Dynamic hysteresis in the rheology of complex fluids

Recently, rheological hysteresis has been studied systematically in a wide range of complex fluids combining global rheology and time-resolved velocimetry. In this paper we present an analysis of the roles of the three most fundamental mechanisms in simple-yield-stress fluids: structure dynamics, viscoelastic response, and spatial flow heterogeneities, i.e., time-dependent shear bands. Dynamical hysteresis simulations are done analogously to rheological ramp-up and -down experiments on a coupled model which incorporates viscoelasticity and time-dependent structure evolution. Based on experimental data, a coupling between hysteresis measured from the local velocity profiles and that measured from the global flow curve has been suggested. According to the present model, even if transient shear banding appears during the shear ramps, in typical narrow-gap devices, only a small part of the hysteretic response can be attributed to heterogeneous flow. This results in decoupling of the hysteresis measured from the local velocity profiles and the global flow curve, demonstrating that for an arbitrary time-dependent rheological response this proposed coupling can be very weak.Peer reviewe

Patterns, Entropy, and Predictability of Human Mobility and Life

Cellular phones are now offering an ubiquitous means for scientists to observe life: how people act, move and respond to external influences. They can be utilized as measurement devices of individual persons and for groups of people of the social context and the related interactions. The picture of human life that emerges shows complexity, which is manifested in such data in properties of the spatiotemporal tracks of individuals. We extract from smartphone-based data for a set of persons important locations such as “home”, “work” and so forth over fixed length time-slots covering the days in the data-set (see also [1], [2]). This set of typical places is heavy-tailed, a power-law distribution with an exponent close to −1.7. To analyze the regularities and stochastic features present, the days are classified for each person into regular, personal patterns. To this are superimposed fluctuations for each day. This randomness is measured by “life” entropy, computed both before and after finding the clustering so as to subtract the contribution of a number of patterns. The main issue that we then address is how predictable individuals are in their mobility. The patterns and entropy are reflected in the predictability of the mobility of the life both individually and on average. We explore the simple approaches to guess the location from the typical behavior, and of exploiting the transition probabilities with time from location or activity A to B. The patterns allow an enhanced predictability, at least up to a few hours into the future from the current location. Such fixed habits are most clearly visible in the working-day length.Peer reviewe

Transient shear banding in viscoelastic Maxwell fluids

The fluidization of complex fluids is studied in the context of a Maxwell viscoelastic structural fluid model and compared to the purely viscous case. Solving iteratively the structural models together with the NavierStokes equation for the circular Couette flow gives spatially and temporally resolved velocity fields closely resembling those found experimentally for viscoelastic carbopol gels. Namely, transient shear banding is found during the initial fluidization phase. Although both structural models show transient shear bands, the viscoelastic one captures the experimental observations in greater detail, showing, for instance, the elastic backward flows during the transient shear band initialization stage

Transient shear banding in time-dependent fluids

We study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the nonhomogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of transient shear banding in time-dependent fluids. Extensions to more complicated rheological systems are considered

Kompleksisten nesteiden spatiaalinen reologiamallinnus

Many complex fluids show yield stress behavior. However, the term yield stress has been subject of much controversy. The separation of yield stress fluids into thixotropic and simple ones resolves many of these issues. This division is mainly driven by experimental results and is suspect to active theoretical development. This thesis addresses yield stress fluids and associated phenomena through continuum modeling for fluids with time dependent structure evolution. In addition to homogeneous laminar shear modeling, the emergence of spatial effects in viscometric flow situations is addressed. Therefore the models are coupled to the creeping flow solution (1-D Stokes equation) of a concentric cylinder geometry, which enables comparisons with experimental observations. Further, the results from thixotropic yield stress fluids are applied to the analysis of rheology measurements of nanocellulose suspensions, which have peculiar rheological properties. In particular, shear rate sweeps are simulated utilizing a structural model for thixotropic yield stress fluids. The results indicate that spatial flow heterogeneities have to be taken into account. Additionally wall slip, which is known to play an important role in the flow of complex fluids is addressed through a simple model. The results in this thesis add to the understanding of nanocellulose suspensions and complex fluids in general.Vaikka monilla kompleksisilla nesteillä on tunnetusti myötöraja, kyseiseen käsitteeseen ja sen määrittelyyn on kuitenkin liittynyt paljon ristiriitoja. Jakamalla myötörajanesteet yksinkertaisiin sekä tiksotrooppisiin, monet käsitteelliset ongelmat poistuvat. Tämä jako on toistaiseksi pitkälti kokeellisten tulosten varassa ja siihen liittyvä teoria on aktiivisen kehityksen alla. Tämä väitöskirja käsittelee myötörajaa ja siihen liittyviä ilmiöitä jatkumotason malleilla sellaisille nesteille, joilla on ajassa kehittyvä rakenne. Homogeenisen, laminaarisen leikkausvirtauksen lisäksi käsitellään spatiaalisten ilmiöiden syntyä. Tätä varten mallit yhdistetään sylinteri-sylinteri geometrian yksiulotteisen Stokesin virtauksen ratkaisuun mahdollistaen samalla vertailun kokeellisten havaintojen kanssa. Tiksotrooppisten myötörajanesteiden tuloksia sovelletaan nanoselluloosasuspensioden reologian analysointiin. Näiden suspensioden kokeellisia leikkausvirtauspyyhkäisyjä simuloidaan tiksotrooppisella rakennemallilla. Tulosten perusteella virtauksen  geometrisiä epähomogenisuuksia on otettava huomioon. Lisäksi kokeellisesti tärkeäksi havaittua liusumisilmiöta (wall slip), käsitellään yksinkertaisen mallin avulla. Tämän väitöskirjan tulokset edistävät nanoselluloosasuspensioden sekä yleisesti kompleksisten nesteiden ymmärrystä

Tilapäisten leikkausvöiden mallintaminen Couette-geometriassa

Long-living transient shear banding in a simple yield stress fluid has recently been observed in Couette rheometer. Here a computational model which shows qualitatively similar behavior to the experimental findings is investigated in detail. The model is based on population balance equations which are frequently used to model colloids. An attempt is made to fit the experimental results and to reproduce the experimentally found relationship between the characteristic scalings between the shear rate and stress controlled fluidization times and the Herschel - Bulkley fit to the steady state flow curve. No exact fit could be obtained and the relationship could not be unambiguously reproduced with the model. At low shear rates deviations from the power-law scaling in the fluidization times were obtained. These are most likely due to the geometry induced stress heterogeneity and the stress plateau in the flow curve.Pitkäikäistä, tilapäistä leikkausjuovaisuutta (transient shear banding) on äskettäin kokeellisesti havaittu yksinkertaisissa myötörajanesteissä Couette reometrissä. Tässä työssä on yksityiskohtaisesti tutkittu laskennallista mallia, joka osoittaa laadullisesti samanlaista käyttäytymistä kokeellisten havaintojen kanssa. Malli pohjautuu populaatiotasapainoyhtälöihin (PBE), joita käytetään usein kolloidien mallinnuksessa. Työssä mallia yritetään sovittaa koetuloksiin ja toistaa kokeellisesti todettu suhde nesteytymisaikojen (fluidization time) skaalauseksponenttien ja virtauskäyrän Herschel - Bulkley sovituksen parametrien välillä. Tarkka sovitus ei ollut mahdollinen, eikä kokeellista riippuvuutta eksponenttien välillä voitu yksiselitteisesti todentaa mallilla. Alhaisilla leikkausnopeusarvoilla havaittiin poikkeama nesteytymisaikojen potenssilakiskaalautuvuudesta. Poikkeamat ovat peräisin todennäköisesti geometriasta johtuvasta stressin heterogeenisuudesta ja virtauskäyrän muodosta

Apparent wall slip in non-Brownian hard-sphere suspensions

We analyze apparent wall slip, the reduction of particle concentration near the wall, in hard-sphere suspensions at concentrations well below the jamming limit utilizing a continuum level diffusion model. The approach extends a constitutive equation proposed earlier with two additional potentials describing the effects of gravitation and wall-particle repulsion. We find that although both mechanisms are shear independent by nature, due to the shear-rate-dependent counter-balancing particle migration fluxes, the resulting net effect is non-linearly shear dependent, causing larger slip at small shear rates. In effect, this shows up in the classically measured flow curves as a mild shear thickening regime at the transition from small to intermediate shear rates

Start-up inertia as an origin for heterogeneous flow

For quite some time nonmonotonic flow curve was thought to be a requirement for shear banded flows in complex fluids. Thus, in simple yield stress fluids shear banding was considered to be absent. Recent spatially resolved rheological experiments have found simple yield stress fluids to exhibit shear banded flow profiles. One proposed mechanism for the initiation of such transient shear banding process has been a small stress heterogeneity rising from the experimental device geometry. Here, using computational fluid dynamics methods, we show that transient shear banding can be initialized even under homogeneous stress conditions by the fluid start-up inertia, and that such mechanism indeed is present in realistic experimental conditions.Peer reviewe