Numerous physical models have been proposed to explain how cell motility emerges from internal activity, mostly focused on how crawling motion arises from internal processes. Here we offer a classification of self-propulsion mechanisms based on general physical principles, showing that crawling is not the only way for cells to move on a substrate. We consider a thin drop of active matter on a planar substrate and fully characterize its autonomous motion for all three possible sources of driving: (i) the stresses induced in the bulk by active components, which allow in particular tractionless motion, (ii) the self-propulsion of active components at the substrate, which gives rise to crawling motion, and (iii) a net capillary force, possibly self-generated, and coupled to internal activity. We determine travelling-wave solutions to the lubrication equations as a function of a dimensionless activity parameter for each mode of motion. Numerical simulations are used to characterize the drop motion over a wide range of activity magnitudes, and explicit analytical solutions in excellent agreement with the simulations are derived in the weak-activity regime.

中文翻译：

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细胞可以移动多少种方式：主动液滴的自我推进方式

已经提出了许多物理模型来解释细胞动力如何从内部活动中出现，主要集中在爬行运动如何从内部过程中产生。在这里，我们根据一般物理原理对自推进机制进行了分类，表明爬行不是细胞在基质上移动的唯一途径。我们考虑了在平面基板上的一小滴活性物质，并充分表征了其对所有三种可能的驱动源的自主运动：（i）活性成分在主体中产生的应力，这尤其允许无牵引运动；（ii）基材上活性成分的自我推进，这会引起爬行运动；（iii）可能是自生的并与内部活动相关的净毛细作用力。我们根据每种运动模式的无量纲活动参数确定润滑方程的行波解。数值模拟被用来表征在很大的活动量范围内的跌落运动，并且在弱活动状态下得到了与模拟非常一致的显式解析解。