Millimetre Length Scales Research Articles

Bespoke periodic topography in hard polymer films by infrared radiation-assisted evaporative lithography

Polymer coatings with periodic topographic patterns, repeating over millimetre length scales, are created from lateral flows in an aqueous dispersion of colloidal particles. The flow is driven by differences in evaporation rate across the wet film surface created by IR radiative heating through a shadow mask. This new process, which we call IR radiation-assisted evaporative lithography (IRAEL), combines IR particle sintering with the concept of evaporative lithography. We show that the height of the surface features increases with an increase in several key parameters: the initial thickness of the film, the volume fraction of particles, and the pitch of the pattern. The results are interpreted by using models of geometry and particle transport. The patterned coatings can function as “paintable” microlens arrays, applicable to nearly any surface. Compared with existing methods for creating textured coatings, IRAEL is simpler, inexpensive, able to create a wide variety of bespoke surfaces, and applicable to nearly any substrate without prior preparation.

A scanning fluid dynamic gauging technique for probing surface layers

Fluid dynamic gauging (FDG) is a technique for measuring the thickness of soft solid deposit layers immersed in a liquid environment, in situ and in real time. This paper details the performance of a novel automated, scanning FDG probe (sFDG) which allows the thickness of a sample layer to be monitored at several points during an experiment, with a resolution of ±5 µm. Its application is demonstrated using layers of gelatine, polyvinyl alcohol (PVA) and baked tomato purée deposits. Swelling kinetics, as well as deformation behaviour—based on knowledge of the stresses imposed on the surface by the gauging flow—can be determined at several points, affording improved experimental data. The use of FDG as a surface scanning technique, operating as a fluid mechanical analogue of atomic force microscopy on a millimetre length scale, is also demonstrated. The measurement relies only on the flow behaviour, and is thus suitable for use in opaque fluids, does not contact the surface itself and does not rely on any specific physical properties of the surface, provided it is locally stiff.

YIG magnonics

Early experiments in magnonics were made using ferrite samples, largely due to the intrinsically low magnetic (spin-wave) damping in these materials. Historically, magnonic phenomena were studied on micrometre to millimetre length scales. Today, the principal challenge in applied magnonics is to create sub-micrometre devices using modern polycrystalline magnetic alloys. However, until certain technical obstacles are overcome in these materials, ferrites—in particular yttrium iron garnet (YIG)—remain a valuable source of insight. At a time when interest in magnonic systems is particularly strong, it is both useful and timely to review the main scientific results of YIG magnonics of the last two decades, and to discuss the transferability of the concepts and ideas learned in ferrite materials to modern nano-scale systems.

Self-Assembly of Porphyrins on a Single Crystalline Organic Substrate

Layers of a free base porphyrin have been deposited from n-heptane solution onto single crystalline potassium acid phthalate substrates and the self-assembled structures have been characterized by atomic force microscopy and X-ray diffraction. Depending on the concentration of the porphyrin solution, different anisotropic structures are found that are epitaxially related to the substrate, with the length axis parallel to the [001] direction of the (010) substrate surface. For undersaturated solutions, islands, cellular structures, and a hole pattern of monolayers of approximately 2.5 nm height are formed, which correspond to porphyrin molecules stacking approximately perpendicular to the substrate surface. From supersaturated solutions, multilayers are formed. At the highest concentrations, nanosized needles develop, which cover the whole substrate, creating a "monocrystalline" film with a length scale of several millimeters.

Investigation of the Neumann triangle for dodecane liquid lenses on water

Liquid lenses of dodecane resting on a water surface are imaged and analyzed computationally. The equilibrium condition for the three-phase line is a vector balance of three interfacial tensions known as the Neumann triangle. Measurement of the interfacial tensions, and two angles, allows the relation to be verified. Despite use of purified dodecane and the highest accuracy methods currently available at the millimetre length scale, a discrepancy with the Neumann triangle is found. The results, as well as literature data, are not consistent with interpretation as a thermodynamic line tension or as a pseudo-line tension. Alternative possibilities are briefly discussed.

Applicability of averaged concentrations in determining geochemical reaction rates in heterogeneous porous media

This work examines the applicability of averaged concentrations, a mathematical analog of field-measured solute concentrations averaged over a large number of pores, in determining mineral reaction rates in heterogeneous porous media. Pore-scale network models were used to represent sandstones with anorthite and kaolinite as reactive minerals that are heterogeneously distributed in space. Reaction rates calculated from averaged concentrations were compared to true reaction rates that take into account variabilities in individual pore properties. Simulations were run under the highly acidic conditions relevant to geological CO2 sequestration in deep brine formations under various mineralogical and flow conditions. Results show that, under conditions where incomplete mixing arises, the averaged concentrations and analogously the field-measured concentrations, do not accurately reflect reaction progress. Over the length scale of several millimeters, the anorthite dissolution rates can be overestimated by a factor of three. For kaolinite, due to its highly nonlinear reaction rate law, even the reaction direction may be incorrectly determined, with precipitation predicted as dissolution. The extent of errors introduced depends on the extent of incomplete mixing. Conditions that homogenize the concentration fields, such as small reactive mineral clusters, abundant reactive minerals, and very fast or very slow flow rates, minimize errors introduced from averaging. These results indicate that the averaging scheme may partly contribute to the often-cited laboratory-field rate discrepancy and have important implications for the interpretation of concentration data obtained from field investigation.

Limitations for the trapped field in large grain YBCO superconductors

The actual limitations for the trapped field inYBa2Cu3O7−δ (YBCO) monoliths are discussed. The influence of the sample geometry and of the criticalcurrent density on the trapped field is investigated by numerical calculations. The fielddependence of the critical current density strongly influences the trapped field. A nonlinearrelationship between the sample size, the critical current density and the resultingtrapped field is derived. The maximum achievable trapped field in YBCO at77 K is found to be around 2.5 T. This limit is obtained for reasonable geometriesand high but realistic critical current densities. Such high fields have not beenreached experimentally so far, due to non-optimized flux pinning and materialinhomogeneities. These inhomogeneities can be directly assessed by the magnetoscantechnique, and their influence is discussed. Significant differences between thea- andthe c-growth sectors were found. Limitations due to cracks and non-superconducting inclusions(e.g. 211 particles) are estimated and found to be candidates for variations ofJc on a millimetre length scale, as observed in experiments.

Relativity and photons in laser-plasmas and z-pinches

An electron interacting with an intense, planar electromagnetic field leads to relativistic quiver motion. Under such conditions a host of phenomena not normally associated with light waves occur when the wave interacts with a plasma. These include the production of extreme magnetic fields as high as 700 MG and the acceleration of electrons and protons to 100's of MeV over micron to millimetre length scales. Wire array z-pinches have been shown to be a source of ultra powerful X-ray pulses (>230 TW) and record ion temperatures of 2 billion Kelvin.

Development and experimental validation of a continuum micromechanics model for the elasticity of wood

This contribution covers the development and validation of a microelastic model for wood, based on a four-step homogenization scheme. At a length scale of several tens of nanometers, hemicellulose, lignin, and water are intimately mixed, and build up a polymer (polycrystal-type) network. At a length scale of around one micron, fiberlike aggregates of crystalline and amorphous cellulose are embedded in an contiguous polymer matrix, constituting the so-called cell wall material. At a length scale of about one hundred microns, the material softwood is defined, comprising cylindrical pores (lumen) in the cell wall material of the preceding homogenization step. Finally, at a length scale of several millimeters, hardwood comprises larger cylindrical pores (vessels) embedded in the softwood-type material homogenized before. Model validation rests on statistically and physically independent experiments: The macroscopic stiffness values (of hardwood or softwood) predicted by the micromechanical model on the basis of tissue-independent (‘universal’) phase stiffness properties of hemicellulose, amorphous cellulose, crystalline cellulose, lignin, and water (experimental set I) for tissue-specific composition data (experimental set IIb) are compared to corresponding experimentally determined tissue-specific stiffness values (experimental set IIa).

Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica

Femtosecond laser radiation tightly focused in bulk fused silica is used to generate self-ordered nanogratings when the sample is translated under the lens at constant speed. The nanogratings are preserved over a length scale of millimeters. We demonstrate that nanogratings are formed for all pulse durations tested, ranging from 40to500fs, and that the pulse energy threshold for this phenomenon increases with decreasing pulse duration. We use high spatial resolution diagnostics based upon selective chemical etching followed by atomic force microscopy and scanning electron microscopy to reveal the morphology of the nanogratings.

Lower critical dimensions for superconducting long-range order in type-II superconductors.

It is shown that phase fluctuations destroy off-diagonal long-range order (phase coherence) in the Meissner phase below two dimensions, below four dimensions in the conventional Abrikosov flux-lattice phase, and below three dimensions in the limit of infinite Ginzburg ratio \ensuremath{\kappa}. The destruction of long-range order in the flux lattice is due to phase changes induced by shear motions of the flux lines. The phase coherence decays over a long length scale (of order millimeters) in three-dimensional systems.