Jamie K. Hobbs

Supramolecular dendritic liquid quasicrystals

A large number of synthetic and natural compounds self-organize into bulk phases exhibiting periodicities on the 10-8–10-6 metre scale as a consequence of their molecular shape, degree of amphiphilic character and, often, the presence of additional non-covalent interactions. Such phases are found in lyotropic systems (for example, lipid–water, soap–water), in a range of block copolymers and in thermotropic (solvent-free) liquid crystals. The resulting periodicity can be one-dimensional (lamellar phases), two-dimensional (columnar phases) or three dimensional (‘micellar’ or ‘bicontinuous’ phases). All such two- and three-dimensional structures identified to date obey the rules of crystallography and their symmetry can be described, respectively, by one of the 17 plane groups or 230 space groups. The ‘micellar’ phases have crystallographic counterparts in transition-metal alloys, where just one metal atom is equivalent to a 103 - 104-atom micelle. However, some metal alloys are known to defy the rules of crystallography and form so-called quasicrystals, which have rotational symmetry other than the allowed two-, three-, four- or six-fold symmetry. Here we show that such quasiperiodic structures can also exist in the scaled-up micellar phases, representing a new mode of organization in soft matter.

A mechanical microscope: High-speed atomic force microscopy

An atomic force microscope capable of obtaining images in less than 20ms is presented. By utilizing a microresonator as a scan stage, and through the implementation of a passive mechanical feedback loop with a bandwidth of more than 2MHz, a 1000-fold increase in image acquisition rate relative to a conventional atomic force microscope is obtained. This has allowed images of soft crystalline and molten polymer surfaces to be collected in 14.3ms, with a tip velocity of 22.4cms−1 while maintaining nanometer resolution.

Cell wall peptidoglycan architecture in Bacillus subtilis

The bacterial cell wall is essential for viability and shape determination. Cell wall structural dynamics allowing growth and division, while maintaining integrity is a basic problem governing the life of bacteria. The polymer peptidoglycan is the main structural component for most bacteria and is made up of glycan strands that are cross-linked by peptide side chains. Despite study and speculation over many years, peptidoglycan architecture has remained largely elusive. Here, we show that the model rod-shaped bacterium Bacillus subtilis has glycan strands up to 5 μm, longer than the cell itself and 50 times longer than previously proposed. Atomic force microscopy revealed the glycan strands to be part of a peptidoglycan architecture allowing cell growth and division. The inner surface of the cell wall has a regular macrostructure with ≈50 nm-wide peptidoglycan cables [average 53 ± 12 nm (n = 91)] running basically across the short axis of the cell. Cross striations with an average periodicity of 25 ± 9 nm (n = 96) along each cable are also present. The fundamental cabling architecture is also maintained during septal development as part of cell division. We propose a coiled-coil model for peptidoglycan architecture encompassing our data and recent evidence concerning the biosynthetic machinery for this essential polymer.

Ultrahigh-speed scanning near-field optical microscopy capable of over 100 frames per second

Scanning near-field optical microscopy is a powerful technique offering subdiffraction-limit optical resolution. However, the range of applications is limited by slow image acquisition rates. In this letter we demonstrate an implementation of a near-field optical microscope capable of scan speeds of 150 mm/s producing images of an area 20 μm2 in less than 10 ms, i.e., over 100 frames/s. To achieve this, a method of measuring the optical near-field intensity with a high bandwidth of greater than 1 MHz has been developed. A second original aspect is that the scan system uses a mechanical resonance of the probe to address the sample. The presented microscope is over 1000 times faster than a conventional scanning near-field optical microscope and ∼10 times faster than any scanning probe microscope to date.

Direct observations of the growth of spherulites of poly (hydroxybutyrate-co-valerate) using atomic force microscopy

Abstract Atomic force microscopy (AFM) has been used to observe, in real time, the growth of two-dimensional poly(hydroxybutyrate-co-valerate) (PHB/V) ‘spherulites’ in thin films. The AFM permits us to image the growth over a wide range of magnifications, from the macroscopic spherulitic growth down to observations of growth of individual lamellae. The lamellar growth images are obtained using a special, high resolution, phase-imaging technique. Low magnification images show, in common with optical microscope techniques, sharp circular growth fronts which move at a constant growth rate. At higher magnifications the rough nature of the growth front on a lamellar scale is clearly revealed with dominant lamellae leading the growth. The most remarkable observation is that these dominant lamellae do not grow at a fixed, constant rate, as predicted by most growth theories, but rather they initially spurt forwards at a rate substantially faster than the macroscopic growth rate, and then slow down or stop. A new theory, in which the spherulite growth rate is controlled not by the growth rate of the individual lamellae, but rather by the rate at which new lamellae nucleate on existing, dormant lamellae, is suggested. It is believed that these observations, although only made on one system, may be more widely applicable.

On the origin of the multiple endotherms in PEEK

Abstract Two models have been proposed to explain the two melting endotherms commonly observed in polyethers: (1) the crystals with the lower melting temperature may transform into a higher melting form during heating; (2) the two melting peaks come from separate populations of crystals. Our TEM studies of thin films of PEEK have shown two crystal populations, with widely differing lamellar thicknesses, which can occur in quite separate areas of the samples. These results strongly support the second model.

Super‐resolution microscopy reveals cell wall dynamics and peptidoglycan architecture in ovococcal bacteria

Cell morphology and viability in Eubacteria is dictated by the architecture of peptidoglycan, the major and essential structural component of the cell wall. Although the biochemical composition of peptidoglycan is well understood, how the peptidoglycan architecture can accommodate the dynamics of growth and division while maintaining cell shape remains largely unknown. Here, we elucidate the peptidoglycan architecture and dynamics of bacteria with ovoid cell shape (ovococci), which includes a number of important pathogens, by combining biochemical analyses with atomic force and super‐resolution microscopies. Atomic force microscopy analysis showed preferential orientation of the peptidoglycan network parallel to the short axis of the cell, with distinct architectural features associated with septal and peripheral wall synthesis. Super‐resolution three‐dimensional structured illumination fluorescence microscopy was applied for the first time in bacteria to unravel the dynamics of peptidoglycan assembly in ovococci. The ovococci have a unique peptidoglycan architecture and growth mode not observed in other model organisms.

Complex Multicolor Tilings and Critical Phenomena in Tetraphilic Liquid Crystals

X-shaped molecules undergo reversible thermal transitions between phase-separated and mixed states. T-shaped molecules with a rod-like aromatic core and a flexible side chain form liquid crystal honeycombs with aromatic cell walls and a cell interior filled with the side chains. Here, we show how the addition of a second chain, incompatible with the first (X-shaped molecules), can form honeycombs with highly complex tiling patterns, with cells of up to five different compositions (“colors”) and polygonal shapes. The complexity is caused by the inability of the side chains to separate cleanly because of geometric frustration. Furthermore, a thermoreversible transition was observed between a multicolor (phase-separated) and a single-color (mixed) honeycomb phase. This is analogous to the Curie transition in simple and frustrated ferro- and antiferromagnets; here spin flips are replaced by 180° reorientations of the molecules.

Parallel scanning near-field photolithography: The snomipede

The “Millipede”, developed by Binnig and co-workers (Bining, G. K.; et al. IBM J. Res. Devel. 2000, 44, 323.), elegantly solves the problem of the serial nature of scanning probe lithography processes, by deploying massive parallelism. Here we fuse the “Millipede” concept with scanning near-field photolithography to yield a “Snomipede” that is capable of executing parallel chemical transformations at high resolution over macroscopic areas. Our prototype has sixteen probes that are separately controllable using a methodology that is, in principle, scalable to much larger arrays. Light beams generated by a spatial modulator or a zone plate array are coupled to arrays of cantilever probes with hollow, pyramidal tips. We demonstrate selective photodeprotection of nitrophenylpropyloxycarbonyl-protected aminosiloxane monolayers on silicon dioxide and subsequent growth of nanostructured polymer brushes by atom-transfer radical polymerization, and the fabrication of 70 nm structures in photoresist by a Snomipede probe array immersed under water. Such approaches offer a powerful means of integrating the top-down and bottom-up fabrication paradigms, facilitating the reactive processing of materials at nanometer resolution over macroscopic areas.

Cracking in spherulites of poly(hydroxybutyrate)

Poly(hydroxybutyrate) (PHB) is a biodegradable thermoplastic. Melt-cast PHB sheets normally become brittle on storage. The embrittlement has been associated with cracks formed during cooling due to differences in radial and circumferential thermal expansion coefficients. We show that the cracks are in fact due to differences in thermal expansion between the PHB film and the constraining glass slides. The circumferential features, previously identified as cracks, are shown to be surface features formed due to volume reduction during crystallization constrained by the substrate.