Dynamic arrest and non-equilibrium behaviour in suspensions of deformable colloids

The molecules of a liquid move easily and a snap-shot of the molecular arrangements reveal complete disorder. As a liquid is cooled, it often undergoes crystallization, where the molecules arrange into an ordered pattern. The most common example is water turning into ice. However, such crystallization can be avoided by cooling at a fast rate. The liquid molecules then move slower and slower upon cooling and if the cooling rate is high enough there is not enough time to rearrange into an ordered pattern before all long-range motions come to a halt; thus, the structure remains disordered and liquid-like but the material is hard - the resulting solid is called a glass.
Glassy materials are important in a wide range of man-made materials and applications including battery electrolytes and electrodes, solar cells, pharmaceuticals and most plastics. Neither the microscopic mechanisms involved in glass-formation nor the behaviour of the glassy state are well understood; this is remarkable since humans have been producing glass for thousands of years and glasses have been naturally formed by geological processes for millions of years. Thus, reaching an understanding of glasses and their formation is a key unsolved problem in both fundamental science and technology.
Remarkably, an analogy can be drawn between and the molecules (size: 0.1-10 nm) of molecular glass-formers and the behaviour of particles (size: 0.1-10 microns) suspended in a liquid, so called colloids. For colloids, glass-formation is controlled by the concentration of particles within a certain volume; for low particle concentrations the system is a liquid but as the concentration is increased the system gets crowded, which leads to the formation of a glass. Practical examples include paints, emulsions, lubricants and thickeners. The advantage of using colloids as a model system to study glass-formation is the large particle size, which means that the colloid motions can be studied using light as a probe, together with the great control of properties such as colloid size, elasticity and inter-particle interactions.
In this work we will use a versatile colloidal model system consisting of gel particles swollen in a solvent, so called microgels. In addition to their role as model systems, such microgel suspensions are important in applications including biosensing and medical diagnostics, chemical separation technologies, oil recovery, pharmaceutical delivery, and switchable materials.
We will synthesize microgel particles with varying mechanical properties, by controlling the cross-linking of the particle gels. Each microgel batch will be characterized with regards to particle size, gel structure and mechanical properties. We will then study how these microgel suspensions form glasses as the particles crowd the volume upon concentration. Both the arrangement and the motions of the microgel particles will be studied as the glassy state is approached, using light scattering and rheology techniques. Light scattering studies yield information about the individual microgel structure, the microgel particle arrangements and the microgel motions over a wide range of time-scales (10 ns-1000 s). With rheology, the response of the material to a mechanical disturbance is investigated.
Specific aims of the study are to (i) find the relationship between single microgel properties and the corresponding suspension arrangements and motions as the glassy state is approached (ii) determine which types of microgel motions are relevant to the glass formation process and how these motions are inter-related (iii) investigate how an applied shear affects and eventually 'melts' a microgel glass.
This work addresses questions that are key to an understanding of glassy materials in general. By systematic studies of an excellent model system, we aim to form a benchmark for future glass-transition work.

Disordered solids are common and important both in technological applications and in nature and include molecular glasses, colloidal suspensions, granular systems, emulsions, foams, pastes, polymer gels and biological tissues. Thus, disordered materials impact applications including drug delivery systems, amorphous pharmaceuticals, biosensors, industrial emulsions, paints, lubricants and thickeners and energy related materials such as polymer battery electrolytes, fuel and solar cells. Despite their importance, we generally neither understand their behaviour nor the microscopic mechanisms responsible for their formation. To resolve these questions would both lead to technological innovations and to improved and more environmentally friendly every-day industrial products. Moreover, the model system chosen to study dynamic arrest and non-equilibrium behaviour in disordered solids in this work, microgels, are key elements in medical, biosensing and diagnostics applications, in chemical separation technologies, oil recovery, pharmaceutical delivery systems, in switchable materials and in novel optical techniques. They are also regularly used to control the rheological properties of industrial products such as paints, motor oils, foods, cosmetics and inks. Thus, a better understanding of microgel suspensions alone would strongly benefit systems of key scientific and industrial interest within the UK.

Generally, the work outlined in this proposal will be of importance to a vast range of present and future applications and products both within industry and academia, both within the UK and globally. The research outcome could impact any area that use disordered materials and particularly microgels in materials or applications development and production and proper understanding of the materials would create new avenues for innovative materials design. We particularly stress the important role of disordered systems in the development of new materials for energy-related and medical applications, which are both key strategic areas for research and development in the UK. Moroever, the strategic importance of understanding the physics of non-equilibrium matter in general is highlighted by the EPSRC signposting on "Matter far from equilibrium". The importance of this project for the UK in general is further strengthened by the fact that it directly impacts 3 out of 4 of the recently identified EPSRC Physics Grand Challenges: "Emergence and Physics Far from Equilibrium", "Nanoscale Design of Functional Materials" and "Understanding the Physics of Life". It is also important to point out the close relation between the physics of non-equilibrium solids and non-equilibrium behaviour in large-scale systems such as traffic systems, animal populations, human societies or financial markets, all of which are areas of direct importance to the UK society.

The outcomes of the grant work will be directly fed into teaching in several undergraduate modules at the University of Leeds and used in University open day presentations, which will further spread the knowledge and understanding of disordered materials and their behaviour within the UK.