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We use high-speed photography to observe the dendritic freezing of ice between two closely spaced parallel plates. Measuring the propagation speeds of dendrites, we investigate whether there is a confinement-induced thermal influence upon the speed beyond that provided by a single surface. Plates of thermally insulating plastic and moderately thermally conductive glass are used alone and in combination, at temperatures between −10.6 and −4.8 °C, with separations between 17 and 135 μm wide. No effect of confinement was detected for propagation on glass surfaces, but a possible slowing of propagation speed was seen between insulating plates. The pattern of dendritic growth was also studied, with a change from curving to straight dendrites being strongly associated with a switch from a glass to a plastic substrate.

Fluid-driven grain flow through a confined channel filled with non-buoyant grains is herein observed to exist in three regimes according to total imposed flow rate. (1) At low imposed flow rates, no grain flow occurs as the fluid stress is insufficient to mobilise the grains and Darcy flow is observed. (2) At a sufficient imposed flow rate, grains begin to flow at the top of the channel with self-similar Gaussian velocity profiles that become faster and encroach deeper into the channel with increased flow rate. (3) At high flow rates, significant grain flow occurs at the base of the channel, distorting the Gaussian profile, resulting in a gradual transition towards a more symmetric, full-channel flow. Each regime, and the transitions between them, is discussed in relation to experimental grain velocity measurements.

Silo discharge has been extensively studied for decades although questions remain regarding the nature of the velocity field, particularly for submerged systems. In this work, fluid-driven granular drainage was performed in a quasi-two-dimensional silo with grains submerged in fluid. While the observed Gaussian velocity profiles were generally consistent with current diffusion models, the diffusion length was found to significantly decrease with height in contrast to the increases previously seen in dry silos. We propose a phenomenological anomalous diffusion model for the spreading of the flow upwards in the cell, with the fluid-driven flows we study here falling in the category of subdiffusive behavior. As the viscous characteristics of the system were amplified, the diffusion length increased and the shape of the flowing zone in the silo changed, deviating further from the parabolic form predicted by traditional normal diffusion models, in effect becoming more subdiffusive as quantified by a decreasing diffusion exponent.

Recent theoretical and experimental work suggests a frictionless-frictional transition with increasing inter-particle pressure explains the extreme solid-like response of discontinuous shear thickening suspensions. However, analysis of macroscopic discontinuous shear thickening flow in geometries other than the standard rheometry tools remain scarce. Here we use a Hele-Shaw cell geometry to visualise gas-driven invasion patterns in discontinuous shear thickening cornstarch suspensions. We plot quantitative results from pattern analysis in a volume fraction-pressure phase diagram and explain them in context of rheological measurements. We observe three distinct pattern morphologies: viscous fingering, dendritic fracturing, and system-wide fracturing, which correspond to the same packing fraction ranges as weak shear thickening, discontinuous shear thickening, and shear-jammed regimes. The microscopic mechanisms underlying the discontinuous shear thickening transition in dense granular systems are still under debate. Here, the authors explore this transition by characterizing the shape of invasion patterns in Hele-Shaw cell experiments with confined cornstarch suspensions.

Granular flows such as landslides, debris flows and avalanches are systems of particles with a large range of particle sizes that typically segregate while flowing. The physical mechanisms responsible for this process, however, are still poorly understood, and there is no predictive framework for ascertaining the segregation behaviour of a given system of particles. Here, we provide experimental evidence of individual large intruder particles being attracted to a fixed point in a dry two-dimensional flow of particles of otherwise uniform size. A continuum theory is proposed which captures this effect using only a single fitting parameter that describes the rate of segregation, given knowledge of the bulk flow field. Predictions of the continuum theory are compared with the experimental findings, both for the typical location and velocity field of a range of intruder sizes. For large intruder particle sizes, the continuum model successfully predicts that a fixed point attractor will form, where intruders are drawn to a single location.

Confined fluid-driven granular flows are present in a plethora of natural and industrial settings, yet even the most fundamental of these is not completely understood. While widely studied grain flows such as bed load and density-matched Poiseuille flows have been observed to exhibit exponential and Bingham style velocity profiles, respectively, this work finds that a fluid-driven bed of non-buoyant grains filling a narrow horizontal channel—confined both from the sides and above—exhibits self-similar Gaussian velocity profiles. As the imposed flow rate is increased and the grain velocity increases, the Gaussian flow profiles penetrate deeper into the packing of the channel. Filling fractions were observed to be also self-similar and qualitatively consistent with granular theory relating to the viscous number I, which at a given position on the self-similar Gaussian curve is found to be generally constant regardless of the imposed flow rate or velocity magnitude. An empirical description of the flow is proposed, and local velocity and filling fraction measurements were used to obtain the local grain flux and accurately recover a total grain flow rate.

International audience; The motion of a capillary interface across a deformable granular material in a confined geometry shows the complex interplay between viscous forces, solid friction, and capillary forces. In a horizontal quasi-one-dimensional geometrical confinement, a millifluidic tube, the displacement of a three-phase flow consisting of two fluids and a mobile granular phase exhibits viscous or frictional displacement regimes, as shown in [Phys. Rev. Lett. 117, 028002 (2016).]. In the present paper we explore in detail the dynamics in both regimes by making use of a new set of data. The viscous displacement regime which is characterized by a fluidization of the immersed granular material dragged by the flow driving the displacement of the capillary interface is interpreted from a rheological point of view. The frictional displacement regime which displays a self-structuring of the granular material left in the tube behind the invading capillary interface is interpreted by using a model based on the Janssen law to predict the typical size of the plugs obtained.

Frictional finger trees are patterns emerging from non-equilibrium processes in particle-fluid systems. Their formation share several properties with growth algorithms for minimum spanning trees (MSTs) in random energy landscapes. We propose that the frictional finger trees are indeed in the same geometric universality class as the MSTs, which is checked using updated numerical simulation algorithms for frictional fingers. We also propose a theoretical model for anomalous diffusion in these patterns, and discuss the role of diffusion as a tool to classify geometry.

Commercial aluminosilicate proppant particles have been coated with Ca-Si oxides, with the aim to provide an in-situ increase in the angularity (decrease in Krumbein roundness value) to facilitate their immobilization. Ca-Si oxide systems have been synthesized via sol-gel, cured, and sintered at 1200 °C using (a) CaCO3, (b) CaCO3 + orthosilicic acid (Si(OH)4, SA), and (c) CaCO3 + fused silica (SiO2, FS). When the proppant is cured in the presence of CaCO3 and silicic acid the coatings undergo a significant compositional change, while sintering results in the conversion of the cured samples to ceramic agglomerates with the desired “popcorn” shapes. The best results are obtained in the presence of Si reagents, and hydration of these sintered proppants allows for a distinct increase in the angularity, which is the desired transformation to allow the proppant to be locked-in-place once located in the reservoir. The samples have been characterized at each stages of preparation by scanning electron microscopy (SEM) with associated energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and infrared (IR) spectroscopy.

Aligned finger structures, with a characteristic width, emerges during the slow drainage of a liquid/granular mixture in a tilted Hele-Shaw cell. A transition from vertical to horizontal alignment of the finger structures is observed as the tilting angle and the granular density are varied. An analytical model is presented, demonstrating that the alignment properties is the result of the competition between fluctuating granular stresses and the hydrostatic pressure. The dynamics is reproduced in simulations. We also show how the system explains patterns made during dyke formations., 20 pages, 6 figures

We study experimentally the flow and patterning of a granular suspension displaced by air inside a narrow tube. The invading air-liquid interface accumulates a plug of granular material that clogs the tube due to friction with the confining walls. The gas percolates through the static plug once the gas pressure exceeds the pore capillary entry pressure of the packed grains, and a moving accumulation front is reestablished at the far side of the plug. The process repeats, such that the advancing interface leaves a trail of plugs in its wake. Further, we show that the system undergoes a fluidization transition-and complete evacuation of the granular suspension-when the liquid withdrawal rate increases beyond a critical value. An analytical model of the stability condition for the granular accumulation predicts the flow regime.

When a compressible gas displaces a granular mixture in a quasi 2D space, friction causes stick slip motion. A localized slip event at the interface allows expansion of the compressed gas and a sudden forward motion of the front. The reservoir volume of gas determines the available elasticity in the system, and thereby the size of the expansion. Large expansions take the shape of bubbles, and in this paper we study the accumulation of granular material around the inflating gas bubble, and the rheological response of the system during this short expansion phase.

International audience; Fluid flow in transforming porous rocks, fracture networks, and granular media is subject to considerable current interdisciplinary research activity in Physics, Earth Sciences, and Engineering. Examples of natural and engineered processes include hydrocarbon recovery, carbon dioxide geo-sequestration, soil drying and wetting, pollution remediation, soil liquefaction, landslides, dynamics of wet or dry granular media, dynamics of faulting or friction, volcanic eruptions, gas venting in sediments, karst development and speleogenesis, ore deposit development, and radioactive waste disposal. Hydrodynamic flow instabilities and pore scale disorder typically result in complex flow patterning. In transforming media, additional mechanisms come into play: compaction, de-compaction, erosion, segregation, and fracturing lead to changes in permeability over time. Dissolution, precipitation, and chemical reactions between solutes and solids may gradually alter the composition and structure of the solid matrix, either creating or destroying permeable paths for fluid flow. A complex, dynamic feedback thus arises where, on the one hand, the fluid flow affects the characteristics of the porous medium, and on the other hand the changing medium influences the fluid flow. This Research Topic Ebook presents current research illustrating the depth and breadth of ongoing work in the field of flow and transformation in porous media through 15 papers by 72 authors from around the world. The body of work highlights the challenges posed by the vast range of length-and timescales over which subsurface flow processes occur. Importantly, phenomena from each scale contribute to the larger-scale behavior. The flow of oil and gas in reservoirs, and the flow of groundwater on catchment scale is sensitively linked to pore scale processes and material heterogeneity down to the micrometer scale. The geological features of the same reservoirs and catchments evolved over millions of years, sometimes as a consequence of cracking and fracture growth occurring on the time scale of microseconds. The research presented by the authors of this Research Topic represents a step toward bridging the separation of scales as well as the separation of scientific disciplines so that a more unified picture of flow and transformation in porous media can start to emerge.

Hensikten med denne masterutredningen er å undersøke hvorvidt SAS benytter fordelsprogrammet Eurobonus aktivt i forhold til konkurranseforholdene, forretnings- og fritidssegmentet og innlands- og utenlandsmarkedet. For å avdekke dette tar vi utgangspunkt hvor mye et bonuspoeng koster for medlemmene av Eurobonus for et utvalg av flyruter både innenlands og utenlands. Temaet er spennende med tanke på litteraturen som foreligger om bonusprogrammenes innlåsningseffekt av passasjerene. Vi ønsker derfor å undersøke om SAS tar nytte av bonusprogrammets effekter. Oppgavens tema belyses ved å ta utgangspunkt i tidligere litteratur og i tillegg gjennomføre en empirisk analyse av innlands- og utenlandsmarkedet. Vi starter med en introduksjon av SAS og Norwegian ved å se på forretningsstrategier, nettverksstrukturer, strategiske allianser og oppkjøp. Deretter ser vi på passasjerutvikling, prisutvikling og markedsandeler for både innlands- og utenlandsmarkedet. Videre vil vi se på hvordan bonusprogrammer kan påvirke konkurranseforholdene i et marked. I denne delen vil vi presentere det vesentligste av litteratur som foreligger på området, og omfatter innlåsningseffekten, etableringshindringer, kapasitetsnedleggelse og prinsipal-agent problematikken i forhold til forretningssegmentet. Essensen her er at bonusprogrammenes tilstedeværelse fører til at medlemmenes byttekostnader øker, noe som gir en innlåsningseffekt av passasjerene. Dette fører dermed til en konkurransedempende effekt i markedet grunnet økt lojalitetsgrad til selskapene med bonusprogrammer. Vi viser deretter til mekanismene som lå til grunn for bonusforbudet på flyrutene innenlands, og opphevelsen av denne. Til slutt i denne delen presenterer vi bonusprogrammene til SAS og Norwegian. Resultatene fra den empiriske analysen viser at større markedsandeler for SAS har en positiv effekt på bonusprisen. I tillegg viser det seg at bonusprisen og den relative prisforskjellen mellom SAS og Norwegian korrelerer negativt med hverandre. Dette betyr at jo større prisavstanden er mellom aktørene gitt at Norwegian tilbyr billigste billett, jo høyere er bonusprisen. Konkurransen i forhold til avganger og antall konkurrenter virker ikke til å ha en påvirkning på bonusprisen. Videre finner vi ingen støtte for at SAS benytter avreisetidspunkt og reiseformål for å sette bonusprisen, men finner det sannsynlig at de benytter billettypene for å skille mellom forretnings- og fritidsreisende. Vi finner også støtte for at bonusprisen er satt systematisk lavere for utenlandsreiser enn innenlandsreiser. nhhmas

International audience; Experiments on confined two-phase flow systems, involving air and a dense suspension, have revealed a diverse set of flow morphologies. As the air displaces the suspension, the beads that make up the suspension can accumulate along the interface. The dynamics can generate " frictional fingers " of air coated by densely packed grains. We present here a simplified model for the dynamics together with a new numerical strategy for simulating the frictional finger behavior. The model is based on the yield stress criterion of the interface. The discretization scheme allows for simulating a larger range of structures than previous approaches. We further make theoretical predictions for the characteristic width associated with the frictional fingers, based on the yield stress criterion, and compare these to experimental results. The agreement between theory and experiments validates our model and allows us to estimate the unknown parameter in the yield stress criterion, which we use in the simulations.

Understanding how gases escape from particle-rich suspensions has important applications in nature and industry. Motivated by applications such as outgassing of crystal-rich magmas, we map gas migration patterns in experiments where we vary (1) particle fractions and liquid viscosity (10 Pa s – 500 Pa s), (2) container shape (horizontal parallel plates and upright cylinders), and (3) methods of bubble generation (single bubble injections, and multiple bubble generation with chemical reactions). We identify two successive changes in gas migration behavior that are determined by the normalized particle fraction (relative to random close packing), and are insensitive to liquid viscosity, bubble growth rate or container shape within the explored ranges. The first occurs at the random loose packing, when gas bubbles begin to deform; the second occurs near the random close packing, and is characterized by gas migration in a fracture-like manner. We suggest that changes in gas migration behavior are caused by dilation of the granular network, which locally resists bubble growth. The resulting bubble deformation increases the likelihood of bubble coalescence, and promotes the development of permeable pathways at low porosities. This behavior may explain the efficient loss of volatiles from viscous slurries such as crystal-rich magmas.

International audience; Submerged granular material exhibits a wide range of behavior when the saturating fluid is slowly displaced by a gas phase. In confined systems, the moving interface between the invading gas and the fluid/grain mixture can cause beads to jam, and induce intermittency in the dynamics. Here, we study the stability of layers of saturated jammed beads around stuck air bubbles, and the deformation mechanism leading to air channel formations in these layers. We describe a two-dimensional extension of a previous model of the effective stress in the jammed packing. The effect of the tangential stress component on the yield stress is discussed, in particular how arching effects may impact the yield threshold. We further develop a linear stability analysis, to study undulations which develop under certain experimental conditions at the air-liquid interface. The linear analysis gives estimates for the most unstable wavelengths for the initial growth of the perturbations. The estimates correspond well with peak to peak length measurements of the experimentally observed undulations.

Thermal energy can be stored in supercooled liquids where the material is in thermal equilibrium with its surroundings. The stored latent heat of fusion is released by triggering the crystallization of the supercooled substance. In this study, enthalpy–temperature curves including the effect of supercooling are measured for some well known supercooling salt hydrates (disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate and STL-47). A series of properties relevant for supercooling energy storage applications are identified, including the optimal working temperature range for the materials, temperature increase during crystallization as a function of degree of supercooling, available enthalpy at different temperatures, and fraction of enthalpy lost in the initial supercooling phase. The enthalpy–temperature curves are measured by a simple and inexpensive method.

We performed synchrotron X-ray scattering studies of the dynamics of the water intercalation front in a Na–Fluorohectorite clay. Like other smectite clays, fluorohectorite particles can swell due to intercalation of successive water layers. Monitoring the intensities of Bragg peaks of the known 1- and 2-water-layer hydration states at different positions in the sample enabled spatial and temporal measurement of the proportions of the different hydration states. From experiments with controlled temperature and an imposed humidity gradient on a quasi one-dimensional powder sample, we were able to localize the intercalation front and demonstrate that the width of this front was smaller than 2 mm after penetrating 9 mm into the sample. The speed at which the intercalation front advanced through the sample during the diffusion process was shown to decrease with time. The diffraction signature of random water intercalation in the vicinity of the intercalation front also provided information on the changes in the water content of the mesopores around clay particles.

A calorimetric method is applied to determine the solar system performance by in-situ measured data. The heat store is interpreted as a calorimeter and the information on charge and discharge is extracted from the shape analysis of the heat store’s temperature profile. The method is applicable to any heating system which includes a heat store. In the present paper, this procedure is presented and applied to determine the solar gain for a large solar system for domestic hot water preparation and a solar combisystem. The uncertainty of this method is in the range of ±10%. The results obtained by the calorimetric method are compared to TRNSYS simulations.

A polymer solar heat collector was combined with single-crystal silicon PV cells in a hybrid energy-generating unit that simultaneously produced low temperature heat and electricity. The PV/T unit was tested experimentally to determine its thermal and photovoltaic performance, in addition to the interaction mechanisms between the PV and thermal energy systems. Thermal efficiency measurements for different collector configurations are compared, and PV performance and temperature readings are presented and discussed. An analytical model for the PV/T system simulated the temperature development and the performance of both the thermal and photovoltaic units.

The influence of faunal activity and particle type on sediment-mixing processes of the particle-bound organic contaminant di(2-ethylhexyl) phthalate (DEHP) were examined over a 48 d experimental period with natural box-cosm sediments. A dual-labelling radiotracer technique using 14 C-DEHP and 51 Cr determined the fate and utilization of DEHP associated with either sediment or phytoplankton particles. Particle-mixing was estimated using a 1-dimensional transient-state biodiffusion model and expressed as Db coefficients. Sediment 51 Cr depth profiles fitted this simple biodiffusion model well, indicating randomized, vertical particle mixing. Particle reworking rates were approximately 9-fold higher in faunated treatments compared to defaunated control sediments, thus quantifying the importance of benthic fauna as agents of physical transport. Biodiffusion rates in faunated box-cosms were significantly higher in phytoplankton compared to sediment 14 C-DEHP particle treatments (p < 0.05). This was attributed to an experimental urchin effect resulting from natural variations in the density of the large macrofaunal urchin Echinocardium cordatum between particle treatments. Sediment-mixing rates were independent of particle type when this urchin effect was removed. Examination of 14 C and 51 Cr relative depth profiles showed the 2 tracers to be coupled in defaunated sediments and decoupled with animals present. This effect was also independent of particle-type, indicating no selective transport associated with the particle-type treatments. The decoupling of tracers in faunated treatments, with 14C-DEHP having consistently higher depth-weighted mean values than the 51 Cr tracer, suggested a decrease in surface 14 C-DEHP concentrations through degradation processes at the sediment-water interface. Sediment-mixing rates increased significantly with increasing total community biomass, indicating that size (biomass) may be the single most important community parameter determining sediment-mixing intensity. Downward particle transport was strongly correlated with E cordatum abundance (p < 0.01), the dominant species in the benthic community in terms of size. E. cordatum densities also correlated strongly with the number of benthic infaunal species present in the box-cosms, with maximum infaunal species numbers occurring at intermediate E. cordata densities. This is in accordance with theories on intermediate disturbance. The 2 14 C-DEHP particle-type treatments had no significant effect on E. cordata body sizes or DEHP body burden. The large urchin E. cordatum clearly dominated sediment-mixing, and did so in a 'non-selective' manner.

The key objective of this study was to compare tracer-based methods that measure bioturbation and irrigation rates with predictive models based on benthic community parameters such as animal size and abundance. The individual parameters of each model were investigated separately to assess their relative influence on model output. An important aspect of this study was its focus on benthic processses in natural marine sediments, where the well-defined model assumptions are not necessarily accomodated. Faunal parameters were incorporated into a biodiffusion model proposed to describe bioturbation, and direct comparisons were made to tracer-based depth distributions of an inert particle tracer, 51 Cr. The results showed that this biodiffusion model under-estimated bioturbation rates (Db) compared to 51 Cr tracer measurements. It was shown that the model predictions were highly sensitive to estimates of faunal weight and sediment reworking depth. The model suggests that large, deposit-feeding animals dominate biogenous mixing and that mixing rates are more influenced by animal size than density. In the box-cosm system, the large urchins Echinocardium cordatum did in fact dominate sediment-mixing processes. Irrigation was modelled by a combination of the non-local transport model and the radial-diffusion model, using animal size and density, and was compared to down-core distributions of a dissolved, conservative tracer bromide (Br - ). Modelled irrigation rates increased as a function of burrow radius and animal density, with the strongest dependency on animal density. Given the simplicity of the assumptions for the idealized radial-diffusion model, agreement between measured and predicted rates was relatively good in 50% of the box-cosms. Over-estimation in the remainder may be attributable to unrealistic assumptions that all individuals inhabit vertical burrows and irrigate them continuously.

An experimental observation of the structuring of a granular suspension under the progress of a gas/liquid meniscus in a narrow tube is reported here. The granular material is moved and compactifies as a growing accumulation front. The frictional interaction with the confining walls increases until the pore capillary entry pressure is reached. The gas then penetrates the clogged granular packing and a further accumulation front is formed at the far side of the plug. This cyclic process continues until the gas/liquid interface reaches the tube’s outlet, leaving a trail of plugs in the tube. Such 1D pattern formation belongs to a larger family of patterning dynamics observed in 2D Hele-Shaw geometry. The cylindrical geometry considered here provides an ideal case for a theoretical modelling for forced granular matter oscillating between a long frictional phase and a sudden viscous fluidization.

This paper addresses the scar tissue maturation process that occurs stepwise, and calls for reliable classification. The structure of collagen imaged by nonlinear optical microscopy (NLOM) in post-burn hypertrophic and mature scar, as well as in normal skin, appeared to distinguish these maturation steps. However, it was a discrimination analysis, demonstrated here, that automated and quantified the scar tissue maturation process. The achieved scar classification accuracy was as high as 96%. The combination of NLOM and discrimination analysis is believed to be instrumental in gaining insight into the scar formation, for express diagnosis of scar and surgery planning.

Experiments on granular-fluid systems in confined geometries have been shown to produce labyrinth patterns. This is the case when the fluid from a particle/fluid dispersion in a Hele-Shaw cell is slowly withdrawn, resembling a drying process. Based on a model of capillary and friction forces, we present a simulator that reproduces the experimental behaviour.

When a droplet of silica particles suspended in water dries out on a horizontal surface, one can observe an emerging pattern of interconnected stripes of sedimented mass. From experimental observations and numerical simulations we find that the mechanism for the pattern formation is a combination of meniscus shape perturbations caused by pinned particles and a reorganization of the evaporation driven flow.

We present a new pattern formation process where labyrinthine structures emerge during slow drainage of a confined granular-fluid mixture. Capillary and frictional forces govern the process, and the resulting pattern has a characteristic wavelength that is a function of both the initial volume fraction of granular material in the mixture, and also the system thickness.

Flexing a metallic disk triggers the crystallization of the supercooled sodium acetate solution contained in commercial heat pads. Many mechanisms have been proposed to explain the apparent nucleation of crystalline material. In this paper a simple experiment is described that demonstrates that nucleation is triggered by preserving seed crystals clamped between opposing metal surfaces. An explanation for the retention of the crystalline particles is the elevated melting point caused by very high local pressures. A series of thermophysical properties of the sodium acetate solution is also measured, including the available enthalpy upon crystallization of the supercooled substance, and liquid and solid phase specific heat capacities.

Pattern forming processes are abundant in nature. Here, we report on a particular pattern forming process. Upon withdrawal of fluid from a particle-fluid dispersion in a Hele-Shaw cell, the particles are shown to be left behind in intriguing mazelike patterns. The particles, initially being uniformly spread out in a disc, are slowly pulled inwards and together by capillary and pressure forces. Invading air forms branching fingers, whereas the particles are compiled into comparably narrow branches. These branches are connected in a treelike structure, taking the form of a maze. The characteristic length scale within the structure is found to decrease with the volume fraction of the particles and increase with the plate separation in the Hele-Shaw cell. We present a simulator designed to simulate this phenomenon, which reproduces qualitatively and quantitatively the experiments, as well as a theory that can predict the observed wavelengths.

A random, labyrinthine pattern emerges during slow drainage of a granular-fluid system in two-dimensional confinement. Compacted grains are pushed ahead of the fluid-air interface, which becomes unstable due to a competition between capillary forces and the frictional stress mobilized by grain-grain contact networks. We reproduce the pattern formation process in numerical simulations and present an analytical treatment that predicts the characteristic length scale of the labyrinth structure. The pattern length scale decreases with increasing volume fraction of grains in the system and increases with the system thickness.

We introduce a retroemitter (REM) device comprising a planar glass bead set placed on a luminescent material substrate, which converges an excitation beam into a set of foci (voxels). The in-voxel emission is collimated by the beads, and propagates upstream over the long range, unlike the out-of-voxel emission spreading in all angles. The REM signal contrast is characterized as a function of incidence and observation angles and propagation distance. REM signal contrasts of approximately 20 and 1600 were found for the organic fluorescent dye and upconverting phosphor substrates, respectively. In the latter case, nonlinear optical signal enhancement plays a role in addition to the retroemission effect. This allows centimeter-scale REM patterns to be read out at the meter-scale distance using eye-safe sub-mW/cm(2) excitation intensities.

Pattern-forming processes in simple fluids and suspensions have been studied extensively, and the basic displacement structures, similar to viscous fingers and fractals in capillary dominated flows, have been identified. However, the fundamental displacement morphologies in frictional fluids and granular mixtures have not been mapped out. Here we consider Coulomb friction and compressibility in the fluid dynamics, and discover surprising responses including highly intermittent flow and a transition to quasi-continuodynamics. Moreover, by varying the injection rate over several orders of magnitude, we characterize new dynamic modes ranging from stick-slip bubbles at low rate to destabilized viscous fingers at high rate. We classify the fluid dynamics into frictional and viscous regimes, and present a unified description of emerging morphologies in granular mixtures in the form of extended phase diagrams., Pattern-forming processes in simple fluids and suspensions are well understood, but displacement morphologies in frictional fluids and granular mixtures have not been studied extensively. Sandnes et al. consider the effects of Coulomb friction and compressibility on the fluid dynamics of granular mixtures.

The effects of non-ablative infrared (IR) laser treatment of collagenous tissue have been commonly interpreted in terms of collagen denaturation spread over the laser-heated tissue area. In this work, the existing model is refined to account for the recently reported laser-treated tissue heterogeneity and complex collagen degradation pattern using comprehensive optical imaging and calorimetry toolkits. Patella ligament (PL) provided a simple model of type I collagen tissue containing its full structural content from triple-helix molecules to gross architecture. PL ex vivo was subjected to IR laser treatments (laser spot, 1.6 mm) of equal dose, where the tissue temperature reached the collagen denaturation temperature of 60 ± 2°C at the laser spot epicenterin the first regime, and was limited to 67 ± 2°C in the second regime. The collagen network was analyzed versus distance from the epicenter. Experimental characterization of the collagenous tissue at all structural levels included cross-polarization optical coherence tomography, nonlinear optical microscopy, light microscopy/histology, and differential scanning calorimetry. Regressive rearrangement of the PL collagen network was found to spread well outside the laser spot epicenter (>2 mm) and was accompanied by multilevel hierarchical reorganization of collagen. Four zones of distinct optical and morphological properties were identified, all elliptical in shape, and elongated in the direction perpendicular to the PL long axis. Although the collagen transformation into a random-coil molecular structure was occasionally observed, it was mechanical integrity of the supramolecular structures that was primarily compromised. We found that the structural rearrangement of the collagen network related primarily to the heat-induced thermo-mechanical effects rather than molecular unfolding. The current body of evidence supports the notion that the supramolecular collagen structure suffered degradation of various degrees, which gave rise to the observed zonal character of the laser-treated lesion.