Postgraduate research opportunities What makes particle packings stable? Explaining and engineering random loose packing

How particles pack together to make stable structures is a puzzle that has been addressed by many illustrious scientists and engineers, including Kepler, Coulomb and James Clerk Maxwell. Recent interest has grown from a need to design porous packings or materials for a range of critical applications, such as absorption and molecular storage for CO2 and H2, effective filtration eg in water treatment, understanding sediment formation and soil instabilities in natural phenomena and for climate resilience, improving particulate processes such as tableting in foods and pharmaceuticals, and designing innovative materials such as structured low-density solids for construction, aerospace, transport and packaging. Of particular interest is the case of small particles, where thermal energy fluctuations (Brownian motion) play an important role: this case includes packings made from chemical and physical flocculation/gelation, with applications such as manufacture of adsorbents and porous materials; and natural sedimentation processes involving microscopic particles such as fines, micropollutants and algae. A much studied starting point for particle packings is the simplified model of a packing of spherical, monosized, rigid particles. A well-defined limit in this case is so-called random close packing (RCP), the maximum solids volume fraction that can be achieved with randomly positioned spheres, found from models and experiments to be around 64% solids. Less clear is so-called random loose packing (RLP). Depending on conditions, it is found that packings can be stable (against further compression or sedimentation under gravity) even though much more dilute than RCP, experiments identifying a random loose packing limit around 55%. However the reasons for this limit and for the existence of multiple quasi-stable loose packing states between RCP and RLP are not understood. The aim of this PhD project is therefore to explain how structure and dynamics lead to so-called loose packing; and to build on this understanding to enhance structural design control over packings, opening up new routes to innovative materials. The engineering goal is to be able to obtain stable packings at any chosen particle content by altering the process conditions of formation. But such design is only possible if we understand the structural and dynamic reasons for the emergence of stability in this loose packing regime. How stability arises as a connected solid ‘network’ of particles forms is a critical question also in related areas such as the flow of high solids-content suspensions (eg pastes, blood, cement), particulate processing and transport, non-Newtonian rheological materials such as yield stress and shear-thickening suspensions, flow and deformation in geological systems, and even more generally, basic theories of jamming in traffic and pedestrian flow. Understanding what structural and dynamic effects are responsible for the onset of stable packing is thus a critical question with wide-ranging implications for science and engineering across many lengthscales. The goal of the project is to build this understanding using recent developments in modelling and analysis methods, whose advantage is that they are simple to implement and to draw insight from, while retaining features consistent with real experimental materials.

In addition to undertaking cutting edge research, students are also registered for the Postgraduate Certificate in Researcher Development (PGCert), which is a supplementary qualification that develops a student’s skills, networks and career prospects.

• Chemical and Process Engineering
• PhD Chemical Engineering

Further information

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