ALBERT

Description: We review a powerful regularization method, known as CONTIN, for obtaining the size distribution of colloidal suspensions from dynamic light scattering data. We show that together with the so-called L-curve criterion for selecting the optimal regularization parameter, the method correctly describes the average size and size distribution of microgel suspensions independently characterized using small-angle neutron scattering. In contrast, we find that when using the default regularization process, where the regularizer is selected via the “probability to reject” method, the results are not as satisfactory.

Description: Author(s): B. Sierra-Martín, Y. Laporte, A. B. South, L. A. Lyon, and A. Fernández-Nieves We report measurements of the bulk modulus of individual poly( N -isopropylacrylamide) microgels along their swelling transition. The modulus is determined by measuring the volume deformation of the microgel as a function of osmotic pressure using dextran solutions. We find that the modulus softens th... [Phys. Rev. E 84, 011406] Published Thu Jul 28, 2011

Description: Author(s): E. Pairam, H. Le, and A. Fernández-Nieves We study the stability of toroidal droplets inside a yield stress material. Similar to toroidal droplets in a viscous liquid, the slenderness of the torus controls whether it breaks into spherical droplets or grows thicker towards its center to coalesce onto itself and form a single spherical drople... [Phys. Rev. E 90, 021002] Published Thu Aug 28, 2014

Description: We study the form factor of thermoresponsive microgels based on poly( N -isopropylacrylamide) at high generalized volume fractions, ζ, where the particles must shrink or interpenetrate to fit into the available space. Small-angle neutron scattering with contrast matching techniques is used to determine the particle form factor. We find that the particle size is constant up to a volume fraction roughly between random close packing and space filling. Beyond this point, the particle size decreases with increasing particle concentration; this decrease is found to occur with little interpenetration. Noteworthily, the suspensions remain liquid-like for ζ larger than 1, emphasizing the importance of particle softness in determining suspension behavior.

Description: Glass formation in colloidal suspensions has many of the hallmarks of glass formation in molecular materials. For hard-sphere colloids, which interact only as a result of excluded volume, phase behaviour is controlled by volume fraction, phi; an increase in phi drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When phi increases above phi* approximately 0.53, the viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the glass transition, phi(g) approximately 0.58, where particle crowding greatly restricts structural relaxation. The large particle size makes it possible to study both structure and dynamics with light scattering and imaging; colloidal suspensions have therefore provided considerable insight into the glass transition. However, hard-sphere colloidal suspensions do not exhibit the same diversity of behaviour as molecular glasses. This is highlighted by the wide variation in behaviour observed for the viscosity or structural relaxation time, tau(alpha), when the glassy state is approached in supercooled molecular liquids. This variation is characterized by the unifying concept of fragility, which has spurred the search for a 'universal' description of dynamic arrest in glass-forming liquids. For 'fragile' liquids, tau(alpha) is highly sensitive to changes in T, whereas non-fragile, or 'strong', liquids show a much lower T sensitivity. In contrast, hard-sphere colloidal suspensions are restricted to fragile behaviour, as determined by their phi dependence, ultimately limiting their utility in the study of the glass transition. Here we show that deformable colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility. Colloidal suspensions may thus provide new insight into glass formation in molecular systems.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Mattsson, Johan -- Wyss, Hans M -- Fernandez-Nieves, Alberto -- Miyazaki, Kunimasa -- Hu, Zhibing -- Reichman, David R -- Weitz, David A -- England -- Nature. 2009 Nov 5;462(7269):83-6. doi: 10.1038/nature08457.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Physics and Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. johanm@chalmers.se〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19890327" target="_blank"〉PubMed〈/a〉

Description: Author(s): A. Scotti, U. Gasser, E. S. Herman, Jun Han, A. Menzel, L. A. Lyon, and A. Fernandez-Nieves We investigate the phase behavior of suspensions of poly( N -isopropylacrylamide) (pNIPAM) microgels with either bimodal or polydisperse size distribution. We observe a shift of the fluid-crystal transition to higher concentrations depending on the polydispersity or the fraction of large particles in ... [Phys. Rev. E 96, 032609] Published Mon Sep 25, 2017

Description: Author(s): U. Gasser, J.-J. Lietor-Santos, A. Scotti, O. Bunk, A. Menzel, and A. Fernandez-Nieves We present a small-angle x-ray scattering study of crystals formed by temperature-sensitive, swollen microgel particles consisting of poly( N -isopropylacrylamide) copolymerized with acrylic acid and 5 mol % of a cross-linker. As for hard spheres, the random hexagonal close-packed structure is predomi... [Phys. Rev. E 88, 052308] Published Wed Nov 13, 2013

Description: Author(s): A. A. Fragkopoulos, A. Aizenman, and A. Fernández-Nieves We show that charged toroidal droplets can develop fingerlike structures as they expand due to Saffman-Taylor instabilities. While these are commonly observed in quasi-two-dimensional geometries when a fluid displaces another fluid of higher viscosity, we show that the toroidal confinement breaks th... [Phys. Rev. Lett. 118, 264501] Published Mon Jun 26, 2017

Description: Author(s): M. Pelaez-Fernandez, Anton Souslov, L. A. Lyon, P. M. Goldbart, and A. Fernandez-Nieves We study ionic microgel suspensions composed of swollen particles for various single-particle stiffnesses. We measure the osmotic pressure π of these suspensions and show that it is dominated by the contribution of free ions in solution. As this ionic osmotic pressure depends on the volume fraction ... [Phys. Rev. Lett. 114, 098303] Published Fri Mar 06, 2015