The occurrence of coiled or helical morphologies is common in nature, from plant roots to DNA packaging into viral capsids, as well as in applications such as oil drilling processes. In many examples, chiral structures result from the buckling of a straight fibre either with intrinsic twist or to which end moments have been applied in addition to compression forces. Here, we elucidate a generic way to form regular helicoidal shapes from achiral straight filaments transported in viscous flows with free ends. Through a combination of experiments using fluorescently labelled actin filaments in microfluidic divergent flows and two distinct sets of numerical simulations, we demonstrate the robustness of helix formation. A nonlinear stability analysis is performed, and explains the emergence of such chiral structures from the nonlinear interaction of perpendicular planar buckling modes, an effect that solely requires a strong compressional flow, independent of the exact nature of the fibre or type of flow field. The fundamental mechanism for the uncovered morphological transition and characterization of the emerging conformations advance our understanding of several biological and industrial processes and can also be exploited for the controlled microfabrication of chiral objects.

从植物根部到将DNA包装到病毒衣壳中，以及在诸如石油钻探过程等应用中，盘绕或螺旋形形态的发生在自然界都是很常见的。在许多示例中，手性结构是由于具有固有扭曲或由于施加了压缩力而施加了端力矩的直纤维的屈曲而产生的。在这里，我们阐明了一种一般方法，该方法可以通过以自由端的粘性流形式传输的非手性直丝形成规则的螺旋形状。通过在微流体发散流中使用荧光标记的肌动蛋白丝进行实验的结合以及两组不同的数值模拟，我们证明了螺旋形成的稳健性。进行非线性稳定性分析，并解释了从垂直平面屈曲模态的非线性相互作用中出现的这种手性结构，这种效应仅需要强大的压缩流，而与纤维的确切性质或流场类型无关。未发现的形态转变和新兴构象表征的基本机制使我们对几种生物学和工业过程有了更深入的了解，也可用于手性对象的受控微加工。