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1. Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals

   https://doi.org/10.3390/ma14051055  

   Yu, Huaizhe; Wang, Kunlun; Szilvási, Tibor; Nayani, Karthik; Bao, Nanqi; Twieg, Robert J.; Mavrikakis, Manos; Abbott, Nicholas L. (March 2021, Materials)

   null (Ed.)

   Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments. 

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2. Calamitic ligand design for quantum dot assembly

   https://doi.org/10.1063/5.0263810  

   Liba, Samia_I; Wheeler, Alauna_C; Ochoa, Jocelyn; Licauco, Nicholas_Javier_C; Reyes, Isabella_C; Stokes, Benjamin_J; Atherton, Timothy_J; Hirst, Linda_S (June 2025, Journal of Applied Physics)

   We report the design and use of calamitic ligands for quantum dot surface modification and nanoparticle assembly. Ligands incorporating a rigid aromatic rod-like core have previously been shown to facilitate the formation of porous nanoparticle-based structures, such as solid-walled capsules and multi-compartment quantum dot foams and networks via liquid crystal phase transition templating—a process in which the host phase is quenched through the isotropic-nematic phase transition. The effect of the calamitic ligand structure on particle dispersion, transport, and subsequent assembly, however, requires further investigation, particularly in the case of anisotropic liquid crystal solvents. In this report, we vary the structure of six new calamitic ligands and characterize quantum dot size and packing into superstructures when modified with each ligand. Dynamic light scattering is used to measure the effective nanoparticle size for each ligand in dilute toluene solution. Transmission electron microscopy reveals nanoparticle distribution in dense drop-cast films for each ligand, and small-angle x-ray scattering is used to measure interparticle separations in the assembled porous structures. Together, these methods provide a full picture of particle packing for each ligand. Notably, our findings demonstrate that while longer, more rigid aromatic cores promote a closer packing structure in drop-cast films (a slow quasi-equilibrium process)—such effects are not evident using a rapid quenching method. This study highlights the fact that when nanoparticles are formed into macroscopic assemblies, both ligand design and the particular method of assembly can contribute significantly to the final packing structure. 

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3. Focal conic flowers, dislocation rings, and undulation textures in smectic liquid crystal Janus droplets

   https://doi.org/10.1039/D1SM01623G  

   Wei, Wei-Shao; Jeong, Joonwoo; Collings, Peter J.; Yodh, A. G. (June 2022, Soft Matter)

   Liquid crystalline phases of matter often exhibit visually stunning patterns or textures. Mostly, these liquid crystal (LC) configurations are uniquely determined by bulk LC elasticity, surface anchoring conditions, and confinement geometry. Here, we experimentally explore defect textures of the smectic LC phase in unique confining geometries with variable curvature. We show that a complex range of director configurations can arise from a single system, depending on sample processing procedures. Specifically, we report on LC textures in Janus drops comprised of silicone oil and 8CB in its smectic-A LC phase. The Janus droplets were made in aqueous suspension using solvent-induced phase separation. After drop creation, smectic layers form in the LC compartment, but their self-assembly is frustrated by the need to accommodate both the bowl-shaped cavity geometry and homeotropic (perpendicular) anchoring conditions at boundaries. A variety of stable and metastable smectic textures arise, including focal conic domains, dislocation rings, and undulations. We experimentally characterize their stabilities and follow their spatiotemporal evolution. Overall, a range of fabrication kinetics produce very different intermediate and final states. The observations elucidate assembly mechanisms and suggest new routes for fabrication of complex soft material structures in Janus drops and other confinement geometries. 

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4. Disorder and demixing in bidisperse particle systems assembling bcc crystals

   https://doi.org/10.1063/5.0219037  

   Kennard, Jasmin J; Zelaya_Solano, H Jonathan; Biddulph, Caleb D; Prager, Ryan C; Dshemuchadse, Julia (August 2024, The Journal of Chemical Physics)

   Colloidal and nanoparticle self-assembly enables the creation of ordered structures with a variety of electronic and photonic functionalities. The outcomes of the self-assembly processes used to synthesize such structures, however, strongly depend on the uniformity of the individual nanoparticles. Here, we explore the simplest form of particle size dispersity—bidispersity—and its impact on the self-assembly process. We investigate the robustness of self-assembling bcc-type crystals via isotropic interaction potentials in binary systems with increasingly disparate particle sizes by determining their terminal size ratio—the most extreme size ratio at which a mixed binary bcc crystal forms. Our findings show that two-well pair potentials produce bcc crystals that are more robust with respect to particle size ratio than one-well pair potentials. This suggests that an improved self-assembly process is accomplished with a second attractive length scale encoded in the particle–particle interaction, which stabilizes the second-nearest neighbor shell. In addition, we document qualitative differences in the process of ordering and disordering: in bidisperse systems of particles interacting via one-well potentials, we observe a breakdown of order prior to demixing, while in systems interacting via two-well potentials, demixing occurs first and bcc continues to form in parts of the droplet down to low size ratios. 

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5. Nematic colloids at liquid crystal–air interfaces via photopolymerization

   https://doi.org/10.1039/d0sm01311k  

   Wei, Xiaoshuang; Sbalbi, Nicholas; Bradley, Laura C. (October 2020, Soft Matter)

   null (Ed.)

   We demonstrate the preparation of colloidal crystals at nematic liquid crystal–air interfaces by simultaneous photopolymerization and assembly. Polymer colloids are produced by polymerization-induced phase separation of 2-hydroxyethyl methacrylate in the non-reactive liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) using an open-cell setup. Colloids adsorbed to the nematic 5CB–air interface form non-close-packed hexagonal crystals that cover the entire interface area. We examine the mechanism of growth and assembly for the preparation of LC-templated interfacial colloidal superstructures. 

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