Medically Advanced Devices Laboratory, Center for Medical Device Engineering and Biomechanics, Department of Mechanical and Aerospace Engineering, Jacobs School of Engineering, University of California San Diego, La Jolla, California 92093-0411, United States.
Materials Science and Engineering Program, Jacobs School of Engineering, University of California San Diego, La Jolla, California 92093-0411, United States.
Langmuir. 2023 Mar 14;39(10):3699-3709. doi: 10.1021/acs.langmuir.2c03403. Epub 2023 Mar 1.
Remarkably, the interface of a fluid droplet will produce visible capillary waves when exposed to acoustic waves. For example, a small (∼1 μL) sessile droplet will oscillate at a low ∼10 Hz frequency when weakly driven by acoustic waves at ∼10 Hz frequency and beyond. We measured such a droplet's interfacial response to 6.6 MHz ultrasound to gain insight into the energy transfer mechanism that spans these vastly different time scales, using high-speed microscopic digital transmission holography, a unique method to capture three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that low-frequency capillary waves are driven into existence via a feedback mechanism between the acoustic radiation pressure and the evolving shape of the fluid interface. The acoustic pressure is distributed in the standing wave cavity of the droplet, and as the shape of the fluid interface changes in response to the distributed pressure present on the interface, the standing wave field also changes shape, feeding back to produce changes in the acoustic radiation pressure distribution in the cavity. A physical model explicitly based upon this proposed mechanism is provided, and simulations using it were verified against direct observations of both the microscale droplet interface dynamics from holography and internal pressure distributions using microparticle image velocimetry. The pressure-interface feedback model accurately predicts the vibration amplitude threshold at which capillary waves appear, the subsequent amplitude and frequency of the capillary waves, and the distribution of the standing wave pressure field within the sessile droplet responsible for the capillary waves.
令人惊讶的是，当液滴的界面暴露在声波中时，会产生可见的表面张力波。例如，当小（∼1 μL）的液滴在∼10 Hz 的频率下受到弱声波驱动时，它会以低∼10 Hz 的频率振荡。我们使用高速微观数字传输全息术测量了这种液滴对 6.6 MHz 超声的界面响应，以深入了解跨越这些差异很大的时间尺度的能量传递机制，这种方法是一种独特的方法，可以在纳米级空间和微秒级时间分辨率下捕捉三维表面动力学。我们表明，低频表面张力波是通过声辐射压力和流体界面不断变化的形状之间的反馈机制产生的。声压分布在液滴的驻波腔中，并且随着界面上的分布压力响应改变流体界面的形状，驻波场也改变形状，反馈产生腔中声辐射压力分布的变化。提供了一个明确基于该机制的物理模型，并使用它进行了模拟，模拟结果与全息术直接观察到的微尺度液滴界面动力学以及使用微粒子图像测速法测量的内部压力分布进行了验证。压力-界面反馈模型准确预测了表面张力波出现的振动幅度阈值、随后的表面张力波幅度和频率，以及驻波压力场在液滴中的分布，这是产生表面张力波的原因。
