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Overview

Our laboratory seeks to understand how molecules encode intermolecular interactions, how those interactions give rise to supramolecular organization, and the properties that emerge from that organization.  We are particularly interested in the ordering of molecules at interfaces, and associated interfacial properties.  In some instances, the interfaces are macroscopic, such as the surface of a polymeric film.  In other cases, the surfaces are buried and nanoscopic, such as interfaces defined by the self-assembly of surfactant molecules into micelles. 

The molecular materials that we study are held together by non-covalent interactions, and thus they tend to be soft and readily reorganized by external stimuli. For example, we are exploring the properties of liquid crystals.  As the name suggests, molecules within liquid crystals possess the mobility of liquids yet exhibit the long-range order typical of crystals.  We are studying the ordering of liquid crystals at interfaces, and how chemical, physical and biomolecular interactions can trigger changes in the ordering of liquid crystals.  

We are also investigating the ordering of water at interfaces, with a focus on understanding how nanoscopic chemical patterns lead to interactions between molecules dispersed in water. This research is providing new design rules for hydrophobically-driven self-assembly. 

A third area that we are studying involves molecular self-assembly of amphiphilic molecules.  We are designing surfactants that incorporate redox-active and light-sensitive groups and exploring how changes in the states of these molecules influence their self-assembly in solution and at interfaces.  We are also exploring how topological defects formed in liquid crystals can serve as virtual templates for the self-assembly of amphiphilic molecules.  
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Our research projects typically require collaboration, as we address challenges that benefit from multiple perspectives and approaches.  We work closely, in particular, with collaborators with expertise in electronic structure calculations, molecular simulations and meso-scale modeling.  In some cases, theory is used to interpret experimental observations and, in other instances, it serves to guide experiments.  We also collaborate with researchers from chemistry, physics, biochemistry, microbiology, applied mathematics, pharmacy, veterinary medicine and medicine. 

Current Projects

Elastic Interactions of Colloids in Liquid Crystals

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Summary: Liquid crystals (LCs) are anisotropic fluids that combine the long-range order of crystals with the mobility of liquids. We are exploring how the elastic strain of LCs around colloidal inclusions (e.g., microdroplets) can trap and eject colloids from LCs. We have shown that the elasticity of LCs provides new opportunities to program the ejection of colloids from LCs in response to a range of stimuli. For example, we have designed LC systems that are triggered by the touch of a human finger (physiological temperature). The system optically reports the triggering event and dispenses a precise dose of chemical microcargo.


Self-Regulating Liquid Crystals that Report and Respond to Bacterial Motility

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Summary: We have designed liquid crystals (LCs) that are triggered by the swimming motion of motile bacteria (e.g. Escherichia coli). Interfacial shear stresses generated by bacteria swimming near the LC interface cause a change in orientation of the LC. The change in LC orientation can be observed optically and also release colloidal microcargo (micrometer-sized water droplets containing bioactive agents) from the LC into the aqueous phase containing the bacteria. We have performed experiments in which the bioactive agents are antibacterial. The LC systems do not release antibacterial agents in the absence of bacteria and release only the minimum amount of biocidal agent required to kill the bacterial cells. The materials provide a self-regulated release of chemoactive agents in response to mechanical forces generated by living cells.


Molecular Self-Assembly Templated by Liquid Crystals

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Summary: Topological defects are present in the equilibrium states of many confined liquid crystals (LCs). However, little is understood about the nanoscopic organization of molecules within LC topological defects. We have recently found that the nanoscopic environments defined by LC topological defects can selectively trigger processes of molecular self-assembly. By using fluorescence microscopy, cryogenic transmission electron microscopy and super-resolution optical microscopy, we have observed signatures of molecular self-assembly of amphiphilic molecules in topological defects, including cooperativity, reversibility and controlled growth. We have used the molecular self-assembly process to provide new insights into the nanoscopic structure of LC topological defects. Overall, our results reveal that, in analogy to other classes of macromolecular templates such as polymer-surfactant complexes, topological defects in LCs are a versatile class of three-dimensional, dynamic and reconfigurable templates that can direct processes of molecular self-assembly.


Chemoresponsive Liquid Crystals

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Summary: We are exploring how molecular transformations at surfaces, including molecular binding events and chemical reactions, can trigger ordering transitions in liquid crystals (LCs) at surfaces. For example, we have designed chemical functional groups into LCs that bind to transition metal cations at surfaces and thereby orient the LC: Displacement of the ligand of the LC by a targeted chemical species can trigger an ordering transition in the LC that can be seen with the naked eye and quantified. We are also using electronic structure calculations to design noble metal surfaces that bind with and transform targeted molecules, including CO, O3, NO2 and other important atmospheric gases. This ongoing work holds promise for the development of wearable sensors for quantification of personal exposure to chemical environments.

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Stimuli-Responsive Liquid Crystal Emulsions

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Summary: Past studies have established that the interfacial energetics, and presence of adsorbates that change interfacial energies, can play a central role in the orientational ordering of liquid crystals (LCs). We have discovered that ordering transitions in micrometer-sized LC droplets dispersed in water can be triggered by pg/ml concentrations of specific bacterial glycophospholipids (endotoxins), as well as lipid A (the lipid component of endotoxin that possesses six tails) in the water. We have also found that these ordering transitions are not due to adsorbate-induced changes in interfacial energetics, but rather are due to the assembly of lipid A with defects (so-called boojums) of the LC droplets. The central role of the topological defects in the ordering transitions is demonstrated by an extreme dependence on geometry of the LC system; the response of LC to lipid A changes by 6 orders of magnitude with geometry. Our results establish a new mechanism by which very low concentrations of lipids can trigger ordering transitions in LC systems.​


Complex Liquid Crystal Emulsions

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Summary: The internal organization of a liquid crystal droplet reflects a delicate balance of contributions to its free energy, including orientation-dependent interfacial free energies, and energies associated with elastic strain of the liquid crystal and the presence of singular topological defects. We are exploring the partial miscibility of mixtures of perfluorocarbons and nematic liquid crystals to design complex emulsions with liquid crystalline and isotropic compartments. We are elucidating the rules that govern their formation and investigating their use for the design of responsive emulsions at equilibrium and beyond equilibrium.


Biomolecular Self-Assembly at Aqueous Interfaces of Liquid Crystals

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Summary: This work is inspired by the observation that most biomolecular interactions at biological membranes are accompanied by reorganization of the proteins and lipids that constitute the membranes. We exploit that reorganization to drive orientational transitions in thermotropic liquid crystals (LCs) at aqueous interfaces that are decorated with phospholipids, thus providing a general means of amplifying and transducing a range of important biomolecular interactions. We have shown that it is possible to use the patterned orientational response of the LCs to follow a wide range of specific binding events at these interfaces, including binding events involving proteins with dissociation constants (Kd) that vary from Kd=10-14 M to Kd=10-4 M. It is also possible to use the LC to follow the lateral organization of proteins and functionalized-lipids following their self-association at the fluid interface.


Blue Phases

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Summary: Blue phases are defect-rich liquid crystal phases consisting of double-twist cylinders of LC that assemble into cubic lattices with periodicities on the order of 100 nm that Bragg reflect visible light. Blue phases form from a precarious balance of elastic, defect and interfacial energetics. We have found that exposure of blue phases to low concentrations of volatile organic compounds (VOCs) can trigger changes in the lattice spacings, resulting in changes in the optical appearance of the blue phase. These materials appear promising as the basis of passive methods for reporting VOCs.


Active Matter

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Summary: Materials driven out of thermal equilibrium by the conversion of external energy into net forces via local chemical or physical gradients are often described as active matter. Examples of active matter include biological organisms such as bacteria or protozoa and synthetic colloids that are self-propelled by local chemical reactions. We are exploring how the anisotropic viscoelastic environment of a liquid crystal (LC) can change the behaviors of active colloids. We find, for example, that the elasticity of the LCs can profoundly change active colloidal behaviors. Whereas passive colloids exhibit preferred motion in the direction of minimum viscous resistance, we find that active colloids can move preferentially in the direction of maximum viscous resistance. We are also exploring cooperative behaviors of multiple colloids in liquid crystals.


Liquid Crystal Templated Colloidal Synthesis and Self-Assembly

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Summary: We are exploring optically anisotropic polymer microspheres that are templated from liquid crystals (LCs) with the long-term goal of understanding intermolecular forces that dictate colloidal stability and self-assembly of soft matter systems. In contrast to optically isotropic colloids that have been widely studied in the past, we are testing the hypothesis that the anisotropic structure of LC-templated polymeric particles can encode orientation-dependent van der Waals forces. We are also investigating how anisotropic van der Waals forces encoded by these distinct configurations can guide the hierarchical ordering of polymeric assemblies.

Effects of Proximal Charges on Hydrophobic Forces

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Summary: We have discovered a new way of tuning (strengthening or weakening) hydrophobic interactions between non-polar interfaces in water based in the incorporation of charges into the interfaces. The results are of practical and fundamental importance because control of hydrophobic interactions underlies a wide range of colloidal and biophysical phenomena, including adhesion, protein folding and molecular self-assembly, and in the majority of these systems, non-polar domains are present along with charged functional groups. Our results are yielding new, general and practical design rules for hydrophobically-driven assembly processes. ​

Active Control of Surfactants

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Summary: We are exploring how redox-active and light-sensitive groups can be incorporated into the designs of surfactants to enable surfactant-based properties (bulk and interfacial) to be manipulated by electrochemical or photophysical processes. For example, we have recently reported on the self-assembly of redox-active surfactants at chemically functionalized electrodes. We have found oxidation state-dependent self-assembly of the redox surfactants to offer the basis of new principles for controlling charge transfer at electrode/solution interfaces. We have demonstrated the potential utility of this approach by using redox-active surfactants to reduce recombination reactions within a solar-energy harvesting device.

​Abbott Group Guidance for Research

Documentation Link: https://cornell.box.com/s/gyimx8s6dw8r4xrt8l2c0rpj4ifdrp9s
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