Porter Daniel, Savage John R, Cohen Itai, Spicer Patrick, Caggioni Marco
Department of Physics, Cornell University, Ithaca, New York 14853, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2012 Apr;85(4 Pt 1):041701. doi: 10.1103/PhysRevE.85.041701. Epub 2012 Apr 6.
Droplet breakup of many Newtonian fluids is well described by current experiments, theory, and simulations. Breakup in complex fluids where interactions between mesoscopic structural features can affect the flows remains poorly understood and a burgeoning area of research. Here, we report on our investigations of droplet breakup in thermotropic liquid crystals. We investigate breakup in the smectic, nematic, and isotropic phases of 4-cyano 4-octylbiphenyl (8CB) and the nematic and isotropic phases of 4-cyano 4-pentylbiphenyl (5CB). The experiment consists of varying the ambient temperature to control liquid crystalline phase and imaging breakup using a fast video camera at up to 110000 frames/s. We expand on previous work [John R. Savage et al., Soft Matter 6, 892 (2010)] that shows breakup in the smectic phase is symmetric, producing no satellite droplets, and is well described by a similarity solution for a shear-thinning power-law fluid. We show that in the nematic phase the breakup occurs in two stages. In the first stage, the breakup is symmetric and the power-law exponent for the minimum radius dependence on the time left to breakup is 1.2<n<1.9. In the second stage the drop develops two minima and the minimum radii shrink with a power-law exponent 0.6<n<1. We find that the exponents vary with temperature across the nematic phase. These results are surprising because rheological measurements of 8CB and 5CB in the nematic phases indicate Newtonian behavior that cannot account for the observed breakup dynamics. Finally, in the isotropic phase, the exponents are consistent with theoretical predictions and experiments for Newtonian fluid breakup in the inertial viscous regime.
目前的实验、理论和模拟很好地描述了许多牛顿流体的液滴破裂。在介观结构特征之间的相互作用会影响流动的复杂流体中，破裂现象仍未得到充分理解，是一个新兴的研究领域。在这里，我们报告了我们对热致液晶中液滴破裂的研究。我们研究了4-氰基-4-辛基联苯（8CB）的近晶相、向列相和各向同性相以及4-氰基-4-戊基联苯（5CB）的向列相和各向同性相中的破裂情况。实验包括改变环境温度以控制液晶相，并使用高达110000帧/秒的高速摄像机对破裂过程进行成像。我们扩展了之前的工作[约翰·R·萨维奇等人,《软物质》6, 892 (2010)]，该工作表明近晶相中的破裂是对称的，不会产生卫星液滴，并且可以用剪切变稀幂律流体 的相似解很好地描述。我们表明，在向列相中，破裂分两个阶段发生。在第一阶段，破裂是对称的，最小半径对剩余破裂时间的幂律指数为1.2 < n < 1.9。在第二阶段，液滴形成两个最小值，最小半径以幂律指数0.6 < n < 1收缩。我们发现，这些指数在向列相范围内随温度变化。这些结果令人惊讶，因为8CB和5CB在向列相中的流变学测量表明其具有牛顿流体行为，无法解释观察到的破裂动力学。最后，在各向同性相中，这些指数与惯性粘性区域中牛顿流体破裂的理论预测和实验结果一致。
