Interfacial Effects and Mechanics of Liquid Crystalline Dispersions

含有液晶分散相的多相体系在显示技术、自增强复合材料和电流变学应用等方面具有广泛的应用价值。液晶结构不仅通过分散相的本体性质表现出来，而且(有时主要是)通过液晶对纳米界面区域和界面性质的影响来表现。这项研究旨在研究液晶/各向同性液体界面对形态和流动的影响，特别是在近晶体系中。将近晶联苯基碳腈(8CB)的少量相与聚二甲基硅氧烷混合，得到了纳米尺度的双连续凝胶状液晶分散体。凝胶在向列相区域是亚稳定的，然后在向列相-各向同性转变时经历自发转变为分散的液滴形态。随着温度的降低，液滴的形态是亚稳定的，不会形成凝胶。我们计划进行以下实验，以加深对这一非凡的动力学捕获系统的理解，并将液晶分散的界面和整体力学置于胶体凝胶的更广泛的背景下：(I)光学显微镜；(Ii)非线性流变学和双连续形态动力学的测定；(Iii)凝胶和液滴形态的宽带介电实验。共混物中近晶相的界面机制基本上是未知的。我们计划测量近晶8CB与PDMS的界面张力，并对PDMS基质中各向同性、向列相和近晶区8CB液滴的变形和恢复进行系统的研究。这些力学机制最终由表面有序性和液晶取向弹性体应力之间的相互作用所决定。我们将使用模拟退火法来确定液晶相中的取向分布。在8CB/PDMS体系中，凝胶分散液滴转变的机理似乎是表面张力驱动的现象；在近晶向列相转变时不发生转变，因为体系似乎被动力学捕获在亚稳态，并且类似的动力学陷阱似乎阻止了自发的形态转变以恢复液晶凝胶。我们将研究嵌入在凹凸体中的延展的和球形的同向同性液晶相之间的界面张力驱动的转变的机制。拟议活动的广泛影响在促进教学、培训和学习的同时促进发现和理解。校长调查员有培训博士生和博士后研究员以及将本科生纳入他的研究小组的既定记录。该研究小组是本杰明·莱维奇物理化学流体力学跨学科研究所和NSF-CREST介观建模与模拟中心的一部分，来自化学工程、生物医学工程、机械工程和物理系的教职员工和学生之间有相当大的互动。Denn教授是NSF-IGERT软材料多尺度现象项目的首席研究员，他的研究小组成员通过定期联合研讨会和一系列新的软材料课程与IGERT参与者互动，其中包括两个实验室课程。扩大代表性不足群体的参与。城市学院是一所城市院校，85%的学生是少数族裔，其中包括许多回国的学生，他们往往肩负着家庭责任。本科生定期参加软材料研究活动。我们将寻求每年包括最多两名成功完成第三年运输序列第一部分并希望进行本科生独立研究或暑期研究的本科生。我们将从校园中许多已经开始运作的项目中受益，这些项目旨在加强科学和工程领域代表性不足群体的参与，包括为有意攻读博士学位的本科生设立的CUNY管道项目。对社会有益。拟议的研究解决了与各种重要技术相关的问题，并提供了软材料这一重要领域的培训机会。

Intellectual Merit of Proposed ActivityMultiphase systems containing liquid crystalline dispersed phases are of interest for a variety oftechnological applications, including display technology, self-reinforced composites, and electrorheologicalapplications. The liquid crystalline structure manifests itself not only through the bulk properties of thedispersed phase, but also (and sometimes primarily) through the impact of liquid crystallinity on thenanoscopic interfacial region and the interfacial properties. The proposed research addresses the effect ofthe liquid crystal/isotropic liquid interface on morphology and flow, particularly in smectic systems.Experimental. A bicontinuous gel-like liquid crystalline dispersion with nanoscale dimension is obtainedwhen a minor phase of a smectic biphenylcarbonitrile (8CB) is mixed with poly(dimethyl siloxane). Thegel is meta-stable through the nematic regime and then undergoes a spontaneous transition to a disperseddroplet morphology at the nematic-isotropic transition. The droplet morphology is meta-stable and does notreform to the gel with lowering of the temperature. We plan to carry out the following experiments, with agoal of developing an understanding of this remarkable kinetically-trapped system and placing theinterfacial and bulk mechanics of the liquid crystal dispersion in the broader context of colloidal gels: (i)optical microscopy; (ii) nonlinear rheology and determination of the dynamics of the bicontinuousmorphology; (iii) broadband dielectric experiments in the gel and droplet morphologies. The interfacialmechanics of the smectic phase in blends are essentially unknown. We plan to measure the interfacialtension of smectic 8CB against PDMS and to carry out a systematic study of droplet deformation andrecovery for 8CB droplets in isotropic, nematic, and smectic regimes in a PDMS matrix.Theoretical. The mechanics are ultimately determined by the interplay between surface ordering and bulkstresses from the liquid crystalline orientational elasticity. We will employ a simulated annealing method todetermine orientation distribution in the liquid crystalline phase. The mechanics of the gel-disperseddroplet transition in the 8CB/PDMS system seems to be a surface tension-driven phenomenon; thetransition does not occur at the smectic-nematic transition because the system appears to be kineticallytrapped in a meta-stable state, and a similar kinetic trapping appears to prevent a spontaneous morphologytransformation to recover the liquid crystalline gel. We will study the mechanics of the interfacial tensiondriven transition between extended and spherical homeotropic liquid crystalline phases embedded in aviscous matrix.Broader impacts of the proposed activitiesAdvance discovery and understanding while promoting teaching, training, and learning. The PrincipalInvestigator has an established record of training Ph.D. students and postdoctoral fellows and ofincorporating undergraduate students into his research group. The research group is a part of theinterdisciplinary Benjamin Levich Institute for Physico-Chemical Hydrodynamics and the NSF-CRESTCenter for Mesoscopic Modeling and Simulation, and there is considerable interaction among the facultyand students, who come from the Departments of Chemical Engineering, Biomedical Engineering,Mechanical Engineering, and Physics. Professor Denn is the Principal Investigator for an NSF-IGERTprogram on Multiscale Phenomena in Soft Materials, and members of his research group interact withIGERT participants through regular joint seminars and a body of new soft materials courses, including twolaboratory courses. Broaden Participation of Underrepresented Groups. City College is an urban institution with an 85% minority student body that includes many returning students, often with family responsibilities. Undergraduate students participate regularly in the soft materials research activities. We will seek to include up to two undergraduates annually who have successfully completed the first part of the third-year transport sequence and wish to pursue undergraduate independent research or summer research. We will profit in reaching out to undergraduates from the presence on the campus of many programs already in operation to enhance the participation of members of groups underrepresented in science and engineering,including the CUNY Pipeline Program for undergraduates interested in pursuing a Ph.D. program.Benefits to society. The proposed research addresses problems that are relevant to a variety of importanttechnologies, as well as providing training opportunities in the important area of soft materials.