Complex Systems and Materials (CSM)

Antti Puisto as an independent Academy Research Fellow carries out research on computational rheology, specifically focusing on:

Rheological characterization: We apply continuum fluid models to simulate the apparent behaviors of various complex fluid types in experimentally applied geometries (concentric cylinders, cone-and-plate, and plate-and-plate comparing the apparent (as obtained by the device) and intrinsic (as input by the model) to develop suitable correction algorithms and schemes to properly address the non-linear rheology of such materials in experiments.

Rheology of unstable colloids: Different levels of numerical tools from particle based dynamics to continuum simulations are used to study particle interactions under flow. The aim is to consider the consequences of particle cluster elasticity to the fluids continuum “apparent” elastic behavior.

Shear thickening multiphase flows: Suspensions containing high solid fractions of repulsively interacting particles exhibit shear thickening. This is especially true for non-spherical particles. Inter-particle locking and friction have been suggested to drive such anomaly. To resolve the fundamental origin of this requires further theoretical development.

Rheology of loaded foams and emulsions: Loaded foams are special kind of foams consisting of gas, liquid, and solid phases, in the order of decreasing volume fraction, respectively. We consider the role of particles in such a multiphase system with respect to the foam configuration, stability, and rheology.

Department of Applied Physics
Aalto University
P.O.Box 11100
FI-00076 AALTO Finland

Office: Y430b
Email: [email protected]
Tel: +35850 4333828

Academy Research Fellow Lasse Laurson carries out independent research activities related to dynamical processes in various material science applications. His current research consists mainly of the following focus areas:

Domain wall dynamics in low-dimensional ferromagnets (thin films and nanowires). This activity is pursued via a Helsinki Institute of Physics (HIP) Theory Programme Project Domain wall dynamics which Laurson leads. The research is done in collaboration both locally within Aalto University (e.g. with the NanoSpin group of Prof. Sebastiaan van Dijken) and across Europe, in particular via an associated partnership in the European Commission ITN project WALL, with Laurson the Scientist in Charge of the project at Aalto.

Right: Domain wall (DW) dynamics in a model of in-plane magnetized thin ferromagnetic films with different relative strengths of dipolar interactions (increasing from left to right). The DW morphology (example DW configurations shown in black) evolves from rough to zigzag, and the Barkhausen avalanches (regions with different colours) exhibit a cross-over between two universality classes. See also Phys. Rev. B 89, 104402 (2014).

Left: A domain wall separating two out-of-plane domains in a thin ferromagnetic film, including an internal domain wall (or a Bloch line), separating regions of opposite in-plane magnetisation  within the domain wall; magnetisation direction is indicated by the arrows, and the different colours. Our results show how such internal walls can be nucleated and subsequently displaced along the main domain wall by applying magnetic fields or spin-polarised electric currents. We show that due to topological protection of the internal domain wall structure by the surrounding out-of-plane domains, Walker breakdown is absent in the dynamics of the internal walls; this could lead to interesting possibilities to develop spintronics applications where domain walls would serve as guides for fast internal domain wall propagation (Figure: Touko Herranen). See also Phys. Rev B 92, 100405(R) (2015).

Plastic deformation of crystalline solids, or dislocation dynamics, is studied in close collaboration with other members of the CSM group, and with collaborators across Europe. Here, the key questions relate to the character of the “yielding transition”, and to that of the ensuing deformation bursts or avalanches, see e.g. Phys. Rev. Lett. 112, 235501 (2014), and Scientific Reports 5, 10580 (2015).

Friction between two surfaces, with or without a lubricant layer in between. A related Aalto Science InstituteThematic Research Programme Machine learning strategies for optimising frictional properties of materials is about to start during autumn 2014, with Laurson the coordinator of the Programme. The research is also related to a COST network Understanding and Controlling Nano and Mesoscale Friction. For an example of our recent friction research, see e.g. Phys. Rev. Lett. 114, 095502 (2015).

General physics of avalanching systems, with the aim of understanding the fundamental nature of the bursty dynamics encountered in various driven systems (for examples, see above). For such studies, we consider simple minimal models, such as driven elastic interfaces in random media.

Left: The average shape of the activity bursts or avalanches in driven interfaces in random media, with different ranges of the elastic interactions, showing how the average avalanche shape depends on the universality class of the avalanche dynamics. In particular, we find a small temporal asymmetry of the average shapes, evolving with the universality class, and reflecting a broken time-reversal symmetry in the avalanche dynamics, see Nature Communications 4, 2927 (2013).

For more papers, have a look at the list of publications.

Department of Applied Physics

Aalto University 
P.O.Box 11100
FI-00076 AALTO
Finland

Office: Y417b, main building (Otakaari 1), 4th floor
E-mail: [email protected]
Tel: +358-50-4333671

FiDiPro – the Finland Distinguished Professor Programme enables distinguished researchers, both international and expatriates to work and team up with the ‘best of the best’ in Finnish academic research. Led and financed by the Academy of Finland and Tekes, FiDiPro provides competitive grants to projects recruiting highly merited scientists, who are able to commit to long-term cooperation with a Finnish university or research institute.

The project on Noise and fluctuations in materials was awarded to Stefano Zapperi and Mikko J. Alava at the Department of Applied Physics of Aalto University.

Next time Stefano Zapperi will be at COMP/Aalto is from October 27 to November 7, 2014. Also Alessandro Luigi Sellerio (November 2-9) and Giulio Costantini (October 27-November 8) will be visiting us.

Coworkers

Research

Research activities  revolve around the following themes:

1. Plasticity and dislocation dynamics in micrometer size samples: Plastic forming processes are a key issue of materials and manufacturing technology. When the sample size is reduced, smooth plastic flow described by conventional plasticity gives way to spatio-temporal intermittency. Here we will address this by dislocation dynamics simulations and network models to understand the universality of plastic fluctuations.

2. Irreversible deformation of amorphous materials: In amorphous materials such as glasses, pastes or foams, in absence of an underlying crystalline lattice, plasticity cannot be described in terms of dislocations.  We will address size effects and strain bursts in the irreversible deformation of amorphous materials by experiments and numerical simulations of atomistic and mesoscopic models. The rheology of yielding materials (foams, pastes) is another key area, of importance at Aalto for the research in forest-based materials.

3. Strength fluctuations and size effects in fracture: The fracture strength of materials depends in a non-trivial manner on the characteristic length-scales of the specimen and represents a fundamental open problem of science and engineering. This size effect can be understood considering that some form of disorder, such as dislocations, grains or microcracks, is always present in materials. To address this issue, we will develop a renormalization theory of fracture size effects taking into account explicitly interactions and disorder. In addition, we will experimentally test size effects and strength fluctuations by experiments on fiber-based materials.

4. Fluctuations in sliding friction and nanotribology: While the laws of friction at the macroscopic level are well known, the fundamental understanding at the atomic scale is still unsatisfactory.  At small length scales, the simple empirical laws of macroscopic friction turn out to be inadequate since surface inhomogeneities lead to strong local fluctuations of the friction coefficient. We will perform an analysis of friction fluctuations and stick-slip dynamics at the nanoscale by numerical simulations.
 

5. Hysteresis and noise in ferromagnetic/ferroelectric thin films and nanostructures: Understanding magnetic hysteresis has important technological implications for magnetic devices. From a purely theoretical point of view, the dynamics of disordered magnetic systems represents a central issue in non-equilibrium statistical mechanics. Due to disorder the hysteresis loop typically displays magnetization jumps and noise, known as the Barkhausen effect, commonly used as a non-destructive tool for magnetic material testing. Similar physics is found in ferroelectric systems.

6. Mechanical and statistical properties of biology-related systems: Biological systems provide extremely interesting examples where methods of statistical physics developed to study disordered elastic systems can find fruitful applications. We plan to study the mechanics and dynamics of systems of biological interest. Examples include the kinetics of cell colony growth, the mechanics of tumors in tissues and the mechanics of cell division. Another example is the deformation-dependent growth of cell cultures.