Dr. Laura Filion

Ornstein Laboratory, room 21
Princetonplein 1,  3584 CC Utrecht
P.O. Box 80 000, 3508 TA Utrecht
The Netherlands
phone: +31 (0)30 253 3519
secretariat: +31 (0)30 253 2952
e-mail: l.c.filion@uu.nl

Research

Overview

We study the self-organization of colloidal particles using computer simulations. We focus on simple model systems where the interaction between the colloids is treated using coarse-grained potentials. Broadly, our research can be subdivided into two main categories: “passive” colloidal particles which self-assemble purely due to Brownian motion, and “active” colloidal particles which absorb energy from their environment in order to self-propel. Using a wide range of computational and theoretical techniques, we aim to predict and characterize the equilibrium and steady-state structures formed when these particles self-assemble. Moreover, we investigate the nucleation processes which govern the formation of these structures.

We work closely with the experimentalists in the Soft Condensed Matter group, as well as the Condensed Matter and Interfaces group and the Van ‘t Hoff Laboratory. Some research highlights are detailed below.

Research Highlights:

Removing grain boundaries from three-dimensional colloidal crystals using active dopants

GrainBoundaryFIGSelf-propelled particles, also known as active particles, incessantly convert energy into self-propulsion, and as such are intrinsically out-of-equilibrium. While traditionally such particles occurred solely within the purview of natural systems (e.g. bacteria), recent experimental breakthroughs have led to many novel types of synthetic colloidal swimmers. These systems exhibit a wealth of new phase behaviour, including motility-induced phase separation into dense and dilute phases, giant density fluctuations, and swarming.
Moreover, experimental and simulation studies have shown that the dynamics of a passive
system can be altered dramatically by incorporating as little as 1% of active particles into the
system.  At these concentrations, the self-propelled particles can be viewed as active “dopants”, which like passive dopants can strongly alter the properties (e.g. dynamics) of the underlying passive system. 

We use simulations to explore the behaviour of two- and three- dimensional colloidal (poly)crystals doped with active particles. We show that these active dopants can provide an elegant new route to removing grain boundaries in polycrystals.

For more information see:
Removing grain boundaries from three-dimensional colloidal crystals using active dopants
B. van der Meer, M. Dijkstra and L. Filion, Soft Matter 12, 5630-5635, (2016)

Self assembly of active attractive spheres

One of the most exciting recent developments in the field of colloidal self-assembly is the realization of synthetic, self-propelled colloidal particles. Like living organisms, these “active” colloidal particles convert energy from their environment into directed motion. As a result, these systems are inherently out-of-equilibrium, and are not bound by the laws of equilibrium statistical physics – making them both very intriguing and challenging to study.

VassilisSoftMatterHere we used Brownian Dynamics simulations to examine one of the simplest examples of such an active system: namely a system of active, attractive spheres in three dimensions, where the attraction was modeled using the Lennard-Jones potential. We extracted how the state diagram was affected by the speed with which the particles rotated. Specifically, for slow rotational diffusion, we showed that the gas-liquid coexistence is transformed into a novel percolating network state. Interestingly, this state exhibits high degrees of local orientational alignment of the self-propulsion axis, despite the absence of an aligning force in the model. We also propose a simple mechanism which explains how this alignment and simultaneously rationalizes how activity changes the state diagram.

 

 

For more information see:
Self-assembly of active attractive spheres
V. Prymidis, H. Sielcken, L. Filion,  Soft Matter 11, 4158 (2015)

 

Erasing no-man’s land by thermodynamically stabilizing the liquid-liquid transition in tetrahedral particles

 

LiquidLiquidFIGOne of the most fascinating yet controversial hypotheses for explaining the origin of the numerous thermodynamic anomalies characterizing liquid water postulates the presence of a metastable, second-order, liquidliquid critical point (LLCP) . Located in the so-called ´´no-man´s land´´, where spontaneous crystallization obscures the liquidliquid phase transition, it is impossible to directly access the LLCP experimentally in order to conclusively prove its existence. Here, we use a simple, single-component patchy-particle model to identify two key ingredients controlling the existence of a LLCP, namely the softness of the interparticle interaction and the flexibility of the bond orientation. We systematically explore the phase behavior of this model, mapping out the competition between crystallization and liquidliquid phase separation. We show that for certain choices of the interaction parameters, the liquidliquid phase transition can be made thermodynamically stable, enabling the study of this phenomenon without interference of crystallization at any temperature. Realizing these conditions in soft-matter systems would open up the possibility to experimentally probe liquidliquid phase transitions, shedding new light on the phase behavior of water and similar tetrahedral liquids.

For more information see:
Erasing no-man’s land by thermodynamically stabilizing the liquid-liquid transition in tetrahedral particles
F. Smallenburg, L. Filion, F. Sciortino,  Nature Physics 10,  653 (2014).


Vacancy-stabilized crystalline order in hard cubes

cubesWe examined the effect of vacancies on the phase behavior and structure of hard cubes using event-driven molecular dynamics and Monte Carlo simulations. We found a first-order phase transition between a fluid and a simple cubic crystal phase which is stabilized by an amazingly large number of vacancies: near bulk coexistence the net vacancy concentration is approximately 6.4%, which is more than two orders of magnitude higher than that of hard spheres. Remarkably, we also found that vacancies increased the positional order in the system. A closer examination of the vacancies showed that they were delocalized and extended over several lattice sites. A snapshot of a typical configuration is shown below where the vacancies and the particles which surround the vacancy have been highlighted.

For more information see :
Vacancy-stabilized crystalline order in hard cubes,
F. Smallenburg, L. Filion, M. Marechal and M. Dijkstra, PNAS, 109, 17886 (2012),
and a commentary on this article by Daan Frenkel “Colloidal crystals full of invisible vacancies”.