Orientation in a Fatty Acid Monolayer: Effect of Flow Type

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• 作者：T. Maruyama ; J. Lauger ; G. G. Fuller ; C. W. Frank ; and C. R. Robertson
• 刊名：Langmuir
• 出版年：1998
• 出版时间：March 31, 1998
• 年：1998
• 卷：14
• 期：7
• 页码：1836 - 1845
• 全文大小：216K
• 年卷期：v.14,no.7(March 31, 1998)
• ISSN：1520-5827

文摘

The two-dimensional fluid dynamics of the different phases of afatty acid monolayer (docosanoic acid)were examined. Using Brewster angle microscopy, we studied thepolydomain structure of two liquidcondensed phases (the L₂ and L'₂ phases) andthe "solid", S, phase in situ during the application ofextensionalflow and simple shear flow. We found that only the L₂phase deformed nearly reversibly with a liquid-likeresponse. Nonsymmetric domain deformations were, however, foundfor that phase at the lowest rate ofstrain studied with a four-roll mill. At higher strain rates,plots of the evolution of strain against timecollapsed onto a single curve so that the rate of strain wasindependent of roller speed. Furthermore, acritical strain _c exists above which the rate of strainshows a stepwise increase despite the constantvelocity of the rollers. The two other phases, the L'₂and S phases, experienced flow-induced reorientationof the lattice onto which the molecules are arranged. Thereorientation process was accompanied by theappearance of shear bands in the monolayers at ±45 to theextension axis of both types of flow. The shearbands observed in the L'₂ phase were modeled as a plasticflow accompanying the molecular tilt reorientationdeveloped within elastic regions of the monolayer. This modeldescribes the time evolution of the bandwidthquite well and provides strong evidence of the existence of anadditional phase within the conventionalL'₂ phase region. In simple shear flow, the velocityprofile for the L₂ phase across the gap of theshearingcell showed a nonlinear distribution of shear rates, which were highestat the center of the gap. Flow-induced breakup of domains was observed in the L'₂ phasesubject to simple shear.