self-cleaning, antierosion or anti-icing. Recent studies have used the texturing of surfaces to split droplets during impact or inducing asymmetric spreading, but these require specifically designed substrates that cannot be easily reconfigured. A key challenge is to realize an effective reduction in contact time during droplet impingement on a smooth surface without texturing but with active and programmable control. Our experimental results show that surface acoustic waves (SAWs), generated at a location distant from a point of droplet impact, can be used to minimize contact time by as much as 35% without requiring a textured surface. Additionally, the ability to switch on and off the SAWs means that a reduction in droplet impact contact time on a surface can be controlled in a programmable manner. Moreover, our results show that, by applying acoustic waves, the impact regime of the droplet on the solid surface can be changed from deposition or partial rebound to complete rebound. To study the dynamics of droplet impact, we develop a numerical model for multiphase flow and simulate different droplet impingement scenarios. Numerical results reveal that the acoustic waves can be used to modify and control the internal velocity fields inside the droplet. By breaking the symmetry of the internal recirculation patterns inside the droplet, the kinetic energy recovered from interfacial energy during the retraction process is increased, and the droplet can be fully separated from the surface with a much shorter contact time. Our work opens up opportunities to use SAW devices to minimize the contact time, change the droplet impact regime, and program or control the droplet’s rebounding on smooth or planar and curved surfaces, as well as rough or textured surfaces

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

Research for a Better Tomorrow: Communicating with and engaging non expert audiences

This is a book of abstracts from Northumbria University's Faculty of Engineering and Environment's Postgraduate Research Conference, held on 21st June 2018

Wide range of droplet jetting angles by thin-film based surface acoustic waves

Nozzleless jetting of droplets with different jetting angles is a crucial requirement for 2D and 3D printing/bioprinting applications, and Rayleigh mode surface acoustic waves (SAWs) could be a potential technique for achieving this purpose. Currently, it is critical to vary the jetting angles of liquid droplets induced by SAWs and control the liquid jet directions. Generally, the direction of the liquid jet induced by SAWs generated from a bulk piezoelectric substrate such as LiNbO3 is along the theoretical Rayleigh angle of ~22o. In this study, we designed and manufactured thin-film SAW devices by depositing ZnO films on different substrates (including silicon and aluminium) to realize a wide range of jetting angles from ~16o to 55o using propagating waves generated from one interdigital transducer (IDT). We then systematically investigated different factors affecting the jetting angles, including liquid properties, applied SAW power and SAW device resonant frequency. Finally, we proposed various methods using thin-film SAW devices together with different transducer designs for realizing a wide range of jetting angles within the 3D domain

Droplet impact dynamics on hydrophobic and slippery liquid-infused porous surfaces controlled by propagating surface acoustic waves

Controlling droplet impact on different solid surfaces is an essential topic in developing diverse types of engineering surfaces. Slippery liquid infused porous surfaces (SLIPs) has recently been used in self-cleaning 1–3 and anti-icing surfaces . This paper introduces a new method to actively control the droplet impact on SLIPs surfaces using surface acoustic wave (SAW) technology. To do that, we designed and fabricated SLIPs on ZnO/Al thin film SAW devices. Our experimental results showed that by applying a SAW wave, the droplets' spreading, and retractions are altered, leading to a change in droplet impact regime