Current Research Projects

Stress responsive polymer materials

The development and design of highly specific responsive materials has great potential in many applications, ranging from self-healing or self-strengthening materials, shape recovering materials, chemical sensing or damage sensing, and bio-related control and release systems. Our group explores the fundamental questions of how macroscopic forces are transmitted to the micro-scale in heterogeneous, complex environments. We will use the generated fundamental knowledge to help design novel responsive materials.

Polymers for tuning crystallization

Small organic molecules are used in electronics, solar cells, pharmaceutics, and as biodegradable coatings. For many of these applications to be effective, it is crucial to be able to control crystallization behavior and morphology. Polymer additives can be used to achieve this control. For this purpose, it is essential to understand the non-equilibrium pathways for crystallization in complex systems. Automated screening with efficient computer simulations can be used to gain systematic insight into how polymers affect crystallization behavior of small molecules.

Past Research Projects

Star polymers under shear flow

We explored the tunable, responsive character of telechelic star polymers as models for soft patchy particles. We focus on the simplest possible system: a star comprising three asymmetric block copolymer arms with solvophilic inner and solvophobic outer blocks. Clusters comprising multiple stars are formed and we employ simulations to investigate flow properties of the system at high shear rates, revealing shear thinning behavior caused by the breakup of interstar associations under flow.

Related publication:

  1. Self-Organization and Flow of Low-Functionality Telechelic Star Polymers with Varying Attraction, E Moghimi, I Chubak, A Statt, MP Howard, D Founta, G Polymeropoulos,  K Ntetsikas, N Hadjichristidis, A Z Panagiotopoulos, C N Likos, D Vlassopoulos, ACS Macro Letters 8, 766-772

Evaporation of polymer solvent mixtures

We showed with nonequilibrium molecular dynamics simulations of an implicit model (without hydrodynamics) and an explicit model (with hydrodynamics) that models lacking hydrodynamic interactions do not faithfully represent stratification (separation into vertical layers) in drying polymer mixtures. We also investigated the effect of solvent quality on already formed films.

Related publications:

  1. Influence of hydrodynamic interactions on stratification in drying mixtures, A Statt, MP Howard, AZ Panagiotopoulos, The Journal of Chemical Physics 149 (2)
  2. Solvent quality influences surface structure of glassy polymer thin films after evaporation, A Statt, MP Howard, AZ Panagiotopoulos

Instability of Shear Flows in Spatially Periodic Domains

When applying unidirectional shear flow to a simple model fluid, we found unexpected shear flow patterns, where a secondary flow perpendicular to the flow direction developed. Periodic Couette and Poiseuille flows were unstable at Reynolds numbers two orders of magnitude smaller than their aperiodic equivalents because the periodic boundaries impose fundamentally different constraints.

Related publications:

  1. Unexpected secondary flows in reverse nonequilibrium shear flow simulations A Statt, MP Howard, AZ Panagiotopoulos Physical Review Fluids 4 (4), 043905, 2019
  2. Instability of Shear Flows in Spatially Periodic Domains MP Howard, A Statt, HA Stone, TM Truskett arXiv preprint arXiv:1907.07086, 2019

Binary hard sphere mixtures

For binary fluid mixtures of spherical particles the dominant wavelength of oscillations of the pair correlation functions is predicted to change along a sharp crossover line in the phase diagram. Using particle-resolved colloid experiments in 3d we demonstrate that crossover exists and that its location in the phase diagram is in quantitative agreement with the results of both theory and our Monte-Carlo simulations. In contrast with previous work where a correspondence was drawn between crossover and percolation of both species, in our 3d study we found that structural crossover is unrelated to percolation.

Related publication:

  1. Direct observation in 3d of structural crossover in binary hard sphere mixtures A Statt, R Pinchaipat, F Turci, R Evans, CP Royall The Journal of chemical physics 144 (14), 144506

Nucleation of colloidal crystals

Homogeneous and heterogeneous nucleation happens frequently all around us and is relevant for many technological processes. Nucleation rates are usually estimated in terms of an Arrhenius law involving the nucleation barrier. The nucleation barrier is given by the free energy gain from the nucleus volume and the surface free energy costs. For crystal nuclei this “classical nucleation theory” is hampered by the problem that the nucleus in general is non spherical. We developed a method to determine the nucleation barrier without needing to determine the nucleus size or shape.

Related publications:

  1. Finite-size effects on liquid-solid phase coexistence and the estimation of crystal nucleation barriers A Statt, P Virnau, K Binder Physical review letters 114 (2), 026101
  2. Estimation of Nucleation Barriers from Simulations of Crystal Nuclei Surrounded by Fluid in Equilibrium, A Statt, P Koß, P Virnau, K Binder, High Performance Computing in Science and Engineering 16, 49-59 2016
  3. Monte Carlo simulation of crystal-liquid phase coexistence, A Statt, F Schmitz, P Virnau, K Binder, High Performance Computing in Science and Engineering 15, 75-87, 2016
  4. Crystal nuclei in melts: A Monte Carlo simulation of a model for attractive colloids, A Statt, P Virnau, K Binder, Molecular Physics 2015

Phase separation of colloids in confinement

Nanomaterials and nanofluidic devices are more and more relevant in applications, which emphasizes the need for a better understanding of the phase behavior of materials on small, confined length scales. Phase transitions are rounded and shifted and affected by boundary effects in confined systems. We investigated polymer colloid mixtures between planar walls and in spherical confinement. We observed both “Janus” and “core-shell” morphologies by tuning the wall properties, which are of interest in applications.

Related publications:

  1. Computer simulation of heterogeneous nucleation of colloidal crystals at planar walls, BJ Block, D Deb, F Schmitz, A Statt, A Tröster, A Winkler, T Zykova-Timan, The European Physical Journal Special Topics 223 (3), 347-361, 2014
  2. Computer simulations of structure, dynamics, and phase behavior of colloidal fluids in confined geometry and under shear, A Winkler, D Winter, P Chaudhuri, A Statt, P Virnau, J Horbach, K Binder The European Physical Journal Special Topics 222 (11), 2787-2801, 2013
  3. Phase transitions and phase equilibria in spherical confinement,A Winkler, A Statt, P Virnau, K Binder,Physical Review E 87 (3), 032307,2013
  4. Phase Separation of Colloid Polymer Mixtures Under Confinement,A Statt, A Winkler, P Virnau, K Binder,High Performance Computing in Science and Engineering ‘13, 19-31, 2013
  5. Controlling the wetting properties of the Asakura–Oosawa model and applications to spherical confinement,A Statt, A Winkler, P Virnau, K Binder,Journal of Physics: Condensed Matter 24 (46), 464122, 2012