Particulate and continuum mechanics of microgel pastes: effect and non-effect of compositional heterogeneity
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ORIGINAL CONTRIBUTION
Particulate and continuum mechanics of microgel pastes: effect and non-effect of compositional heterogeneity Fany Di Lorenzo & Sebastian Seiffert
Received: 2 May 2013 / Revised: 19 June 2013 / Accepted: 12 July 2013 / Published online: 11 August 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract Microgels are deformable colloids that can be packed by external compression; such packing transforms a suspension of loose microgels into a viscoelastic paste with mechanical properties controlled by the elasticity of the constituent particles. We aim to understand how the presence of microgel particles with different individual elastic moduli affects this interplay in heterogeneous microgel packings. We do this by preparing microgel pastes that contain both soft, loosely cross-linked and stiff, densely cross-linked microgel particles and probe their shear elasticity. We consider particle packing fractions that cover the range from particles at the onset of contact to particles that are strongly packed, deformed, and deswollen to investigate the transition from a particulate suspension to a macrogel-type system. These studies reveal that the elasticity of heterogeneous microgel suspensions at low packing is due to the response of the soft, easily deformable microgel particles alone, whereas at high packing both soft and stiff microgels linearly add to the paste elasticity. This fundamental difference is due to the fundamentally different origin of elasticity at different microgel packing; whereas the soft particle interaction potential dominates the suspension mechanics at low microgel packing, rubber-like elasticity that equally reflects both soft and stiff contributions governs the mechanics of the same samples at high microgel packing.
Keywords Elasticity . Heterogeneity . Mechanics . Microgel . Particulate . Paste F. Di Lorenzo : S. Seiffert (*) F-ISFM Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany e-mail: [email protected] S. Seiffert e-mail: [email protected] S. Seiffert Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany
Introduction Microgels are micrometer- to submicrometer-sized particles that consist of cross-linked and swollen polymer networks [1]. These colloids can be both deformed and compressed to a degree that is determined by their cross-linking density and polymer–solvent interaction [2, 3]. This circumstance allows microgels to be packed to effective packing fractions greater than the close-packing limit for hard spheres [3, 4]. In the dilute limit, microgel particles are undeformed and behave like hard spheres in a suspension [5]. At packing fractions higher than 0.5, microgels interact with one another by soft interaction potentials that become noticeable when the particles are close to contact, eventually forming a glassy “jammed” phase with solid-like mechanics [5]. At concentrations above close packing, microgel particles adapt th
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