8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.112Wed 711:10-14:00Escaping the squeeze: soft particles at high effectivevolume fractionsPriti Mohanty, 1 Jérôme Crassous, 2 Divya Paloli, 3 Kitty Gruijthuijsen, 2 MarcObiols-Rabasa, 3 Anna Stradner, 2 Urs Gasser, 4 Juan-Jose Lietor-Santos, 5 AlbertoFernandez-Nieves, 5 Emily Herman, 5 Andrew Lyon, 5 Emanuela Zaccarelli, 6 andPeter Schurtenberger 31 Lund University, Getingevägen 60, Box 124 221 00, Lund, Sweden2 University of Fribourg, Fribourg, Switzerland3 Lund University, Lund, Sweden4 Paul Scherrer Institut, Villigen, Switzerland5 Georgia Institute of Technology, Atlanta, United States of America6 Università di Roma La Sapienza, Roma, ItalyWe report on the influence of a soft repulsive potential in concentrated particle dispersions to studythe phase behaviour of concentrated suspensions of Poly (N-isopropylacrylamide) (PNIPAM) microgelparticles. PNIPAM particles are very interesting as they show a thermoresponsive size,where the particle swells at low and collapses at high temperatures. The size and thus the effectivevolume fraction of the particles can therefore be tuned through a variation of the temperature([1, 2]). We use PNIPAM microgels as convenient model systems ([2, 3, 4]) to investigate the glasstransition that occurs at volume fractions Φ ≈ 0.6. We also look at the influence of the intrinsicsoftness of these particles on dynamical arrest, and investigate the nature of the dense phases thatexist at ultra-high densities far beyond close packing. We use confocal laser scanning microscopy(CLSM) that allows us to track the particles in real time and determine quantities such the pair correlationfunction or the mean square displacement of the particles as a function of effective volumefraction. We combine these experiments with static (SLS) and dynamic (DLS) light scattering,small-angle neutron (SANS) and x-ray scattering (SAXS) and rheology to obtain a full characterizationof the structural and dynamic properties of these suspensions at all relevant length and timescales. We report in particular a special variant of SANS, experiments under so-called zero averagecontrast conditions ([5]), to extract the size and shape of the microgel particles at all densitieswhich allows us to completely decouple interactions and particle size and shape experimentally forthe first time.[1] H. Senff et al. J. Chem. Phys. 111, 1705 (1999).[2] M. Stieger et al. Langmuir 20, 7283 (2004).[3] Z. Zhang et al. Nature 230, 459 (2009).[4] F. Scheffold et al. PRL 104, 128304 (2010).[5] M. Willner et al. Macromolecules 27, 3821 (1994).112