IOP Talks : "Colloidal musical chairs"

Organisator Institute of Physics (IOP)

ma 14 april 2014 tot wo 16 april 2014 12:30

Locatie Homerton College, Cambridge, UK

Laboratory of Physical Chemistry and Colloid Science will join IOP Conference: The Physics of Soft and Biological Matter which takes place in Homerton College, Cambridge University in 14-16 April with two talks and two poster presentations.

Talks will be given by staff member Sprakel with the title "Colloidal musical chairs" and by post-doc researcher V.D.Nguyen with the title "Kinetic control over out-of-equilibrium self-assembled hydrogels". Abstracts can be found below.

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"Colloidal musical chairs"

We explore the mechanisms with which internal stresses are relaxed in fragile, low-density, colloidal crystals; using a combination of optical tweezer experiments and computer simulations. Unlike the typical dislocation mechanisms found when hard crystals are deformed, these soft and low-density solids exhibit a very rich collection of collective dynamics upon mechanical perturbation from within. The string- and loop-like collective rearrangements that result from the optically-enhanced vibrations of a single particle within the lattice, are mediated either by rotations in transiently created circular grain boundaries, or from the motion of dissociated vacancy-interstitial pairs. We show that these stress relaxation modes in ultrasoft crystals, which are only marginally stable against mechanical perturbations, are highly cooperative and long ranged; the active excitation of a single particle with an amplitude of only a single lattice spacing, can cause the irreversible displacement of several hundreds of particles in the crystal spanning several tens of lattice constants. These results shed new light on how activated and collective rearrangements can result from localised perturbations and enhance the stability of low-density solid states.

"Kinetic control over out-of-equilibrium self-assembled hydrogels"

Self-assembly under thermodynamic control gives a well-defined single state due to minimizing the Gibb’s free energy. In contrast, kinetic control over self-assembly processes allows us to access many different states of a system. However, our limited understanding of the kinetics of self-assembly process restricts our ability to design kinetically controlled self-assembly routes. Here we explore a new method to kinetically control multi-step self-assembly using dynamic combinatorial chemistry. The first step starts with a dynamic combinatorial library of macrocycles functionalised with different numbers of peptide side-groups. The macrocycles are held together by reversible disulfide covalent bonds. Then the peptides start to interact to assemble macrocycles into fibres, which  form a hydrogel. We tune the physical properties of the hydrogel using various methods such as adding cross-linker, varying salt concentration and controlling the  length of the fibres. We relate the gel properties to their structure by performing rheology measurement and direct visualization techniques including Cryo-TEM and AFM. Our results suggest that adding cross-linker and increasing the fibre length can enhance the gel strength notably. While by adding salt we can weaken the gel strength significantly. These results open up new possibilities in kinetic control of self-assembly process