Nathan A. Lockwood

Bio: Nathan A. Lockwood is an academic researcher from University of Wisconsin-Madison. The author has contributed to research in topics: Liquid crystal & Thermotropic crystal. The author has an hindex of 7, co-authored 10 publications receiving 553 citations.

Influence of surfactant tail branching and organization on the orientation of liquid crystals at aqueous-liquid crystal interfaces.

Abstract: We have examined the influence of two aspects of surfactant structuretail branching and tail organizationon the orientational ordering (so-called anchoring) of water-immiscible, thermotropic liquid...

Thermotropic Liquid Crystals as Substrates for Imaging the Reorganization of Matrigel by Human Embryonic Stem Cells

Abstract: We have investigated the culture of human embryonic stem cells (hESCs) on interfaces of the thermotropic liquid crystal, TL205, that are decorated with thin films of the extracellular matrix, Matrigel. hESCs seeded at the liquid-crystal/Matrigel interface survive for weeks, and cell colonies grow over this time. The cells show levels of differentiation comparable to that observed for cells on Matrigel-coated glass controls. Polarized and fluorescence microscopy reveal that the orientational order of the liquid crystal is coupled to the presence and organization of Matrigel. This enables straightforward imaging of the reorganization of Matrigel by the hESCs through changes in the appearance of the liquid crystal when observed using polarized light microscopy. The coupling between Matrigel and TL205 thus provides a simple tool for monitoring the reorganization of the Matrigel film over time. Our results suggest new approaches to the culture of cells and measurements of cell–extracellular-matrix interactions.

Self-assembly of surfactants and phospholipids at interfaces between aqueous phases and thermotropic liquid crystals

Abstract: Recent studies have reported on the self-assembly of surfactants and phospholipids at interfaces between aqueous phases and thermotropic liquid crystals. These studies have been enabled by an experimental system that permits the preparation of stable and easily-imaged interfaces between liquid crystals and aqueous phases. The influence of the molecular structure of surfactants and their interfacial organization on the ordering of the liquid crystals has been revealed. Because the liquid crystals reorder on time-scales of seconds or less, the time-dependent behavior of the liquid crystals can be used to follow dynamic phenomena at these interfaces, including the adsorption and desorption of surfactants. At these same aqueous-liquid crystal interfaces, phospholipids have been shown to organize into assemblies that possess the mobility of biological membranes and are coupled to the order within the liquid crystal. Specific protein binding events and enzyme-catalyzed reactions at phospholipid-decorated interfaces of liquid crystals lead to changes in the organization of the phospholipids that are “reported” as ordering transitions in the liquid crystal.

Detection of organophosphorous nerve agents using liquid crystals supported on chemically functionalized surfaces

Abstract: We report use of thin films of the nematic liquid crystal E7 supported on chemically functionalized surfaces to indicate the presence of vapors of the organophosphorous nerve agents sarin (GB), soman (GD), tabun (GA) and VX. The surfaces were prepared by the deposition of metal perchlorate salts onto carboxylic acid-terminated self-assembled monolayers. When using surfaces prepared from aluminum perchlorate salts, the nematic film of E7 underwent a transition from a perpendicular orientation to a tilted or planar orientation upon exposure to vapors of GB, GD, GA or VX generated from a small drop of agent placed on a piece of filter paper. The orientational transition of the liquid crystal was readily apparent as an optical signal that was visible to the naked eye. The agent VX, which has the lowest vapor pressure of these organophosphorous nerve agents (∼140 ppb), was reported by the liquid crystal within 60 s. The agents GB, GD, and GA, which have higher vapor pressures, triggered responses in the liquid crystal that were evident within 15 s, 15 s, and 60 s, respectively. By preparing surfaces from perchlorate salts of aluminum(III), zinc(II) and iron(III) it was possible to distinguish between GB, VX and either GD or GA, thus demonstrating the feasibility of designing surfaces patterned with metal salts to detect and positively identify chemical warfare agents.

Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions.

Abstract: We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water−surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with ∼3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over ∼6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelect...

Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions

Abstract: We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water-surfactant solutions. Optical characterisation of these suspensions allowed the partial optimisation of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with ~3% of flakes consisting of monolayers. These flakes are stabilised against reaggregation by Coulomb repulsion due to the adsorbed surfactant. However, the larger flakes tend to sediment out over ~6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides. The deposited films are reasonably conductive and are semi-transparent. Further improvements may result in the development of cheap transparent conductors.

Template Synthesis of Nanostructured Materials via Layer-by-Layer Assembly†

Abstract: The layer-by-layer (LbL) templating technique has attracted significant interest as a simple, highly versatile approach that has been widely used to prepare nanostructured materials with tailored properties. The process involves the sequential deposition of species, such as polymers, nanoparticles, lipids, proteins, and dye molecules, onto various templates, which are subsequently removed to yield free-standing structures. Although fine control of the material properties (e.g., size, composition, thickness, permeability, function) is afforded by the type of species LbL-assembled, the morphology and composition of the templates also play a crucial role in determining the properties, and hence potential applications, of the materials generated. In this review, we focus on the two main classes of templates that have been employed to prepare nanostructured materials: planar and colloidal. The use of porous planar and colloidal substrates in LbL templating synthesis is also presented, as this offers opportunit...

Liquid-crystal materials find a new order in biomedical applications.

Abstract: With the maturation of the information display field, liquid-crystal materials research is undergoing a modern-day renaissance. Devices and configurations based on liquid-crystal materials are being developed for spectroscopy, imaging and microscopy, leading to new techniques for optically probing biological systems. Biosensors fabricated with liquid-crystal materials can allow label-free observations of biological phenomena. Liquid-crystal polymers are starting to be used in biomimicking colour-producing structures, lenses and muscle-like actuators. New areas of application in the realms of biology and medicine are stimulating innovation in basic and applied research into these materials.