Hydrodynamic stability and turbulence in fibre suspension flows

QC 2012061

Transitional and turbulent fibre suspension flows

In this thesis the orientation of macro-sized fibres in turbulent flows is studied, as well as the effect of nano-sized fibrils on hydrodynamic stability. The focus lies on enabling processes for new materials where cellulose is the main constituent. When fibres (or any elongated particles) are added to a fluid, the complexity of the flow-problem increases. The fluid flow will influence the rotation of the fibres, and therefore also effect the overall fibre orientation. Exactly how the fibres rotate depends to a large extent on the mean velocity gradient in the flow. In addition, when fibres are added to a suspending fluid, the total stress in the suspension will increase, resulting in an increased apparent viscosity. The increase in stress is related to the direction of deformation in relation to the orientation of the particle, i.e. whether the deformation happens along the long or short axis of the fibre. The increase in stress, which in most cases is not constant neither in time nor space, will in turn influence the flow. This thesis starts off with the orientation and spatial distribution of fibres in the turbulent flow down an inclined plate. By varying fibre and flow parameters it is discovered that the main parameter controlling the orientation distribution is the aspect ratio of the fibres, with only minor influences from the other parameters. Moreover, the fibres are found to agglomerate into streamwise streaks. A new method to quantify this agglomeration is developed, taking care of the problems that arise due to the low concentration in the experiments. It is found that streakiness, i.e. the tendency to agglomerate in streaks, varies with Reynolds number. Going from fibre orientation to flow dynamics of fibre suspensions, the influence of cellulose nanofibrils (CNF) on laminar/turbulent transition is investigated in three different setups, namely plane channel flow, curved-rotating channel flow, and the flow in a flow focusing device. This last flow case is selected since it is can be used for assembly of CNF based materials. In the plane channel flow, the addition of CNF delays the transition more than predicted from measured viscosities while in the curved-rotating channel the opposite effect is discovered. This is qualitatively confirmed by linear stability analyses. Moreover, a transient growth analysis in the plane channel reveals an increase in streamwise wavenumber with increasing concentration of CNF. In the flow focusing device, i.e. at the intersection of three inlets and one outlet, the transition is found to mainly depend on the Reynolds number of the side flow. Recirculation zones forming downstream of two sharp corners are hypothesised to be the cause of the transition. With that in mind, the two corners are given a larger radius in an attempt to stabilise the flow. However, if anything, the flow seems to become unstable at a smaller Reynolds number, indicating that the separation bubble is not the sole cause of the transition. The choice of fluid in the core flow is found to have no effect on the stability, neither when using fluids with different viscosities nor when a non-Newtonian CNF dispersion was used. Thus, Newtonian model fluids can be used when studying the flow dynamics in this type of device. As a proof of concept, a flow focusing device is used to produce a continuous film from CNF. The fibrils are believed to be aligned due to the extensional flow created in the setup, resulting in a transparent film, with an estimated thickness of 1 um.QC 20141003</p

Interfacial Behavior of Recombinant Spider Silk Protein Parts Reveals Cues on the Silk Assembly Mechanism

The mechanism of silk assembly, and thus the cues for the extraordinary properties of silk, can be explored by studying the simplest protein parts needed for the formation of silk-like materials. The recombinant spider silk protein 4RepCT, consisting of four repeats of polyalanine and glycine-rich segments (4Rep) and a globular C-terminal domain (CT), has previously been shown to assemble into silk-like fibers at the liquid-air interface. Herein, we study the interfacial behavior of the two parts of 4RepCT, revealing new details on how each protein part is crucial for the silk assembly. Interfacial rheology and quartz crystal microbalance with dissipation show that 4Rep interacts readily at the interfaces. However, organized nanofibrillar structures are formed only when 4Rep is fused to CT. A strong interplay between the parts to direct the assembly is demonstrated. The presence of either a liquid-air or a liquid-solid interface had a surprisingly similar influence on the assembly.Peer reviewe

Silk Assembly against Hydrophobic Surfaces?Modeling and Imaging of Formation of Nanofibrils

A detailed insight about the molecular organization behind spider silk assembly is valuable for the decoding of the unique properties of silk. The recombinant partial spider silk protein 4RepCT contains four poly-alanine/glycine-rich repeats followed by an amphiphilic C-terminal domain and has shown the capacity to self-assemble into fibrils on hydrophobic surfaces. We herein use molecular dynamic simulations to address the structure of 4RepCT and its different parts on hydrophobic versus hydrophilic surfaces. When 4RepCT is placed in a wing arrangement model and periodically repeated on a hydrophobic surface, fi-sheet structures of the poly-alanine repeats are preserved, while the CT part is settled on top, presenting a fibril with a height of similar to 7 nm and a width of similar to 11 nm. Both atomic force microscopy and cryo-electron microscopy imaging support this model as a possible fibril formation on hydrophobic surfaces. These results contribute to the understanding of silk assembly and alignment mechanism onto hydrophobic surfaces

Recombinant Spider Silk Forms Tough and Elastic Nanomembranes that are Protein‐Permeable and Support Cell Attachment and Growth

Biologically compatible membranes are of high interest for several biological and medical applications. Tissue engineering, for example, would greatly benefit from ultrathin, yet easy‐to‐handle, biodegradable membranes that are permeable to proteins and support cell growth. In this work, nanomembranes are formed by self‐assembly of a recombinant spider silk protein into a nanofibrillar network at the interface of a standing aqueous solution. The membranes are cm‐sized, free‐standing, bioactive and as thin as 250 nm. Despite their nanoscale thickness, the membranes feature an ultimate engineering strain of over 220% and a toughness of 5.2 MPa. Moreover, they are permeable to human blood plasma proteins and promote cell adherence and proliferation. Human keratinocytes seeded on either side of the membrane form a confluent monolayer within three days. The significance of these results lays in the unique combination of nanoscale thickness, elasticity, toughness, biodegradability, protein permeability and support for cell growth, as this may enable new applications in tissue engineering including bi‐layered in vitro tissue models and support for clinical transplantation of coherent cell layers.QC 20200819</p

Recombinant Spider Silk Forms Tough and Elastic Nanomembranes that are Protein‐Permeable and Support Cell Attachment and Growth

Biologically compatible membranes are of high interest for several biological and medical applications. Tissue engineering, for example, would greatly benefit from ultrathin, yet easy‐to‐handle, biodegradable membranes that are permeable to proteins and support cell growth. In this work, nanomembranes are formed by self‐assembly of a recombinant spider silk protein into a nanofibrillar network at the interface of a standing aqueous solution. The membranes are cm‐sized, free‐standing, bioactive and as thin as 250 nm. Despite their nanoscale thickness, the membranes feature an ultimate engineering strain of over 220% and a toughness of 5.2 MPa. Moreover, they are permeable to human blood plasma proteins and promote cell adherence and proliferation. Human keratinocytes seeded on either side of the membrane form a confluent monolayer within three days. The significance of these results lays in the unique combination of nanoscale thickness, elasticity, toughness, biodegradability, protein permeability and support for cell growth, as this may enable new applications in tissue engineering including bi‐layered in vitro tissue models and support for clinical transplantation of coherent cell layers.QC 20200819</p

Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments

Cellulose nanofibrils can be obtained from trees and have considerable potential as a building block for biobased materials. In order to achieve good properties of these materials, the nanostructure must be controlled. Here we present a process combining hydrodynamic alignment with a dispersion–gel transition that produces homogeneous and smooth filaments from a low-concentration dispersion of cellulose nanofibrils in water. The preferential fibril orientation along the filament direction can be controlled by the process parameters. The specific ultimate strength is considerably higher than previously reported filaments made of cellulose nanofibrils. The strength is even in line with the strongest cellulose pulp fibres extracted from wood with the same degree of fibril alignment. Successful nanoscale alignment before gelation demands a proper separation of the timescales involved. Somewhat surprisingly, the device must not be too small if this is to be achieve