Scheme: University Research Fellowship
Organisation: University of Nottingham
Dates: Oct 2012-Sep 2017
Summary: Concentrated suspensions are fluids containing lots of small particles (100nm - 10µm). These kinds of fluids are very common: toothpaste, mayonnaise, cement, blood, printer inks. Under the correct conditions flows of these particulate fluids may change from free flowing to jammed and solid. Understanding this process applies to, for example, inkjet printing or why blood vessels become blocked. The nature of jammed states are however poorly understood. Are there general principles that apply to all jammed systems as there would be for example when say a material melts or is jamming just a word to describe an array of different processes with little commonality.
Using model systems, where the interactions between particles are well understood we measure the response of suspensions to suddenly imposed flows. As these fluids jam they can fracture. Using vibrations under certain conditions we can suppress this jamming and thus keep a fluid flowing. Careful investigation enables us to distinguish different types of jammed state and understand why they behave differently when subjected to vibrations.
As particle fluids jam they try to release energy through various mechanisms such as crack formation. An important additional mechanism is known as ‘shear banding; in which the flow spontaneously separates into two regions which flow at very different speeds. During film formation (e.g paint drying) instabilities can lead to defects in the coating which are highly undesirable. Understanding the origin of shear banding has enabled us to describe the conditions under which such behaviour is expected.
We are also investigating granular fluids in which small particles move randomly due to vibrations imposed on the system. This enables us to track the motion of every particle in a way that is difficult in large systems of colloidal particles. This complementary approach is being developed to understand dynamics of particles in a concentrated fluid.