Charge dispersity of size disperse colloidal spheres

• Applicant: Professor Dr. Thomas Palberg
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 513654570

Synthetic colloids inevitably show some spread in size and shape. Further their charge is non-uniform. Theory and experiment reveal a manifold and pronounced influence of both dispersities on the structure, phase behaviour and dynamics of colloidal suspensions aswell as on material properties like shelf life and rheology. Size and shape are practically fixed by synthesis. By contrast, charging is a complex process depending on the particle chemistry, on salt concentration, pH, particle concentration, and particle size. The thus adjusted bare surface charge is related in a known way to different experimentally accessible effective charges,. These in turn determine the system properties and behaviour. However, the relations between size and effective charge, between size dispersity and charge dispersity as well as their dependence on the experimental conditions are still poorely understood. While these are discussed in various theoretical approaches, the main obstacle appears to be a lack of a reliable access and characterization of effective charge dispersity. If available, it could be compared to the easily accessible size dispersity. This would reveal the connection between the two dispersites in much detail. It further will assist the discrimination of the influences and the relative importance of the two dispersities on suspension behaviour. Here we suggest to study on the charge dispersity of particles with well-characterized size dispersity under systematic variation of experimental boundary conditions. We willcombine an established light scattering technique (super-heterodyne dynamic light scattering, SH-DLS) with the precise adjustment of experimental parameters under conductometric control. SH-DLS records Doppler spectra corresponding to the velocity distribution of colloidal particles subjected to a homogeneous DC electric field. Thespectra show a diffusive line broadening but also a field-dependent broadening. The latter is directly related to the dispersity of mobilities and hence the charge dispersity. Other additional broadening mechanisms will be either avoided (e.g. taylor dispersion by working with a special cell design suppressing solvent flows) or independently characterized and accounted for in evaluation (like temporal velocity fluctuations at moderate interaction strength). We will study spheres of diferent average size, different size distribution widths, and different surface chemistry. Complementary experiments under systematical variation of experimental boundary conditions will provide access to the coupling between size and charge dispersity. The obtained datasets can then be compared to theoretical models. This shall deepen our understanding of chargedispersity and its relation to size dispersity. In the long run, our studymay assist the targeted control of system properties via size andcharge dispersity.

• DFG Programme: Research Grants