Viscosity is an important property of out-of-equilibrium systems such as active biological materials and driven non-Newtonian fluids, and for fields ranging from biomaterials to geology, energy technologies and medicine. Non-invasive viscosity measurements typically require integration times of seconds. Here, we demonstrate measurement speeds reaching 20 μs, with uncertainty dominated by thermal molecular collisions for the first time. We achieve this using the instantaneous velocity of a trapped particle in an optical tweezer. To resolve the instantaneous velocity we develop a structured-light detection system that allows particle tracking over femtometre length scales and 16-ns timescales. Our results translate viscosity from a static averaged property to one that may be dynamically tracked on the timescales of active dynamics. This opens a pathway to new discoveries in out-of-equilibrium systems, from the fast dynamics of phase transitions to energy dissipation in motor molecule stepping and to violations of fluctuation laws of equilibrium thermodynamics.

粘度是非平衡系统（如活性生物材料和驱动的非牛顿流体）的重要属性，适用于从生物材料到地质学、能源技术和医学等领域。非侵入式粘度测量通常需要几秒钟的积分时间。在这里，我们首次展示了达到 20 μs 的测量速度，不确定性由热分子碰撞主导。我们使用光镊中捕获的粒子的瞬时速度来实现这一点。为了解决瞬时速度，我们开发了一种结构光检测系统，该系统允许在飞米长度尺度和 16 纳秒时间尺度上进行粒子跟踪。我们的结果将粘度从静态平均属性转换为可以在主动动力学的时间尺度上动态跟踪的属性。