Abstract
This study reports the development of a three-dimensional numerical model for acoustic interactions with a microscale sessile droplet under surface acoustic wave (SAW) excitation using the lattice Boltzmann method (LBM). We first validate the model before SAW interactions are added. The results demonstrate good agreement with the analytical results for thermodynamic consistency, Laplace law, static contact angle on a flat surface, and droplet oscillation. We then investigate SAW interactions on the droplet, with resonant frequencies ranging 61.7-250.1 MHz. According to our findings, an increase in wave amplitude elicits an increase in streaming velocity inside the droplet, causing internal mixing, and further increase in wave amplitude leads to pumping and jetting. The boundaries of wave amplitude at various resonant frequencies are predicted for mixing, pumping, and jetting modes. The modeling predictions on the roles of forces (SAW, interfacial tension, inertia, and viscosity) on the dynamics of mixing, pumping, and jetting of a droplet are in good agreement with observations and experimental data. The model is further applied to investigate the effects of SAW substrate surface wettability, viscosity ratio, and interfacial tension on SAW actuation onto the droplet. This work demonstrates the capability of the LBM in the investigation of acoustic wave interactions between SAW and a liquid medium.
Highlights
Over the past few years, there has been a move towards integrating complete laboratory chemical analysis procedures onto the surface of a microfluidic chip, known as lab on a chip (LOC) [1,2,3]
This study reports the development of a three-dimensional numerical model for acoustic interactions with a microscale sessile droplet under surface acoustic wave (SAW) excitation using the lattice Boltzmann method (LBM)
To validate the SAW interactions in the LB model, the simulation results are quantitatively compared with experimental data, which are taken from the selected records of previous experiments described in Ref. [56]
Summary
Over the past few years, there has been a move towards integrating complete laboratory chemical analysis procedures onto the surface of a microfluidic chip, known as lab on a chip (LOC) [1,2,3]. SAW parameters (amplitude and frequency) are investigated and their influences on mixing, pumping, and jetting are analyzed It is the objective of this study, and in addition the model development, to provide a comprehensive understanding of the roles that each parameter plays on SAW induced streaming and deformation. Where Cf and CR are the sound velocity in the liquid and the Rayleigh SAW velocity on the SAW substrate, respectively [5,18] This initiates complex streaming patterns to emerge which can cause mixing, translation, or jetting of the droplet. Additional benefits of the LBM are its ability to model complex boundary conditions, predict interfacial dynamics, and implement external forces with ease This could be beneficial in the exploration of SAW-based microfluidics involving curved or complex shaped devices. Numerical results will be compared with experimental observations, validating the constructed model and culminating in a study of various LB parameters to determine their impact on droplet shape and motion
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