Pulsatile pressure gradients are widely present in microfluidic multiphase systems where the movement of fluids is affected by the presence of fluid-fluid interfaces. We present a simple theoretical model that incorporates dynamic interfacial curvatures produced as a response to pulsatile external forcing. Our equations make evident a singular character of the dynamics at low frequencies, due to surface tension. Analytical solution of the model shows the emergence of a resonant behavior for the dynamic permeability. We have designed and implemented microfluidic experiments to observe both the low-frequency dynamics and the resonance. We have studied a fluid slug whose length was chosen in order to look for resonances as predicted by our theoretical model, in the range of operational frequencies of our piezoelectric actuator. We have obtained the experimental dynamic permeability for water and a 70.0% glycerol solution in water and observed agreement with theoretical findings. Our model, validated by experiments, allows us to understand differences of several orders of magnitude in the amplitude of flow velocity at low frequencies, between systems with and without interfaces.

中文翻译：

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空气-流体界面的动态弯曲导致微流体脉动流的奇异行为

脉动压力梯度广泛存在于微流体多相系统中，其中流体的运动受流体-流体界面的存在影响。我们提出了一个简单的理论模型，该模型结合了动态界面曲率，作为对脉动外力的响应而产生。由于表面张力，我们的方程式清楚地表明了低频动力学的奇异特性。该模型的解析解显示了动态渗透率的共振行为的出现。我们已经设计并实施了微流体实验，以观察低频动力学和共振。我们已经研究了一种流体塞，选择其长度是为了在我们的压电执行器的工作频率范围内寻找由我们的理论模型预测的共振。我们已经获得了水和70.0％甘油水溶液在水中的实验动态渗透率，并观察到与理论结果相符的结果。通过实验验证的我们的模型使我们能够了解带有接口和不带有接口的系统之间低频流动速度幅值的几个数量级的差异。