Interfacial Effects and Mechanics of Liquid Crystalline Dispersions

包含液晶分散相的多相系统对各种技术应用很感兴趣，包括显示技术、自增强复合材料和电流变学应用。液晶结构不仅通过分散相的体积性质表现出来，而且（有时主要）通过液态结晶度对纳米界面区域和界面性质的影响表现出来。提出的研究解决了液晶/各向同性液体界面对形貌和流动的影响，特别是在近晶系统中。将半联苯碳腈（8CB）的小相与聚二甲基硅氧烷混合，得到了具有纳米级尺寸的双连续凝胶状液晶分散体。凝胶在向列向体系中是亚稳定的，然后在向列向-各向同性过渡中自发地转变为分散的液滴形态。液滴形态是亚稳定的，不随温度的降低而转变为凝胶。我们计划进行以下实验，目的是了解这个非凡的动力学捕获系统，并将液晶分散的界面和体力学置于更广泛的胶体凝胶背景下：(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）凝胶和液滴形貌的宽带介电实验。共混物中近晶相的界面力学基本上是未知的。我们计划测量近晶8CB与PDMS的界面张力，并对PDMS矩阵中各向同性、向列和近晶制度下8CB液滴的变形和恢复进行系统的研究。其力学性质最终由液晶取向弹性产生的表面有序和体积应力之间的相互作用决定。我们将采用模拟退火方法来确定液晶相中的取向分布。凝胶分散液滴在8CB/PDMS体系中的转变机理似乎是一种表面张力驱动的现象；这种转变不会发生在微向列相转变，因为系统似乎被动力学捕获在一个亚稳定状态，类似的动力学捕获似乎阻止了自发的形态转变，以恢复液晶凝胶。我们将研究嵌入粘性基质中的扩展和球面各向同性液晶相之间的界面张力驱动转变的力学。拟议活动的更广泛影响：在促进教学、培训和学习的同时，促进发现和理解。首席研究员在培养博士生和博士后以及将本科生纳入他的研究小组方面有着良好的记录。该研究小组隶属于Benjamin Levich物理化学流体动力学研究所和nsf - crest介观建模与仿真中心，来自化学工程系、生物医学工程系、机械工程系和物理系的教师和学生之间有相当多的互动。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的联合研讨会和一系列新的软材料课程（包括两个实验室课程）与igert参与者进行互动。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，他们通常有家庭责任。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本科生，他们已经成功完成了第三年运输序列的第一部分，并希望进行本科生独立研究或暑期研究。我们将从校园中已经开展的许多项目中受益，这些项目旨在提高在科学和工程领域代表性不足的群体成员的参与度，包括针对有兴趣攻读博士学位的本科生的纽约市立大学管道项目。造福社会。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.