Organisms often swim through density-stratified fluids. Here, we investigate the dynamics of active particles swimming in fluid density gradients and report theoretical evidence of taxis as a result of these gradients (densitaxis). Specifically, we calculate the effect of density stratification on the dynamics of a force- and torque-free spherical squirmer and show that density gradients induce reorientation that tends to align swimming either parallel or normal to the gradient depending on the swimming gait. In particular, swimmers that propel by generating thrust in the front (pullers) rotate to swim parallel to gradients and hence display (positive or negative) densitaxis, while swimmers that propel by generating thrust in the back (pushers) rotate to swim normal to the gradients. This work could be useful to understand the motion of marine organisms in ocean or be leveraged to sort or organize a suspension of active particles by modulating density gradients.

中文翻译：

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密度轴：密度梯度中的活跃粒子运动


生物体经常在密度分层的液体中游动。在这里，我们研究了在流体密度梯度中游动的活性粒子的动力学，并报告了这些梯度（密度趋向）导致滑行的理论证据。具体来说，我们计算了密度分层对无力和无扭矩球形蠕动器动力学的影响，并表明密度梯度会引起重新定向，根据游泳步态，倾向于使游泳平行或垂直于梯度。特别是，通过在前面产生推力（拉力）来推进的游泳者旋转以平行于梯度游泳，因此显示（正或负）密度轴，而通过在后面产生推力（推动者）来推进的游泳者旋转以垂直于梯度游泳。梯度。这项工作可用于了解海洋生物在海洋中的运动，或可用于通过调节密度梯度来分类或组织活性颗粒的悬浮液。