Effects of contact angle hysteresis on drop manipulation using surface acoustic waves

Surface acoustic waves have gained much attention in flow control given the effects arising from acoustic streaming. In this study, the hydrodynamic interference of a drop under surface acoustic waves is comprehensively investigated and the contact angle hysteresis effects are considered, too. This paper reveals the effects of some control parameters such as wave amplitude and wave frequency on the dynamical behaviors of drop. For these purposes, a multiple-relaxation-time color-gradient model lattice Boltzmann method is developed. In these case studies, wave frequency and amplitude were in the ranges of 20-60 MHz and 0.5-2 nm, respectively. In addition, the density ratio of 1000, the kinematic viscosity ratio of 15, Reynolds numbers of 4-24, Capillary numbers of 0.0003-0.0008 and Weber numbers of 0-0.4 were considered. Results show that drop would not move, but would incline in the direction of wave propagation equal to radiation angle when the wave amplitude is low. However, the drop will initiate to move as wave amplitude is progressively augmented. Meanwhile, the increase in frequency leads to an increment of required power to change the modes of the system from streaming to pumping or jetting states. The obtained results clearly show that a reduction in viscosity and an increase in surface tension coefficient significantly influence the flow control system and enhance its sensitivity. Also, the contact angle hysteresis modeling can improve the numerical results by up to 20%.