Oscillating modes of driven colloids in overdamped systems.
Oscillating modes of driven colloids in overdamped systems.
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DOI：
10.1038/s41467-018-03345-2
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发表时间：
2018-03-08
影响因子：
16.6
通讯作者：
Bechinger C
中科院分区：
文献类型：
--
作者：
Berner J;Müller B;Gomez-Solano JR;Krüger M;Bechinger C
悬浮在液体中的微观胶体颗粒是过阻尼系统的一个突出例子，其中粘性力比惯性效应占主导地位。通常，胶体被用作敏感探针，例如，在生物物理应用中，从分子力推断。对这类实验的解释基于这样的假设：即使粒子被驱动，液体仍保持平衡。在这里，我们通过实验证明，这对粘弹性流体中的颗粒是无效的。即使在很小的驱动力下，我们也能观察到粒子在几十秒内的振荡。它们归因于被粒子运动激发的流体的非平衡波动。振荡动力学在定量上符合过阻尼朗之万方程，负摩擦记忆项相当于随机驱动的欠阻尼振荡器。这种振荡模式有望扩大胶体作为模型系统的使用，但也必须在胶体探针实验中加以考虑。假定悬浮在液体中的微粒的运动受粘性力支配。在这里，Berner等人通过观察粘弹性流体中的欠阻尼粒子振荡，并将其归因于粒子激发的液体的非平衡波动，挑战了这一共识。
Microscopic colloidal particles suspended in liquids are a prominent example of an overdamped system where viscous forces dominate over inertial effects. Frequently, colloids are used as sensitive probes, e.g., in biophysical applications from which molecular forces are inferred. The interpretation of such experiments rests on the assumption that, even when the particles are driven, the liquid remains in equilibrium. Here we experimentally demonstrate that this is not valid for particles in viscoelastic fluids. Even at small driving forces, we observe particle oscillations with several tens of seconds. They are attributed to non-equilibrium fluctuations of the fluid being excited by the particle’s motion. The oscillatory dynamics is in quantitative agreement with an overdamped Langevin equation with negative friction-memory term being equivalent to a stochastically driven underdamped oscillator. Such oscillatory modes are expected to widen the use of colloids as model systems but must also be considered in colloidal probe experiments. The motion of microparticles suspended in liquids is assumed to be dominated by viscous forces. Here, Berner et al. challenge this consensus by observing underdamped particle oscillations in a viscoelastic fluid and attributing it to the non-equilibrium fluctuations of liquid excited by particles.
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