The helical self-assembly of cholesteric liquid crystals is a powerful motif in nature, enabling exceptional performance in many biological composites. Attempts to mimic these remarkable materials by drying cholesteric colloidal nanorod suspensions often yield films with a non-uniform mosaic-like character, severely degrading optical and mechanical properties. Here we show—using the example of cellulose nanocrystals—that these problems are due to rod length dispersity: uncontrolled phase separation results from a divergence in viscosity for short rods, and variations in pitch can be traced back to a twisting power that scales with rod length. We present a generic, robust and scalable method for fractionating nanorod suspensions, allowing us to interrogate key aspects of cholesteric self-assembly that were previously hidden by colloid dispersity. By controlled drying of fractionated suspensions, we can obtain mosaic-free films that are uniform in colour. Our findings unify conflicting observations and open routes to biomimetic artificial materials with performance that can compete with that of nature’s originals.

胆甾型液晶的螺旋自组装是自然界的有力基元，可在许多生物复合材料中发挥出色的性能。尝试通过干燥胆甾型胶体纳米棒悬浮液来模仿这些非凡的材料，通常会产生具有不均匀马赛克样特征的薄膜，从而严重降低光学和机械性能。在这里，我们以纤维素纳米晶体为例，表明这些问题归因于杆长度的分散性：短杆的粘度差异导致不受控制的相分离，并且节距的变化可以追溯到随杆成比例的扭曲力长度。我们提出了一种分级分离纳米棒悬浮液的通用，可靠且可扩展的方法，使我们可以查询以前由胶体分散性隐藏的胆甾型自组装的关键方面。通过分级干燥悬浮液的受控干燥，我们可以获得颜色均匀的无马赛克薄膜。我们的发现统一了相互矛盾的观察结果，并开辟了仿生人造材料的开路，其性能可与自然界的原始材料相媲美。