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Modelling and Simulation of Structure Formation in Multiphase Flows

Speaker: Omar Matar, Imperial College, London
Series: CBE Departmental Seminars
Location: Elgin Room (E-Quad A224)
Date/Time: Wednesday, November 14, 2018, 4:00 p.m. - 5:00 p.m.

Multiphase flows have been a constant source of fascination for centuries. The complex dynamics of interfaces are at the heart of so many industrial, biomedical, natural, and daily-life applications, and span decades of length and time scales. These include nano- and microfluidics, coating flow technology, intensive processing, pipeline transportation of oil-and-gas, reaction engineering, separations, and manufacturing. These flows exhibit complex dynamics and pattern formation, which are complicated further by the presence of particulate phases, complex rheology, heat and mass transfer, phase change, surfactants, applied fields, and turbulence. In this talk, we consider two rather distinct problems, which pose considerable challenges for modelling and simulation. 

We first examine the drying of complex fluids, such as polymer solutions or colloidal dispersions, whereby a liquid phase is transformed into a solid. Compared to the case of a pure drop of volatile liquid, the presence of a particulate phase leads to appreciable changes in the evaporation process, and gives rise to hydrodynamical and mechanical instabilities, often resulting in cracking. The onset of cracking during thin-film deposition, photolithography, and colloidal assembly can ultimately lead to failure of the fabricated product. Cracking also plays an important role in many other applications ranging from affordable medical diagnostics to high-resolution nano-patterning. We propose a hierarchy of models, which account for a range of effects including compaction, evaporation, and substrate interactions. We show that the interplay between these physical mechanisms leads to a variety of crack patterns, which are in good agreement with experimental observations.

We then consider a multiphase problem of central importance to the engineering of new materials for energy storage and membrane-separation applications, involving the design and creation of novel porous structures. A physical understanding of the relevant continuous manufacturing techniques is difficult to capture, particularly as micro-scale structure formation can be disrupted by the presence of macro-scale flows. Comprehensive modelling of such systems remains a challenge due to length scale disparity: any included transport models must be solved at the unit-operation scale, while simultaneously capturing structure formation at the level of the product material. We present results of simulations of spontaneous structure formation within flowing polymer blends. Detailed pattern formation is modelled via a coupling of the Navier–Stokes and multi-component Cahn–Hilliard equations. To cascade physical information across scales, we consider a moving frame formulation in which characteristic time-dependent shear fields are sampled from accompanying reactor-scale flow simulations. This approach enables a closed-loop design whereby macro-scale design parameters, including reactor geometry, can be tuned such that micro-scale properties are optimised for each target application.