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Project

A High Frequency Rheometrical Study on Colloidal Suspensions

Colloidal suspensions are present in a wide range of natural and synthetic applications. Their colloidal nature leads to a complex interplay between different interaction forces and defines their structure. In order to understand and manipulate the behaviour of a suspension, knowledge on the relationship between structure and properties is hence crucial. Rheology is particularly useful for deriving such relationships and allows to investigate suspensions directly at relevant concentrations. Small amplitude oscillatory measurements are of particular interest: the potential of tuning the frequency of the oscillation makes it possible to vary both the time and length scale that is probed and allows to obtain an integrated fingerprint of the material. Specifically, this thesis focuses on the potential role of rheology in determining local scale parameters of the colloidal building blocks in a suspension. In order to obtain information on the local scale, the applied frequency must be larger than the characteristic time scale of the colloidal suspension, which is typically reached from frequencies larger than 1 kHz. 

The first major part of this work concerns the development of a homebuilt piezo shear rheometer. A sensitive mechanical alignment method is installed for a fast and accurate gap control and is combined with a careful signal analysis method for accurate measurements between 10 - 2000 Hz. The instrument directly extends the accessible frequency range of conventional rotational instruments into the low kHz region, and can hence be used to study the continuous transition of colloidal suspensions into the high-frequency regime. By applying a pure shear flow, material properties can be derived in a straightforward manner. The second part of this work focuses on stabilised suspensions, and more particularly on the interplay between the surface topology of colloids and the local stress contributions in a suspension. Model particles with different types of surface topologies are used, ranging from colloids with grafted polymer layers of varying layer thicknesses to raspberry-like particles with controllable surface roughness. At high frequencies, when the response probes contributions close to the particle surface, the results reveal an intricate interplay between hydrodynamic lubrication stresses, repulsive interactions and surface topology. The third part discusses the use of high-frequency rheology to assess the local structure in partially dispersed suspensions. The degree of dispersion of colloidal particles in a suspension is crucial for many applications, and is for instance an important parameter for the performance of nanomaterials. The high-frequency response is compared to predictions based on a hydrodynamic viscosity model for the derivation of a dispersion quality index. This index can for instance be used to follow the evolution of the dispersion quality in a dispersion process. 

The findings presented in this thesis illustrate that high-frequency rheology is a sensitive tool to probe local scale properties in colloidal suspensions. By focusing on specific stress contributions, interaction parameters or local structural properties can directly be derived. Such relationships are useful in two ways. On one hand, knowledge on the properties of the colloidal building blocks will lead to an improved understanding of the complex phenomena that colloidal dispersions display, and will pave the way for improved model predictions. On the other hand, rheology is a versatile technique that allows to study suspensions directly at relevant concentrations. The possibility for using it as a probe to quantify local structural properties is highly relevant for many practical applications. 

 

 

Date:2 Sep 2014 →  26 Sep 2018
Keywords:colloidal suspensions, rheology, rheometry
Disciplines:Condensed matter physics and nanophysics, Catalysis and reacting systems engineering, Chemical product design and formulation, General chemical and biochemical engineering, Process engineering, Separation and membrane technologies, Transport phenomena, Other (bio)chemical engineering, Polymeric materials
Project type:PhD project