Aaltodoc Repository :: Browsing by Author "Koivisto, Juha"

Browsing by Author "Koivisto, Juha"

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Akustiseen emissioon perustuva ohjaus paperin rasituskokeissa
(2011-08-31) Pursiainen, Eetu; Koivisto, Juha; Perustieteiden korkeakoulu; Alava, Mikko
Applications of C++ as a Specification Language
(1996) Koivisto, Juha; Tietotekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Martikainen, Olli
Cellulose foams as scalable templates for phase change materials
(Elsevier BV, 2023-12-10) Miranda-Valdez, Isaac Y.; Yazdani, Maryam Roza; Mäkinen, Tero; Coffeng, Sebastian; Viitanen, Leevi; Koivisto, Juha; Alava, Mikko J.; Department of Applied Physics; Department of Mechanical Engineering; Complex Systems and Materials; Energy Conversion and Systems; Department of Applied Physics
Cellulose foams produced by wet-templating fibers and surfactants offer an unlimited creative space for the design of green functional materials with a wide range of energy-related applications. Aiming to reduce plastic pollution, cellulose foams promise to replace plastic foams after tailoring physical functionalities into their structures. Here, this work demonstrates that cellulose foams made of methylcellulose and cellulose fibers can exhibit a solid–liquid phase change functionality by adding a phase change material (PCM) during the foam-forming process. The resulting foam composites, termed cellulose phase change foams (PCFs), exhibit a tenth of cellulose's density (134.7 kg m−3) yet a high Young's modulus (0.42MPa). They are also dimensionally stable over a wide range of temperatures while absorbing up to 108 kJ kg−1 as latent heat when the PCM confined to the foam experiences a solid-to-liquid transition at ∼60 °C, and releasing 108 kJ kg−1 as latent heat when changing from liquid to solid at ∼40 °C. Such phase change transition opens up broad applications for the PCFs as thermal insulators. For example, by further tuning the transition temperature, the PCFs can exploit their phase change and reduce the heat flow rate through their radial direction at specified temperatures. This article showcases the versatility of the foam-forming process of cellulose to accommodate physical functionalities in materials with complex architectures. Furthermore, thanks to the advances in cellulose foam-forming, such foams are recyclable, industrially scalable, and can be exploited as heat storage materials.
Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams
(ROYAL SOC CHEMISTRY, 2021-01-07) Tainio, Oskar; Sohrabi, Fereshteh; Janarek, Nikodem; Koivisto, Juha; Puisto, Antti; Viitanen, Leevi; Timonen, Jaakko; Alava, Mikko; Department of Applied Physics; Active Matter; Complex Systems and Materials
Unicellular Chlamydomonas reinhardtii micro-algae cells were inserted into a quasi-2D Hele-Shaw chamber filled with saponin foam. The movement of the algae along the bubble borders was then manipulated and tracked. These self-propelled particles generate flow and stresses in their surrounding matter. In addition, the algae possess the capability of exerting forces that alter bubble boundaries while maintaining an imminent phototactic movement. We find that by controlling the gas fraction of the foam we can change the interaction of the algae and bubbles. Specifically, our data expose three distinct swimming regimes for the algae with respect to the level of confinement due to the Plateau border cross-section: unlimited bulk, transition, and overdamped regimes. At the transition regime we find the speed of the algae to be modeled by a simple force balance equation emerging from the shear inside the Plateau border. Thus, we have shown that it is possible to create an algae-friendly foam while controlling the algae motion. This opens doors to multiple applications where the flow of nutrients, oxygen and recirculation of living organisms is essential.
Concepts for Actuator Driven Lens Adjustment of a Medical Endoscope
(2002) Kauhanen, Petteri; Koivisto, Juha; Konetekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Ekman, Kalevi
Constriction Flow of Cellulose Laden Air-Aqueous Foam
(NORTH CAROLINA STATE UNIV DEPT WOOD & PAPER SCI, 2019) Viitanen, Leevi; Halonen, Alisa; Fristrom, Eira; Koivisto, Juha; Korhonen, Marko; Puisto, Antti; Alava, Mikko; Department of Applied Physics; Department of Computer Science; Complex Systems and Materials; Department of Applied Physics; Department of Computer Science
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.
Constriction flow of particle laden foams in a Hele-Shaw cell
(2017-12-04) Friström, Eira; Koivisto, Juha; Perustieteiden korkeakoulu; Alava, Mikko
Contamination detection by optical measurements in a real-life environment
(WILEY-VCH VERLAG, 2020-01-01) Inkinen, Jenni; Ahonen, Merja; Iakovleva, Evgenia; Karppinen, Pasi; Mielonen, Eelis; Mäkinen, Riika; Mannonen, Katriina; Koivisto, Juha; Department of Applied Physics; Satakunta University of Applied Sciences; Complex Systems and Materials
Organic dirt on touch surfaces can be biological contaminants (microbes) or nutrients for those but is often invisible by the human eye causing challenges for evaluating the need for cleaning. Using hyperspectral scanning algorithm, touch surface cleanliness monitoring by optical imaging was studied in a real-life hospital environment. As the highlight, a human eye invisible stain from a dirty chair armrest was revealed manually with algorithms including threshold levels for intensity and clustering analysis with two excitation lights (green and red) and one bandpass filter (wavelength λ = 500 nm). The same result was confirmed by automatic k-means clustering analysis from the entire dirty data of visible light (red, green and blue) and filters 420 to 720 nm with 20 nm increments. Overall, the collected touch surface samples (N = 156) indicated the need for cleaning in some locations by the high culturable bacteria and adenosine triphosphate counts despite the lack of visible dirt. Examples of such locations were toilet door lock knobs and busy registration desk armchairs. Thus, the studied optical imaging system utilizing the safe visible light area shows a promising method for touch surface cleanliness evaluation in real-life environments.
Crack growth and energy dissipation in paper
(2018-12-01) Hanifpour, Maryam; Mäkinen, Tero; Koivisto, Juha; Ovaska, Markus; Alava, Mikko J.; Department of Applied Physics; Complex Systems and Materials
Here, we follow the stable propagation of a roughening crack using simultaneously Digital Image Correlation and Infra-Red imaging. In a quasi-two-dimensional paper sample, the crack tip and ahead of that the fracture process zone follow the slowly, diffusively moving “hot spot” ahead of the tip. This also holds when the crack starts to roughen during propagation. The well-established intermittency of the crack advancement and the roughening of the crack in paper are thus subject to the dissipation and decohesion in the hot spot zone. They are therefore not only a result of the depinning of the crack in a heterogeneous material.
Crossover from mean-field compression to collective phenomena in low-density foam-formed fiber material
(ROYAL SOC CHEMISTRY, 2020-08-07) Mäkinen, Tero; Koivisto, Juha; Pääkkönen, Elina; Ketoja, Jukka A.; Alava, Mikko J.; Department of Applied Physics; Complex Systems and Materials; VTT Technical Research Centre of Finland
We study the compression of low-weight foam-formed materials made out of wood fibers. Initially the stress-strain behavior follows mean-field like response, related to the buckling of fiber segments as dictated by the random three-dimensional geometry. Our Acoustic Emission (AE) measurements correlate with the predicted number of segment bucklings for increasing strain. However, the experiments reveal a transition to collective phenomena as the strain increases sufficiently. This is also seen in the gradual failure of the theory to account for the stress-strain curves. The collective avalanches exhibit scale-free features both as regards the AE energy distribution and the AE waiting time distributions with both exponents having values close to 2. In cyclic compression tests, significant increases in the accumulated acoustic energy are found only when the compression exceeds the displacement of the previous cycle, which further confirms other sources of acoustic events than fiber bending.
Detecting organic contaminants on surfaces by hyperspectral imaging and visible light
(2023-01-23) Paasilahti, Filimon; Koivisto, Juha; Insinööritieteiden korkeakoulu; Ekman, Kalevi
Food contamination within factories is still an issue. There is a need for a product on the market that can combat this problem. The technology of hyperspectral imaging coupled with an artificial intelligence is turned into a prototype through the process of product development. The focus of tackling the spreading of the contamination is hand hygiene of the factory workers. Hands are our medium with which we interact with the world and with which the foods are being made. The most common medium which spreads contamination is faeces. The result is a working prototype that can detect faeces on hands and alert the user of the contamination.
Erratum: Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams (Soft Matter (2021) 17 (145–152) DOI: 10.1039/D0SM01206H)
(ROYAL SOC CHEMISTRY, 2021-07-21) Tainio, Oskar; Sohrabi, Fereshteh; Janarek, Nikodem; Koivisto, Juha; Puisto, Antti; Viitanen, Leevi; Timonen, Jaakko V.I.; Alava, Mikko; Department of Applied Physics; Complex Systems and Materials; Active Matter; Department of Applied Physics
The authors regret that two of the concentrations in section 2.1 of the Methods section of the original article were incorrect. The first error is in the number concentrations of cells in the cultures used in the experiments. In the original article, the authors stated: ‘‘The number density of cells in the cultures used for experiments was around 106 cells per ml.’’ However, the correct concentration was 106 cells per ml, so the unit in the original sentence is wrong. The second error is in the concentration of saponin. The authors stated: "The solution consisted of high salt medium, active C. reinhardtii cells and saponin (c = 0.5 g ml-1)." The original article is missing one zero in the saponin concentration and the correct concentration here should be c = 0.05 g ml-1)." . The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.
Fatigue crack growth in an aluminum alloy
(American Physical Society, 2021-11-19) Lomakin, Ivan V.; Mäkinen, Tero; Widell, Kim; Savolainen, Juha; Coffeng, Sebastian; Koivisto, Juha; Alava, Mikko J.; Department of Applied Physics; Aalto University; Complex Systems and Materials; Department of Mechanical Engineering
In fatigue fracture the crack growth is slow and in many materials exhibits apparent self-similarity as expressed by the dependence of the growth velocity on a stress intensity factor that grows with the crack size. We study the intermittency of fatigue crack dynamics in aluminium alloys by optical tracking. A power-law distribution of crack tip jumps is found with an exponent close to two and a cutoff which increases with time or crack propagation. The cutoff is related to the crack velocity. We show how such a distribution evolves or coarse grains with the scale of observation or time window. The correlations of the crack propagation imply short-range memory effects in the underlying dynamics. Our results show universal features of fatigue cracks and how these lead to the crack growth and failure in material samples.
The flow of foam around an obstacle in Hele-Shaw cell
(2017-11-07) Viitanen, Leevi; Koivisto, Juha; Puisto, Antti; Perustieteiden korkeakoulu; Alava, Mikko
This study investigates the flow of a two-dimensional foam between two parallel plates. The project includes building the appropriate experimental apparatus, which would allow studying the flow field around a circular obstacle by filming the bubbles with a regular camera. Furthermore, it has the required flexibility to study the effect of boundary conditions, induced by the cell, to the foam rheology are studied by varying the flow driving. The experiments show that the driving of the flow, whether it is by pressure or moving the obstacle, has a significant effect to the flow field and thus to the foam rheology. The distinct values of static and dynamic friction between the bubbles and the top and bottom plate are expected to be the cause of the difference in the data obtained with different boundary conditions. Also the foam flow around the obstacle is found to be influenced by the foam’s liquid-gas ratio. Finally, the velocity field is compared to the changes in the bubble contacts. Despite the difficulties in the analysis, the results indicate that bubbles rearrange more frequently at the parts of the device where the spatial variation of the velocity is rapid.
Fluctuations and Scaling in Creep Deformation
(American Physical Society (APS), 2010) Rosti, Jari; Koivisto, Juha; Laurson, Lasse; Alava, Mikko J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
The spatial fluctuations of deformation are studied in the creep in Andrade’s power law and the logarithmic phases, using paper samples. Measurements by the digital image correlation technique show that the relative strength of the strain rate fluctuations increases with time, in both creep regimes. In the Andrade creep phase characterized by a power-law decay of the strain rate ϵt∼t−θ, with θ≈0.7, the fluctuations obey Δϵt∼t−γ, with γ≈0.5. The local deformation follows a data collapse appropriate for a phase transition. Similar behavior is found in a crystal plasticity model, with a jamming or yielding transition.
Foam flows through a local constriction
(2017) Chevalier, Thibaud; Koivisto, Juha; Shmakova, N.; Alava, Mikko; Puisto, Antti; Raufaste, C.; Santucci, Stephane; Department of Applied Physics; Complex Systems and Materials; RAS - Lavrentyev Institute of Hydrodynamics, Siberian Branch; Université Côte d'Azur
We present an experimental study of the flow of a liquid foam, composed of a monolayer of millimetric bubbles, forced to invade an inhomogeneous medium at a constant flow rate. To model the simplest heterogeneous fracture medium, we use a Hele-Shaw cell consisting of two glass plates separated by a millimetric gap, with a local constriction. This single defect localized in the middle of the cell reduces locally its gap thickness, and thus its local permeability. We investigate here the influence of the geometrical property of the defect, specifically its height, on the average steady-state flow of the foam. In the frame of the flowing foam, we can observe a clear recirculation around the obstacle, characterized by a quadrupolar velocity field with a negative wake downstream the obstacle, which intensity evolves systematically with the obstacle height.
Foam-formed biocomposites based on cellulose products and lignin
(SPRINGER, 2023-03) Miranda-Valdez, Isaac Y.; Coffeng, Sebastian; Zhou, Yu; Viitanen, Leevi; Hu, Xiang; Jannuzzi, Luisa; Puisto, Antti; Kostiainen, Mauri A.; Mäkinen, Tero; Koivisto, Juha; Alava, Mikko J.; Department of Applied Physics; Department of Bioproducts and Biosystems; Complex Systems and Materials; Biohybrid Materials; Department of Applied Physics; Complex Systems and Materials; VTT Technical Research Centre of Finland
Abstract: Foam-formed cellulose biocomposites are a promising technology for developing lightweight and sustainable packaging materials. In this work, we produce and characterize biocomposite foams based on methylcellulose (MC), cellulose fibers (CF), and lignin (LN). The results indicate that adding organosolv lignin to a foam prepared using MC and CF moderately increases Young’s modulus, protects the foam from the growth of Escherichia coli bacteria, and improves the hydrophobicity of the foam surface. This article concludes that organosolv lignin enhances many properties of cellulose biocomposite foams that are required in applications such as insulation, packaging, and cushioning. The optimization of the foam composition offers research directions toward the upscaling of the material solution to the industrial scale. Graphical abstract: [Figure not available: see fulltext.].
Fractional rheology of colloidal hydrogels with cellulose nanofibers
(Springer, 2024-02) Miranda-Valdez, Isaac Y.; Sourroubille, Marie; Mäkinen, Tero; Puente-Córdova, Jesús G.; Puisto, Antti; Koivisto, Juha; Alava, Mikko J.; Department of Applied Physics; Complex Systems and Materials; Complex Systems and Materials; Universidad Autonoma de Nuevo Leon; VTT Technical Research Centre of Finland
Abstract: Colloidal gels are soft solids composed of particles dispersed in a fluid phase. Their rheological behavior highly depends on the particle concentration, but establishing a relationship can be challenging. This article showcases the potential of fractional rheology to model and predict linear viscoelastic responses of colloidal hydrogels containing TEMPO-oxidized cellulose nanofibers. Cellulose nanofiber hydrogels are soft solids whose rheology is directly related to the particle concentration. Therefore, this work defined the rheological behavior of the hydrogels using a fractional order derivative analytically solved to determine rheological responses in frequency, stress relaxation, and creep. Using two parameters, it evaluated the rheology of cellulose nanofiber hydrogels and established tests that predict rheological behaviors for given particle concentrations. The findings suggested that the fractional approach could become a standard method for characterizing cellulose nanofiber hydrogels in the reported concentration regime. The two parameters of the fractional model build a comparison framework to assess the rheology of different viscoelastic materials. Graphic abstract: [Figure not available: see fulltext.]
Fracture propagation and prediction in heterogeneous materials
(Aalto University, 2013) Koivisto, Juha; Alava, Mikko, Prof., Aalto University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex Systems and Materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Finland
The topic of this thesis is fracture and damage. It is discussed with an experimental point of view in multiple geometries. In Publications I and II a 1D fracture line is driven through elastic medium that is paper. The fracture line propagates with jerky motion producing audible avalanches. These acoustic emission events obey Gutenberg-Richter statistics for energy and Omori law for waiting times. In Publications III and IV digital image correlation technique is used to measure local strains in paper creep experiments. The significance of local fluctuations increases during the experiment as the fluctuations decay slower than the average strain rate. The global strain rate is found to have a Monkman-Grant relation. In Publication V the global and local strains are used to predict the failure in the paper fatique experiment. The system is compared to a standard fiberbundle model with viscous components. Chapter 4 relates to unpublished work of two interacting cracks. Here, it is shown that curved cracks follow the local symmetry path where the shear stress intensity factor KII = 0.
Friction controls even submerged granular flows
(ROYAL SOC CHEMISTRY, 2017-09-15) Koivisto, Juha; Korhonen, Marko; Alava, Mikko; Ortiz, Carlos P.; Durian, Douglas J.; Puisto, Antti; Department of Applied Physics; Complex Systems and Materials; University of Pennsylvania
We investigate the coupling between the interstitial medium and granular particles by studying the hopper flow of dry and submerged systems experimentally and numerically. In accordance with earlier studies, we find that the dry hopper empties at a constant rate. However, in the submerged system we observe the surging of the flow rate. We model both systems using the discrete element method, which we couple with computational fluid dynamics in the case of a submerged hopper. We are able to match the simulations and the experiments with good accuracy by fitting the particle–particle contact friction for each system separately. Submerging the hopper changes the particle–particle contact friction from µvacuum = 0.15 to µsub = 0.13, while all the other simulation parameters remain the same.

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