Natural sediment flocs are highly porous particulate aggregates composed of biogenic and minerogenic materials. They can be an important component of suspended sediment load in rivers, estuaries and the marine environment and modelling floc dynamics and behaviour is very important for many aquatic industries, maintenance of waterways and conservation and management of aquatic water bodies. X-ray computed microtomography has recently been applied to quantify the complex three-dimensional (3D) geometry of natural sediment flocs. Here, X-ray images of 3 selected natural millimetre-sized flocs sampled from the Thames River have been digitalised and converted into geometries used in Stokesian Dynamics calculations of the hydrodynamic properties of the flocs, where each floc is represented as a rigid ensemble of spherical beads moving in the creeping flow regime. Our approach is a substantial step from previous attempts in which synthetic fractal structures were simulated. In addition to describe the complex dynamics of floc settling, we compute: (i) the hydrodynamic radius of the flocs; (ii) the floc mobility and resistance tensors; and (iii) the relation between sedimentation velocity and fractal dimension. The simulations show that the coupling of gravitational forces with lateral velocities, which we analysed by examining the cross-components of the mobility matrix, produces a helical motion of the flocs as they settle. We argue that this lateral motion may lead to an enhancement of floc–floc aggregation by differential sedimentation due to an increase in the effective collisional area. Furthermore, the simulations demonstrate significant differences in the dynamics of settling between the three flocs despite a similar gross shape. Our work exemplifies how high-resolution X-ray techniques can be coupled with accurate particle-resolved simulations to understand the settling dynamics of real (as opposed to synthetic) flocs collected from estuarine, coastal or waste-water environments.

The density of individual particles is commonly assessed experimentally by quantifying the settling velocity of a collection of particles transferred into a settling column and allowed to settle under the action of gravity. The individual settling velocities of the particles are recorded close to the bottom of the settling column, in a region where it is assumed that the particles have reached their Stokes terminal velocity after the particle cloud has broken up. In the present study we use numerical particle-based simulations in the Stokes regime to demonstrate that this fundamental assumption might not be fulfilled in practice. Even at low volume fraction of monodisperse spheres, a large deviation from the Stokes settling velocity was found. In the case of a collection of polydisperse spheres, a distinction could be made between particles belonging to a cloud, and particles trailing the cloud. It was found that the velocity of the largest trail particles is reasonably close to their Stokes settling velocity. However, the particles close to the core of the cloud can have velocities more than ten times their Stokes velocities, making the use of the single-particle Stokes velocity based on the core particle not suitable to extract the particle density without corrections. An expression based on the local volume fraction, the cloud radius and the particle settling velocity in the cloud is proposed to estimate the single-particle Stokes settling velocity, and therefrom the particle density.

2D materials are promising high-performance sheet-like nanomaterials with unique properties. Liquid-phase exfoliation (LPE) is a scalable and cost-effective process to produce 2D materials on large scales. However, the product of LPE is highly polydispersed. An efficient procedure to fractionate 2D materials is the liquid cascade centrifugation (LCC), which is currently done by trial and error. Moreover, 2D materials are easily deformed when processed in liquids because of their low bending rigidities. To exploit the unique properties of 2D materials, it is essential to control the sizes and morphologies of the nanosheets.
To provide insights for the rational design of the LCC procedure and the understanding of deformation of nanosheets in the shear flow, this thesis tackles two relevant fluid dynamics problems: (i) sedimentation of polydisperse suspensions, and (ii) buckling of flexible particles in the shear flow, both in the Stokes flow regime. The approaches adopted in this thesis are mainly numerical, including Stokesian dynamics and boundary integral method, which are efficient methods to simulate particle dynamics in Stokes flow. Moreover, collaborations with experimentalists have been established during this thesis. The code developed has been used to answer practical questions.
Overall, this thesis contributes to the understanding of particle dynamics in Stokes flow, including the settling of polydisperse suspensions and buckling of flexible sheets in the shear flow, utilizing the theories and numerical approaches of microhydrodynamics. Results of this thesis can be used to optimize the procedures of liquid processing of 2D nanomaterials and in other relevant applications.

Hyaluronic acid (HA), a linear polysaccharide composed of alternating β-1,3-glucuronic acid (GlcA) and β-1,4-N-acetylglucosamine (GlcNAc) disaccharide units, is widely utilized in food, pharmaceutical, and cosmetic industries. Conventional in vitro HA biosynthesis is hindered by the reliance on costly nucleotide sugar precursors (UDP-GlcA and UDP-GlcNAc) and inefficient multienzyme coordination. To address these challenges, this study established a cell-free enzymatic cascade system integrating HA de novo synthesis with nucleotide recycling through eight pathway enzymes. By leveraging nucleotide sugar salvage pathways, UDP-GlcA and UDP-GlcNAc were efficiently synthesized from inexpensive monosaccharides, thereby bypassing energy-intensive de novo routes. Soluble expression of Pasteurella multocida hyaluronan synthase (PmHAS) was achieved by truncating its membrane-associated domains to enable sequential glycosyl transferase activity in aqueous systems. A dual ATP/UTP regeneration strategy was further implemented to sustain nucleotide supply, eliminating costly downstream purification. Under optimized conditions, the system produced 1.28 g/L HA within 24 h, with a molecular weight range of 1.28 × 10⁴to 1.02 × 10⁶Da and a substrate conversion yield of 65.9%. This work not only provides an economical platform for scalable HA synthesis but also establishes a modular enzymatic blueprint for engineering complex biopolymers, demonstrating broad applicability in synthetic glycobiology.

Settling velocity statistics for dilute, non-Brownian homogeneous suspensions of polydisperse spheres having a log-normal size distribution are generated from Stokesian dynamics simulations, as a function of the total volume fraction and normalised width of the particle size distribution. Several hundred instantaneous configurations are averaged to obtain reliable statistics. The paper reports data for the average and fluctuating settling velocity of each particle class in a suspension that is widely polydisperse - previous work was limited to only two or three classes, and the average settling velocity of each particle class was in most cases not reported - and provides an assessment of the accuracy of the analytical models proposed by Batchelor, Richardson & Zaki, Davis & Gecol and Masliyah-Lockett-Bassoon in predicting the simulation data. A limited comparison with dynamic simulations in which the particle microstructure is allowed to evolve in time is also included.

Buckling induced by viscous flow changes the shape of sheetlike nanomaterial particles suspended in liquids. This instability at the particle scale affects collective behavior of suspension flows and has many technological and biological implications. Here, we investigated the effect of viscous hydrodynamic interactions on the morphology of flexible sheets. By analyzing a model experiment using thin sheets suspended in a shear cell, we found that a pair of sheets can bend for a shear rate ten times lower than the buckling threshold defined for a single sheet. This effect is caused by a lateral hydrodynamic force that arises from the disturbance flow field induced by the neighboring sheet. The lateral hydrodynamic force removes the buckling instability but massively enhances the bending deformation. For small separations between sheets, lubrication forces prevail and prevent deformation. Those two opposing effects result in a nonmonotonic relation between distances and shear rate for bending. Our study suggests that the morphology of sheetlike particles in suspensions is not purely a material property but also depends on particle concentration and microstructure.