Experimental investigation of thermal and kinetic impacts of surface acoustic waves on water droplet

Abstract

An analysis model for investigation of the coupling of kinetic and thermal impacts of surface acoustic wave (SAW) on a microscale droplet is proposed. The model is based on mass, momentum, and energy conservation principles with assistance from experimental observations. The investigation is carried out on a 25 µl water droplet placed on the SAW device fabricated on an aluminium (Al) plate substrate with deposition of 5 µm thick zinc oxide (ZnO) as a top layer. The devices have a thickness of 200 µm and 600 µm with the wavelength (λ) of 100 µm and 200 µm using Rayleigh, Sezawa, and a Lamb and Rayleigh hybridized mode. The SAW input power values are from 0.30 W to 4.0 W with a temperature range of 5-30 °C in this study. A charged-coupled device (CCD) camera has been employed to monitor streaming inside the droplet. To visualise the streaming, 10 µm red polystyrene particles have been used whereas the velocity of particles estimated using particle image velocimetry (PIV). An infrared (IR) thermal camera has been used to detect the droplet surface temperature. However, temperature distributions of fluid layers of the droplet are estimated by developing a MATLAB code. The data has been used in the implementation of the analysis model to interpret the coupling mechanism inside the droplet. The thermal impact includes energy absorbed by the droplet, heat transfer from the substrate to the droplet, from the droplet to the air and the waves penetrated to the droplet (radiation). Whereas kinetic impact involves energy transferred by the streaming and friction inside the droplet. Since this study is based on temperatures much lower than the boiling temperature, no phase change or evaporation observed, therefore, no significant mass transfer has been observed either with Rayleigh or Sezawa. However, at input power (Pin) of 4.0 W using Rayleigh wave (R-wave) where the droplet slightly moves away on the surface in the direction of the waves. Since Sezawa waves (S-waves) travel in the interlayers, they have less SAW force because of which droplet sticks to the surface and does not move away even at higher input power. It has been observed that the thermal impacts of the SAW are more dominant than the kinetic when considering both Rayleigh and Sezawa wave modes. However, the streaming plays a key role in enhancing the heat transfer inside the droplet by internal convection. The major source of thermal impact is the radiation of SAW power (~ 0.05 W to 0.20 W) penetrated to the droplet at input power (Pin) ranging from 0.96 W to 3.2 W, while, at the power of 0.38 W or lower, is from both the SAW radiation (~0.025 W) and hot substrate (~0.01 W) using Rayleigh waves. Inverse heat flux from droplet to the substrate is observed after ‘reverse time’ at Pin > 0.50 W and Pin > 1.0 W for Rayleigh and Sezawa, respectively. Heat always transfers from droplet to the air since this is the heat leaving the system. The thermal impacts of both Rayleigh and Sezawa modes on the droplet showed an exactly similar trend when compared to each other based on the results from the analysis model and experimental data. However, the thermal impacts of SAW on the droplet with a Sezawa wave is slightly less as compared to the Rayleigh. The thermal energy absorbed by the droplet using Rayleigh waves is 4% more as compared to the Sezawa at an input power of 0.96 W. However, this dropped to 1.5% at a higher input power of 3.2 W after the same time frame. It has been found that using the same wave mode, temperature rise inside the droplet is directly proportional to the resonant frequency of the device. Furthermore, Lamb and Rayleigh hybridized waves generate intense thermal impacts, showed 3.5 times and 2.5 times higher temperature as compared to the pure Rayleigh mode at Pin of 2.2 W and 3.2 W, respectively. The same trend and difference have been observed when the hybridized mode is compared with pure Sezawa mode.