Multiscale Statistical and Quantum Physics (MSP)

Metal nanoparticles have fascinating optical properties that differ greatly from the ones of their bulk counterparts. Metal nanoparticles absorb light at specific wavelengths due to (localized) surface plasmon resonances (SPR). SPR have a high optical selectivity and thus metal nanoparticles can be used for applications in various fields from biomedicine to energy technology.

The intensity and position of the SPR are affected by the shape, size, and material of the nanoparticles and the refractive index of the medium in which the nanoparticles are embedded. Various geometries, such as spheres, spheroids and shells, and sizes of metal nanoparticles have been studied to adjust the SPR into the desired wavelength region without a considerable decrease in the intensity. Moreover, different nanoparticle concentrations and media have been used to adjust the SPR. Coating a metallic sphere with a high index of refraction dielectric shell has been shown to redshift the SPR up to hundreds to 1000 nm into the IR and NIR regions [1]. Another interesting option is the opposite case- that is, a metal shell with a dielectric core. For these metal-dielectric shell-core particles redshifts up to 1500 nm can be achieved by decreasing the shell thickness down to about 1 nm with a high refractive index core [2]. Semi-conductors offer still more capabilities to adjust the properties and to tune of the SPR peak to precise wavelengths. Such microinclusions have been shown to reflect nearly 70% by heat radiation in the IR region giving them excellent promise for controlling the spectrum of radiation reaching a sensor [3].

The traditional theory of thermodynamics is one of the most useful and universal tool in every physicist’s toolbox. However, the traditional formulation is at its best in describing macroscopic systems where the thermodynamic limit is reached. Recently, a great effort has been made towards extending the fundamental laws of thermodynamics for small systems in or out of equilibrium.

At the moment there exists a range of remarkable out of equilibrium equalities describing fluctuating quantities such as entropy, work and heat, commonly called as fluctuation relations. In the thermodynamic limit these equalities are replaced by inequalities, of which the second law of thermodynamics is the best known one.

In addition to interest towards the general theory of thermodynamics of small systems, we study thermodynamics in the context of single electron tunneling. In these interesting systems, the microscopic violations of the second law of thermodynamics can be seen. Thanks to close collaboration with PICO and QCD groups at Aalto SCI, the project includes contact to physical world trough experimental studies.

Another interesting side of the project is so called stochastic thermodynamics and thermodynamics of small systems under feedback control. In these systems, for example the famous Maxwell’s demon can be realized.

Dynamics of the Phase Field Crystal models

Phase Field Crystal (PFC) models extend the traditional Phase Field (PF) models to account for microscopic details of crystalline solids such as elasticity or grain boundary energies. They bring diffusive time scales into the scope allowing for studying of such phenomena as vacancy diffusion, crystal growth or grain rotation.

The dynamics of the PFC models are normally taken to be diffusive slow dynamics. This creates certain ambiguities since the model contains elastic excitations that promote fast dynamics. Recent work consists on quantifying and separating the fast and slow dynamics in a consistent way. This is important in any applications in which elastic excitations and the decay of these excitations plays an important role. These systems include for example the aforementioned grain rotation and almost any crystalline systems actively driven out of equilibrium.

Modelling Strained Metallic Overlayers

Thin heteroepitaxial overlayers have been proposed as a route to generate stable, self-organized nanostructures at large length scales. Such structures could be used as templates for a variety of important technological applications. Modeling strain-driven self-organization has remained a formidable challenge due to different length scales involved, ranging from atomic to micrometer scales.

By means of an Amplitude Expansion of Phase Field Crystal model, the relationship of the equilibrium surface patterns of the film to the adsorbate-substrate adhesion energy, as well as to the mismatch between the adsorbate and the substrate bulk lattice parameters are obtained in both both the tensile and the compressive regimes. The approach used captures pattern periodicities over large length scales, up to several hundreds of nm, retaining atomistic resolution. Thus the results can be directly compared with experimental data, in particular for systems such as Cu/Ru(0001) and Ag/Cu(111).

Three nontrivial, stable superstructures for the overlayer, namely stripe, honeycomb and triangular, are identified that closely resemble those observed experimentally. Simulations in non-equilibrium conditions are performed, as well, to identify metastable structural configurations and the dynamics of ordering of the overlayer.

Water mediated electrostatic interactions are omnipresent in various nanoscale phenomena. These interactions regulate several biological and industrial processes, to name but a few,  the ionic selectivity of biological and artificial nanopores, the rigidity of charged proteins,  and the functioning of new generation energy storage devices such as supercapacitors.  Despite their importance, electrostatic effects in nanoscale systems have been modeled for several decades within mean-field level dielectric continuum formulations, such as the Poisson-Boltzmann (PB) approach, which neglects electrostatic correlations (i.e. many-body effects), the charge structure of water solvent, and ion specific effects.

Using field-theoretic formulations, we incorporate into the PB formalism the complications associated with correlations and solvent charge structure.  We develop  self-consistent approaches that allow to consider many-body interactions and excluded volume effects in inhomogeneous media, such as electrolytes at membrane surfaces or in solid nanopores [1,2,3].  We also extend the dielectric continuum formulation of electrostatics by developing charged liquid models where the charge structure of the solvent is taken into account on the same footing as the ions [4,5,6].

Our research deals with complex fluid flow in confinement with an emphasis on coarse-grained solvent models that incorporate thermal fluctuations. We have developed a hybrid method that couples continuum fluctuating hydrodynamics to molecular dynamics. Our method features a hydrodynamically consistent coupling to a particle phase, which allows discrete objects of arbitrary shape and stiffness, such as colloids and polymers, to interact with the solvent. The model offers quantitative accuracy without fitting parameters. Currently we are investigating how it can be extended to fluctuating, energy-conserving flows in order to study the Soret effect/thermophoresis.

Interestingly, the same model without thermal fluctuations can be used to study the behavior of porous particles in microfluidic networks. We have calculated theoretic predictions for the hydrodynamic drag experienced by porous spheres and, using computer simulations, we have examined how colloids move and interact in a T-shaped microfluidic junction.

Goal of the current work on the polymer escape problem is to study the dynamics of the polymer escaping from the metastable potential well and to understand better which parameters affect to the crossing rate and how. Various numerical methods and theories such as Molecular Dynamics (MD), Path Integral Hyperdynamics (PIHD), Transition State Theory (TST), Nudged Elastic Band (NEB) with different polymer models (ideal, harmonic, FENE-LJ) and problems are studied. Possible future applications of the results are in the gene studies and biotechnology applications. Work is done in collaboration with Hannes Jónsson from University of Iceland.

We present a modeling approach that enables numerical simulations of a boiling Van der Waals fluid based on the diffuse interface description. A boundary condition is implemented that allows in and out flux of mass at constant external pressure. In addition, a boundary condition for controlled wetting properties of the boiling surface is also proposed. We present isothermal verification cases for each element of our modeling approach.

By using these two boundary conditions we are able to numerically access a system that contains the essential physics of the boiling process at microscopic scales. Evolution of bubbles under film boiling and nucleate boiling conditions are observed by varying boiling surface wettability. We observe flow patters around the three-phase contact line where the phase change is greatest. For a hydrophilic boiling surface, a complex flow pattern consistent with vapor recoil theory is observed.