Intellectual Merit of Proposed Activity Multiphase systems containing liquid crystalline dispersed phases are of interest for a variety of technological applications, including display technology, self-reinforced composites, and electrorheological applications. The liquid crystalline structure manifests itself not only through the bulk properties of the dispersed phase, but also (and sometimes primarily) through the impact of liquid crystallinity on the nanoscopic interfacial region and the interfacial properties. The proposed research addresses the effect of the 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 obtained when a minor phase of a smectic biphenylcarbonitrile (8CB) is mixed with poly(dimethyl siloxane). The gel is meta-stable through the nematic regime and then undergoes a spontaneous transition to a dispersed droplet morphology at the nematic-isotropic transition. The droplet morphology is meta-stable and does not reform to the gel with lowering of the temperature. We plan to carry out the following experiments, with a goal of developing an understanding of this remarkable kinetically-trapped system and placing the interfacial 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 bicontinuous morphology; (iii) broadband dielectric experiments in the gel and droplet morphologies. The interfacial mechanics of the smectic phase in blends are essentially unknown. We plan to measure the interfacial tension of smectic 8CB against PDMS and to carry out a systematic study of droplet deformation and recovery 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 bulk stresses from the liquid crystalline orientational elasticity. We will employ a simulated annealing method to determine orientation distribution in the liquid crystalline phase. The mechanics of the gel-dispersed droplet transition in the 8CB/PDMS system seems to be a surface tension-driven phenomenon; the transition does not occur at the smectic-nematic transition because the system appears to be kinetically trapped in a meta-stable state, and a similar kinetic trapping appears to prevent a spontaneous morphology transformation 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 a viscous matrix.
Broader impacts of the proposed activities Advance discovery and understanding while promoting teaching, training, and learning. The Principal Investigator has an established record of training Ph.D. students and postdoctoral fellows and of incorporating undergraduate students into his research group. The research group is a part of the interdisciplinary Benjamin Levich Institute for Physico-Chemical Hydrodynamics and the NSF-CREST Center for Mesoscopic Modeling and Simulation, and there is considerable interaction among the faculty and students, who come from the Departments of Chemical Engineering, Biomedical Engineering, Mechanical Engineering, and Physics. Professor Denn is the Principal Investigator for an NSF-IGERT program on Multiscale Phenomena in Soft Materials, and members of his research group interact with IGERT participants through regular joint seminars and a body of new soft materials courses, including two laboratory 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 important technologies, as well as providing training opportunities in the important area of soft materials.
