Phototactic Behaviour of Active Fluids: Effects of Light Perturbation on Diffusion Coefficient of Bacterial Suspensions

T. Vourc’h, J. Léopoldès, H. Peerhossaini
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Abstract

Active fluids refer to the fluids that contain self-propelled particles such as bacteria or micro-algae, whose properties differ fundamentally from the passive fluids. Such particles often exhibit an intermittent motion; with high-motility “run” periods separated by low-motility “tumble” periods. The average motion can be modified with external stresses, such as nutrient or light gradient, leading to a directed movement called chemotaxis and phototaxis, respectively. Using cyanobacterium Synechocystis sp.PCC 6803, a model micro-organism to study photosynthesis, we track the bacterial response to light stimuli, under isotropic and non-isotropic conditions. In particular, we investigate how the intermittent motility is influenced by illumination. We find that just after a rise in light intensity, the probability to be in the run state increases. This feature vanishes after a typical time of about 1 hour, when initial probability is recovered. Our results are well described by a model based on the linear response theory. When the perturbation is anisotropic, the characteristic time of runs is longer whatever the direction, similar to what is observed with isotropic conditions. Yet we observe a collective motion toward the light source (phototaxis) and show that the bias emerges because of more frequent runs towards the light.
活性流体的光致行为:光扰动对细菌悬浮液扩散系数的影响
主动流体是指含有自驱微粒(如细菌或微藻)的流体,其性质与被动流体有根本区别。这类粒子通常表现为间歇运动;高运动性的“跑”期和低运动性的“翻”期分开。平均运动可以被外部压力(如营养或光梯度)改变,导致定向运动,分别称为趋化性和光趋化性。利用研究光合作用的模式微生物蓝藻Synechocystis sp.PCC 6803,研究了各向同性和非各向同性条件下细菌对光刺激的响应。特别是,我们研究间歇性运动如何受到光照的影响。我们发现,光强增加后,处于运行状态的概率增加。当初始概率恢复时,该特征在典型时间约1小时后消失。我们的结果很好地描述了基于线性响应理论的模型。当扰动是各向异性时,无论向哪个方向,运行的特征时间都更长,与各向同性条件下观察到的情况相似。然而,我们观察到朝向光源的集体运动(趋光性),并表明偏倚的出现是因为更频繁地奔向光源。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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