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ICS-3 Vortrag: Dr. Priti Mohanty,
University of Lund

"Phase transitions in soft-repulsive colloids"

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20.Nov.2012 10:30
venue
Seminarraum A1-A3, Building 04.6

Soft colloids, where the interparticle distance, as can be smaller than the particle diameter s, and where the interaction potential shows a finite repulsion at or beyond contact, have recently attracted considerable interest from the experimental and theoretical soft matter community. It is due to this softness that particles can be packed to an effective volume fraction feff far above close packing fcp, with enormous consequences for the resulting structural and dynamical properties. In this talk, we will use cross-linked poly(N-isopropylacrylamide) (PNIPAM) based microgels as a model for soft colloids, where we actively control the interparticle interactions between the particles. We demontrate that we can vary it from short-range to long-range repulsive, and add an additional  dipolar component in order to explore the very rich and complex phase behaviour at high effective feff. The resulting structural ordering and phase behavior is probed by real space methods using confocal laser scanning microscopy.

In the first part of my talk, we will look at the density dependent phase behavior of neutral microgels interacting via a short-range Hertzian potential both below and above closed packing fcp [1]. We find clear evidence of anomalous structural features in the pair correlation function above the glass transition, which we interpret as being due to a competition between the entropy and potential energy of the overlapping soft particles.

 In the second part, we will move to ionic microgels, where the effective pair potential crosses over from a Yukawa-type at large distances to a Hertzian-type repulsive interaction at short distances [2]. Here, we also use an alternating electric field to induce an additional tunable dipolar contribution [3]. Novel field-induced structural transitions have been found at low as well as at very high effective volume fractions. At feff = 0.1 we observe a transition from an isotropic to a string fluid. At feff = 0.85, there is a reversible transition from an amorphous to a dipolar crystalline state, followed by the onset of a gas–(string) solid coexistence. At feff = 1.6 and 2.0, i.e. far above close packing, evidence for a field-induced arrested phase separation is found.

References:

 1)   Priti Mohanty, Divya Paloli, Jerome Crassous, and Peter Schurtenberger, in “Hydrogel Micro-and Nanoparticles”, L. Andrew Lyon and Michael J. Serpe, Eds., Wiley VCH, ISBN 978-3-527-33033-1, pp 369 - 396 (2012).

2)      Jonas Riest, Priti S. Mohanty, Peter Schurtenberger and Christos N. Likos, Zeitschrift für Physikalische Chemie,  2012, 226, 711.

3)      Priti S. Mohanty, Anand Yethiraj and Peter Schurtenberger, Soft Matter, 2012, 8, 10819.


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