Rakenteellisten kompleksisten nesteiden reologia

Foams are present in many applications of modern-day life, however, one seldom stops to admire their beautiful structure and fascinating dynamics. These properties make foams a subject of extensive scientific research ranging from elementary dynamics of amorphous materials to applications in industry and consumer products. The present article dissertation addresses the deformation, yielding and flow of foams using experimental methods. The emphasis is on predicting the deformation and yielding, and modifying the flow properties of foam with varying experimental parameters. The four articles reveal the importance of the local configuration and dynamics in defining the macroscopic flow response and how tuning the local interactions alters the mechanical deformation of foams.         Publications I and II adapt view from amorphous solids where the foam deforms plastically in local topological transformations and these events define the macroscopic dynamics. The center of this view is the ability to predict these events from the past configuration of the foam. Both of the publications apply machine learning tools to large data sets describing foam motion and classify locations with high propensity to yield. Publication I reveals that the vertex, transforming in the yield event, results in the best predictions of the yielding. In addition, Publication I characterizes the essential time scale for predictions to be around 1 second for the used system. Publication II studies the essential features of the yielding vertex in more detail. The analysis reveals the essential predictors for yield events which are the angles of films joining in the vertex and the lengths of the films joining in the vertex.         Publication III studies the effect of external mechanical vibration on the flow of foam. An external pressure drives foam flow in an experimental cell which is vibrated. The results showed that the vibration enables flow on small driving pressure below yielding and enhances the flow velocity on small pressures that exceed the yielding. Publication III provides evidence that the vibration increases the probability of yield events which then decreases the effective viscosity.         Publication IV modifies the flow properties of foam with fibers in the continuous medium. The fibers decrease the flow velocity of foam. Although, the fibers form a viscoelastic fluid, the fibers in the continuos medium do not enhance the elastic response of the foam indicating that the origin of the elasticity of the foam remains in the bubble matrix. Giesekus model describes well the foam flow, and the addition of the fibers may be incorporated to the model with increasing the ratio of viscous contribution to the elastic contribution.Vaahdot ovat monille tuttuja materiaaleja erilaisista jokapäiväisistä sovelluksista. Silti niiden kiehtovaan rakenteeseen ja ominaisuuksiin kiinnitetään harvemmin huomiota. Vaahdon ominaisuudet tekevät niistä kuitenkin erittäin kiinnostavia tutkimuskohteita lähtien kuplatason perustutkimuksesta aina teollisiin ja kaupallisiin sovelluksiin asti. Tämä neljään artikkeliin perustuva väitöskirja tarkastelee kokeellisesti vaahtojen muodonmuutosta, myötämistä ja virtausta. Rajatummat kiinnostuksen kohteet ovat vaahtojen paikallinen myötäminen sekä virtaus käyttäytymisen muokkaus muuttamalla erilaisia koesysteemin parametreja. Tämä työ demonstroi kuinka paikallinen rakenne ja sen muutokset määrittävät vaahdon virtausta makroskooppisella tasolla.         Artikkelit I ja II tarkastelevat vaahtoja samoista lähtökohdista, joista amorfisten aineiden muodon muutosta yleensä tutkitaan. Tässä tarkastelussa keskeisiä ovat paikalliset järjestyksen rikkovat muutokset rakenteessa, jotka määrittävät materiaalin mekaanisia ominaisuuksia. Molemmissa artikkeleissa hyödynnetään koneoppimista paikallisten muutosten ennustamiseen rakenteen perusteella. Artikkeli I osoittaa, että neste kalvojen solmukohta, jonka ympärillä kuplat uudelleen järjestyvät paikallisessa muutoksessa, tarjoaa parhaimpiin tuloksiin johtavan ennusteen. Tässä artikkelissa myös ennusteiden luotettavuuden todetaan säilyvän noin 1 sekunnin pituisella ennustus horisontilla. Seuraavaksi artikkeli II tarkasteli rakenteellisia ominaisuuksia, jotka ennustavat parhaiten solmukohdan myötämistä. Tärkeimmät ominaisuudet ennusteelle ovat kalvojen väliset kulmat solmukohdassa ja näiden kalvojen pituudet.         Artikkeli III tutkii, miten mekaaninen värähtely vaikuttaa vaahdon virtaukseen. Näissä kokeessa vaahtoa pusketaan kaksiulotteiseen koekammioon ulkoisella paineella samalla, kun kammiota värisytetään matala taajuisilla ääniaalloilla. Tutkimuksessa havaittiin, että vaahto saadaan virtaamaan värinän ansiosta myös pienillä paineilla, jotka muuten eivät riittäisi ylittämään vaahdon myötörajaa. Samaten pystyttiin toteamaan värinän nopeuttavan virtausta pienillä nopeuksilla. Tutkimus indikoi värinän lisäävään paikallisten myötötapahtumien todennäköisyyttä ja siten laskevan vaahdon efektiivistä viskositeettia.         Artikkeli IV tutkii sellukuitujen vaikutusta vaahdon virtaukseen. Kuidut hidastavat virtausta. Vaikka kuitususpensiot itsessään ovat elastisia nesteitä, lisättynä vaahdon neste komponenttiin ne eivät voimista vaahdon elastista vastetta. Tämä osoittaa vaahtojen elastisuuden johtuvan pääasiassa neste-kaasu kalvoista ja niiden pintajännitteestä. Giesekus malliin perustuneet simulaatiot tarjoavat menetelmän sellukuidun vaikutuksen mallintamiseen vaahtojen virtauksessa. Kuitujen lisäyksen aiheuttamat ilmiöt pystyttiin toistamaan simulaatiossa kasvattamalla viskoosin ja elastisen komponentin suhdetta

Probing the local response of a two-dimensional liquid foam

Aqueous foams are viscoelastic yield stress fluids. Due to their complex rheology, foam flow around an obstacle embedded in a 2D Hele-Shaw cell has been widely studied. Typically, in such geometry in the moving flow reference frame the flow field of viscoelastic fluids exhibit a quadrupolar structure characterized by a negative wake. Here, we introduce a measuring geometry, new in this context, whereby instead of flowing the foam around the obstacle, we move the obstacle as an intruder inside the foam. The proposed setup makes it possible to independently control the driving velocity and the liquid foam properties, such as the gas fraction and polydispersity. We show that the liquid foam velocity field around the intruder is similar to the one observed in viscoelastic fluids, in particular the emergence of a negative wake, e.g. a velocity overshoot downstream side of the obstacle. However, surprisingly, the intensity of this velocity overshoot decreases with the number of intruder passes, probably related to the evolution of the local disordered structure of the liquid foam.Peer reviewe

Measuring biofoam anisotropy using optical coherence tomography

Optical coherence tomography enables quick scans of translucent objects in a simple environment. Here, we apply this technique to wood-based biofoam. We measure the geometrical properties of the foam, such as bubble eccentricity and density fluctuations, in addition to characterising the possible orientation of fibres. We find that the wood-based foams are extremely suitable for optical coherence tomography due to their translucent nature and large changes of optical density between air-filled bubbles and solid films. Measurement of bubble eccentricity revealed a reasonably high aspect ratio of 1:2, enabling the orientation of long cellulose fibres if added to the mixture. The results demonstrate an effective method to characterise foamlike metamaterials. Furthermore, focusing on eccentricity enables the adjustment of the foam’s manufacturing method and, in turn, helps to produce anisotropic structures.Peer reviewe

Machine learning and predicting the time dependent dynamics of local yielding in dry foams

The yielding of dry foams is enabled by small elementary yield events on the bubble scale, “T1”s. We study the large-scale detection of these in an expanding two-dimensional (2D) flow geometry using artificial intelligence (AI) and nearest neighbor analysis. A good level of accuracy is reached by the AI approach using only a single frame, with the maximum score for vertex centered images highlighting the important role the vertices play in the local yielding of foams. We study the predictability of T1s ahead of time and show that this is possible on a timescale related to the waiting time statistics of T1s in local neighborhoods. The local T1 event predictability development is asymmetric in time, and measures the variation of the local property to yielding and similarly the existence of a relaxation timescale after local yielding.Peer reviewe

Thermal gelation of cellulose based suspensions

Funding Information: Open Access funding provided by Aalto University. J. K. acknowledges funding from Academy of Finland (308235). M. J. A., A. P. and J. K. acknowledge support from FinnCERES flagship (151830423), Business Finland (211835) and Future Makers (Älyä Vaahtoihin) programs. Publisher Copyright: © 2023, The Author(s).A more sustainable future calls for bio-based alternatives to replace plastic foams for various applications, such as packaging, insulation and cushioning. Some bio-based foams emerging in scientific publications are fabricated using liquid foam templating and methyl cellulose as well as fibers as main constituents. Scaling up of the production, however, requires a comprehensive understanding of the rheology of the foam during the shaping and drying processes. In this article, we report rheological studies of cellulose based systems in the context of thermal gelation. In more precise terms, we study how the presence of cellulose fibers and other additive materials influences the thermal gelation properties of methyl cellulose. We observe that the rheological properties, while heavily dependent on the material composition, are reasonably adjustable by appropriate material choices. The fibers are seen to decrease the temperature required for methyl cellulose to undergo a viscoelastic transition which is useful in the solid foam fabrication process. We anticipate that in the present application, the fibers increase the stability of the desired structure during the drying stage of the foam.Peer reviewe

Vibration controlled foam yielding

In rheological terms, foams are time independent yield stress fluids, displaying properties of both solid and liquid materials. Here we measure the propagation of a 2D dry foam in a radially symmetric Hele-Shaw cell forcing local yielding. The yield rate is manipulated by mechanical vibration with frequencies from 0 to 150 Hz. The flow speed is then extracted from the video stream and analyzed using digital image correlation software. The data are modeled analytically by a Guzman–Arrhenius type of energy landscape where the local yielding of foam correlates with the number of oscillations, i.e. attempts to cross the energy barrier. The model is confirmed in an auxiliary experiment where the vibrated foam stays in its flowing state at the same small driving pressures, where the flow of the unvibrated foam ceases. We conclude that the yield stress behaviour of foams under an external perturbation can be summarized using a simple energy landscape model. The vibration affects the films causing the stress to occasionally and locally exceed the yield threshold. This, thus, prevents the foam from jamming as in a static configuration even when the global driving is below the yield point of the foam.Peer reviewe

Constriction Flow of Cellulose Laden Air-Aqueous Foam

Foams are encountered in everyday life across wide applications, e.g., in foods and cleaning products. They have also been widely used in different industries in processes such as flotation and oil recovery. The application of bio-based materials is a novel interest, and foam forming enables these materials to be used more flexibly. For efficient industrial usage, the flow of such materials must be well understood and characterized. This work measured the velocity field of nanocellulose laden foam in a two-dimensional Hele-Shaw cell with a constriction, using optical imaging and particle image velocimetry. The measurements showed that the addition of cellulose increased the effective viscosity of the liquid films. In a numerical simulation using the Giesekus polymer model, the experimental trend was reproduced through increasing the fluid's viscosity. Adding highly viscoelastic nanofibrillated cellulose suspension to foam affected only the viscous component of the foam. The delayed elastic response did not change.Peer reviewe

Predicting effect of fibers on thermal gelation of methylcellulose using Bayesian optimization

Understanding of the viscoelastic behavior of a polymer is a prerequisite for its thermomechanical processing beyond laboratory scale. Utilizing rheological characterization is a powerful tool to comprehend the complex nature and time-dependent properties of macromolecular materials. Nevertheless, it consumes time as rheometry involves iterating experiments under several conditions to visualize the non-linear behavior of materials under varying conditions. The work hereunder examines the rheology of cellulosic aqueous suspensions prepared using cellulose fibers as the dispersed phase (Refcell and Storacell) and methylcellulose (MC) as the polymeric matrix. Interfacial phenomena between MC and cellulose fibers arise in particle laden systems with supramolecular structures formed by non-covalent interactions. Therefore, this study elucidates the rheological evolution of these interactions as a function of temperature and fiber concentration. This study displays how researchers may reduce the number of rheological experiments and save time utilizing a novel method based on a Bayesian optimization with Gaussian processes.Peer reviewe

Local time-dependent microstructure of aging TEMPO nanofibrillated cellulose gel

The aging behavior of TEMPO-CNF suspensions is investigated using rheological measurements and experiments involving the free-falling solid sphere. The properties of CNF suspensions, which are formed of water and cellulose fibers with a large aspect ratio, are fundamentally different from those commonly known as model low-density gels. We characterize their aging utilizing SAOS rheometry following a period of shear rejuvenation, measuring the effect of aging time on the observed stress overshoot, and additionally measure the rheological hysteresis under continuous shear. In addition to such tests, which probe the sample at the bulk level, we study their local aging behavior via the Stokes' experiment. During SAOS, the aging of the material results in a logarithmic growth of the elastic modulus over the first 50 minutes. In the Stokes' experiment, depending on the sphere size, we find a systematic decrease in the sphere velocity with aging times up to 16 days and identify a range of intermittent particle motion. Based on our experimental evidence, we propose that the aging effect within the TEMPO-CNFs occurs due to the restructuring of fibrous elements by Brownian diffusion and that the aging of the system does not develop homogeneously across the whole sample as a consequence of the wide size distribution of CNFs. Finally, we note that this may be one of the primary reasons why the rheological data on even the same batch of CNFs show a large scatter.Peer reviewe