Mechanical Science and Engineering , University of Illinois Urbana-Champaign , 1206 W. Green St. , Urbana , Illinois 61801 , United States.
The Beckman Institute for Advanced Science and Technology , University of Illinois Urbana-Champaign , 405 N. Mathews Ave , Urbana , Illinois 61801 , United States.
Langmuir. 2019 Oct 15;35(41):13421-13426. doi: 10.1021/acs.langmuir.9b01441. Epub 2019 Oct 1.
We investigate the self-assembly of hair-like fibers into twisted helices as they are pulled through the liquid interface at a controlled rate. Capillary-induced spontaneous fiber twisting phenomena are observed from the nano- to the millimeter scale. Here, we control the drain rate of the liquid and observe two regimes of self-assembly of long hairs. At low drain rates, the hairs coalesce radially to form a dense aggregate. At higher drain rates, spontaneous hair twisting occurs. We find that the drain rate corresponding to the twisting threshold scales with the characteristic velocity of fiber coalescence set by a balance between liquid viscosity μ and surface energy σ and reads ∼(σ/μ)·(/) where and are the spacing between hairs and their length, respectively. At drain rates higher than this threshold, liquid is entrained between the hairs as they emerge from the liquid surface, forming a circular liquid column. Twisting is induced by the fast radial shrinking of this liquid column, combined with the nonlinear resistance to the hairs' radial versus tangential coalescence. Understanding the kinetics is crucial to control this complex self-assembly and to engineer fiber drying processes at various length scales.
我们研究了在受控速率下通过液体界面拉动时，毛发状纤维自行组装成扭曲螺旋的过程。从纳米到毫米尺度都观察到了毛细诱导的自发纤维扭曲现象。在这里，我们控制液体的排出速率，并观察到长纤维的两种自组装状态。在低排出速率下，毛发径向聚集形成密集的聚集体。在较高的排出速率下，自发的毛发扭曲发生。我们发现，与由液体粘度μ和表面能σ之间的平衡决定的纤维聚集特征速度相对应的排出速率与扭曲阈值成比例，其表达式为∼(σ/μ)·(/)，其中 和 分别是毛发之间的间距和它们的长度。在高于这个阈值的排出速率下，当毛发从液体表面伸出时，液体被卷入毛发之间，形成一个圆形的液体柱。这种液体柱的快速径向收缩以及对毛发径向与切向聚集的非线性阻力共同诱导了扭曲。了解动力学对于控制这种复杂的自组装以及在各种长度尺度上设计纤维干燥过程至关重要。
