Soft colloids, e.g. polymer-coated silica particles, block copolymer micelles, star polymers etc., are hybrids between (soft) polymer chains and (hard) colloids. Due to this hybrid nature, solutions of soft colloids macroscopically show interesting phenomena resulting from its unique microscopic structure.
The combination of polymer-like properties, i.e. the formation of (transient) geometric constraints due to overlapping polymeric coronas and direct colloidal interactions due to the (hard) core in particular affects flow properties and non-equilibrium behaviour of soft colloids. Therefore soft colloids are frequently used in many technical applications (paints, shampoos, motor oils, polymer nano-composites etc.). The task of fundamental research on soft colloids is to investigate the basic principles of the so-called structure-property-relationship (SPR) that finally enables tailoring material properties for technical applications. However, for such comprehensive studies excellent model systems are prerequisite.
Structure and Interactions:
The macroscopic softness of soft colloids results from two microscopic contributions i. the deformability of the individual colloidal particle, its intramolecular softness, and ii. the form of particle interactions in terms of an effective potential (its intermolecular softness). Small Angle Neutron Scattering (SANS) combined with contrast variation is an excellent tool to investigate structure and interaction soft colloids on a microscopic level.
(Phys. Rev. Letters, 94, 195504, 2005, Phys. Rev. E, 76, 041503, 2007)
Flow Behaviour and Rheology:
The effect of external shear on the structure of soft colloids is investigated in situ by Small Angle Neutron Scattering, Rheo-SANS, within the framework of the Transregional Collaborative Research Centre SFB TR 6 “Physics of Colloidal Dispersions in External Fields“, subproject A2 ”Tunable Model Systems for Soft Colloids”.
Shear induced crystallisation followed by shear induced melting in concentrated solutions of soft colloids as observed by Rheo-SANS (J. Phys.: Condens. Matter 20 (2008) 404206).
Soft Colloid/Polymer Mixtures:
A microscopic understanding that allows the tailoring of macroscopic material properties is one of the great challenges for complex soft matter systems. Ideally, such an understanding is based on experiment and theory, both quantitatively calibrated against each other. We performed such a benchmark study by investigating mixtures of ultrasoft colloids and polymer chains using rheology, small angle neutron scattering and liquid state theory. We show that experimental data can be described by employing recently-developed effective interactions in which both components are modelled by a coarse-grained approach. Quantitative, parameter-free agreement between experiment and theory for pair correlations, phase behaviour and concentration dependence of the interaction length σs is achieved.
B. Lonetti et al., Phys. Rev. Lett.106 (2011) 228301.
Figure 1: Phase diagram: Symbols denote our experimental path (open symbols assuming shrinkage, open squares correspond to glassy samples) whereas lines (GL, HV, and GL, see text) are given by theory. Full lines correspond to size ratio \xi =0.3 (the actual experimental one), while the dashed lines provide a comparison for \xi =0.27.