Tailoring flow behavior of colloidal dispersions with short-range repulsive interactions using depletion forces

• Applicant: Professor Dr. Norbert Willenbacher
• Subject Area: Mechanical Process Engineering
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 443694127

Weak attractive interactions introduced by non-adsorbing polymer added to the fluid phase, are a powerful tool to tune flow properties of colloidal dispersions in a wide range according to the demands during processing and application. Our preliminary results, however, show that there is no trivial relationship between attraction strength and macroscopic flow behavior. Detailed knowledge of the microstructure of such dispersions and corresponding rheological properties are the key requisite for a targeted product design. Fundamental knowledge about these phenomena is available for ideal hard sphere systems. Here we will focus on technically relevant systems stabilized by short-range repulsive interactions among particles. We want to understand how the range of repulsion controls the dispersions microstructure, and finally the complex relationship between phase behavior, structure and flow properties when additional attractive depletion forces are present. Our preliminary work demonstrated that multi particle tracking (MPT) microrheology is a unique tool to characterize microstructure and microscopic heterogeneities in such turbid, aqueous colloidal dispersions which are of utmost technical relevance. The dispersions phase composition in the fluid/crystalline co-existence regime can be determined including size and shape of the crystalline regions, even the shear modulus of these micro-crystals is accessible. Dynamic and static heterogeneities in highly concentrated arrested states can be analyzed regarding characteristic time and length scales. Various heterogeneous gel states with different local particle mobility and bulk flow behavior could be distinguished, so far not observed in true hard sphere systems. Preliminary investigations also revealed that compared to hard sphere systems much stronger attraction is needed to induce transitions from fluid to fluid/crystalline or gel states and dispersions can be fluidized via weak depletion forces at particle loadings far above the hard sphere glass transition. MPT experiments will be combined with classical steady and oscillatory shear rheometry to study microstructure, local particle mobility, and dispersion flow systematically. Furthermore, a microfluidic flow channel combined with PIV data analysis will be employed to study shear flow of highly concentrated systems in glass and gel states on a microscopic level. A recently suggested non-local rheological model describing the flow in confined geometries via cooperative motion of clusters will be evaluated based on heterogeneity length scale data from MPT. Combining these experimental approaches will correlate parameters of established rheological models and continuum mechanical constitutive equations to microstructure and colloidal dispersion properties. Thus we will be able to predicting the flow behavior of dispersions with short-range particle repulsion when additional weak depletion forces are present solely from sample composition.

• DFG Programme: Research Grants