Browsing by Author "Koivisto, Juha, Dr., Aalto University, Finland"

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    Collective phenomena in deformation
    (Aalto University, 2020) Mäkinen, Tero; Laurson, Lasse, Assoc. Prof., Tampere University, Finland; Koivisto, Juha, Dr., Aalto University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex Systems and Materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics ,Finland
    In this work the collective phenomena in material deformation, here meaning the collective dynamics of crystal defects (dislocations) or cellulose fibers, are studied experimentally using various techniques, which include imaging techniques, such as laser speckle technique and Digital Image Correlation, and also Acoustic Emission monitoring. Due to the nature of these collective bursts, or avalanches, they can be modelled with mesoscopic models and here each of the experimental systems is compared to simulations based on one of these simple models. In Publication I the dynamics of Portevin–Le Chatelier (PLC) deformation bands are studied using statistical methods. The results are mapped to a model based on the well-known model of mean-field depinning, the Alessandro–Beatrice–Bertotti–Montorsi (ABBM) model. This shows that a PLC deformation band, consisting of a large number of dislocations, behaves essentially as a single excitation of a quasiparticle. In Publication II the creep deformation of a quasi-2D fibrous material, paper, was studied close to failure. The localization of strain in tertiary creep was observed, concomitantly with scalefree behavior, divergence, of associated quantities. A serial fiber bundle model qualitatively reproduces this behavior and this implies that the behavior is not due to correlated avalanche phenomena. In Publication III the compression of foam-formed fibrous materials is studied. It is compared with the predictions of a mean-field model of fiber buckling, which reproduces the observed phenomena up to a certain strain, at which a crossover to collective phenomena is observed. The exponents of the acoustic emission energy and waiting time distributions seem to belong to a novel universality class.
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    Rheological properties of structured complex fluids
    (Aalto University, 2022) Viitanen, Leevi; Koivisto, Juha, Dr., Aalto University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex systems and materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics, Finland
    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.