This work discusses the flow of granular materials (e.g. sand). Even though a single particle is a simple object, the collective behavior of billions of particles can be very complex. In a... Show moreThis work discusses the flow of granular materials (e.g. sand). Even though a single particle is a simple object, the collective behavior of billions of particles can be very complex. In a surprisingly large amount of cases, it is not exactly known how a granular material behaves, and this while these kinds of materials are omnipresent in everyday life, industry, and nature Similar to materials such as water, which can occur as ice, liquid water and vapor, sand can also exist in different phases of matter. If you for instance walk on the beach, sand behaves like a solid, but if you pour it out of your shoes afterwards, it flows like a liquid. This thesis is dedicated to experiments where we investigate what happens when you try to "liquefy" sand by weakly vibrating it. We use an experimental setup which enables us to study how much stress is required to make sand flow, depending on the desired flow rate and the amount of vibrations. These experiments reveal several physical principles that turn out to be important towards understanding the unique behavior of these kinds of materials Show less
When soft, repulsive particles, like foam bubbles or emulsion droplets, are sheared, they show interesting scaling behaviour. We develop a simple scaling model that captures the rheological... Show moreWhen soft, repulsive particles, like foam bubbles or emulsion droplets, are sheared, they show interesting scaling behaviour. We develop a simple scaling model that captures the rheological behavior starting from three assumptions that explicitly depend on the microscopic interactions. This model starts from three ingredients: energy conservation, the concept of an effective steady state strain in our flowing system and a constitutive elasticity equation linking the effective strain to the shear stress. Our model allows for non-linear microscopic particle interactions and it predicts that the global rheological behaviour depends on the details of the microscopic interactions between the particles - in contrast to standard critical scaling theory. We test our model in computer simulations of soft, massless particles under steady shear and find that the numerics are broadly consistent with our model. jamming, rheology, foam, critical scaling Show less
This thesis presents two lines of research. On the one hand, we investigate heterogeneity in supercooled glycerol by means of rheometry, small-angle neutron scattering, and fluorescence imaging. We... Show moreThis thesis presents two lines of research. On the one hand, we investigate heterogeneity in supercooled glycerol by means of rheometry, small-angle neutron scattering, and fluorescence imaging. We find from the rheological experiments that supercooled glycerol can behave like weak solids at temperatures well above the glass transition. This is very surprising because glycerol is supposed to be purely liquid-like in this temperature range. However, the structural origin of this solid-like state of glycerol still remains unclear. The preliminary results from small-angle neutron scattering show that the solidified glycerol is structurally different from both the supercooled liquid and the crystal. In addition, fluorescence imaging of a thin film of glycerol doped with fluorescent probes reveals heterogeneous patterns of the fluorescence intensity, which is related to long-lived and micrometer scale density fluctuations in supercooled glycerol. All these results will contribute to understanding heterogeneity or even glass transition of supercooled liquids. On the other hand, we study the conformational dynamics of polyprolines by single-molecule FRET (F_rster resonance energy transfer) combined with temperature-cycle microscopy, a novel technique developed in our group, and demonstrate the potential of this new method to address complex molecular dynamics, for example the dynamics of protein-folding, at the single-molecule level. Show less
This thesis is about weakly driven granular flows and suspensions. Chapter 1 is an overview of the current knowledge of slow granular flows in so-called split-bottom geometries, which in essence... Show moreThis thesis is about weakly driven granular flows and suspensions. Chapter 1 is an overview of the current knowledge of slow granular flows in so-called split-bottom geometries, which in essence consist of a disk rotating at the bottom of a container. In chapter 2 we study dry granular flows in this split-bottom geometry, both in the frictional, slow, rate-independent regime, and in the liquid-like, rate dependent regime which is reached for faster flows. Chapters 3-5 deal with the flow of suspensions in the same geometry. We improve the so-called index matched scanning technique, that allows 3D imaging of the suspensions. Also for the suspension we study both the slow, rate independent and the faster, rate dependent regime. In all cases we combine 2D and 3D imaging of the flow with rheological measurements. Chapter 6 is devoted to the rheology of dry, weakly vibrated granular media. In chapter 7 we revisit a classic experiment on the compaction of granular media. Show less
We study the shear flow of two-dimensional foams, i.e., a monolayer of bubbles floating on a soapy solution. We successfully connect local and global flow behaviour
In the first part of this thesis, polyelectrolyte copolymer micelles are studied to elucidate the role of the micelle concentration on the intermicelle correlation and the extension of the... Show moreIn the first part of this thesis, polyelectrolyte copolymer micelles are studied to elucidate the role of the micelle concentration on the intermicelle correlation and the extension of the polyelectrolyte chains in the coronal layer. With increasing packing fraction the corona shrinks and/or interpenetrates in order to accommodate the micelles in the increasingly crowded volume. It is shown that interpenetration of the polyelectrolyte brushes controls the fluid rheology: the viscosity increases dramatically and the dynamic moduli show the formation of a physical gel. In the second part, we study giant vesicles as a superstructure self-assembled by dsDNA fragments, pUC18 or pEGFP-N1 and oppositely charged cationic diblock copolymer poly(butadiene-b-N-methyl 4-vinyl pyridinium. Under a preparation, the PBd brush collapses and a capsule is formed. The compaction of DNA is shown by the appearance of liquid crystalline textures under crossed polarizers and the increase in fluorescence intensity of labelled DNA. To form vesicles, the capsules are dispersed in aqueous medium supported by an osmotic agent. The integrity of the DNA after encapsulation and subsequent release was confirmed. We demonstrate ``reverse'' transfection of in vitro cultured HeLa cancer cells growing on plasmid-copolymer vesicles by the expressed green fluorescent protein in cultured cells. Show less
