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Ronald G Larson - One of the best experts on this subject based on the ideXlab platform.
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Stress Gradient induced polymer migration in taylor couette flow
Soft Matter, 2017Co-Authors: Elnaz Hajizadeh, Ronald G LarsonAbstract:We apply our recent continuum theory for Stress-Gradient-induced migration of polymers in solution (G. Zhu et al., J. Rheol., 2016, 60, 327-343) to rotational shearing flow in the gap between concentric cylinders (the so-called Taylor-Couette flow), where we have also accounted for the effect of polymer depletion from the solid boundaries on migration patterns. The steady-state distribution of dilute solutions of polymer dumbbells, obtained both using a systematic perturbation analysis in terms of the Weissenberg number (Wi) and by solving numerically the transport problem coupled to the upper-convected Maxwell equation, shows accumulation of polymers near the inner cylinder. This accumulation becomes so strong that most polymers are driven near the inner cylinder once Wi > 4. We also show that there is no first-order contribution to the polymer migration in Taylor-Couette flow due to the absence of a velocity component in the r-direction. Brownian dynamics (BD) simulations for a Hookean dumbbell give a concentration distribution in good agreement with the theoretical predictions of our theory, confirming the accuracy of the theory when the dumbbell radius of gyration is around an order of magnitude or much smaller than the gap. The demonstration of the accuracy of our continuum theory by direct molecular simulation opens the door to application of the theory to journal bearing and other lubrication flows containing polymers that may migrate due to Stress Gradients.
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the effect of wall depletion and hydrodynamic interactions on Stress Gradient induced polymer migration
Soft Matter, 2016Co-Authors: Hossein Rezvantalab, Guorui Zhu, Ronald G LarsonAbstract:We generalize our recent continuum theory for the Stress-Gradient-induced migration of polymers [Zhu et al., J. Rheol., 2016, 60, 327-343] by incorporating the effect of solid boundaries on concentration variations. For a model flow in a channel with periodic slip wall velocity, which can in principle be produced by an electric field in the presence of a sinusoidal wall charge, we obtain theoretical results for the steady-state distribution of dilute solutions of polymer dumbbells using a systematic perturbation analysis in Weissenberg number Wi. We find that the presence of a thin wall depletion zone changes the lowest order solution from second to first in Wi and drastically affects the concentration field far from the depletion layer, due both to a coupling of the second derivative of the velocity field to the concentration Gradient, and to convection of the polymer-depleted fluid in this layer into the bulk of the fluid. Additional effects induced by wall hydrodynamic interaction (HI) are assessed by incorporating polymer flux from the wall-HI migration theory of Ma and Graham into our continuum theory. We establish the range of validity of our theory by comparing the theoretical results with Brownian dynamics (BD) simulations: excellent agreement is achieved for relatively small molecules, while the theory breaks down when the Gradient number Gd is greater than 0.5, where Gd is the ratio of polymer coil size to the length scale over which the velocity Gradient changes. The BD simulations are also extended to the case of long Hookean chains with numbers of springs per chain ranging from 1 to 32, where it is found that for fixed Gd and Wi, the results are nearly identical, showing that all important phenomena are captured by a simple dumbbell model, thus supporting the continuum theory which was derived for the case of dumbbells. In addition, the Stochastic Rotation Dynamics (SRD) method is employed to evaluate the role of HI on the migration pattern, producing effects consistent with the continuum theory incorporating the wall-migration flux. In general, we demonstrate that the polymer concentrates in drastically different regions of the channel depending on Gd and Wi.
Olivier Dangles - One of the best experts on this subject based on the ideXlab platform.
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testing the Stress Gradient hypothesis with aquatic detritivorous invertebrates insights for biodiversity ecosystem functioning research
Journal of Animal Ecology, 2012Co-Authors: Vincent Fugere, Patricio Andino, Rodrigo Espinosa, Fabien Anthelme, Dean Jacobsen, Olivier DanglesAbstract:Summary 1. The Stress-Gradient hypothesis (SGH) states that environmental Stress modulates species interactions, causing a shift from negative interactions to net positive interactions with increasing Stress. 2. Potentially, this modulation of species interactions could in turn influence biodiversity-ecosystem function (B-EF) relationships along Stress Gradients. Although the SGH has been extensively discussed in plant community ecology in the past two decades, it has received little attention from animal ecologists. 3. To explore whether the SGH could be applied to animal communities, we conducted a litter decomposition experiment with aquatic detritivorous invertebrates in which we manipulated litter quality and measured species interactions along this resource quality Gradient. Litter quality was manipulated by presenting detritivores with leaves of plant species varying in specific leaf area and decomposition rate in streams. 4. We found a switch from negative to neutral interactions with increasing resource quality Stress, in line with the SGH. However, by re-examining other published results with aquatic detritivores from the perspective of the SGH, we found that a diversity of patterns seem to characterize detritivore interactions along Stress Gradients. 5. Although the basic pattern proposed by the SGH may not apply to animal systems in general, we show that aquatic detritivore interactions do change along Stress Gradients, which underlines the importance of incorporating environmental Stressors more explicitly in B-EF research.
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unexpected mechanisms sustain the Stress Gradient hypothesis in a tropical alpine environment
Journal of Vegetation Science, 2012Co-Authors: Fabien Anthelme, Olivier Dangles, Belen Buendia, Charlotte MazoyerAbstract:AbstractQuestions: Does facilitation among plants increase with elevation in a humidtropical alpine system in which climatic and ecological conditions differ fromotheralpineenvironments?Whatmechanismsareinvolvedintheinteractions?Location: Volcano Antisana, Eastern Cordillera of the Ecuadorian Andes(00 °28 ′S,78 09W).Methods: We selected the cushion-forming Azorella aretioides as a potentialnurse plant along an altitudinal Gradient (4400, 4550 and 4700 m) in the hightropical Andes. We quantified its effects on other plants at species and commu-nity levels by comparing the product vegetation cover 9 number of individualsof every vascular species found inside and outside 265 cushions, using the rela-tive interaction index. We inferred potential mechanisms behind the interac-tions through analysis of microclimate, soil moisture and soil nutrientmeasurementsinsideandoutsidecushions.Results: Predictions of the Stress Gradient hypothesis (SGH) were corroboratedat community level, with transition from competitive or neutral effects of A.aretioides at 4400 and 4550 m to facilitative effects at 4700 m. Strong species-specific effects were observed along the altitudinal Gradient, with a substantialeffect of local habitat disturbance on the outcome of plant–plant interactions.Surprisingly, cushions lowered air and soil temperatures and air humidity,whichreducedathigherelevations.Facilitationappearedtobecausedbyhighersoilmoistureandnutrientcontentbeneathcushions.Conclusions:OurdataextendtheframeworkoftheSGHbycorroboratingitforthe first time in a tropical alpine system. However, the mechanisms underlyingplant–plantinteractionsdifferedfromthosegenerallyreportedfromalpineenvi-ronments,withfacilitationvaryingaccordingtoresource-mediatedStress(nutri-ents).Itremainstobetestedwhetherthisisspecifictotropicalalpinesystems.IntroductionSince the seminal paper of Bertness & Callaway (1994),the StressGradient hypothesis (SGH) has imposed itselfasamajorconceptinunderstandingtheeffectsofenvironmen-talStressonplantcommunitydistributioninawidevarietyofenvironments(Callaway2007;Brookeret al.2008;Lor-tie 2010). As recently proposed by Maestre et al. (2009), itis crucial to determine the relative importance of resource(e.g. nutrients, light, water) vs. non-resource Stress (e.g.temperature, wind, salinity) in order to achieve a betterunderstanding of the high variability in the outcome ofplant–plant interactions at the extremities of Stress gradi-ents (Michalet 2007; Brooker et al. 2008; Maestre et al.2009; le Roux & McGeoch 2010). This issue has been par-ticularly addressed in alpine environments where plant–plantinteractionshavebeensuggestedtobemainlydrivenby non-resource Stress (Maestre et al. 2009), therebyexplaining why facilitation processes are generally intenseattheendofStressGradientsintheseecosystems.Most(ifnotall)ofwhatweknowaboutSGHpredictionsinalpinesystemshasbeenlearnedfrommountainsintem-perate and sub-polar latitudes. A world map view of loca-tions of the many SGH studies performed so far in alpineenvironments shows that tropical region virtually remainsa ‘SGH terra incognita’ (Fig. 1; but see Cavieres & Badano
Castrenze Polizzotto - One of the best experts on this subject based on the ideXlab platform.
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a unifying variational framework for Stress Gradient and strain Gradient elasticity theories
European Journal of Mechanics A-solids, 2015Co-Authors: Castrenze PolizzottoAbstract:Abstract Stress Gradient elasticity and strain Gradient elasticity do constitute distinct continuum theories exhibiting mutual complementary features. This is probed by a few variational principles herein presented and discussed, which include: i) For Stress Gradient elasticity, a (novel) principle of minimum complementary energy and an (improved-form) principle of stationarity of the Hellinger–Reissner type; ii) For strain Gradient elasticity, a (known) principle of minimum total potential energy and a (novel) principle of stationarity of the Hu–Washizu type. Additionally, the higher order boundary conditions for Stress Gradient elasticity, previously derived by the author (Polizzotto, Int. J. Solids Struct. 51, 1809–1818, (2014)) in the form of higher order boundary compatibility equations, are here revisited and reinterpreted with the aid of a discrete model of the body's boundary layer. The reasons why the latter conditions need to be relaxed for beam and plate structural models are explained.
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Stress Gradient versus strain Gradient constitutive models within elasticity
International Journal of Solids and Structures, 2014Co-Authors: Castrenze PolizzottoAbstract:Abstract A Stress Gradient elasticity theory is developed which is based on the Eringen method to address nonlocal elasticity by means of differential equations. By suitable thermodynamics arguments (involving the free enthalpy instead of the free internal energy), the restrictions on the related constitutive equations are determined, which include the well-known Eringen Stress Gradient constitutive equations, as well as the associated (so far uncertain) boundary conditions. The proposed theory exhibits complementary characters with respect to the analogous strain Gradient elasticity theory. The associated boundary-value problem is shown to admit a unique solution characterized by a Hellinger–Reissner type variational principle. The main differences between the Eringen Stress Gradient model and the concomitant Aifantis strain Gradient model are pointed out. A rigorous formulation of the Stress Gradient Euler–Bernoulli beam is provided; the response of this beam model is discussed as for its sensitivity to the Stress Gradient effects and compared with the analogous strain Gradient beam model.
Guorui Zhu - One of the best experts on this subject based on the ideXlab platform.
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the effect of wall depletion and hydrodynamic interactions on Stress Gradient induced polymer migration
Soft Matter, 2016Co-Authors: Hossein Rezvantalab, Guorui Zhu, Ronald G LarsonAbstract:We generalize our recent continuum theory for the Stress-Gradient-induced migration of polymers [Zhu et al., J. Rheol., 2016, 60, 327-343] by incorporating the effect of solid boundaries on concentration variations. For a model flow in a channel with periodic slip wall velocity, which can in principle be produced by an electric field in the presence of a sinusoidal wall charge, we obtain theoretical results for the steady-state distribution of dilute solutions of polymer dumbbells using a systematic perturbation analysis in Weissenberg number Wi. We find that the presence of a thin wall depletion zone changes the lowest order solution from second to first in Wi and drastically affects the concentration field far from the depletion layer, due both to a coupling of the second derivative of the velocity field to the concentration Gradient, and to convection of the polymer-depleted fluid in this layer into the bulk of the fluid. Additional effects induced by wall hydrodynamic interaction (HI) are assessed by incorporating polymer flux from the wall-HI migration theory of Ma and Graham into our continuum theory. We establish the range of validity of our theory by comparing the theoretical results with Brownian dynamics (BD) simulations: excellent agreement is achieved for relatively small molecules, while the theory breaks down when the Gradient number Gd is greater than 0.5, where Gd is the ratio of polymer coil size to the length scale over which the velocity Gradient changes. The BD simulations are also extended to the case of long Hookean chains with numbers of springs per chain ranging from 1 to 32, where it is found that for fixed Gd and Wi, the results are nearly identical, showing that all important phenomena are captured by a simple dumbbell model, thus supporting the continuum theory which was derived for the case of dumbbells. In addition, the Stochastic Rotation Dynamics (SRD) method is employed to evaluate the role of HI on the migration pattern, producing effects consistent with the continuum theory incorporating the wall-migration flux. In general, we demonstrate that the polymer concentrates in drastically different regions of the channel depending on Gd and Wi.
Francisco I Pugnaire - One of the best experts on this subject based on the ideXlab platform.
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does the Stress Gradient hypothesis hold water disentangling spatial and temporal variation in plant effects on soil moisture in dryland systems
Functional Ecology, 2016Co-Authors: Bradley J Butterfield, Cristina Armas, John B Bradford, Ivan Prieto, Francisco I PugnaireAbstract:Summary The nature of the relationship between water limitation and facilitation has been one of the most contentious debates surrounding the Stress-Gradient hypothesis (SGH), which states that plant-plant interactions shift from competition to facilitation with increasing environmental Stress. We take a closer look at the potential role of soil moisture in mediating plant-plant interaction outcomes by assessing effects of climate and soil texture on plant modulation of soil moisture. Using an empirically-parameterized soil moisture model, we simulated soil moisture dynamics beneath shrubs and in un-vegetated coarse and fine soils for 1000 sites in the Western United States with <700 mm mean annual precipitation. This threshold reflects the transition from dryland (<600 mm precipitation) to mesic ecosystems. Positive effects of shrubs on shallow soil moisture (i.e. the difference between shrub and interspace soil moisture) decreased along the aridity Gradient when long-term average conditions were considered, contrary to expectations based on the SGH. Negative effects of shrubs on deeper soil moisture also increased with aridity. However, when extreme years were considered, positive effects of shrub on soil moisture were greatest at intermediate points along the spatial aridity Gradient, consistent with a hump-backed model of plant-plant interactions. When viewed through time within a site, shrub effects on shallow soil moisture were positively related to precipitation, with more complex relationships exhibited in deeper soils Taken together, the results of this simulation study suggest that plant effects on soil moisture are predictable based on relatively general relationships between precipitation inputs and differential evaporation and transpiration rates between plant and interspace microsites that are largely driven by temperature. In particular, this study highlights the importance of differentiating between temporal and spatial variation in weather and climate, respectively, in determining plant effects on available soil moisture. Rather than focusing on the somewhat coarse-scale predictions of the SGH, it may be more beneficial to explicitly incorporate plant effects on soil moisture into predictive models of plant-plant interaction outcomes in drylands.
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a field test of the Stress Gradient hypothesis along an aridity Gradient
Journal of Vegetation Science, 2011Co-Authors: Cristina Armas, Susana Rodriguezecheverria, Francisco I PugnaireAbstract:Aims: The Stress-Gradient hypothesis (SGH) predicts how plant interactions change along environmental Stress Gradients. We tested the SGH in an aridity Gradient, where support for the hypothesis and the specific shape of its response curve is controversial. Location: Almer´ia, C´ aceres and Coimbra, three sites in the Iberian Peninsula that encompass the most arid and wet habitats in the distribution range of a nurse shrub species - Retama sphaerocarpa L. (Boiss) - in Europe. Methods: We analysed the effect of Retama on its understorey plant commu- nity and measured the biomass and species richness beneath Retama and in gaps. We estimated the frequency (changes in species richness), importance and intensity of the Retama effects, and derived the severity-interaction relationship pattern, analysing how these interaction indices changed along this aridity Gradient. Results and conclusions: The intensity and frequency of facilitation by Retama increased monotonically with increasing environmental severity, and the importance tended to have a similar pattern, overall supporting the SGH. Our data did not support predictions from recent revisions of the SGH, which may not apply to whole plant communities like those studied here or when interactions are highly asymmetrical. Facilitation by Retama influenced com- munity composition and species richness to the point that a significant fraction of species found at the most arid end of the Gradient were only able to survive beneath the nurse shrub, whereas some of these species were able to thrive in gaps at more mesic sites, highlighting the ecological relevance of facilitation by nurse species in mediterranean environments, especially in the driest sites.
