Akbulut Mustafa, Chen Nianhuan, Maeda Nobuo, Israelachvili Jacob, Grunewald Torsten, Helm Christiane A
Departments of Chemical Engineering and Materials and the Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA.
J Phys Chem B. 2005 Jun 30;109(25):12509-14. doi: 10.1021/jp0448200.
It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.
众所周知，当限制表面发生剪切时，受限流体和固体的薄膜结构会发生变化。可能会出现位置和取向的短程或长程重新排序，而这些情况在本体中通常不存在对应物。这些多层、单层甚至亚单层效应对于理解粘附和摩擦过程很重要，但事实证明它们很难测量，部分原因是缺乏实验技术，部分原因是它们明显微妙地依赖于许多实验参数。在此，我们报告了在表面力装置中使用剪切测量和“光吸收光谱法”来测量水溶液中限制在两个剪切云母表面之间的各向异性（染料）分子的剪切诱导相变。我们对薄水膜中高度各向异性的花菁染料分子的剪切诱导排序和摩擦力的研究表明，分子各向异性对剪切诱导排序、摩擦力以及剪切诱导结晶的起始仅产生微弱影响，尽管当受限分子最终结晶时会发生显著变化。
