Rheotaxis is a common phenomenon in nature that refers to the directed movement of micro-organisms as a result of shear flow. The ability to mimic natural rheotaxis using synthetic micro/nanomotors adds functionality to enable their applications in biomedicine and chemistry. Here, we present a hybrid strategy that can achieve both positive and negative rheotaxis of synthetic bimetallic micromotors by employing a combination of chemical fuel and acoustic force. An acoustofluidic device is developed for the integration of the two propulsion mechanisms. Using acoustic force alone, bimetallic microrods are propelled along the bottom surface in the center of a fluid channel. The leading end of the microrod is always the less dense end, as established in earlier experiments. With chemical fuel (H₂O₂) alone, the microrods orient themselves with their anode end against the flow when shear flow is present. Numerical simulations confirm that this orientation results from tilting of the microrods relative to the bottom surface of the channel, which is caused by catalytically driven electro-osmotic flow. By combining this catalytic orientation effect with more powerful, density-dependent acoustic propulsion, both positive and negative rheotaxis can be achieved. The ability to respond to flow stimuli and collectively propel synthetic microswimmers in a directed manner indicates an important step toward practical applications.

中文翻译：

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化学声混合动力驱动的双金属微电机的流变

流变是自然界中的一种普遍现象，是指由于剪切流而导致的微生物的定向运动。使用合成的微/纳米马达模拟自然流变的能力增加了功能，使其能够在生物医学和化学中应用。在这里，我们提出了一种混合策略，通过使用化学燃料和声力的组合，可以实现合成双金属微电机的正流变和负流变。开发了一种用于将两个推进机构整合在一起的声流体装置。仅使用声力，双金属微棒就沿着流体通道中心的底面推进。如先前的实验所确定的那样，微棒的前端始终是密度较小的端。含化学燃料（H ₂ O ₂）时，当存在剪切流时，微棒的阳极端使其自身朝向流。数值模拟证实，这种取向是由微棒相对于通道底表面的倾斜引起的，这是由催化驱动的电渗流引起的。通过将这种催化取向效应与更强大的，取决于密度的声学推进相结合，可以实现正流变和负流变。响应流动刺激并以有针对性的方式共同推动合成微泳的能力表明朝着实际应用迈出了重要的一步。