Interfacial Effects and Mechanics of Liquid Crystalline Dispersions

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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.