The acoustic-induced steady motion of fluids in a confined space is understood by the established acoustic streaming theory but buildup of the streaming receives little attention, especially in microfluidics using surface acoustic waves (SAWs). In this work, we experimentally and numerically studied the temporal acoustic and streaming fields excited by a traveling-SAW lasting for a finite time. Based on the perturbation theory for slow streaming, we propose a concept of unsteady time-averaged large-scale streaming, for which the slow variation of flow velocity is detectable in a time-scale much longer than an acoustic period. Theoretical analysis, numerical calculations, and experiments reveal that the buildup time of the acoustic field, within which the acoustic energy reaches the maximum in a SAW-based device, is about N (the electrode number of interdigital transducers) times of the acoustic period T, while buildup of the streaming field is an acoustic momentum diffusion process. The results show that the geometry of the microchannel determines the characteristic size of the flow, giving a square relationship between the channel height h and buildup time of the streaming. For periodic excitation of SAW pulses, we show the distinct behaviors of the unsteady streaming by a phase diagram. A short pulse duration comparable to the buildup time of the acoustic field makes the streaming fluctuate with unobservable magnitude, whereas microscopic and macroscopic fluctuations are observable for an increasing pulse duration. Based on the separation of the buildup times, we also propose a hybrid time–frequency scheme for efficient finite element analysis, which opens a way to design devices with additional functionality in the time domain.

已建立的声流理论可以理解有限空间中流体的声诱导稳态运动，但流的建立很少受到关注，尤其是在使用表面声波 (SAW) 的微流体中。在这项工作中，我们通过实验和数值方法研究了由持续有限时间的行进 SAW 激发的时间声学和流场。基于慢流的摄动理论，我们提出了非定常时间平均大尺度流的概念，其中流速的缓慢变化可以在比声学周期长得多的时间尺度上检测到。理论分析、数值计算和实验表明，声场的建立时间约为N（叉指换能器的电极数）乘以声周期T，而流场的建立是声动量扩散过程。结果表明，微通道的几何形状决定了流动的特征尺寸，给出了通道高度h之间的平方关系和流的建立时间。对于 SAW 脉冲的周期性激励，我们通过相图显示了非稳态流的不同行为。与声场的建立时间相当的短脉冲持续时间使得流以不可观察的幅度波动，而随着脉冲持续时间的增加，可以观察到微观和宏观的波动。基于构建时间的分离，我们还提出了一种用于高效有限元分析的混合时频方案，这为设计具有时域附加功能的设备开辟了道路。