We present a novel approach for characterizing the properties and performance of active matter in dilute suspension as well as in crowded environments. We use Super-Heterodyne Laser-Doppler-Velocimetry (SH-LDV) to study large ensembles of catalytically active Janus particles moving under UV illumination. SH-LDV facilitates a model-free determination of the swimming speed and direction, with excellent ensemble averaging. In addition, we obtain information on the distribution of the catalytic activity. Moreover, SH-LDV operates away from walls and permits a facile correction for multiple scattering contributions. It thus allows for studies of concentrated suspensions of swimmers or of systems where swimmers propel actively in an environment crowded by passive particles. We demonstrate the versatility and the scope of the method with a few selected examples. We anticipate that SH-LDV complements established methods and paves the way for systematic measurements at previously inaccessible boundary conditions.

中文翻译：

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用外差激光多普勒测速仪表征致密悬浮液中的活性物质

我们提出了一种新方法来表征稀悬浮液和拥挤环境中活性物质的特性和性能。我们使用超外差激光多普勒测速 (SH-LDV) 来研究在紫外线照射下移动的催化活性 Janus 粒子的大型集合。SH-LDV 有助于无模型地确定游泳速度和方向，并具有出色的整体平均。此外，我们还获得了有关催化活性分布的信息。此外，SH-LDV 远离墙壁运行，并允许对多个散射贡献进行轻松校正。因此，它允许研究游泳者的集中悬浮液或游泳者在被动粒子拥挤的环境中积极推进的系统。我们通过一些选定的示例展示了该方法的多功能性和范围。我们预计 SH-LDV 可以补充已建立的方法，并为在以前无法访问的边界条件下进行系统测量铺平道路。