School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138;
Mathematical Institute, University of Oxford, Oxford OX2 6GG, United Kingdom;
Proc Natl Acad Sci U S A. 2017 Aug 29;114(35):9255-9260. doi: 10.1073/pnas.1701739114. Epub 2017 Aug 15.
Many biological systems are appropriately viewed as passive inclusions immersed in an active bath: from proteins on active membranes to microscopic swimmers confined by boundaries. The nonequilibrium forces exerted by the active bath on the inclusions or boundaries often regulate function, and such forces may also be exploited in artificial active materials. Nonetheless, the general phenomenology of these active forces remains elusive. We show that the fluctuation spectrum of the active medium, the partitioning of energy as a function of wavenumber, controls the phenomenology of force generation. We find that, for a narrow, unimodal spectrum, the force exerted by a nonequilibrium system on two embedded walls depends on the width and the position of the peak in the fluctuation spectrum, and oscillates between repulsion and attraction as a function of wall separation. We examine two apparently disparate examples: the Maritime Casimir effect and recent simulations of active Brownian particles. A key implication of our work is that important nonequilibrium interactions are encoded within the fluctuation spectrum. In this sense, the noise becomes the signal.
从主动膜上的蛋白质到受边界限制的微观游泳者。主动浴对包裹体或边界施加的非平衡力通常会调节功能，而且这些力也可以在人工主动材料中得到利用。尽管如此，这些主动力的一般现象仍然难以捉摸。我们表明，主动介质的涨落谱，即能量随波数的分布，控制着力产生的现象学。我们发现，对于一个狭窄的、单峰的谱，非平衡系统对两个嵌入壁施加的力取决于涨落谱中峰的宽度和位置，并随着壁分离而在排斥和吸引之间振荡。我们检查了两个明显不同的例子：海洋 Casimir 效应和最近的主动布朗粒子模拟。我们工作的一个关键含义是，重要的非平衡相互作用被编码在涨落谱中。从这个意义上说，噪声变成了信号。
