Acoustic Waves for Active Reduction of Contact Time in Droplet Impact

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.

Figure 1

Anticipated time evolution of droplet impact on a solid surface in the presence of SAWs propagating on the surface. SAWs can dissipate energy into the liquid droplet during the impact process and break the symmetry in a droplet spreading and the retracting phase, leading to shorter contact time.

Figure 2

Experimental snapshots of a water droplet impinging on the solid surface. (a) Droplet free impact case without application of SAWs, (b) impact on $\mathrm{Zn}\mathrm{O}/\mathrm{Si}$ surface with TSAWs applied to propagate from left to right, and (c) impact on $\mathrm{Zn}\mathrm{O}/\mathrm{Si}$ surface while SSAWs are applied to the surface. For all experiments, the droplet impact velocity and volume are 1.4 m/s and 3.56 µl, respectively. See Videos S1–S3 within the Supplemental Material [62] for experimental movies.

Figure 3

(a) Comparison between numerical and experimental results for normalized contact widths of droplet impact on SAW device for DFI, TSAW, and SSAW scenarios. (b) Maximum contact width versus SAW amplitude for both SSAW and TSAW cases. Impact experiments are carried out for a droplet with a volume of 3.56 µl and an impact velocity of 1.4 m/s. (c) Normalized contact time as a function of SAW amplitude for a droplet with a volume of 3.56 µl and impact velocity of 1.4 m/s impacting on the $\mathrm{Zn}\mathrm{O}/\mathrm{Si}$ SAW device surface with CYTOP surface treatment. (d) Contact time versus impact velocity for a droplet with a volume of 3.56 µl for DFI and TSAW scenarios. Shaded area represents droplet deposition on the surface for DFI cases. Error bars represent SD of the results.

Figure 4

Temporal evolution of contact width for droplets impacting on the SAW device surface with different wettability. In these experiments, droplet volume and impact velocity are kept constant at 3.56 µl and 1.08 m/s, respectively. For each surface coating, experiments are carried out for both DFI and TSAW scenarios. Applied rf power to the IDTs for the TSAW scenario is 27 W. Snapshots of the experimental results confirm that TSAW changes droplet deposition (red dashed line) and breakup (black dashed line) on the surface to complete rebound (solid lines).

Figure 5

Time-evolution images of droplet impact dynamics obtained by numerical simulation. (a) DFI scenario. (b) SSAW propagation on the solid surface during impact. In all simulations, droplet impact velocity and volume are 1.4 m/s and 3.56 µl, respectively.

Figure 6

CFD snapshots of liquid phase overlaid by velocity vectors during impingement process in the presence of TSAWs propagating from left to right. Droplet impact velocity and volume are 1.4 m/s and 3.56 µl, respectively.

Figure 7

(a) Total energy, (b) kinetic energy, and (c) interfacial energy for FI, TSAW, and SSAW scenarios. In the simulation, droplet impact velocity and volume are 1.4 m/s and 3.56 µl, respectively. All energies are normalized by kinetic energy at the onset of impact. (d) Droplet vertical momentum normalized by the vertical momentum at the onset of the impact.

Figure 8

(a) Experimental results of normalized contact time as a function of SAW amplitude for droplets with initial diameters of 1.87, 2.04, and 2.22 mm. Impact velocity for all cases is kept constant at 1.7 m/s. (b) Experimental results of normalized contact time as a function of SAW amplitude for droplets with impact velocities of 1.4, 1.7, and 2.0 m/s. Droplet initial diameter is kept constant at 1.87 mm. Error bars represent SD of contact time.

Figure 9

(a) Experimental results of normalized contact time versus SAW amplitude for $\mathrm{Zn}\mathrm{O}/\mathrm{Si}$ and $\mathrm{Zn}\mathrm{O}/\mathrm{Al}$ SAW devices. For both devices, wavelength of the SAW is 64 µm, and a droplet with a volume of 3.56 µl impacts the surface with a velocity of 1.4 m/s. Contact time, $\tau$_, is normalized with the contact time of DFI case, ${\tau }_0$, for each device. (b) Experimental results of transition distance in x direction before separation as a function of SAW amplitude. (c) Force analysis of impacting droplet.

Figure 10

Experimental results of normalized contact time versus wave amplitude for SAW devices with different frequencies for a droplet with volume of 3.56 µl and impact velocity of 1.4 m/s impacting on the $\mathrm{Zn}\mathrm{O}/\mathrm{Si}$ SAW device surface with CYTOP surface treatment. Solid and dashed lines represent droplet rebounding and breakup regimes, respectively.