Controlling Morphology in Swelling-Induced Wrinkled Surfaces

Wrinkles represent a pathway towards the spontaneous generation of ordered surface microstructure for applications in numerous fields. Examples of highly complex ordered wrinkle structures abound in Nature, but the ability to harness this potential for advanced material applications remains limited. This work focuses on understanding the relationship between the patterns on a wrinkled surface and the experimental conditions under which they form. Because wrinkles form in response to applied stresses, particular attention is given to the nature of the stresses in a wrinkling surface. The fundamental insight gained was then utilized to account for observed wrinkle formation phenomena within more complex geometric and kinetic settings. In order to carefully control and measure the applied stresses on a wrinkling film, a swelling-based system was developed using poly(dimethylsiloxane) (PDMS), surface-oxidized with a UV-ozone treatment. The swelling of the oxidized surface upon exposure to an ethanol vapor atmosphere was characterized using beam-bending experiments, allowing quantitative measurements of the applied stress. The wrinkle morphologies were characterized as a function of the overstress, defined as the ratio of the applied swelling stress to the critical buckling stress of the material. A transition in the dominant morphology of the wrinkled surfaces from dimple patterns to ridge patterns was observed at an overstress value of ~2. The pattern dependence of wrinkles on the ratio of the principal stresses was examined by fabricating samples with a gradient prestress. When swollen, these samples exhibited a smooth morphological transition from non-equibiaxial to equibiaxial patterns, with prestrains as low as 2.5% exhibiting non-equibiaxial characteristics. This transition was seen both in samples with low and high overstresses. To explore the impact of these stress states in more complex geometries, wrinkling hemispherical surfaces with radii of curvature ranging from 50-1000 μm were fabricated using the same material system. Upon wrinkling, the hemispheres formed complex hierarchical assemblies reminiscent of naturally occurring structures. The curvature of a surface exhibited a correlation with its critical buckling stress, independent of other factors. This enables the surface curvature to be used as an independent control over the dimple-to-ridge transition which occurs as a function of overstress. As in the flat buckling surfaces, this transition was shown to occur at an overstress value of ~2. Surface curvature was also shown to improve the observed hexagonal ordering of the dimple arrays, resulting in the formation of regular golf ball structures. Geometric effects in finite flat plates were also examined. Using circular masks during the oxidation process, plates with radii ranging from 0.4-8.6 mm were created. Upon wrinkling, a dimple-to-ridge transition was observed with increasing plate size, with the morphological switch occurring at a radius of ~2 mm. This observed transition was not found to be due to the inherent mechanics of plates of different sizes, but instead to a reduction in the oxide conversion due to shadowing or stagnation caused by the masking process, which lowered the applied overstress. The shape of the finite plate was found to have little impact on the resulting wrinkle morphologies. Kinetic aspects of wrinkling were qualitatively characterized by observing the wrinkling process over the course of swelling. Wrinkling was observed to frontally propagate across the surface, and the ordering of the patterns which developed showed a qualitative correlation with the degree of uniformity in the advancing wrinkle front. Swelling with different solvents was found to lead to the formation of different patterns, based on the swelling kinetics of the UVO-treated PDMS upon exposure to each solvent

Transmission of pathogenic α-synuclein to mice

α-Synuclein is a soluble, cellular protein that in a number of neurodegenerative diseases, including Parkinson's disease, multiple system atrophy, and Lewy body dementia aggregates into pathological protein deposits. Principles how misfolded and aggregated α-synuclein is transmitted within the central nervous system (CNS) causing neurologic disease were found to be similar to those of prions. Misfolded α-synuclein can be transmitted between cells and act as a seed, recruiting native, unfolded α-synuclein to form insoluble aggregates. The mechanisms and the routes through which pathogenic proteins enter the CNS causing progressive disease are still not completely understood. The work in this thesis confirms previous findings indicating that α-synuclein fibrils intracerebrally injected into wild-type mice for α-synuclein can induce neuropathology in interconnected brain regions as similarly observed in sporadic Parkinson's disease. In contrast, α-synuclein fibrils injected into the tongue muscle of wild-type mice for α-synuclein did not neuroinvade the CNS causing α-synuclein pathology. Moreover, the present study is the first to show, that α-synuclein fibrils peripherally injected into the tongue and the peritoneum of mice overexpressing human α-synuclein, can neuroinvade the CNS, cause widespread α-synuclein pathology and induce neurologic symptoms. The induction of neuropathology was accompanied by neuroinflammation as monitored by astrocytic gliosis and microgliosis. In addition, the study presented here indicates that exposure of mice overexpressing human α-synuclein with pathogenic α-synuclein aerosols was not sufficient for α-synuclein prionoids to enter the brain via the olfactory epithelium and induce neuropathology. In summary, these findings corroborate the prionoid character of misfolded α-synuclein using similar routes like prions to neuroinvade brain and spinal cord and induce neurologic disease

Photonic Bloch-dipole-Zener Oscillations in Binary Parabolic Optical Waveguide Arrays

We have studied the propagation and Zener tunneling of light in the binary parabolic optical waveguide array (BPOWA), which consists of two evanescently coupled dissimilar optical waveguides. Due to Bragg reflections, BPOWA attains two minibands separated by a minigap at the zone boundary. Various coherent superpositions of optical oscillations and Zener tunneling occur for different parameters on the phase diagram. In particular, Bloch-Zener oscillation and a different type of Bloch-dipole-Zener oscillation are obtained by the field-evolution analysis. The results may have potential applications in optical splitting and waveguiding devices and shed light on the coherent phenomena in optical lattices.Comment: Submitted to JOSA

Tunable Hybridization at Mid Zone and Anomalous Bloch-Zener Oscillations in Optical Waveguide Ladders

We have studied the optical oscillation and tunneling of light waves in optical waveguide ladders formed by two coupled planar optical waveguide arrays. For the band structure, a mid-zone gap is formed due to band hybridization and its wavenumber position can be tuned throughout the whole Brillouin zone, which is different from the Bragg gap. By imposing a gradient in the propagation constant in each array, Bloch-Zener oscillation (BZO) is realized with Zener tunneling between the bands occurring at mid zone, which is contrary to the common BZO with tunneling at the center or edge of the Brillouin zone. The occurrence of BZO is demonstrated by using the field-evolution analysis. The tunable hybridization at mid zone enhances the tunability of BZO in the optical waveguide ladders. This work is of general and fundamental importance in understanding the coherent phenomena in lattice structures.Comment: Submitted to Optics Letter

Collapse and revival in inter-band oscillations of a two-band Bose-Hubbard model

We study the effect of a many-body interaction on inter-band oscillations in a two-band Bose-Hubbard model with external Stark force. Weak and strong inter-band oscillations are observed, where the latter arise from a resonant coupling of the bands. These oscillations collapse and revive due to a weak two-body interaction between the atoms. Effective models for oscillations in and out of resonance are introduced that provide predictions for the system's behaviour, particularly for the time-scales for the collapse and revival of the resonant inter-band oscillations.Comment: 10 pages, 5 figure

Periodic Patterns and Energy States of Buckled Films on Compliant Substrates

Thin stiff films on compliant elastic substrates subject to equi-biaxial compressive stress states are observed to buckle into various periodic mode patterns including checkerboard, hexagonal and herringbone. An experimental setting in which these modes are observed and evolve is described. The modes are characterized and ranked by the extent to which they reduce the elastic energy of the film–substrate system relative to that of the unbuckled state over a wide range of overstress. A new mode is identified and analyzed having nodal lines coincident with an equilateral triangular pattern. Two methods are employed to ascertain the energy in the buckled state: an analytical upper-bound method and a full numerical analysis. The upper-bound is shown to be reasonably accurate to large levels of overstress. For flat films, except at small states of overstress where the checkerboard is preferred, the herringbone mode has the lowest energy, followed by the checkerboard, with the hexagonal, triangular, and one-dimensional modes lowering the energy the least. At low overstress, the hexagonal mode is observed in the experiments not the square mode. It is proposed that a slight initial curvature of the film may play role in selecting the hexagonal pattern accompanied by a detailed analysis. An intriguing finding is that the hexagonal and triangular modes have the same energy in the buckled state and, moreover, a continuous transition between these modes exists involving a linear combination of the two modes with no change in energy. Experimental observations of various periodic modes are discussed with reference to the energy landscape. Discrepancies between observations and theory are identified and open issues are highlighted.Engineering and Applied Science

Transport behaviour of a Bose Einstein condensate in a bichromatic optical lattice

The Bloch and dipole oscillations of a Bose Einstein condensate (BEC) in an optical superlattice is investigated. We show that the effective mass increases in an optical superlattice, which leads to localization of the BEC, in accordance with recent experimental observations [16]. In addition, we find that the secondary optical lattice is a useful additional tool to manipulate the dynamics of the atoms.Comment: Modified manuscrip

A framework to support the design and cultivation of embedded research initiatives

This work was supported by the National Institute of Health Research (NIHR) Health Services & Delivery Research (HS&DR) programme under grant number 16/52/21.Background: Embedded research involves co-locating researchers within non-academic organisations to better link research and practice. Embedded research initiatives are often complex and emergent with a range of underlying intents, structures and processes. This can create tensions within initiatives and contributes to ongoing uncertainty about the most suitable designs and the effectiveness of different approaches. Aims and objectives: We aimed to devise a practical framework to support those designing and cultivating embedded research by operationalising findings from an extensive study of existing initiatives. Key conclusions: The underpinning research on embedded initiatives – a literature review and scoping exercise of initiatives in health settings across the UK – showed that such initiatives share ten common sets of concerns in relation to their intent, structure and processes. We used these insights during a co-production workshop with embedded researchers and their managers that made use of a range of creative activities. The workshop resulted in a practical framework (and associated web-based tools) that draw on the metaphor of a garden to represent the growing, emergent nature of embedded research initiatives and the active work which individuals and organisations need to put into planning and maintaining such initiatives. Each of the aspects is represented as a separate area within the garden using relevant visual metaphors. Building on this, we also present a series of reflective questions designed to facilitate discussion and debate about design features, and we link these to the wider literature, thereby helping those involved to articulate and discuss their preferences and expectations.Publisher PDFPeer reviewe