Pore wetting induced by surfactants and salt scaling is a major obstacle to the industrial application of membrane distillation (MD). Identifying the transition of wetting stages and achieving early monitoring of pore wetting are crucial for wetting control. Herein, we made a pioneering attempt to use ultrasonic time-domain reflectometry (UTDR) technique to non-invasively detect the pore wetting in a direct contact MD, and explain the UTDR waveform with the help of optical coherence tomography (OCT) imaging. The results showed that the water–vapor interface had a strong reflection to ultrasound (reflection coefficient = 0.9995), while the water-membrane and water-scaling layer interfaces showed relatively weak reflection. Therefore, UTDR could effectively detect the movement of water–vapor interface with the low interference from the signals generated by the membrane and scaling layer. For the surfactant-induced wetting, the occurrence of wetting could be successfully detected by the right-shift in phase and the reduction in amplitude of the UTDR waveform. Moreover, the wetting depth could be accurately calculated by the time of flight (ToF) and ultrasonic velocity. For scaling-induced wetting, the waveform slightly shifted to the left at the beginning due to the growth of scaling layer, then to the right because the left-shift was surpassed by the right-shift of the waveform caused by pore wetting. Both for the surfactant- and scaling-induced wetting, the variation of the UTDR waveform was sensitive to wetting dynamics, and the right-shift of phase and the reduction in amplitude of the waveform could act as early monitoring signals to the occurrence of wetting.

表面活性剂和盐垢引起的孔润湿是膜蒸馏（MD）工业应用的主要障碍。识别润湿阶段的转变并实现对孔隙润湿的早期监测对于润湿控制至关重要。在此，我们开创性地尝试使用超声时域反射计 (UTDR) 技术无创检测直接接触 MD 中的孔隙润湿，并借助光学相干断层扫描 (OCT) 成像解释 UTDR 波形。结果表明，水气界面对超声波有很强的反射（反射系数=0.9995），而水膜和水垢层界面反射相对较弱。所以，UTDR 可以有效检测水气界面的运动，膜和结垢层产生的信号干扰小。对于表面活性剂引起的润湿，可以通过 UTDR 波形的相位右移和振幅减小成功检测润湿的发生。此外，润湿深度可以通过飞行时间（ToF）和超声速度准确计算。对于结垢引起的润湿，由于结垢层的生长，波形在开始时略微向左移动，然后向右移动，因为左移被孔隙润湿引起的波形右移所超过。对于表面活性剂和结垢引起的润湿，UTDR 波形的变化对润湿动力学敏感，