Science and Engineering at The University of Edinburgh

Soft Matter Physics

Decorative Picture: Soft Condensed Matter Images Decorative Picture: Soft Condensed Matter Images


Academics: Profs. Wilson Poon (FRSE) and Peter Pusey (FRS); Drs. Rosalind Allen, Paul Clegg and Cait MacPhee
COSMIC lab manager and scientist in charge: Dr. Jochen Arlt
Senior research associate: Dr. Rut Besseling
Biology laboratory manager: Dr. Angela Dawson
Group synthetic chemist: Dr. Andrew Schofield

About Us

The experimental research in our group has two directions: ‘Soft Matter’, a sub-discipline of physics concerned with the study of colloidal suspensions, surfactants and polymers, and Active Matter (see also here) the active or life counterparts of soft matter, such as bacterial suspensions and biological polymers

Soft matter displays many fascinating properties. One example is shown above, where suspensions of Perspex spheres that act as hard particles (~1 micron diameter), self assemble into ‘colloidal crystals’ at high densities. The right-hand-side movie, taken with confocal microscopy, shows the crystal growth process, while the iridescence in the main picture is due to individual colloidal crystallites Bragg-scattering incident white light. Soft matter also shows interesting mechanical behaviour. A well known ‘kitchen’ example is ‘shear thickening’ - a concentrated corn starch solution gets harder to stir the harder it is stirred. Significantly, biology is almost entirely made up of ‘living soft matter’ – globular proteins are colloids, DNA is a stiff polymer, and the lipids forming cell membranes are essentially surfactants.

Soft matter has been studied by chemists, chemical engineers and biologists for many years. It is increasingly clear, however, that these systems show generic properties independent of chemical details. For example, all polymers share certain properties simply because they are long strings of balls performing Brownian motion. This is the central reason why physicists are getting interested. Moreover, studying the generic properties of soft matter can give fresh insights into a broad range of fundamental questions that cut across the whole of condensed matter physics, e.g. concerning the nature of disordered solids.

Many experimental projects (mostly done in COSMIC, an interdisciplinary lab for physicists, physical chemists and biologists) are closely related to projects in theory and simulation in our group.

Research Interests

Much of our research is funded by successive EPSRC programme grants, the current one running from October 2007 for 4 years. This flexible funding source gives significant scope for exploration of new topics and the exploitation of unexpected opportunities arising from on-going research, and for postdoctoral researchers to work on multiple projects. The main directions of our research are:

Rheophysics of soft matter

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What physical mechanisms govern the yielding and flow of dense colloids, gels and emulsions and how are these related to the ‘dynamical arrest’ or jamming in these systems without flow? We study these problems via confocal microscopy of the flow (particle tracking and velocimetry) with simultaneous rheologicalmeasurement. These experiments give important new insights in this field, sometimes overthrowing accepted ideas and results in concentrated colloid rheology. For example, the flow of a concentrated hard sphere suspension in a rectangular channel (flow profile shown in figure) is not like that of a ‘yield stress fluid’, but behaves instead more like dry grains (click here for the paper). For more details on the various projects, click here.

Physics of barriers in soft matter and biology

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The ‘aging’ of concentrated metastable states in colloids (‘glasses’), the switching between different phenotypes in bacteria, and the nucleation of growth of fibrillar aggregates in certain proteins (see picture, click here for the paper) share a common feature – all three processes involve crossing energy (or free energy) barriers. Experimental work in all three areas will be complemented by novel simulations and analytic theory. Perhaps generic features will emerge despite apparent dramatic differences between these experimental systems.

New soft materials

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An emerging research focus is the study of colloidal particles dispersed in complex solvents, in particular various liquid crystals and phase-separating binary fluids. Often, new soft materials result. Because of the presence of an ‘intermediate length scale’ between colloids and the bulk, the properties of these materials can often be tuned by altering kinetic pathways during their preparation. For example, the picture shows what happens when a binary liquid mixture phase separates under the ‘spinodal’ mechanism in the presence of particles that wets the two phases equally – a bicontinuous jammed emulsion (bijel) is formed (see the paper announcing its discovery and see here for more information). We are also interested in gels formed by fibrillar protein aggregates (see also under barrier crossing). While in vivo these are associated various neurodegenerative diseases, our interest is more in using these as structural elements in building new soft materials.

Physics of cellular motion

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The current experimental focus here is on the motility of bacteria (see picture for E. coli cells with their flagella) in complex polymer media – concentrated polymer solutions as well as porous polymer gels (including agar, a standard microbiological culture medium). We are interested in how the polymer affects the motility of single cells, as well as how various kinds of collective motion may emerge, either ‘on its own’, or under the influence of external fields (such as gravity, or gradients in nutrients, etc.). Bacterial motility in confined geometries, such as inside microfluidics devices, is also of interest. This new area emerged partly because we started thinking about bacteria as colloids – they certainly fit into the colloidal domain in terms of size, with their locomotive ability adding extra features. (Click here for a chapter on bacteria as colloids published in the 39th IFF Spring School 2008 at Juelich.)

Staff-research interest

Dr. Rosalind Allen
Development of simulation methodology for rare events in nonequilibrium systems and its application to colloidal systems. Current work includes simulating crystal nucleation under shear in simplified model systems such as Ising lattices and Lennard-Jones fluids.
Dr. Jochen Arlt (COSMIC laboratory manager)
Dynamics of colloids and bacteria studied using optical tweezers.
Dr. Rut Besseling
Rheology an imaging studies of colloids and emulsions under external fields, in particular various kinds of flow.
Dr. Paul Clegg
Experimental studies of interfacially jammed emulsions, ‘bijels’, and nanoparticle-liquid crystal composites
Dr. Cait MacPhee
Peptide and protein self-assembly.
Prof. Wilson Poon
Experimental investigation, using direct imaging and rheo-optical techniques, of colloids and colloid-polymer mixtures in simple liquids and complex dispersion media, concentrating on various dynamically-arrested states (glasses and gels) in quiescent states as well as under external deformation; bacteria as colloids.
Prof. Peter Pusey
Phase diagram of hard sphere colloidal systems, dynamic light scattering in non-ergodic systems including colloid-polymer mixtures.
Andrew Schofield
Preparation and characterisation of novel and model soft matter systems.

Some of our collaborators

Prof. Bernie Binks (Hull, UK)
Surfactants and colloids
Prof. Stefan Egelhaaf (Duesseldorf, Germany)
Self-assembling systems, colloid/surfactant mixtures
Prof. Eric Weeks (Emory, USA)
Confocal microscopy of jammed and glassy systems
Dr. George Petekidis and Prof. Dimitris Vlassopoulos (FORTH, Crete)
Colloidal systems in non-ergodic states
Prof. Michael Solomon (Michigan USA)
Colloidal assembly
Prof. Daan Frenkel (Cambridge University)
Computational Physics
Prof. Matthias Fuchs (Konstanz, Germany)
Mode Coupling Theory extended to sheared and driven systems
Dr. J. Bergenholtz (Goeteborg, Sweden)
Colloid dynamics
Prof. Ron Larson (Michigan USA)
Theoretical rheology
Dr. S. Majumdar (Paris-Sud, France)
Stochastic processes in soft matter
Prof. David Mukamel(Weizmann, Israel)
Nonequilibrium statistical mechanics
Dr. E. Orlandini(Padova, Italy)
Lattice Boltzmann simulation
Dr. I. Pagonabarraga(Barcelona, Spain)
Lattice Boltzmann simulation
Dr. A. Puertas (Almeria, Spain)
Molecular dynamics simulations of colloidal gels and glasses
Prof. S. Ramaswamy (IISc Bangalore, India)
Glass transitions, driven systems
Dr. Pieter Rein ten Wolde (AMOLF, Amsterdam).
Simulation of biochemical networks