Department of Physics, Indian Institute of Science, Bangalore 560012, India.
Proc Natl Acad Sci U S A. 2013 Sep 10;110(37):14849-54. doi: 10.1073/pnas.1304777110. Epub 2013 Aug 28.
We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature (T(K)°) in weakly swollen isotropic (Li) and lamellar (La) mesophases with bilayers formed in a cationic-anionic mixed surfactant system. Synchrotron rheological X-ray diffraction study reveals the crystallization transition to be reversible under shear (i.e., on stopping the shear, the nonequilibrium crystalline phase Lc melts back to the equilibrium mesophase). This is different from the shear-driven crystallization below T(K)°, which is irreversible. Rheological optical observations show that the growth of the crystalline phase occurs through a preordering of the Li phase to an La phase induced by shear flow, before the nucleation of the Lc phase. Shear diagram of the Li phase constructed in the parameter space of shear rate (γ) vs. temperature exhibits Li → Lc and Li → La transitions above the equilibrium crystallization temperature T(K)°, in addition to the irreversible shear-driven nucleation of Lc in the Li phase below T(K)°. In addition to revealing a unique class of nonequilibrium phase transition, the present study urges a unique approach toward understanding shear-induced phenomena in concentrated mesophases of mixed amphiphilic systems.
我们在阳离子-阴离子混合表面活性剂体系中形成双层的弱溶胀各向同性（Li）和层状（La）中间相，在平衡冻结温度（T(K)°）以上演示了一种独特的剪切诱导结晶现象。同步流变学 X 射线衍射研究表明，在剪切下结晶转变是可逆的（即停止剪切时，非平衡晶相 Lc 会回熔到平衡中间相）。这与低于 T(K)°的剪切驱动结晶不同，后者是不可逆的。流变光学观察表明，在 Lc 相成核之前，通过剪切流诱导 Li 相向 La 相的预有序化，晶相的生长发生。在剪切速率（γ）与温度的参数空间中构建的 Li 相剪切图显示，在平衡结晶温度 T(K)°以上，除了低于 T(K)°的 Li 相中不可逆的剪切驱动 Lc 成核外，还存在 Li → Lc 和 Li → La 转变。本研究不仅揭示了一类独特的非平衡相变，还促使人们对混合两亲体系的浓中间相中的剪切诱导现象有了独特的理解方法。
